DieselZyklus

The Diesel cycle, known in German as the Dieselzyklus, is an idealized thermodynamic cycle used to model compression-ignition internal combustion engines. Developed by Rudolf Diesel in the late 19th century, it describes a constant-pressure heat addition following an isentropic compression of the working fluid.

In the ideal Diesel cycle, four processes connect four states: 1-2 isentropic compression of the working fluid; 2-3 heat addition at approximately constant pressure due to fuel combustion; 3-4 isentropic expansion; 4-1 heat rejection at constant volume. The compression ratio r is V1/V2, and the cut-off ratio rc = V3/V2 describes the volume change during combustion.

The cycle yields higher thermal efficiency than the Otto cycle at similar peak temperatures and air–fuel ratios, particularly at high compression ratios. Efficiency increases with compression ratio and decreases as the cut-off ratio increases. However, the idealized model neglects several real-process effects, such as heat transfer during compression and finite-rate heat release during combustion.

In practice, diesel engines approximate the Diesel cycle, with fuel injected during late compression to maintain near-constant pressure during combustion. Real engines exhibit deviations and may be described by dual-cycle models or more complex simulations. The Diesel cycle remains a foundational reference in thermodynamics and automotive engineering for understanding compression-ignition operation and the limits of efficiency.