Explain the Diesel cycle and its key difference from Otto cycle.

The Diesel cycle is the ideal cycle for compression-ignition engines, featuring isentropic compression, constant-pressure heat addition, isentropic expansion, and constant-volume heat rejection. Unlike the Otto cycle which has constant-volume combustion, the Diesel cycle has constant-pressure combustion due to fuel injection over a period. This allows higher compression ratios (14-22) compared to Otto cycle (8-12), resulting in better thermal efficiency.