Understanding how engines work - advanced

Engine efficiency

 

There are different areas of efficiency that relate to engines - Does it develop a lot of power for itís size, does it use a lot of fuel for the power that it develops, is it heavy for the power it develops, does it create a lot of harmful emissions for the power it develops. Here we will be concerned with how much power it develops for itís size and how much fuel it uses for the power it develops. The main factors that affect these two areas of efficiency are other more specific areas of efficiency:

 

Volumetric efficiency

If you consider a two litre petrol engine, the total volume of the engineís cylinders that is stroked by the pistons will equal two litres. If you remember how the four stroke cycle works, where there will only be an intake stroke for every second revolution of the engine, you would expect the engine to be able to suck in one litre of air per engine revolution at full throttle. This would be true with a 100% volumetric efficiency, but in practice this is hard to achieve. Restrictions to airflow will usually mean that a figure of less than 100% will be achieved. In the case of turbo or supercharged engines, more than 100% volumetric efficiency can be achieved because the air is pressurised before it flows into the engine.

 

Charge efficiency

The engine valves cannot be made to fully open or close instantaneously, it is impossible for anything to do this, so the valves take time to open or close during which time the piston will be moving. Even if we could get the valves to open instantaneously we wouldnít really want them to on many engines because to fully open the valve at the correct time would mean it hitting the top of the piston. In fact, we donít really want the inlet valve to open at exactly the same time as the piston begins itís inlet stroke or to close at exactly the same time as the piston ends itís inlet stroke. And we donít want the exhaust valve to open at the exact same time as the piston begins itís exhaust stroke or to close at the exact same time it finishes the exhaust stroke. This is because of how the flow of air into and exhaust gas out of the cylinders behave, both of which are very complicated due to the high gas speeds involved. In simple terms, the volume of air or exhaust gas flowing through the inlet or exhaust ports takes time to stop flowing and time to start flowing again when a valve opens or closes. If we start to open the inlet valve just before the inlet stroke starts, actually towards the end of the exhaust stroke, we might be able to get some air into the cylinder even before the start of the inlet stroke. But why doesnít exhaust gas flow out into the intake system through the inlet valve if we do this? Well at low engine speeds it would, but at higher engine speeds and high airflows it wonít because the volume of air that is rushing through all the pipework of the inlet system towards the inlet valve has inertia, it doesnít want to stop and has the slight effect of being able to push itís way into the cylinder. Another advantage considering the valves cannot move instantaneously, is that the inlet valve will be further open at an earlier point during the inlet stroke to allow for easier passage of the air to enter the engine. If the inlet valve is still open during the very start of the power stroke, some air may still flow into the engine at the start of the power stroke before the quickly rising pressure from the exploding fuel prevents it from doing so. Similar factors relate to the exhaust valve, except if we begin opening the exhaust valve just before the start of the exhaust stroke, it will actually be towards the end of the power stroke and you would expect the high pressure in the cylinder during the power stroke to rush out through the exhaust valve under these conditions anyway. Now, remembering that the inlet stroke follows the exhaust stroke, if the valves open for a short time before the start of their respective strokes and close the valves a little after their respective strokes, you can see that there will be a time when both valves are slightly open at the end of the exhaust stroke and the start of the inlet stroke. This is called ívalve overlapí and can further aid volumetric efficiency because the inertia of the gas exiting through the exhaust valve can help to suck the new air and fuel mixture into the engine through the inlet valve. The best moments for the inlet and exhaust valves to open and close in relation to the position of the piston is a key factor in the performance and characteristics of the engine. Usually, the higher the engine speed the more advantage there is in having the valves opening earlier and closing later as there is more time for air and fuel to flow into the engine and for the exhaust gas to flow into the exhaust. At lower engine speeds there may be more advantage to having the valves open and close when the position of the piston is nearer the actual start and stop positions of the relevant stroke because with longer valve opening times and low engine speeds some exhaust gas might have time to flow into the inlet system, inlet into the exhaust and some of the inlet charge that has entered through the inlet valve might be pushed back through the inlet valve as the piston begins to rise on the compression stroke while the inlet valve is still open, any of these would lower the amount of air/fuel that is trapped in the cylinder ready to burn on the power stroke. As with many aspects of the engine, valve timing is a compromise. Older engines had set valve timing and newer engines attempt to address the issues by using various means to try to adjust the point at which the valves start to open and how long they stay open for in relation to the position of the piston and engine speed. If we can do this, then we can make the engine make more power over a wider range of rpm. More modern engine designs often employ systems, of varying complexity, to adjust the amount that valves open, the time that they start to open, the length of time they are open for, or a combination of these, but valve operation is still always a compromise and must be designed to work with other aspects of engine design which will also be a compromise.†

