Combustion speed

As we all know, a gasoline engine ignites the air-fuel mixture via a spark plug, and the flame propagates outward from the spark, burning all of the mixture, whereas in a diesel engine the fuel is ignited almost instantly when it is injected into the combustion chamber, allowing complete combustion in a shorter time.

By observing piston motion, we can see that in a gasoline engine the piston has already moved downward a considerable distance before the mixture has fully burned, while in a diesel engine the piston moves only a short distance before the fuel has completely burned. The result is that the combustion flame in a diesel engine has more time to push the piston downward, converting into more useful work and greater torque. In a gasoline engine, if combustion is still occurring after the piston has moved away from top dead center, that portion of the flame can push the piston only a short distance; if it occurred closer to top dead center, it could push the piston over a longer distance. Therefore, if a gasoline engine had the same near-instant combustion characteristics as a diesel engine, it could also maximize torque output.

Bore-to-stroke ratio

In fact, diesel engines tend to favor a long stroke and small bore, while gasoline engines tend to favor a large bore and short stroke. Since torque equals force x distance, diesel applications benefit not only from greater force but also from a longer distance (a longer crank throw) to amplify that force, which ultimately translates into higher torque.

For example, suppose there are two 0.6L single-cylinder engines. The diesel engine has a bore of 80mm and a stroke of 120mm; the gasoline engine has a bore of 100mm and a stroke of 80mm. Both have a displacement of 0.6L, but the diesel engine has a much longer stroke.

It is also worth noting that because diesel engines burn much faster, the timing of combustion does not occur at the ideal crank angle, so a long stroke is clearly helpful for generating more torque. More importantly, the higher compression ratio of diesel engines, combined with near-instant combustion and turbocharging, is what drives their high torque output.The longer stroke also limits diesel engine speed, because with a longer stroke the piston has to move faster at the same rpm.

Diesel engines are better suited to turbocharging

Diesel engine components are stronger. The strength of cast-iron cylinder blocks, pistons, connecting rods, and crankshafts is higher. This makes the engine heavier, but it also allows it to withstand greater torque.

Another feature of diesel engines is lean combustion. The air-fuel ratio of a diesel engine can reach 18:1~70:1. Through lean combustion, more air is available for expansion and useful work, so engine efficiency is higher. Achieving this effect requires the help of a turbocharger, which compresses as much additional air as possible for efficient combustion.

At the same time, pumping losses in diesel engines are lower than in gasoline engines because diesel engines do not need a throttle, or more precisely, they have a throttle that is almost always fully open. By contrast, gasoline engines have a much narrower air-fuel ratio range, and boost pressure is not as high, because production gasoline engines are not designed like diesel engines to withstand very high boost. In general, higher boost pressure plus more fuel means more horsepower, and therefore more torque.

Diesel fuel has higher energy density

For the same fuel volume, whether 1L of diesel or 1L of gasoline, diesel contains about 10-15% more energy than gasoline, so injecting the same amount of fuel allows diesel to produce more energy, which means more torque.

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