A gasoline engine uses spark ignition. Air and fuel are mixed together before entering the cylinder. The piston compresses this mixture, and a spark plug ignites it at a precise time. The explosion pushes the piston down, producing mechanical work. A diesel engine uses compression ignition instead. In a diesel cycle, only air is drawn into the cylinder during the intake stroke. The piston then compresses that air to an extremely high pressure and temperature—much higher than in gasoline engines. Near the top of the compression stroke, diesel fuel is injected directly into the hot air. Because the air is already hot enough, the fuel auto-ignites without a spark plug. This difference creates several important engineering consequences. First, diesel engines operate at much higher compression ratios (often 14:1 to 25:1 compared with ~8:1 to 12:1 in gasoline engines). Higher compression increases thermal efficiency, meaning more of the fuel’s chemical energy is converted into useful wor...