The Fuel and Engine Bible - how engines work including 2-stroke, 4-stroke and wankel (rotary) engines, fuel, octane rating, power, bhp, gas types and grades, carburettors, fuel injection, tuning, tweaking, nitrous, turbos, superchargers, chipping, hybrids, how to keep your engine running at peak fitness and much more.
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Suck, squeeze, bang, blow.
Not a sexual maneuver, but rather the common description for how an internal combustion engine works. The basic way all internal combustion engines work is to suck in a mixture of fuel and air, compress it, ignite it either with a spark plug or by self-igntion (in the case of a diesel engine), allow the explosion of combusting gasses to force the piston back down and then expel the exhaust gas. The vertical movement of the piston is converted into rotary motion in the crank via connecting rods. The crank then goes out to the gearbox via a flywheel and clutch, and the gearbox sends the rotary motion to the wheels, driving the vehicle forwards.
The following diagram is for reference for the technical jargon that will pop out on the rest of this page. It shows an inline-4 engine with dual overhead cams.
Nikolaus Otto
If you want to be pedantic, the suck-squeeze-bang-blow cycle of a 4-stroke engine should be called the Otto Cycle, after its inventor Nikolaus Otto. The development of the internal combustion engine is quite interesting, and rather than add even more clutter to this page, enquiring minds can read about the history of the internal combustion engine here. The rest of us will carry on....
Engine layouts
Below are some illustrations of the most common types of cylinder layout you'll find in engines today. Singles are typically used in motorbikes, snowblowers, chainsaws etc. V-twins are also found in motorbikes. The triple is almost unique to Triumph motorbikes where they call it the Speed Triple, or the 675. Inline-fours are the mainstay of car engines, as well as being found in some motorbikes too such as the BMW K1200S. Inline fives used to be used a lot in Audis but have found a new home in current Volvos. The V5 is something you'll find in some VWs. The V6 has the benefits of being smoother than an inline-four but without the fuel economy issues of a V8. Boxer engines are found in BMW motorbikes (twins) and Porsches and Subarus (fours and sixes). You had no idea, did you?
The difference between 4-stroke and 2-stroke engines
First, some basic concepts. Well one basic concept really - the most common types of internal combustion engine and how they work. It's worth reading this bit first otherwise the whole section on octane later in the page will seem a bit odd. Almost every car sold today has a 4-stroke engine. So do a lot of motorbikes, lawnmowers, snowblowers and other mechanical equipment. But there are still a lot of 2-stroke engines about in smaller motorbikes, smaller lawnmowers, leaf-blowers, snowblowers and such.
The difference between the two engine types is the number of times the piston moves up and down in the cylinder for a single combustion cycle. A combustion cycle is the entire process of sucking fuel and air into the piston, igniting it and expelling the exhaust.
2-stroke engines
A 2-stroke engine is different from a 4-stroke engine in two basic ways. First, the combustion cycle is completed within a single piston stroke as oppose to two piston strokes, and second, the lubricating oil for the engine is mixed in with the petrol or fuel. In some cases, such as lawnmowers, you are expected to pre-mix the oil and petrol yourself in a container, then pour it into the fuel tank. In other cases, such as small motorbikes, the bike has a secondary oil tank that you fill with 2-stroke oil and then the engine has a small pump which mixes the oil and petrol together for you.
The simplicity of a 2-stroke engine lies in the reed valve and the design of the piston itself. The picture on the right shows a 4-stroke piston (left) and a 2-stroke piston (right). The 2-stroke piston is generally taller than the 4-stroke version, and it has two slots cut into one side of it. These slots, combined with the reed valve, are what make a 2-stroke engine work the way it does. The following animation shows a 2-stroke combustion cycle. As the piston (red) reaches the top of its stroke, the spark plug ignites the fuel-air-oil mixture. The piston begins to retreat. As it does, the slots cut into the piston on the right begin to align with the bypass port in the cylinder wall (the green oblong on the right). The receding piston pressurises the crank case which forces the reed or flapper valve (purple in this animation) to close, and at the same time forces the fuel-air-oil mixture already in the crankcase out through the piston slots and into the bypass port. This effectively routes the mixture up the side of the cylinder and squirts it into the combustion chamber above the piston, forcing the exhaust gas to expel through the green exhaust port on the left. Once the piston begins to advance again, it generates a vacuum in the crank case. The reed or flapper valve is sucked open and a fresh charge of fuel-air-oil mix is sucked into the crank case. When the piston reaches the top of its travel, the spark plug ignites the mixture and the cycle begins again.
For the same cylinder capacity, 2-stroke engines are typically more powerful than 4-stroke versions. The downside is the pollutants in the exhaust; because oil is mixed with the petrol, every 2-stroke engine expels burned oil with the exhaust. 2-stroke oils are typically designed to burn cleaner than their 4-stroke counterparts, but nevertheless, the 2-stroke engine can be a smoky beast. If, like me, you grew up somewhere in Europe where scooters were all the rage for teenagers, then the mere smell of 2-stroke exhaust can bring back fond memories. The other disadvantage of 2-stroke engines is that they are noisy compared to 4-stroke engines. Typically the noise is described as "buzzy".
4-stroke engines
4-stroke engines are typically much larger capacity than 2-stroke ones, and have a lot more complexity to them. Rather than relying on the simple mechanical concept of reed valves, 4-stroke engines typically have valves at the top of the combustion chamber. The simplest type has one intake and one exhaust valve. More complex engines have two of one and one of the other, or two of each. So when you see "16v" on the badge on the back of a car, it means it's a 4-cylinder engine with 4 valves per cylinder - two intake and two exhaust - thus 16 valves, or "16v". The valves are opened and closed by a rotating camshaft at the top of the engine. The camshaft is driven by either gears directly from the crank, or more commonly by a timing belt.
The following animation shows a 4-stroke combustion cycle. As the piston (red) retreats on the first stroke, the intake valve (left green valve) is opened and the fuel-air mixture is sucked into the combustion chamber. The valve closes as the piston bottoms out. As the piston begins to advance, it compresses the fuel-air mix. As it reaches the top of it's stroke, the spark plug ignites the fuel-air mix and it burns. The expanding gasses force the piston back down on its second stroke. At the bottom of this stroke, the exhaust valve (right green valve) opens, and as the piston advances for a second time, it forces the spent gasses out of the exhaust port. As the piston begins to retreat again, the cycle starts over, sucking a fresh charge of fuel-air mix into the combustion chamber.
Because of the nature of 4-stroke engines, you won't often find a single-cylinder 4-stroke engine. They do exist in some off-road motorbikes but they have such a thump-thump-thump motion to them that they require some large balancing shafts or counterweights on the crank to try to make the ride smoother. They also take a little longer to start from cold because you need to crank the single piston at least twice before a combustion cycle can start. Any more than one piston and the engine gets a lot smoother, starts better, and is nowhere near as thumpy. That's one of the advantages of V-6 and V-8 engines. Apart from the increased capacity, more cylinders typically means a smoother engine because it will be more in balance.
Geek trivia: Mercedes-Benz needed to increase the performance of their diesel passenger cars back in the 70's as their market share in the US was increasing. As professionals with big V-8 luxury cars were trading them in for 2.4l diesels, the demand for performance had to be addressed. Mercedes did not want to retool their 114/115 series chassis and there wasn't enough room in the engine bay for a six cylinder diesel. There was, however, room for a straight-5. Benz engineers just hung another cylinder on the back of the 4 cyl block and presto! The five cylinder engine was born. This engine acquired a lot of status among the high line car owners. When Audi introduced the C2 series cars (the 5000 in America, the 100 in Europe) in 1976, they offered a 5-cylinder petrol engine too. It was basically a 1.8 litre 4-cylinder engine with an extra cylinder. That took it up to 2.0 litres but the fifth piston made such an enormous difference to the smoothness of the engine that it was often mistaken for a V6 or V8. Why only 5 cylinders instead of going for a V6? Partly for the same rationale as Mercedes (and it was a really tight fit) but primarily because Benz had made the straight-5 configuration fashionable. A straight-5 was also more fuel-efficient than a V6. It's also worth pointing out that nowadays, both Audi and VW have V5 engines with three cylinders in one bank and two in the other. Same smoothness, better gas-mileage.
