Diesel cycle, cooling, advantages and disadvantages

Diesel cycle

The diesel cycle is the ideal (air-standard) cycle of a compression-ignition engine, in which heat is added at constant pressure. It has four processes:

  p
  |  2 _______ 3
  |   |        \
  |   |         \        1-2 isentropic compression
  |    \         \       2-3 heat added at const. p
  |     \         + 4    3-4 isentropic expansion
  |      `.       |      4-1 heat rejected at const. V
  |        `-._   |
  |            `--+ 1
  +--------------------- V

1. 1–2 Isentropic compression: air alone is compressed (ratio $r$ = 14 to 22), raising its temperature above the fuel's self-ignition point.
2. 2–3 Constant-pressure heat addition: fuel is injected and burns as the piston moves out.
3. 3–4 Isentropic expansion: power stroke.
4. 4–1 Constant-volume heat rejection: exhaust.

Efficiency, with cut-off ratio $\rho = V_3/V_2$:

$$\eta = 1 - \frac{1}{r^{\gamma-1}}\left[\frac{\rho^\gamma - 1}{\gamma(\rho - 1)}\right]$$

How the engine is cooled

About 30 % of the fuel heat goes to the cylinder walls, so cooling keeps the lubricating oil film and metal parts safe:

• Air cooling: fins and a fan; for small sets only.
• Open water cooling: river/well water passes once through the jacket and is discharged.
• Closed (forced circulation) cooling: soft water is pumped through the jacket and a heat exchanger (or radiator); raw water from a cooling tower or spray pond removes the heat. A thermostat holds the jacket temperature. This is the usual method in diesel power plants.
• The lubricating oil also cools pistons and bearings and is itself cooled in an oil cooler.

Advantages of diesel power plant

• Simple design and layout; small space; can be built quickly.
• Can be located near the load; no large water need.
• Quick starting and loading; good for standby and peak loads.
• Low standby losses; high thermal efficiency at full load (about 35–40 %).
• Low capital cost per kW for small capacities; fewer operating staff.

Disadvantages

• High fuel cost (imported oil in Nepal) and high operating cost.
• Limited unit size (a few MW) and short life.
• High maintenance and lubrication cost.
• Noise, vibration and exhaust pollution.
• Cannot run on sustained overload.