- The big difference between petrol and diesel engines is where the fuel and air mix
In the majority of petrol engines liquid petrol is injected into the inlet airstream before it enters the cylinders. A mixture of fuel vapour (droplets of liquid) and air enters the cylinders, where it is compressed and ignited by a spark. A diesel engine ingests only air. This is compressed well beyond the point where a fuel/air mixture would spontaneously burn. At precise points near the peak of each compression event, measured doses of fuel are injected, which ignite spontaneously. The diesel process is more efficient.
- The piston doesn’t really push spent gasses out past the exhaust valve
Engines aren’t pumps, but they do experience what engineers call ‘pumping losses’. When the fuel/air mixture burns, it expands rapidly, and this expansion is the harbinger of motive power. It shoves the piston down. Before the piston gets right to the bottom of its stroke the exhaust valve opens. The mixture, still expanding rapidly, is expelled through the aperture of the open exhaust valve. In a very real sense, its expansion pushes it out the chamber. Which leads us to…
- The piston doesn’t really suck inlet gasses in past the inlet valve, either
While the exhaust gasses are exiting (see 2), the piston heads north. Before it gets to the top the inlet valve opens, and the exhaust valve remains open. This ‘both valves open’ deal is called ‘overlap’. (Those who subscribe to the ‘push out/suck in’ theory will note an obvious problem during the overlap period: Why doesn’t exhaust flow out the inlet valve?) During overlap, exhaust gasses travelling out the exhaust port have a fair degree of momentum and this sucks inlet mixture into the engine – the process is called ‘scavenging’. (Unfortunately it also means a little unburned mixture leaves via the exhaust, accounting for unburned hydrocarbon emissions.)
- Compression is v-e-r-y important
The piston compresses the fuel/air mix with both valves closed, just before combustion. Doing so consumes a fair bit of energy, so why bother? Why not just suck it all in at low pressure and burn it? The answer is that compressing the mix allows it to expand over a greater range during combustion, resulting in more work being extracted from each stroke. In practice, the highest possible compression ratio is always used, within limitations imposed by the fuel (see 6).
- Fuel/air mix doesn’t really explode inside an engine
It burns. Explosion and combustion are different things. In an explosion (think: the M.O.A.B. – Mother Of All Bombs) the reactive ingredients go bang at once. A great deal of energy is released in a very short time, generating massive destructive power, briefly, which is ideal for destroying something. Combustion is much more gentle and controlled, which is ideal for extracting work without mechanical failure.
- High-octane ‘premium’ fuels and the ‘more energy’ myth
Octane rating is about resistance to ‘knock’. Knock happens if the engine’s compression is too high for the fuel – which burns spontaneously, too early in the process. (That is, before the spark.) It’s destructive and counter-productive. High-octane fuels are required because high-performance engines have higher compression ratios (see 4), which ultimately deliver more Newton-metres. The higher octane rating is required to eliminate knock. (It’s the compression that does the job; the fuel just facilitates combustion by restraining itself.) So tipping high-octane fuel into an engine designed for low-octane fuel realizes benefits in the immeasurable-to-none range. Doing this is merely a waste of money. What can really burn you, however, is running an engine on a lower-than-specified octane fuel. It’s a recipe for engine failure.
- Too cool for fuel
Engines fight a constant battle with temperature. One of the advantages liquid fuels offer is that they enter the combustion chamber as vapours (droplets of liquid) which promptly evaporate into a gas. This chills the process (anyone who ever had petrol on their skin knows it feels cold as it evaporates) and is called the ‘latent heat of vapourisation’. It’s typically chemistry-lab contrarian – calling it a ‘heat’ despite its cooling effect. Sucking heat out of the cylinder increases considerably the density of the mix, another happy outcome. This crams more molecules in the space, delivering more punch. Most LPG (liquefied propane) installations, by comparison, inject gas (not vapour). This hampers performance because propane gas can’t suck heat like petrol.
- The big limitation on performance and fuel efficiency is metallurgy
Combustion is more efficient at higher temperatures. This means more Newton-metres would be produced for every gram of fuel burned if the engine were simply allowed to burn hotter. You’d get either more torque or go further on less fuel. So why don’t we just turn up the heat? The answer is: meltdown. Engines have a cooling system, which is hell bent on conducting heat into water and bleeding it out the radiator. If the cooling system fails, that’s a big problem for your engine’s metallurgy (think: warranty). The radiator can’t even do it on its own. Significantly more fuel is routinely tipped into engines than is needed for efficient combustion just to provide additional cooling (see 7) which keeps operating temperatures below meltdown levels. Unfortunately, attempts to manufacture high-temperature-capable engines using heat-resistant materials have so far failed, and experts say there are no foreseeable breakthroughs on the horizon.