 

Pumping losses

In order to work an engine has to suck air into the cylinders, compress it, and after the power stroke it must push the exhaust gas back out. A lot of power that is gained during the power stroke is therefore used just to power the other three strokes. Under low throttle settings (when you donít have your foot down on the accelerator far), the induction stroke uses more power than at high throttles because the piston moving down in the cylinder will be trying to suck the air past a partly closed throttle valve, so it is pulling the piston away from a vacuum but the compression and exhaust strokes donít sap as much power because there will be lower pressures in the cylinders which the piston is trying to push against. At high throttles the opposite is true because the engine finds it easier to suck air into the cylinders on the intake stroke, but then during the compression stroke a higher pressure will be made in the cylinder which will press down on the piston as it rises and there will be more exhaust gas to push out on the exhaust stroke. Not least to be considered here is the effect of the compression ratio. The air/fuel mixture is compressed to allow for a powerful explosion but by how much is it compressed? There are two different measurements of compression ratio, static and dynamic. Static compression is measured by comparing the amount of space above the piston in the cylinder when the piston is at the bottom of itís stroke with the amount of space above the piston in the cylinder when the piston is at the top of itís stroke. Dynamic compression is more concerned with the actual pressure within the cylinder at the end of the compression stroke and is affected by the amount of air in the cylinder when the inlet valve closes at the end of the inlet stroke combined with the static compression ratio. At a certain rpm, with a fully open throttle and an engine that has 100% volumetric efficiency at that rpm, you might expect static and dynamic compression ratios to be the same but in this case dynamic compression would be above static because the air is heated up during compression. The actual pressure in the cylinder at the end of the compression stroke can also fall much lower with a less than fully open throttle or less than 100% volumetric efficiency, or rise higher in the case of supercharged or turbo charged engines, which have higher than 100% volumetric efficiency because the air is forced into the cylinders already under pressure. We need to remember that static compression is due to basic engine design and never changes. Dynamic compression changes with every different operating circumstance of the engine, the rpm, the amount the throttle (accelerator) is open, static compression and with volumetric efficiency (which also changes with rpm), so it also changes due to valve timing and gas flow characteristics. A good cylinder pressure might be around 170 psi, which because air pressure is around 15.4psi would equal a ratio of around 11:1. To get that 170psi though, we might only need a static compression ratio of about† 8:1 for an engine that can trap a full charge because of the heating effect during the compression stroke.† Most non-turbo and non supercharged (naturally aspirated in other words) engines have a higher static compression ratio than this, probably closer to 10:1, because even at full throttle the volumetric efficiency and charge efficiency of the engine might not allow the cylinder to trap a full charge, even at full throttle. At part throttle, the amount of air flowing into the engine is limited by the throttle anyway, so before compression we start with a cylinder pressure much lower than atmospheric giving a dynamic compression much lower than 170psi. If we started out with a static compression of 8:1 and the cylinder doesnít get filled with air, then after compression the figure will be much lower than the 170psi full throttle pressure. The higher the pressure in the cylinder when the fuel/air ignites the quicker it will burn and because there is not so much space in the cylinder above the piston because of the higher static compression ratio, the pressure in the cylinder will rise to a higher level as it burns and push harder on the piston. When designing an engine, the designers need to know what sort of application the engine will be used for because compression ratio, the design of the combustion chamber (the area above the piston when the piston is at the very top) and valve timing are all compromises and must all compliment each other to give the engine itís required nature. By this we mean we can compromise low rpm power for high rpm power or the other way around. We can compromise fuel efficiency for power or the other way around. Combustion chamber design, compression ratios and valve timings also compromise with emissions, (which we explain on the fuel systems page). Letís look at how engine power is measured, there are two measurements, torque - commonly measured in ft/lbs (foot pounds) is how much turning force the engine is giving regardless of the speed it is turning (rpm). If you put a spanner that is a foot long on a nut so the spanner is horizontal and press down on the end of the spanner with a force of 100lbs, you will be exerting a turning force of 100ft/lbs. If the spanner doesnít move the nut though, you havenít done any work. The amount of work an engine does is measured in break horse power (bhp), which equals torque in ft/lbs, multiplied by rpm, divided by around 5050. So, If you could apply the same turning force on the spanner but spin the nut around 5050 times a minute you would be making 100bhp - and be superhuman! The amount of torque, or turning force, an engine can