4-stroke Diesel Engines
Mechanically, 4-stroke diesel engines work identically to four-stroke petrol engines in terms of piston movement and crank rotation. (To be historically accurate, petrol engines are mechanically similar to diesel engines - diesel engines came first) It's in the combustion cycle where the differences come through. First, during the intake cycle, the engine only sucks air into the combustion chamber through the intake valve - not a fuel/air mix. Second, there is no spark plug. Diesel engines work on self-ignition, or detonation - the one thing you don't want in a petrol engine (see the section on Octane later). At the top of the compression stroke, the air is highly compressed (over 500psi), and very hot (around 700 °C - 1292°F). The fuel is injected directly into that environment and because of the heat and pressure, it spontaneously combusts (this system is known as direct-injection). This gives the characteristic knocking sound that diesel engines make, and is also why pre-igniting petrol engines are sometimes refered to as 'dieseling'.
Petrol engines typically run compression rations around 10:1, with lower end engines down as low as 8:1 and sportier engines up near 12:1. Diesel engines on the other hand typically run around 14:1 compression ratio and can go up as high as 25:1. Combined with the higher energy content of diesel fuel (around 147,000 BTU per gallon versus 125,000 BTU for a gallon of petrol), this means that the typical diesel engine is also a lot more efficient than your common or garden petrol engine, hence the much higher gas-mileage ratings.
Because of the design of the diesel engine, the injector is the most critical part and has been subjected to literally hundreds of variations in both design and position. It has to be able to withstand massive pressures and temperatures, yet still deliver the fuel in a fine mist. One other component that some diesel engines have is a glowplug. From cold, some lower-tech engines can't retard the ignition enough, or get the air temperature high enough on startup for the spontaneous combustion to happen. In those engines, the glowplug is literally a hot wire in the top of the cylinder designed to increase the temperature of the compressed air to the point where the fuel will combust. These engines typically have a pictograph on the dashboard that looks like a lightbulb. When starting the engine cold, you need to wait for that light to go out - basically you're waiting for the glowplugs to get up to temperature. In really old diesel designs, this could be as long as 10 seconds. Nowadays it's nearly instantaneous, or in the case of advanced ECM systems, not needed at all.
2-stroke Diesel Engines
Would you believe there is such a thing as a 2-stroke diesel engine? The two-stroke cycle described above turns out to be highly beneficial for the diesel model, the major difference being the inclusion of exhaust valves at the top of the cylinder. The burn cycle works similarly too. At the top of the piston travel, the air is hot and compressed, just like in a 4-stroke diesel. And like the 4-stroke, the injector sprays fuel in at that point and it self-combusts. As the gasses expand, the piston is forced downwards and towards the bottom of its stroke, the exhaust valves on the top of the cylinder open. Because the gas is still expanding at this point, the combustion chamber empties itself through the open valves. At the very bottom of the power stroke, the piston uncovers the air intake and pressurised air fills the combustion chamber forcing the last remnants of the exhaust gas out. As the piston begins its compression stroke, the exhaust valves close and the air is compressed and voila - a two-stroke diesel engine. The other difference between a 4-stroke and 2-stroke diesel engine is that the 2-stroke variety must have a turbocharger or supercharger; you'll notice I mentioned the air intake fills the cylinder with pressurised air. That doesn't happen by magic.
As with 2-stroke petrol engines, every downward piston stroke is a power stroke, meaning the 2-stroke engine has the potential to product twice as much power as its 4-stroke sibling. Typically you'll find 2-stroke diesels in maritime engines (like those on freighters, tankers and cruise ships) and diesel-electric trains where more power is needed for the same size of engine.
Interference vs. non-interference engines
It's worth mentioning the two sub-types of 4-stroke engine at this point. Because the valves always open inwards, into the combustion chamber, they take up some space at the top of the chamber. In an interference engine, the position of the piston at the top of its stroke will occupy the same physical space that the open valves do whilst the piston is at the bottom of its stroke. It's important to know if your engine is an interference engine because if the timing belt breaks, at least one set of valves will stop in the open position and the momentum of the engine will ram the piston in that cylinder up into the valves requiring a very expensive engine repair or replacement. In a non-interference engine, the valves do not occupy any space that the piston could move into, so if your timing belt snaps on one of these engines, in 99% of cases you won't suffer any valve damage because the piston cannot physically touch the open valves. That is the technical explanation of why its important to get your timing belt changed at the manufacturer-specified mileage.
The following picture shows the difference between the two types. On the left, circled in red is where the open valve interferes with the position of the piston at the top of its travel. On the right, a non-interference engine shows there is still a gap at the same point (exaggerated for my picture).
Top Dead Centre (TDC) and ignition timing
When a piston in an engine reaches the top of its travel, that point is known as Top Dead Centre or TDC. This is important to know because I don't think any engine actually fires the spark plug with the pistons at TDC. More often than not, they fire slightly before TDC. So how does your ignition system work, and what is ignition timing all about?
Well generating the spark is the easy part. The electrical system in your car supplies voltage to your coil and ignition unit. The engine will have a trigger for each cylinder, be it a mechanical trigger (points), electronic module or crank trigger. Whatever it is, at that point, the engine effectively sends a signal to the coil to discharge into the high voltage system. That charge travels into the distributor cap and is routed to the relevant spark plug where it is turned into a spark. The key to this, though, is the timing of the spark in relation to the position of the piston in the cylinder. Hence ignition timing. Having the spark ignite the fuel-air mixture too soon is basically the same as detonation and is bad for all the mechanical components of your engine. Having the spark come along too late will cause it to try to ignite the fuel-air mixture after the piston has already started to recede down the cylinder, which is inefficient and loses power.
Timing the spark nowadays is usually done with the engine management system. It measures airflow, ambient temperature, takes input from knock sensors and literally dozens of sensors all over the engine. It then has an ignition timing map built into its memory and it cross references the input from all the sensors to determine the precise time that it should fire the spark plug, based on the ignition timing map. At 3000rpm, in a 4 cylinder engine, it does this about 100 times a second. In older systems, the spark timing was done using simple mechanical systems which had nowhere near the ability to compensate for the all the variables involved in a running combustion engine.
Typically as an engine revs quicker, the ignition timing needs to advance because the spark needs to get to the cylinder more quickly due to the engine running faster. In modern systems, this is all taken account of in the ignition timing map. On older mechanical system, they used mechanical or vacuum advance systems, so that the more vacuum generated in the intake manifold (due to the engine running quicker), the more advanced the timing became.
Checking ignition timing
Despite the speed that an engine turns, it is possible for mere mortals like you and me to be able to check the ignition timing or an engine using (and you'd have never guessed this) an ignition timing light. Timing lights are typically strobe lights. They work by being connected to the battery directly and then having an induction coil clamped around one of the spark plug leads - normally the first or last cylinder in the engine depending on the manufacturer. When the engine fires the spark plug for that cylinder, the inductive loop detects the current in the wire and flashes the strobe light once. So if the engine is ticking over at 1100rpm, the strobe will flash 550 times a minute (four stroke engine, remember?). Fantastic. So you're now holding a portable rave lighting rig but how does this help you see the timing of an engine? Well it's simple. You must have seen strobe lights working somewhere - a rave, a stage show - they're used to effectively freeze the position of something in time and space by illuminating it only at a certain point and for a fraction of a second. Shooting a strobe at someone walking in a dark room will result in you seeing them walk as if they were a flip-book animation on a reel of film. This effect is what's used to visualise the timing of your engine. Somewhere on the front of the engine there will be a notch near one of the timing belt pulleys and stamped into the metal next to it will be timing marks in degrees. On the pulley itself there will be a bump, recess or white-painted blob. When you point the strobe light down towards the timing belt pulley, remember it fires once for each firing of the cylinders? Each time it fires, the white blob on the pulley should be at the same position in its rotation - the strobe fires once for each ignition spark at which point the mark should be in the same place, and the effect to you is that the whole pulley, timing mark and all, are now standing still in the strobe light. The mark on the pulley will line up with one of the degree marks stamped on the engine, so for example if the white dot always aligns with the 10° mark, it means your engine is firing at 10 degrees before TDC. When you rev the engine, the timing will change so the mark will move closer or further away from the TDC mark depending on how fast the engine is spinning.
