Diesel Cycle

Ideal cycle for compression ignition (CI) reciprocating engines. Models diesel engines. Fuel injected near TDC; heat addition at constant pressure. Named after Rudolph Diesel.

Doesn’t get classic knocking like petrol engines because they don’t premix fuel and air. Higher $r$ possible. Uses less-refined fuel.

Compression ratio is usually between 12 to 24.

For a given compression ratio, OTTO cycle has the high efficiency.

Processes

1 → 2: isentropic compression 2 → 3: constant-pressure heat addition 3 → 4: isentropic expansion 4 → 1: constant-volume heat rejection

For adiabatic process:

$$PV^{\gamma} = \text{constant}\quad \text{and}\quad TV^{\gamma -1} = \text{constant}$$

Cutoff Ratio

Denoted by $r_c$.

$$r_c = \frac{v_3}{v_2}$$

Thermal Efficiency

$$\eta_{diesel} = 1 - \frac{1}{r^{k-1}} \cdot \frac{r_c^k - 1}{k(r_c - 1)}$$