give at any particular rpm is directly related to how much charge of air and fuel can be trapped in the cylinder at that rpm when at full throttle and to a lesser extent thermal efficiency, which is also effected by compression ratio. An engine that is designed to pull a heavy vehicle will be best designed to have a lot of turning force at low rpm. To trap the largest charge at low rpm we need the valves to be open for less time than we would for an engine that is designed for a sports car to make high bhp figures by developing similar torque at high rpm. At low rpm it is easier to get the cylinder closer to being completely full than it is at high rpm, because whatever the valve timing or other aspects of the engine design, there is more time for the air and fuel to get into the cylinder and more time for the exhaust gas to flow out of the cylinder to make space for new air and fuel to enter. Now, when we compress a gas it heats the gas up and if we heat a mixture of air and fuel up too much it will spontaneously ignite without the spark plug even sparking. Just because the pressure in the cylinder might be well below 170psi if the engine is running, say, only at half throttle, doesnít mean that the amount we have compressed it by or the amount we have raised itís temperature is much less, because the amount we have compressed it by will still equal the† compression ratio. So we are limited in how far we can go with both static and dynamic compression ratios before the air and fuel start to spontaneously ignite. Spontaneous ignition in a petrol engine is caused by detonation. An explosion is a fast burning but a detonation is different because it spreads through the fuel as a shockwave, much faster than the fuel would usually burn in a controlled explosion. This is bad for the engine and can cause engine damage similar to putting nitro-glycerine in the engine rather than petrol, bad for power because the fuel is not properly burned but all used before it can do any useful work, and bad for emissions. Another thing that can cause an air/fuel charge in the cylinder to detonate is too far advanced ignition timing, the point at which the spark plug ignites the air/fuel mixture. Because the air and fuel take time to burn, even though this is very fast, the point at which the spark plug ignites the mixture is controlled so that it ignites the mixture before the start of the actual power stroke and actually when the piston is still moving up on the compression stroke. Advancing the ignition timing like this gives better power because the pressure in the cylinder from burning the fuel and air has had time to build, so that more pressure is in the cylinder at the start of the power stroke, where the space above the cylinder is smallest, and where the build in pressure has more effect in pushing the piston back down. If the ignition is advanced too much though, the air and fuel may still detonate because of the rising pressure and temperature due to the start of the explosion. All the above means that an engine that is designed to make more torque at low rpm will have a relatively low static compression, closer to 8:1 because it is easy to completely fill the cylinder at low rpmís, and have inlet and exhaust valve timings that are closer to the actual start and finishes of the respective strokes. The ignition timing on such an engine can still be advanced so the pressure will be optimised when the piston is at the very start of the power stroke. All these things mean that at low rpmís such an engine can make the most torque and hence power at low rpmís. An engine that is designed to make more power at high rpmís has to use longer valve opening times to maximise the amount of air and fuel flowing into the engine and exhaust gas flowing out, but it will still struggle to completely fill the cylinder with a fresh charge of air and fuel. For this reason the dynamic compression of this engine will be lower than the ideal, so the static compression can be set higher to make the most use of the air and fuel that can get into the cylinders, without so much risk of detonation. The ignition timing may be even more advanced because the air/fuel still takes time to burn but the piston will be moving faster towards the growing explosion. This engine would maximise power at high rpmís, but at low rpmís wouldnít make as much power as the other engine. This is because the charge trapping ability at low rpmís will not be able to fill the cylinders with as much air and fuel. Even if it could, the ignition timing would have to be retarded to prevent detonation because of the higher compression ratio causing higher pressure in the cylinder. If the ignition timing is retarded so the air/fuel only start burning when the piston is part way down the cylinder on the power stroke then the pressure will not rise to as high a level or be able to push the cylinder with as much force for any given amount of air/fuel as the engine that is designed for low rpm power. If you wanted to modify any petrol engine to be more economical at low throttle settings but not able to run at high throttles, so limiting the amount of power it could make, one way to do it would be to raise the compression ratio. You would, however, be better off not bothering to modify the engine at all but fitting a smaller engine instead. The reverse is also true - if you want more power but still decent fuel economy and good drivability you might be better off with a bigger engine. Turbocharged and supercharged engines are slightly different but still, if you want to increase power by more than a little you would probably be better off with a bigger engine. Turbochargers especially donít make up for a small engine.

