What type of engine has the highest compression ratio?

The diesel engine has the highest compression ratio among the types listed. This characteristic is fundamental to its operation. Diesel engines compress air to a much higher pressure, which leads to a higher temperature in the combustion chamber. When fuel is injected into this high-pressure, high-temperature air, it ignites spontaneously due to the heat produced from the compression process. This is in contrast to gasoline engines, where ignition is assisted by spark plugs and requires a lower compression ratio to prevent knocking or premature combustion.

The efficiency and power output of a diesel engine benefit significantly from this high compression ratio, as it allows for a more complete combustion of fuel, leading to improved fuel efficiency and greater torque production. Other fuel types like propane and natural gas typically operate at lower compression ratios than diesel, as they utilize different combustion processes. This fundamental difference in operation explains why the diesel engine stands out in this respect.