Note that in some engines, the two marks are simply painted or stamped, and there are no degree markings. In this case, the marks align when the first piston is exactly at TDC.
Check the timing marks first
After all that, it's worth pointing out that crank timing marks can be way off so it's worth confirming that your TDC marker is actually TDC before pratting about with the timing. It's not as bad now as it used to be, but in the bad old days, Rover V8's were particularly bad for this, with some being as much as 12° off! So how you do confirm your TDC really is TDC? Small cameras, a good set of feeler gauges, some cash and someone who knows what they're doing.
Timing marks on cam belt pulleys
The same timing marks exist stamped into the metal near, and on the pulley on the end the cam. Essentially these marks are used to line up the cam to the correct position when you're changing the timing belt. You have to make sure the engine is rotated to TDC and that the cams are properly aligned too. If you don't, the cams will spin permanently out-of-synch with the engine crank and the engine will run badly, if at all.
Spark plugs
And engine without a spark plug is useless, unless it's a diesel engine in which case it uses a glowplug instead. But we're talking about regular petrol engines here so the next topic to get to grips with is the spark plug. It does exactly what it says on the tin - it's a plug that generates a spark. Duh. So why spend time talking about it? Well with apologies to George Orwell not all spark plugs are created equal. Some are more equal than others. They'll all do the job but the more you pay, the better the plug. All spark plugs share the same basic design and construction though.
The high voltage from your vehicle's high-tension electrical system is fed into the terminal at the top of the spark plug. It travels down through the core of the plug (normally via some noise-suppression components to prevent electrical noise) and arrives at the centre electrode at the bottom where it jumps to the ground electrode creating a spark. The crush washer is designed to be crushed by tightening the spark plug down when it's screwed into the cylinder head, and as such, it helps keep the screw threads under tension to stop the spark plug from shaking loose or backing out. The insulator basically keeps the high-tension charge away from the cylinder head so that the spark plug doesn't ground before it gets a chance to generate the spark.
The type of plug I've illustrated here is known as a projected nose type plug, because the tip extends below the bottom of the spark plug itself. The other main type of spark plug has the centre electrode recessed into the plug itself and merely grounds to the collar at the bottom. The advantage of the projected nose type is that the spark is better exposed to the fuel-air mixture.
Ground electrode (ground strap) types.There are plenty of different types of grounding electrodes kicking around in spark plug designs nowadays, from 'Y' shaped electrodes (like SplitFire plugs) to grooved electrodes like you'll find on Champion plugs all the way up to triple-electrode plugs like the high-end Bosch items. They're all designed to try to get a better spark, and to that end, you'll now find all sorts of exotic materials turning up too. Titanium plugs, for example, have better electrical conductivity than brass and steel plugs, and the theory is that they'll generate a stronger, more reliable spark.
Gapping a spark plug. Gapping a spark plug is the process of ensuring the gap between the two electrodes is correct for the type of engine the plug is going to be used in. Too large a gap and the spark will be weak. Too small and the spark might jump across the gap too early. Generally speaking, the factory-set spark plug gap is just fine, but if you're running an older engine, or a highly tuned engine, then you need to pay attention to the gap. Feeler gauges are used to measure the gap, and a gapping tool is used to bend the outer electrode so that the gap is correct.
Heat ranges. Something that is often overlooked in spark plugs is their heat rating or heat range. The term "heat range" refers to the relative temperature of the tip of the spark plug when its working. The hot and cold classifications often cause confusion because a 'hot' spark plug is normally used in a 'cold' (low horsepower) engine and vice versa. The term actually refers to the thermal characteristics of the plug itself, specifically its ability to dissipate heat into the cooling system. A cold plug can get rid of heat very quickly and should be used in engines that run hot and lean. A hot plug takes longer to cool down and should be used in lower compression engines where heat needs to be retained to prevent combustion byproduct buildup.
How does the fuel-air mix happen? Magic?
You keep seeing me talk about fuel-air mix or fuel-air charge on this page, but I thought it wise to explain how this happens because it is pretty fundamental to the operation of internal combustion engines.
The fuel and air are mixed in one of two main ways. The old-school method is to use a carburettor, whilst the new-tech approach is to use fuel injectors. The basic purpose is the same though, and that is to mix the fuel and air together in proportions that keep the engine running. Too little fuel and the engine runs 'lean' which makes it run hot. Too much fuel and it runs 'rich' which conversely makes the engine run cooler. Running rich can also result in fouled up spark plugs, flooded engines and stalling, not to mention wasting fuel. Finding the right balance normally involves about 10 milligrams of petrol for each combustion stroke.
Carburettors
Advantages : analogue and very predictable fuelling behaviour, simple and inexpensive to build and maintain.
Disadvantages : carburettor icing in the venturi, imprecise fuel metering, float chambers don't work well if they're not the right way up.
How they work.
A carburettor is basically a shaped tube. The shape of the tube is designed to swirl the incoming air and generate a vacuum in a section called the venturi pipe (or just the venturi). In the side of the venturi is a fuel jet which is basically a tiny hole connected to the float chamber via a pipe. It's normally made of brass and has a miniscule hole in the end of it which determines the flow of fuel through it. In more complex carburettors, this is an adjustable needle valve where a screw on the outside of the carburettor can screw a needle in and out of the valve to give some tuning control over the fuel flow. The fuel is pulled through the jet by the vacuum created in the venturi. At the bottom of the tube is a throttle plate or throttle butterfly which is basically a flat circular plate that pivots along its centreline. It is connected mechanically to the accelerator pedal or twist-grip throttle via the throttle cable. The more you push on the accelerator or twist open the throttle, the more the throttle butterfly opens. This allows more air in which creates more vacuum, which draws more fuel through the fuel jet and gives a larger fuel-air charge to the cylinder, resulting in acceleration.
When the throttle is closed, the throttle butterfly in the carburettor is also closed. This means the engine is trying to suck fuel-air mix and generating a vacuum behind the butterfly valve so the regular fuel jet won't work. To allow the engine to idle without shutting off completely, a second fuel jet known as the idle valve is screwed into the venturi downwind of the throttle butterfly. This allows just enough fuel to get into the cylinders to keep the engine ticking over.
Float and diaphragm chambers.
To make sure a carburettor has a good, constant supply of fuel to be sucked through the fuel jets, it has a float chamber or float bowl. This is a reservoir of petrol that is constantly topped up from the fuel tank. Petrol goes through an inline filter and a strainer to make sure it's clean of contaminants and is then deposited into the float chamber. A sealed plastic box is pivotted at one end and floats on top of the fuel. Believe it or not, this is called the float. A simple lever connects to the float and controls a valve on the fuel intake line. As the fuel drops in the float chamber, the float drops with it which opens the valve and allows more fuel in. As the level goes up, the float goes up and the valve is restricted. This means that the level in the float chamber is kept constant no matter how much fuel the carburettor is demanding through the fuel jets. The quicker the level tries to drop, the more the intake valve is opened and the more petrol comes in to keep the fuel level up. This is why carburettors don't work too well when they're tipped over - the float chamber leaks or empties out resulting in a fuel spill - something you don't get with injectors. To combat this, another type of chamber is used where carburettors can't be guaranteed to be upright (like in chainsaws). These use diaphragm chambers instead. The principle is more or less the same though. The chamber is full of fuel and has a rubber diaphragm across the top of it with the other side exposed to ambient air pressure. As the fuel level drops in the chamber, the outside air pressure forces the diaphragm down. Because it's connected to an intake valve in the same way that the float is in a float chamber, as the diaphragm is sucked inwards, it opens the intake valve and more fuel is let in to replenish the chamber. Diaphragm chambers are normally spill-proof.
