Otto and Diesel Cycle

An Otto engine completes four strokes: intake, where the piston moves down to draw in a gasoline-air mixture at maximum volume; compression, where adiabatic compression occurs as the piston moves up; power, where the spark plug ignites the mixture, adding heat at constant volume, then the heated gas expands adiabatically; and exhaust, where heat is removed isochorically as gases exit the cylinder before the cycle repeats.

Thermal efficiency in an Otto engine increases with higher compression ratios, calculated from the ratio of specific heat capacities and compression ratio. Typical Otto engines operate at compression ratios of 8–10, while premium gasoline allows up to 13. However, higher compression ratios increase end-of-compression temperatures significantly, risking fuel pre-ignition and explosion, limiting practical efficiency improvements.

Diesel engines use a self-ignition mechanism instead of spark plugs, eliminating pre-ignition risk. Fuel is injected at constant pressure just before the power stroke, ignited by high temperature. This advantage allows compression ratios up to 20, compared to Otto engines' maximum of 13, improving theoretical efficiency to 65%–70% versus Otto's 56% for ideal gases.

The theoretical thermal efficiency of an ideal Otto engine is approximately 56%, assuming the gasoline-air mixture behaves as an ideal gas and neglecting friction and heat loss. However, actual efficiency for a non-ideal gas is around 35% due to real-world losses. Efficiency is always less than 100% and depends on the compression ratio and specific heat capacity ratio.

During adiabatic compression, the piston moves upward, compressing the gasoline-air mixture without heat exchange with surroundings. This process increases both pressure and temperature significantly. The compressed mixture is then ignited by the spark plug, adding heat at constant volume, which drives the subsequent adiabatic expansion and power stroke.

In Otto engines, heat is added at constant volume after spark plug ignition of the compressed mixture. In diesel engines, fuel is injected at constant pressure just before the power stroke, and heat is added during the expansion process as the fuel burns. This pressure-based injection in diesel engines enables higher compression ratios and prevents pre-ignition.

Pre-ignition occurs when fuel spontaneously explodes before the spark plug ignites it, caused by excessive temperature from high compression ratios. This uncontrolled combustion damages the engine and reduces efficiency. Diesel engines avoid this problem through self-ignition control, allowing much higher compression ratios and better thermal performance than Otto engines.