Diesel Cycle

Air-standard analysis of the compression-ignition (diesel) engine. Heat added at constant pressure via fuel injection. γ = 1.4, T₁ = 300 K.

Compression ratio r18

Cutoff ratio r_co2.5

Engine States

1→2 Isentropic compression
(all valves closed)

2→3 Constant-pressure heat addition
(fuel injected at TDC)

3→4 Isentropic expansion
(power stroke)

4→1 Constant-volume heat rejection
(exhaust blowdown)

1→2 Isentropic compression

2→3 Isobaric heat addition (fuel injection)

3→4 Isentropic expansion

4→1 Isochoric heat rejection

State	T (K)	P (kPa)	V (norm.)	s (J/kg·K)	h (kJ/kg)
1	300	101.3	1.000	0	0.0
2	953	5795.6	0.056	0	656.6
3	2383	5795.6	0.139	921	2093.7
4	1082	365.5	1.000	921	785.9

Thermal efficiency η = 60.9%= 1 − (r_co^γ−1) / [γ(r_co−1) · r^γ−1]

Thermal efficiency

η = 1 − (r_co^γ−1) / [γ·(r_co−1)·r^γ−1]

Isobaric heat addition (2→3)

Q_in = c_p(T₃−T₂) | r_co = V₃/V₂ = T₃/T₂

Isentropic compression

T₂/T₁ = r^γ−1 | P₂/P₁ = r^γ

vs Otto at same r

η_Diesel < η_Otto (higher Q_in compensates)