Carb icing.
One of the problems with the spinning, compressing, vacuum-generating properties of the venturi is that it cools the air in the process. Whilst this is good for the engine (colder air is denser and burns better in a fuel-air mix), in humid environments, especially cool, humid environments, it can result in carburettor icing. When this happens, water vapour in the air freezes as it cools and sticks to the inside of the venturi. This can result in the opening becoming restricted or cut off completely. When carbs ice up, engines stop. In aircraft engines, there is a control in the cockpit called "carb heat" which either uses electrical heating elements to heat up the venturi to prevent icing, or reroutes hot air from around the exhausts back into the carburettor intakes. In cars, we don't have "carb heat" but instead there's normally a heat shield over the exhaust manifold connected via a pipe to a temperature-controlled valve at the air filter. When its cold, the valve is open and the air filter draws warm air from over the exhaust manifold and feeds it into the carburettor. As the temperature warms up, the valve closes and the carburettor gets cooler air because the risk of icing has reduced. The symptoms of carb icing are pretty easy to diagnose. First, your engine bogs down at high throttle then it loses power and ultimately could stall completely. You'll stop on the side of the road and wait a couple of minutes, then the engine will start and run normally. This is because with the engine off, the heat from the engine starts to warm up the carbs and melts the ice so that when you try to start it up again, everything is fine.
Complexity for the sake of it.
As car engines evolved, carburettors had to evolve to cope with the various demands. It's not unusual to find five-circuit carburettors which have become so complex that they're a nightmare to design, build and maintain. That flies in the face of one of the carburettor's advantages, which used to be that they were simple. Why five circuits? The main circuit is the one which provides day-to-day running capability. It's augmented by accelerator and load (or enrichment) circuits which can vary the fuelling to accomodate sudden acceleration or the need for more power (like driving uphill). The accelerator circuit also adds a second butterfly valve in most cases which only opens at 70% throttle or more. Then there's the choke circuit designed to provide extra fuel with the throttles closed when the engine is cold, allowing it to start, and finally the idle circuit which does the same thing but when the engine is warm, to keep it going. On top of all of this, with the introduction of stricter emissions requirements came catalytic converters, and these expensive boxes of rare metals just don't work well unless the fuel-air ratio is very carefully controlled. And that's something carburettors just couldn't keep up with. Small wonder then that this mechanical tomfoolery gave way to fuel injection......
Fuel injection
Advantages : precise and variable fuel metering, better fuel efficiency and better emissions.
Disadvantages : Fairly complex engineering that isn't very user-friendly. Binary on/off functionality at low throttles, which is especially noticable on motorbikes where the throttle becomes 'snatchy' and it becomes hard to ride smoothly at low speed.
How it works.
Compared to carburettors, fuel injectors themselves are incredibly simple. They are basically electro-mechanically operated needle valves. The image on the right shows a cutaway of a representative fuel injector. When a current is passed through the injector electromagnetic coil, the valve opens and the fuel pressure forces petrol through the spray tip and out of the diffuser nozzle, atomising it as it does so. When current is removed, the combination of a spring and fuel back-pressure causes the needle valve to close. This gives an audible 'tick' noise when it happens, which is why even a quiet fuel-injected engine has a soft but rapid tick-tick-tick-tick noise as the injectors fire. This on-off cycle time is known as the pulse width and varying the pulse width determines how much fuel can flow through the injectors. When you ask for more throttle either via the accelerator pedal or twist-grip (on a motorbike) you're opening a butterfly valve similar to the one in a carburettor. This lets more air into the intake system and the position of the throttle is measured with a potentiometer. The engine control unit (ECU) gets a reading from this potentiometer and "sees" that you've opened the throttle. In response the ECU increases the injector pulse width to allow more fuel to be sprayed by the injectors. Downwind of the throttle body is a mass airflow sensor. This is normally a heated wire. The more air that flows past it, the quicker it disappates heat and the more current it needs to remain warm. The ECU can continually measure this current to determine if the fuel-air mix is correct and it can adjust the fuel flow through the injectors accordingly. On top of this, the ECU also looks at data coming from the oxygen (lambda) sensors in the exhaust. These tell the ECU how much oxygen is in the exhaust so it can automatically adjust for rich- or lean-running.
Different types of injector systems.
When fuel-injection was first introduced, it was fairly simple and used a single injector in the throttle body. Basically it was a carburettor-derivative but instead of having the induction vacuum suck fuel into the venturi, an injector forced fuel into the airflow. This was known as throttle-body fuel-injection, or single-point fuel-injection.
As engine design advanced, the single-point system was phased out and replaced with multi-point or multi-port fuel-injection. In this design, there is one injector for each cylinder, normally screwed into the intake manifold and aimed right at the intake valve. Because fuel is only sprayed when the intake valve is open, this systems provides more accurate fuel-metering and a quicker throttle response. Typically, multi-point injection systems have one more injector for cold-starting which sprays extra fuel into the intake manifold upstream of the regular injectors, to provide a richer fuel-air mix for cold starting. A coolant temperature sensor feeds information back to the ECU to determine when this extra injector should be used.
As you would expect though, technology marches on with no regard to home mechanics, and the latest technology is direct injection, also known as GDI (gasoline direct injection). This is similar to multi-point injection only the injectors are moved into the combustion chambers themselves rather than the intake manifold. This is nearly identical to the direct injection system used in diesel engines. Essentially, the intake valve only allows air into the combustion chamber and the fuel is sprayed in directly through a high-pressure, heat-resistant injector. The fuel and air mix inside the combustion chamber itself due to the positions of the intake valve, injector tip and top of the piston crown. The piston crown in these engines is normally designed to create a swirling vortex to help mix the fuel and air before combustion, as well as having a cavity in it for ultra-lean-burn conditions (see picture to the left). The ECU controls the amount of fuel injected based on the airflow into the engine and demand, and will operate a direct injection engine in one of three modes: Full power mode is basically foot-to-the-floor driving. The fuel-air ratio is made richer and the injectors spray the fuel in during the piston intake stroke. In stoichiometric mode the fuel-air ratio is leaned off a little. The fuel is still sprayed in during the piston intake stroke but the burn is a lot cleaner and the ECU chooses this mode when the load on the engine is slightly higher than normal, for example during acceleration from a stop. Finally, when you're cruising with very little engine load, for example when you're on wide-open motorway with no traffic (I know that's hard to imagine when you live in England), the ECU will choose an ultra lean mode. In this mode, the fuel is injected later on in the 4-stroke cycle - as the piston is moving up its compression stroke. This forces the fuel-air charge right up next to the tip of the spark plug for the best burn conditions and the combustion itself takes place partly in the cylinder and partly in the shaped piston crown mentioned previously.
ECU maps.
The ECU receives a wide number of sensor readings from all over the engine. Built into the ECU is a fuelling and ignition map which is basically a gigantic table of numbers. It's like a lookup table that the ECU uses to determine injector pulse width, spark timing (and on some engines, the variable valve timing). So the ECU receives a set of values from all its sensors, which it then looks up in the fuelling and ignition map. At the point where all these numbers coincide, there is final number which the ECU then uses to set the injector pulse width. These are the manufacturer's "blessed" fuelling routines, and elsewhere on this page, there's a section dealing with chipping and remapping, whereby aftermarket tuners can alter these mapping tables to make the engine behave differently.
Valves and valve mechanisms.
If you've got this far down the page, hopefully you understand that the valves are what let the fuel-air mixture into the cylinder, and let the exhaust out. Seems simple enough, but there are some interesting differences in the various types of valve mechanism.
