What is the primary difference between a diesel engine and a gasoline engine?

The primary difference between a diesel engine and a gasoline engine lies in the method of ignition used to ignite the fuel-air mixture. Diesel engines operate on the principle of compression ignition, which means that they compress the air within the cylinder to a high degree, raising its temperature. When diesel fuel is injected into this highly compressed, hot air, it ignites spontaneously without the need for a spark plug.

This contrasts with gasoline engines, which rely on spark ignition. In gasoline engines, a mixture of gasoline and air is compressed to a lesser extent and then ignited by a spark from a spark plug. The fundamental difference in ignition methods leads to various operational characteristics, including efficiency, performance, and fuel consumption.

While diesel engines may often be larger and run at different temperatures compared to gasoline engines, these factors are not the defining characteristics that set them apart in terms of their basic operating principles. Thus, the reliance on compression ignition is what fundamentally distinguishes diesel engines from their gasoline counterparts.