 

Thermal efficiency

We want as much of the heat energy from burning the petrol to go into the air in the cylinder as possible, and to stay there for as long as possible during the power stroke, as this will cause the pressure in the cylinder to rise more and the pressure to stay higher for longer. Remember, engines work by using heat from burning petrol to expand the air in the cylinder raising itís pressure so that the increase in pressure pushes the piston down the cylinder to turn the crankshaft and give power. Some of the heat from burning the petrol is conducted or radiated out of the air in the cylinder and into the engine itself and some heat goes straight out of the exhaust. This is why an engine needs a cooling system - most engines circulate water through channels within the engine and through a radiator which passes the heat back to surrounding air, and why the exhaust gets very hot. Any heat that finds itís way into the engine or goes out of the exhaust is wasted energy. On the other hand, when heat does find itís way into the exhaust we want it to stay there, because if hot exhaust gas flows into a cooler exhaust system it will be cooled down and become more dense, so will take more energy to pump out of the exhaust.† So far considering thermal efficiency we have looked at thermal efficiency within the engine. A more basic form of thermal efficiency would be to compare the theoretical amount of heat energy we could get from burning a certain amount of petrol against the mechanical energy that the engine makes as power. No engine is anywhere near 100% efficient in this respect!

 

Frictional and mechanical losses

An engine has many bearings that are constantly turning and pistons that move up and down inside the cylinders. Where there is metal to metal contact these bearings would very quickly be worn out except that all these metal to metal contacts are effectively held apart by pressurised oil. The oil itself has some drag in the bearings and is pressurised and circulated by a pump which uses engine power. The bottom parts of the engine spin very quickly and constantly have to cut through a small amount of oil, some of which falls from the cylinder walls above after lubricating the area of contact between the cylinder wall and piston. The cooling system uses a pump to circulate water around the engine and radiator. The engine valves are held closed usually by strong springs, the pressure of which must be overcome to open a valve. Then there are fan belts causing drag etc. All these things sap some power from the engine, but frictional and mechanical losses are minor compared to, for instance, the power used during the compression strokes.†††