Spring-return valves.
Spring return valves are about the most commonly-used and most basic type of valvetrain in engines today. Their operation is simplicity itself and there are only really three variations of the same style. The basic premise here is that the spinning camshaft operates the valves by pushing them open, and valve return springs force them closed. The cam lobes either operate directly on the top of the valve itself, or in some cases, on a rocker arm which pivots and pushes on the top of the valve. The three variations of this type of valve-train are based on the combination of rocker arms (or not) and the position of the camshaft.
The most basic type has the camshaft at the top of the engine with the cam lobes operating directly on the tops of the valves.
The second more complex type still has the camshaft at the top of the engine, but the cam lobes operate rocker arms, which in turn pivot and operate on the tops of the valves. With some of these designs, the rocker arm is pivoted in the middle (as shown below) and with other designed, it's pivoted at one end and the cam lobe operates on it at the midpoint. Think of a fat bloke bouncing in the middle of a diving board whilst the tip of the board hits a swimmer on the head and you'll get the general idea.
The third type which you'll find in some motorcycle engines and many boxer engines are pushrod-activated valves. The camshaft is actually directly geared off the crank at the bottom of the engine and the cam lobes push on pushrods which run up the sides of the engine. The top of the pushrod then pushes on a rocker arm, which finally pivots and operates on the top of the valve. The image here shows the three derivatives in their most basic form so you can see the differences between them. Note that the pushrod type shows the camshaft in the wrong place simply for the purpose of getting it into the image. In reality the camshaft in this system is right at the bottom of the engine near the crank. The rocker arms shown here are also called fingers, or followers depending on who you talk to.
Tappet Valves
Tappet valves aren't really a unique type of valve but a derivative of spring-return valves. For the most part, the direct spring return valve described above wouldn't act directly on the top of the valve itself, but rather on an oil-filled tappet. The tappet is basically an upside-down bucket that covers the top of the valve stem and contains the spring. It's normally filled with oil through a small hole when the engine is pressurised. The purpose of tappets is two-fold. The oil in them helps quiet down the valvetrain noise, and the top of the tappet gives a more uniform surface for the cam lobe to work on. From a maintenance point of view, tappets are the items which wear and are a lot easier to swap out than entire valve assemblies. The image on the left shows a simple tappet valve assembly. I've rendered the tappet slightly transparent so you can see the return spring inside.
Desmodromic Valves
Desmodromic valve systems are unique to Ducati motorbikes. From the Ducati website: The word 'desmodromic' is derived from two Greek roots, desmos (controlled, linked) and dromos (course, track). It refers to the exclusive valve control system used in Ducati engines: both valve movements (opening and closing) are 'operated." Classy, but what does it mean. Well in both the above systems, the closure mechanism on the valve relies on mechanical springs or hydraulics. There's nothing to actually force the valve to close. With the Ducati Desmodromic system, the camshaft has two lobes per valve, and the only spring is there to take up the slack in the closing system. That's right; Ducati valves are forced closed by the camshaft. The marketing people will tell you it's one of the reasons Ducati motorbike engines have been able to rev much higher than their Japanese counterparts. The idea is that with springs especially, once you get to a certain speed, you're bound by the metallurgy of the spring - it can no longer expand to full length in the time between cylinder strokes and so you get 'valve float' where the valve never truly closes. With Desmodromic valves, that never happens because a second closing rocker arm hooks under the top of the valve stem and jams it upwards to force the valve closed. In fact, the stroke length, rods, and pistons all play their part in valve timing and maximum engine speed - it's not just the springs and valve float. This is why F1 cars use such a small stroke and pneumatic valves springs. In truth, both systems, spring or Desmodromic only work well up to a limit. Newer Japanese bikes have engines that can rev to the same limit as a Ducati just using spring-return valves.
You can see the basic layout of a desmodromic valve on the right. As the cam spins, the opening lobe hits the upper rocker arm which pivots and pushes the valve down and open. As the cam continues to spin, the closing lobe hits the lower rocker arm which pivots and hooks the valve back up, closing it. The red return spring is merely there to hold the valve closed for the next cycle and doesn't provide any springing force to the closing mechanism. This is a fairly simple layout for the purposes of illustration. The real engines have Desmo-due and Desmo-quattro valve systems in them where pairs of valves are opened and closed together via the same mechanism.
Quattrovalvole, 16v and the other monikers you'll find on the back of a car.
In the 80's, the buzzword was 16-valve. If you had a 16-valve engine you were happening. You were the dogs bollocks, the cat's meouw. In Italy, your engine was a quattrovalvole. So what the heck does all this mean? Well it's really, really simple. "Traditional" 4-cylinder in-line engines have two valves per cylinder - one intake and one exhaust. In a 16V engine, you have four per cylinder - two intake and two exhaust. (4 valves) x (4 cylinders) = 16 valves, or 16V. It follows that a 20V engine has 20 valves - 5 per cylinder. Normally three intake and two exhaust. Unless you've got a 5-cylinder Audi or Volvo in which case you've still got 4 valves per cylinder. If you're in America, the thing to have now is 32V - a 32 valve engine. Basically it's a V-8 with 4 valves per cylinder. See - it's all just basic maths.
And what do all these extra valves get you apart from a lot more damage if they ever go wrong? A better breathing engine. More fuel-air mix in, quicker exhaust. When you get further down the page (and if your wife / husband hasn't come and complained to you about spending so damn long reading this stuff so late at night), you'll find some more information on why this is A Good Thing.
"Clean" diesels? Toyota's D-Cat and DPNR
Old-school diesel engines used to sound like tractors when you started them on a cold morning, and they used to spew particulates out of the exhaust to the point where the back of the car went black. Newer generation diesels start much less noisily but for the most part still have some issues with particulates in the exhaust. Toyota claim to have solved this with their D-Cat and DPNR system. D-Cat stands for Diesel Clean Advanced Technology and DPNR stands for Diesel Particulate NOx Reduction. The operating principle is fairly sound. D-Cat is an advanced computer-controlled system for cleaning diesel exhaust gasses which relies on the DPNR catalyser. This is a combination of particle filters and normal gas-reduction catalysing metals that remove particulates, sulphur dioxide (SO2) and nitrogen dioxide (NO2) from the exhaust gasses. A sensor measures can tell when these filters are nearly full at which point a fifth diesel-injector sprays a little fuel directly into the exhaust system. Combined with the exhaust gas recirculation system, this results in all the collected pollutants being burned off, cleaning the filter in the process. DPNR requires ultra-low sulfur diesel (ULSD) to work properly.
This all sounds very good but this system was launched on the D-4D engined 2.0litre Toyota Avensis, and very soon afterwards, the complaints started to come in. Notably, Dutch car magazine AutoWeek (issue 42 / 2006) exposed the problem when their DPNR-equipped Avensis started driving around with a huge cloud of white smoke pouring out of the exhaust. They weren't the only ones to have this problem. Hundreds of complaints have been filed in Germany and other European countries for the same thing. The problem is that the D-Cat/DPNR system needs to 'regenerate' as described above. The particulate and gas filters are cleaned via a combustion mechanism in the exhaust, but this only happens at speeds below 160km/h (99mph), and takes about 20 minutes each time. In Germany especially, where they still have sections of unlimited-speed autobahns, people have been driving well over that speed for miles on end, then stopping and turning the car off, only to repeat the cycle twice a day during their commute. When this happens, the DPNR system never gets time to regenerate normally and the particle filters become clogged. When this happens, the DPNR system forces a clean cycle to happen and the combustion results in white smoke as there are too many pollutants trying to be burned off at the same time. And not just a little white smoke. In the AutoWeek test, they thought their Avensis was on fire it was trailing so much smoke. Toyota has promised to sort this problem out with an improved version of D-Cat, and will only be fitting it to the higher-spec 2.2litre engine.
