A petrol engine draws in a mixture of fuel and air, squeezes it, and then lights it with a spark at a precisely chosen moment. A diesel engine draws in air only, squeezes it much harder, and sprays fuel into it — at which point the fuel ignites on contact, because the air is already hot enough to light it.
No spark is needed because nothing is waiting to be lit. That single difference is where every other difference between the two engines comes from.
Squeezing air makes it hot
Compressing a gas quickly raises its temperature — not because of the fuel, and not because of anything burning, but because you are doing work on the gas and it has nowhere to put that energy. The relationship is steep: temperature climbs with the compression ratio raised to a power.
| Compression ratio | Peak air temperature | Times diesel's autoignition point |
|---|---|---|
| 8:1 | 446 °C | 2.1× |
| 10:1 | 513 °C | 2.4× |
| 12:1 | 573 °C | 2.7× |
| 14:1 | 627 °C | 3.0× |
| 16:1 | 676 °C | 3.2× |
| 18:1 | 722 °C | 3.4× |
| 20:1 | 765 °C | 3.6× |
| 22:1 | 805 °C | 3.8× |
The surprise in that table
Air passes diesel's autoignition point at a compression ratio of about 3.0:1 — which is lower than any real engine ever built. Every engine on that table, the petrol ones included, compresses air hot enough to ignite diesel fuel on contact. A petrol engine at 10:1 reaches 513 °C, more than twice what diesel needs.
So the thing that stops a petrol engine igniting itself is not that its air is too cool. It is the fuel. Petrol is deliberately formulated to resist autoignition — that is precisely what an octane rating measures — and autoignition also takes time, which a charge only briefly at peak temperature does not give it. When petrol does light itself anyway, from a hot spot, an over-advanced engine or fuel with too low an octane rating for it, that is knock: combustion at the wrong moment, and destructive for exactly that reason.
Diesel fuel is engineered for the opposite behaviour, and diesels run at 18:1 and above not to reach the threshold but to clear it with margin — 3.4× over — so ignition is immediate and predictable rather than merely possible. The high ratio pays for itself in thermal efficiency too, which is the other half of why they use it.
Since engine specifications quote intake and coolant temperatures in Celsius, Fahrenheit or Kelvin depending on the source, a temperature converter is worth having open when comparing figures from different manuals.
The fuel arrives at a completely different time
This is the part that is easy to miss. It is not only how the two engines ignite, but when the fuel turns up.
This is what actually caps a petrol engine's compression ratio. Fuel is present for the whole compression stroke, and octane resistance only buys so much time — squeeze harder and the mixture eventually lights on its own before the spark, which is knock. A diesel is compressing plain air, which cannot ignite however hot it gets, so it is free to squeeze as hard as it likes. The fuel only shows up when combustion is wanted.
It also means the two engines are controlled differently. A petrol engine throttles the air to control power, keeping the fuel-air ratio near constant. A diesel takes in a full charge of air every cycle and controls power by how much fuel it injects — which is why diesels have no throttle plate in the classic sense, and why they run very lean at low load.
So what are glow plugs?
Diesel engines have glow plugs, and they are regularly mistaken for spark plugs. They are not igniters. A glow plug is a heating element that warms the combustion chamber before a cold start, and it exists because of the caveat on the figures above: real compression is not adiabatic. A cold engine loses heat to cold cylinder walls, so the air at the top of the stroke is well below the ideal temperature — possibly below the autoignition point. The glow plug makes up the difference for the first few seconds. Once the engine is warm it does nothing, which is why a failing glow plug shows up as hard starting on a winter morning and nothing at all in July.
Why the fuels are not interchangeable
The two fuels are chosen for opposite properties, which is why putting one in the other's tank is such an expensive mistake.
- Petrol resists autoignition on purpose. That is what an octane rating measures. Put petrol in a diesel, and the engine cannot reliably ignite it by compression — and petrol is also a poor lubricant, where diesel fuel lubricates the very high-pressure injection pump it flows through.
- Diesel ignites readily under heat and pressure. That is what a cetane rating measures — effectively the opposite scale. Put diesel in a petrol engine and it will not vaporise or burn properly at all; it mostly fouls the plugs and the exhaust.
The same divide explains the rest of the reputation. Diesel's higher compression ratio gives it better thermal efficiency and its fuel carries more energy per litre, which is the mileage advantage. Igniting fuel the instant it is injected produces a very sharp pressure rise, which is the noise. And burning fuel in pockets of very hot air produces soot and nitrogen oxides, which is why modern diesels carry particulate filters and after-treatment that petrol engines do not need.
Watch the cycle they share
For all these differences, both are four-stroke engines doing the same four jobs in the same order. Our 3D four-stroke engine simulator runs the full 720° cycle with the valves, camshafts and crankshaft in motion — pause it at the top of the compression stroke and you are looking at the exact moment where these two engines part company: one is about to be sparked, the other is about to be sprayed.
If you want the stroke count itself explained, our guide to two-stroke versus four-stroke covers why the extra 360° exists; and turbocharging — which almost every modern diesel relies on — is covered separately.