Why do diesel engines often output larger torque at low speeds?

Diesel engines can output large torque at low speeds, which is closely related to their working principle and structural characteristics.

From the perspective of working principle, diesel engines are compression ignition engines. During the intake stroke, a large amount of air is drawn into the cylinder. During the compression stroke, the piston compresses the air. Due to the high compression ratio of diesel engines, typically between 15-22, the air is compressed to a very small volume, causing a sharp increase in temperature and pressure. When diesel is injected into the cylinder, the high-temperature and high-pressure air causes the diesel to rapidly self ignite. During the combustion process, the energy generated by combustion is released in a short period of time, exerting a strong thrust on the piston. Moreover, at low speeds, the duration of each working cycle is relatively long, giving the fuel more ample time to burn and convert more chemical energy into mechanical energy, resulting in greater torque.

From the perspective of structural characteristics, diesel engines often have relatively large cylinder diameters and longer piston strokes. The larger cylinder diameter provides more space in the combustion chamber to accommodate the mixture of fuel and air, which can generate stronger explosive power during combustion. A longer piston stroke means that in each working cycle, the piston can receive the thrust generated by combustion over a longer distance. According to the torque calculation formula (torque=force x force arm), a longer force arm (piston stroke) helps to generate greater torque.

In addition, the characteristics of diesel fuel itself also play a certain role. Diesel has a higher energy density than gasoline, and the combustion of diesel can release more energy per unit volume. At low speeds, this energy can more effectively drive the crankshaft to rotate, converting it into powerful torque output. This enables vehicles and other equipment to easily cope with situations that require significant traction, such as starting, climbing, or towing heavy objects, thanks to the large torque advantage of diesel engines at low speeds. This characteristic has also made diesel engines widely used in applications such as heavy-duty trucks, construction machinery, and ships that require high torque.