Rotary / Wankel engines
So you've got this far down the page and realised how ridiculously complicated traditional 2-stroke and 4-stroke engines are. The pistons, connecting rods and crank are all there to turn up-down motion into spinning motion. Then there's the complexity of valves and valve trains, timing belts, tappets, springs, fuel delivery systems etc.etc. There is a simpler way. Would you believe the rotary engine essentially has only three moving parts? Conceived in 1957 by Dr. Felix Wankel, the rotary engine (also known as the Wankel Engine or Wankel Motor) works on a very simple principle. The piston isn't a piston at all, but a three-sided convex rotor. The rendering to the right shows a typical example. When spun around a fixed pinion gear inside an epitrochoidal-shaped chamber, the spinning of the rotor creates the suck-squeeze-bang-blow cycle simply by the its position relative to the sides of the chamber. (If you ever used a Spirograph as a kid, you'll have drawn trochoidal shapes without really knowing it). Basically, the combination of the rotor and chamber shapes ensures that the three apexes of the rotor are always in contact with the chamber walls whilst at the same time always creating three different volumes. As the rotor spins, each volume gets larger and smaller in turn, creating the compression and expansion volumes required for the engine to work. But how does the spinning rotor connect to the output shaft? There's an eccentric wheel that sits in a bearing inside the rotor. The spinning rotor transfers its motion to the eccentric wheel and the centre of that wheel is connected to a crank on the output shaft.
A single Wankel rotor could therefore be considered to be the equivalent of three pistons in a regular 4-stroke engine. The image below shows a single chamber of a typical rotary engine. Most rotary engines use two chambers and thus two rotors. Hence the three moving parts - the two rotors and the one output shaft. You can see there are no valves required - the intake and exhaust ports are simple openings in the combustion chamber that are covered and uncovered in the correct sequence by the spinning of the rotor.
At this point you're now asking yourself two questions.
The first is this - "If this is such a simple design, why doesn't everyone use it?"
Well yes, the design is simple. It's also smooth. Both rotors are continuously turning in the same direction so you don't have the violent change of direction problem that a normal engine has (up/down/up/down). As well as that, the design means that the combustion cycle lasts through three quarters of each complete turn of the rotor, as compared to one quarter of every second stroke of a 4-stroke engine. But all this clever design does have some inherent problems. Rotary engines cost more to manufacture because of the engineering tolerances required to make them work. The seals at the rotor apexes have to be very finely manufactured to prevent premature wear. (The apex seals are the equivalent of the piston rings in a normal engine). The low compression ratio and relatively large combustion volumes mean that Wankel engines are also typically less fuel efficient than normal engines, and a side-effect of that it is typically more difficult to get these engines to pass emissions regulations. It's not impossible though. Mazda saw the benefits of rotary engines back in 1961 and to-date have been the only manufacturer willing to spend the time, money and resources required to get a reliable, mass-producable design. Their current generation Renesis (Rotary Engine Genesis) engine powers the Mazda RX-8. Mazda have a plentiful supply of information on the history, design and implementation of their engines. Mazda rotary engines.
The second question is "Can I see an animation of this pinnacle of engineering prowess?". The answer is yes because it won't make much sense otherwise. The easiest way to understand how this all works it to keep your eye on just one of the curved sides of the rotor as it spins and observe the size of the volume between it and the chamber wall. As it passes in the intake port, the volume gets larger, generating a vacuum which pulls air into the chamber. As it passes the top, fuel is injected. As it approaches the left side of the chamber, the volume gets much smaller, creating the compression. At that point, the spark plugs fire. The combustion process causes the expansion of the gas which forces the rotor to continue its motion. Again thinking of just one side of the rotor, you'll see the volume increase in size again (to accomodate the combustion). Finally the leading rotor apex uncovers the exhaust port and as the volume decreases again, the exhaust gasses are forced out. At this point, that one side of the rotor is now ready to start its combustion cycle again. The bigger picture of course is that while the side you were watching was going through its intake cycle, the second side was going through its compression cycle and the third side was going through its exhaust cycle. Hence why a single-rotor wankel engine is the equivalent of a three-cylinder four-stroke engine. During that entire cycle you'll have noticed the eccentric ring spinning in its bearing and in turn spinning the output crank.
Hybrid Engines.
You've no doubt heard of hybrid cars by now, most likely the Toyota Prius. More manufacturers are jumping on the hybrid bandwagon, but just what is a hybrid vehicle? Simply put, it's a vehicle that uses a combination of two technologies to drive it. The most common hybrids are petrol-electric, like the Prius. Hybrid petrol-electric cars use a normal petrol engine, just like you'd find in any other car, but in in addition, there are one are two high-torque electric motor-generators. The motor-generator(s) draw power from a bunch of car batteries stored either in the floorpan of the car (for a low centre of gravity) or in the rear (for convenience). With power supplied to the motor-generator, it behaves like an electric motor. When no power is supplied but the shaft is turning, it becomes a generator to create power. In this mode, you get regenerative braking, where the energy required to slow the vehicle down is all taken up in the motor-generator to re-charge the battery packs. Both the petrol engine and the motor-generator(s) are connected to an onboard computer system which has been programmed by men in white coats to work as efficiently as possible. There are three mainstream technologies in the hybrid market at the time of writing, each championed by a different company or group of companies.
IMA - integrated motor assist (Honda)
The motor-generator (electric motor and regenerative generator) is in-line with the petrol engine, typically built into the bell-housing in front of the gearbox. The motor-generator is used to assist the petrol engine, thus reducing the load on it and allowing it to be smaller than it would otherwise be for a vehicle of the same weight. For example the Civic hybrid uses a 1.3l engine where the non-hybrid uses a 1.8l engine. The motor-generator cannot turn without turning the petrol engine too. First-generation systems didn't have enough power to be able to run the car on electric alone. Current generation ones do through higher powered motors and the ability to shutoff the petrol engine when coasting. Because the motor-generator is in-line, the regenerative braking works very simply - as you start to brake, the motor becomes the generator. Conversely it is also used as the primary starter motor for spinning the petrol engine up quickly after it has been turned off, for example at traffic lights. There is also a backup 'regular' starter motor for cold-starts and emergencies. Of the three mainstream hybrid technologies, IMA is by far the simplest to implement, maintain and repair. In the following images, red is the battery pack, green is motor-generator 1 and blue is motor-generator 2.
Hybrid Synergy Drive (Toyota)
Toyota's take on hybrid drive has a pair of motor-generators, one in-line like the Honda IMA design, one not. The key to its success is the compound planetary gearset in the transmission. In the Toyota system, the petrol engine and one motor-generator are connected to one of the inputs, the second motor-generator to the second input and the wheels to the third. Through a clever use of electronics, the planetary gearbox can be locked and unlocked in various configurations dependent on what is required. For example under modest acceleration, the petrol engine drives the planetary gearbox as well as the first motor-generator. The output from that is fed to the second motor-generator along with the output from the gearbox to drive the wheels. In pure electric mode, the first motor-generator freewheels, the petrol engine is turned off and all the electric power is fed to the second motor-generator. Under regenerative braking the second motor-generator becomes the generator as it does in the IMA system above. The difference is that if the battery pack is full, the energy derived from the second motor-generator is redirected to the first motor-generator which in turn uses it to induce drag in the petrol engine to slow the vehicle down. As a result, the actual brakes in a Toyota Hybrid vehicle do not wear very quickly at all because most of the braking is provided by the motor-generators. Only in severe cases do the brake pads actually engage the brake rotors. This is all made possible by the central engine computer and throttle-by-wire / brake-by-wire system.
Dual-mode or 2-mode Hybrid (GM).
