Diesel Cycle (Compression Ignition)

Reciprocating internal combustion cycle where high-ratio adiabatic air compression ignites injected fuel at constant pressure.

Specifications & Metrics

type: Internal Combustion Gas Cycle
processes: Isentropic air compression (1→2), isobaric fuel injection & combustion (2→3), isentropic expansion (3→4), isochoric heat rejection (4→1)
working fluid: Air (compressed alone) plus atomized liquid fuel injected near top dead center
efficiency formula: η = 1 − [1 / r^(γ−1)] · [(α^γ − 1) / (γ(α − 1))], where α = V₃/V₂ is the cut-off ratio
originator: Rudolf Diesel (1892–1897)
applications: Heavy-duty trucks, marine propulsion, diesel-electric locomotives, and standby generators

Developed by Rudolf Diesel in the 1890s after he initially sought to build a constant-temperature Carnot engine, the Diesel cycle compresses air alone before spraying fuel directly into the hot compressed air at top dead center.
Because no fuel is present during the compression stroke to cause pre-ignition knock, diesel engines operate at much higher compression ratios (15:1 to 22:1) than spark-ignition Otto engines, yielding higher real-world thermal efficiency despite the cut-off ratio penalty.
Large turbo-charged two-stroke marine diesel engines achieve the lowest brake-specific fuel consumption of any single-cycle internal combustion engine (~0.16 kg/kWh, or over 50% thermal efficiency).