Combustion Speed

As we all know, a gasoline engine ignites the air-fuel mixture with a spark plug. The flame spreads outward from the spark and burns the entire mixture, while a diesel engine ignites almost at the instant diesel fuel is injected into the combustion chamber, so combustion can be completed in a shorter time.

By observing piston movement, we can see that a gasoline engine piston has already moved downward a considerable distance before combustion is complete, while a diesel engine piston moves only a very short distance before the fuel is fully burned. As a result, combustion in a diesel engine has more time to push the piston downward, converting more of the energy into useful work and greater torque. In a gasoline engine, if combustion continues after the piston has moved away from top dead center, that portion of the flame can only push the piston over a short distance. If combustion occurs closer to top dead center, it can push the piston over a longer distance. Therefore, if a gasoline engine had the same near-instantaneous combustion characteristics as a diesel engine, it could also maximize torque output.

Bore-to-Stroke Ratio

In fact, diesel engines tend toward a longer stroke and smaller bore, while gasoline engines tend toward a larger bore and shorter stroke. Because 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. Ultimately, all of these factors translate into greater torque.

For example, assume there are two 0.6L single-cylinder engines. The diesel engine has a bore of 80mm and a stroke of 120mm, while 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 combustion is much faster, the timing of combustion does not occur at the ideal crank angle, so a longer stroke clearly helps generate more torque. More importantly, diesel engines have a higher compression ratio, and together with near-instant combustion and turbocharging, this is what drives their high torque output.A longer stroke also limits diesel engine speed, because with a longer stroke, the piston must move faster at the same rpm.

Diesel Engines Are Better Suited to Turbocharging

Diesel engine components are stronger. Cast-iron cylinder blocks, pistons, connecting rods and crankshafts all have high strength. This makes the engine heavier, but it also allows it to withstand greater torque.

Another characteristic of diesel engines is lean combustion. The air-fuel ratio of a diesel engine can range from 18:1 to 70:1. Under lean combustion, more air is available for expansion to do effective work, so engine efficiency is higher. Achieving this effect requires the support of a turbocharger, which compresses as much air as possible for efficient combustion.

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

Diesel 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. Therefore, injecting the same volume of fuel allows diesel to produce more energy, which means more torque.

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