The third hybrid system comes from GM and has two operating modes as oppose to the single mode of IMA or HSD. It again uses two motor-generators. In first and second gears, the first motor-generator sends power to the second motor-generator, and that coupled with the petrol engine provide the power to the wheels. In higher gears or under heavier loads, the petrol engine always runs (as oppose to the IMA and HSD systems where it can be turned off or have cylinders deactivated). The difference is in how the motor-generators work in cooperation with it. As speed increases, the first motor-generator gets to the point where it's providing no useable input to the drivetrain. At this point it begins to freewheel and the second motor-generator begins to act as a generator. As speed increases further, the first motor-generator begins to act as a generator again and at this point its power is once again fed to the second motor-generator which now becomes a motor. Coupled with variable intake timing, direct common-rail injection and a host of other technologies, these all come together to give GM's take on hybrid technology.
Most hybrids have an energy display screen mounted either in the instrument cluster or in the centre console. This is a small LCD which gives you, the driver, information about what mode you're driving in, and where the power is going. Again, the most recognisable and famous of these displays to date is that from the Toyota Prius (see right). The only real problem with these displays is the fascination they provide to the novice hybrid driver. Watching the animations spin around and the energy arrows scroll here and there as you drive is certainly informative but not really conducive to safe driving. One benefit however is the constantly-updated gas-mileage chart. Many Prius owners report that this spurs them to attempt to get videogame-like high scores in their cars, driving them in such a fashion as to get the highest recorded mpg from their cars. If nothing else, the energy display affects most drivers in terms of educating them as to how their driving style directly impacts their gas-mileage.
The battery question
At the time of writing, the estimated lifespan for the batteries in a hybrid car is about seven years. The cost of doing this for the Toyota hybrids is about US$10,000 which is a sizeable percentage of the cost of the entire car. The original theory was that you would have driven enough distance to recoup the extra cost via fuel savings but with the price of petrol where it is now, that is becoming harder and harder to achieve. So far there hasn't been a large recall for batteries for any of the hybrid manufacturers and I've not yet heard of anyone kicking up a stink about the cost. That means one of three things. (1) The batteries are lasting longer than expected, so people haven't had to swap them out yet. (2) They're paying the money but nobody has complained in the press. (3) The manufacturers are doing it free for good publicity.
Plug-in hybrids
It's been said that the reason the all-electric car failed in America is because if people forgot to plug it in overnight, they couldn't drive it the next day. The real reason had more to do with the politics of Big Oil, the California clean air act and GM's unwillingness to promote electric vehicles. Regular petrol-electric hybrids are an excellent choice for people wanting to be more frugal in their gas mileage, but the all-electric mode will only run for a couple of miles before the battery pack is completely drained. In fact, in the US, the Prius has been hobbled by the removal of the all-electric mode completely at the behest of Big Oil. The ideal solution to the pure-electric problem, and the petrol-electric problem is to have a plug-in hybrid. Essentially the idea is very simple. You drive the car as you would normally but you plug it in overnight. And extra set of deep-cycle marine batteries is charged up and can be used to drive in pure electric mode the following day. If the batteries run down, the car reverts to the behaviour of a normal petrol-electric hybrid. If you forget to plug it in overnight, again it behaves like a normal petrol-electric hybrid. In other words, if you choose to plug it in overnight, you buy yourself 30 or 40 miles of driving without using a single drop of petrol. If you forget, no biggie - you can still drive.
Famously, CalCars have converted a regular Prius to be a 100mpg+ vehicle with their plug-in conversion. How is this possible? Well the average commuter typically doesn't drive more than 30 miles a day. With the plug-in conversion, that entire distance is covered on pure electric mode, with the petrol engine only kicking in on a low charge or when it's needed for a burst of acceleration. Because the petrol engine is used so rarely, by the time you fill up, you can easily have covered more than 100 miles on a single gallon of petrol. CalCars will turn any hybrid into a plugin for you, for a price.
Diesel-electric hybrids
Another type of hybrid is the diesel-electric. There are at the time of writing, no production vehicles using this technology, but essentially it works just like a diesel-electric train. The small diesel engine is directly connected to a generator. The generator produces electricity on-demand, which is fed to wheel motors (electric motors built into the wheels) to drive the vehicle forwards.
The cost of hybrids
Because hybrid engine technology is still relatively new, it cost you more to buy a hybrid car than the equivalent petrol-engined car. Some countries, cities and states have incentives to do this, like energy grants, or paying the price difference. Ultimately, if you're willing to write-off the initial extra cost, owning a hybrid is definitely cheaper. If you include the extra cost up front and factor it across the lifetime of the vehicle, you'd need to own a hybrid for about 7 years covering about 15,000 miles a year to break even, given the rising cost of petrol compared to the mpg savings of operating the car. If you choose to go the plug-in hybrid route, you'll be paying even more for a company like CalCars to convert your car for you, but again, over the lifetime of ownership, you can probably recoup the cost within 5 years.
Renting hybrids
At the time of writing (July 2007), Hertz have started to offer hybrids as an option for rental. Some Hertz locations allow you to specify the exact vehicle you want when you rent. There is of course a price premium, but for example if you were to rent from Hertz in England, because the cost of petrol over there is so prohibitively expensive, renting a hybrid will save you money as soon as you go over the 250 mile mark. Up to 250 miles, it's cheaper to rent a regular compact vehicle and fill it with petrol. Over 250 miles, the extra cost of the hybrid is negated by the fuel-saving and you're on your way to a cheaper overall rental.
Engine Cooling Systems
It stands to reason that if you fill a metal engine with fuel and air hundreds of times a second and make it explode, the whole thing is going to get pretty hot. To stop it all from melting into a fused lump of steel and aluminium, all engines have some method of keeping them cool.
Air cooling
You don't see this much on car engines at all now. The most famous cars it was used on were rear-engined boxers like the original VW Beetle, Karmann Ghia, and Porsche Roadsters. It is still used a lot on motorbike engines because it's a very simple method of cooling. For air cooling to work, you need two things - fins (lots of them) and good airflow. An air-cooled engine is normally easy to spot because of the fins built into the outside of the cylinders. The idea is simple - the fins act as heat sinks, getting hot with the engine but transferring the heat to the air as the air passes through and between them. Air-cooled engines don't work particularly well in long, hot traffic jams though, because obviously there's very little air passing over the fins. They are good in the winter when the air is coldest, but that illustrates a weak spot in the whole design. Air cooled engines can't regulate the overall temperature of the cylinder heads and engine, so the temperature tends to swing up and down depending on engine load, air temperature and forward speed. A famous problem with air-cooling is associated with V-twin motorcycles. Because the rear cylinder is tucked in the frame behind the front cylinder, its supply of cool, uninterrupted air is extremely limited and so in these designs, the rear cylinder tends to run extremely hot compared to the front.
The image on the right is ©Ducati and shows the engine from the Monster 695 motorbike. It's a good example of modern air-cooled design and you can see the fins on the engine are all angled towards the direction of travel so the air can flow through them freely.
Oil cooling
To some extent, all engines have oil-cooling. It's one of the functions of the engine oil - to transfer heat away from the moving parts and back to the sump where fins on the outside of the sump can help transfer that heat out into the air. But for some engines, the oil system itself is designed to be a more efficient cooling system. BMW 'R' motorbikes are known for this (their nickname is 'oilheads'). As the oil moves around the engine, at some points it's directed through cooling passageways close to the cylinder bores to pick up heat. From there it goes to an oil radiator placed out in the airflow to disperse the heat into the air before returning into the core of the engine. Actually, in the case of the 'R' motorbikes, they're air- and oil-cooled as they have the air-cooling fins on the cylinders too. For a quick primer on how the radiator itself works, read on....
Water cooling
This is by far and away the most common method of cooling and engine down. With water cooling, a coolant mixture is pumped around pipes and passageways inside the engine separate to the oil, before passing out to a radiator. The radiator itself is made of metal, and it forces the coolant to flow through long passageways each of which have lots of metal fins attached to the outside giving a huge surface area. The coolant transfers its heat into the metal of the radiator, which in turn transfers the heat into the surround air through the fins - essentially just like the air-cooled engine fins. The coolant itself is normally a mixture of distilled water and an antifreeze component. The water needs to be distilled because if you just use tap water, all the minerals in it will deposit on the inside of the cooling system and mess it up. The antifreeze is in the mix, obviously to stop the liquid from freezing in cold weather. If it froze up, you'd have no cooling at all and the engine would overheat and weld itself together in a matter of minutes. The antifreeze mix normally also has other chemicals in it for corrosion resistance too and when mixed correctly it raises the boiling point of water, so even in the warmer months of the year, a cooling system always needs a water / antifreeze mix in it.
