Compression ignition and constant-pressure heat addition
The diesel cycle powers compression-ignition engines: during intake, air alone enters the cylinder (no fuel yet). The piston compresses the air to very high pressure and temperature (typically 15:1 to 25:1 compression ratio), raising temperature above the auto-ignition point of diesel fuel. Fuel then sprays into the hot, compressed air and ignites immediately, releasing heat at roughly constant pressure as the expanding gases push the piston down. This differs from spark-ignition (Otto cycle) engines, which add heat at roughly constant volume.
Pressure-volume analysis on the PV diagram
On a PV diagram, the diesel cycle shows a near-horizontal line during fuel injection and combustion (constant-pressure process) followed by an isentropic expansion to exhaust pressure. The area enclosed by the cycle curve represents net work per cycle. Higher compression ratios and longer constant-pressure phases increase cycle efficiency, making diesels more efficient than Otto engines at high loads. At part load, the fuel spray cuts off earlier, shortening the constant-pressure phase and efficiency drops.
Efficiency gains and noise trade-offs
Diesel efficiency typically reaches 40 to 50 percent in heavy-duty applications, compared to 30 to 35 percent for gasoline engines, because the high compression ratio and expansion ratio are closer to the Carnot limit. The trade-off is mechanical stress and noise: high peak pressure loads the block, crankshaft, and bearings heavily, and combustion knock (uncontrolled pressure rise) produces diesel engine knock. Fuel injection timing and nozzle spray pattern control combustion rate and emissions.