Otto Cycle

Air-standard analysis of the spark-ignition (gasoline) engine. γ = 1.4, T₁ = 300 K, Qin = 800 kJ/kg.

Compression ratio8

Engine States

State 1BDC → CompressState 2TDC → IgniteState 3TDC → ExpandState 4EXBDC → Exhaust
1→2 Isentropic compression
(intake valve closed)
2→3 Constant-volume heat addition
(combustion at TDC)
3→4 Isentropic expansion
(power stroke)
4→1 Constant-volume heat rejection
(exhaust blowdown)

Cycle Diagrams

1→2 Isentropic compression
2→3 Isochoric heat addition
3→4 Isentropic expansion
4→1 Isochoric heat rejection
Volume V (norm.)0101k1.9M2.4M4.9MPressure P (Pa)1234P–V Diagram
0345691Entropy s (J/kg·K)3007002kTemperature T (K)1234T–S Diagram
0345691Entropy s (J/kg·K)0400k1.5MEnthalpy h (J/kg)1234H–S Diagram (Mollier)

State Properties

StateT (K)P (kPa)V (norm.)s (J/kg·K)h (kJ/kg)
1300101.31.00000.0
26891862.30.1250391.2
318034872.90.1256911510.9
4785265.11.000691487.4
Thermal efficiency η = 56.5%= 1 − r1−γ

Key Relations

Thermal efficiency
η = 1 − r1−γ
Isentropic compression
T₂/T₁ = rγ−1  |  P₂/P₁ = rγ
Constant-volume heat add
Qin = cᵥ (T₃ − T₂)
Mean effective pressure
MEP = Wnet / (V₁ − V₂)