The coolant system in a typical car is under pressure once the engine is running, as a byproduct of the water pump and the expansion that water undergoes as it heats up. Because of the coolant mixture, the water in the cooling system can get over 100°C without boiling which is why it's never a good idea to open the radiator cap immediately after you've turned the engine off. If you do, a superheated mixture of steam and coolant will spray out and you'll spend some quality time in a burns unit.
The complexities of water cooling. Water cooling is the most common method of cooling and engine down, but it's also the most complicated. For example you don't want the coolant flowing through the radiator as soon as you start the engine. If it did, the engine would take a long time to come up to operating temperature which causes issues with the emissions systems, the drivability of the engine and the comfort of the passengers. In truly cold weather, most water cooling systems are so efficient that if the coolant flowed through the radiator at startup, the engine would literally never get warm. So this is where the thermostat comes in to play. The thermostat is a small device that normally sits in the system in-line to the radiator. It is a spring-loaded valve actuated by a bimetallic spring. In layman's terms, the hotter it gets, the wider open the valve is. When you start the engine, the thermostat is cold and so it's closed. This redirects the flow of coolant back into the engine and bypasses the radiator completely but because the cabin heater radiator is on a separate circuit, the coolant is allowed to flow through it. It has a much smaller surface area and its cooling effect is nowhere near as great. This allows the engine to build up heat quite quickly. If you look at the first of the two diagrams on the right, you can see the representation of the coolant flow in a cold engine.
As the coolant heats up, the thermostat begins to open and the coolant is allowed to pass out to the radiator where it dumps heat out into the air before returning to the engine block. Once the engine is fully hot, the coolant is at operating temperature and the thermostat is permanently open, redirecting almost all the coolant flow through the radiator. If you look at the second of the two diagrams on the right, you can see the representation of the coolant flow in a cold engine.
It's the action of the thermostat that allows a water-cooled engine to better regulate the heat in the engine block. Unlike an air-cooled engine, the thermostat can dynamically alter the flow of coolant depending on engine load and air temperature to maintain an even temperature.
The radiator fan. In the good old days, car radiators had belt-driven fans that spun behind the radiator as fast as the engine was spinning. The fan is there to draw the warm air away from the back of the radiator to help it to work efficiently. The only problem with the old way of doing it was that the fan ran all the time the engine was running, and stopped when the engine stopped. This meant that the radiator was having air drawn through it at the same rate in freezing cold conditions as it was on a hot day, and when you parked the car, the radiator basically cooked because it had no airflow while it was cooling down. So nowadays, the radiator fan is electric and is activated by a temperature sensor in the coolant. When the temperature gets above a certain level, the fan comes on and because it's electric, this can happen even once you've stopped the engine. This is why sometimes on a hot day, you can park up, turn off, and hear the radiator fan still going. It's also the reason there are big stickers around it in the engine bay because if you park and open the hood to go and start messing with something, the fan might still come on and neatly separate you from your fingers.
The cabin heater. Most water-cooled car engines actually have a second, smaller radiator that the coolant is allowed to flow through all the time for in-car heating. It's a small heat-exchanger in the air vent system. When you select warm air with the heater controls, you will either be allowing the coolant to flow through that radiator via an inline valve in the cooling system (the old way of doing it) or moving a flap to allow the warm air already coming off that radiator to mix in with the cold air from outside.
It's all these combinations and permutations of plumbing in a water-cooled engine that make it so relatively complex. The rendering below shows the basic elements a water-cooled engine.
Overheating on a snow day
If you live anywhere where it snows a lot, you'll have seen hundreds of motorists stranded at the side of the road, hood up, with steam pouring out of their radiators on the worst weather days - when it's snowing hard. It's counterintuitive at first - surely on the coldest, snowiest day of the year, the last thing you'd need to worry about was engine cooling? Well - sort of. If you're going on a long-distance drive - hours on end on the motorway, you probably need to consider covering part of your radiator so it doesn't get too much cold air - otherwise your engine will never quite get hot. That's rare though. More common is the lazy motorist syndrome, where they'll come out to the car park, clear the snow off the driver's side of the windscreen, get in and drive. Ten minutes later, they're standing at the side of the road, freezing, in driving snow, wondering why their engine blew up. Simple. They didn't clear the snow and ice away from in front of the radiator grille on the front of their car. That large lump of snow and ice blocks the airflow to the radiator so the engine just gets hotter and hotter until eventually it overheats and blows the radiator or pressure relief valve. It's not helped by the fact that on a good snow day, you'll be stuck in 5mph traffic anyway so there's not even a chance the snow might dislodge itself. So don't be lazy - spend the extra 2 seconds to brush that stuff away from the front bumper before you get in.
Why is good engine cooling important? Case Study : the BMC Mini Minor
The importance of overall engine design and cooling system design and efficiency is very well illustrated by the fate that befell the original British Motor Corporation Mini Minor. The following contribution is by Rodney Brown - a reader of this site.
In the Morris Mini, the water pump, fan and radiator block were mounted in the same position as they were on the same 948cc engine which was concurrently being used in the more conventional fore & aft engine layout of the Morris Minor 1000 saloon. Both cars were designed by Alec Issegonis, and this was just post-war; England was basically bust, so make do and mend was the order of the day. It took a genius like Alec to make a fore & aft power train work transversely, by folding beneath itself to fit in a very tight space. The Mini had to be kept small to keep development, production and ownership costs down.
Because of all this, whilst the cooling fan and radiator were still where you would expect to find them - at one end of the block, they now closely abutted the nearside front inner wheel arch because the normally fore & aft engine was now turned 90 degrees so it faced across the car. The arch inner flitch panel had suitable slots punched in it and a close fitting cowl enclosed the fan blades on the inner face. Good radiator cooling was possible as the engine was mounted on a sub frame which also carried the suspension components, leaving only a small shock absorber to pass in front of (and obstruct) the slots. The problem was that the Mini's front grille was large - as big an area as the original radiator, but now with no radiator actually behind it - that was on the end of the engine. Without something in the way, it offered little resistence to the flow of cold air onto the engine, (now placed sideways) close behind the grille, with just enough room to take off the distributor cap. (Early on before the cap was covered by a protective boot and plug shrouds fitted, rain would drive through the grille onto the distributor and HT lead plug connections stopping the engine.)
The carburettor and inlet manifold shared the space between the engine and the bulkhead with the exhaust manifold (which only just missed the bulkhead). Therefore when the car was in motion, the whole of the side of the block facing the open grille was bathed in a 30 - 60 mph icy blast whilst the opposite side was baked by convection/radiation/conduction from an ill ventilated exhaust manifold. This is where the problem lay. The side of the piston bores closes to the front of the car remained relatively stable but on the side facing the rear bulkhead, where all the heat built up, it caused the piston bores to expand. So circular piston bores were cold on one side and hot on the other causing uneven distortion. The main effect of this was a poor fit of the piston rings which increased oil consumption, and more disastrously, enabled blow-by for unburnt fuel and combustion gasses which in turn pressurised the sump and gearbox. Remember that space-saving folded design, where the gearbox was folded under the engine? You've got it: the engine oil was also the gearbox oil. The sump/gearbox was not vented initially, but like the engine block above it, was cooled by an icy blast on one side and baked on the other! The consequences for the then-current SAE30 single-weight oils were that the oil was essentially useless after 3,000 miles. This rose to 6,000 miles with the advent of the multi grade oils, and it's interesting to note that the development of these oils in England was prompted by the pressing need to solve the problems posed by the Mini.