--- title: "Diesel cycle, cooling, advantages and disadvantages" subject: "Power Plant Equipment" chapter: 5 key: "semester_7/power_plant_equipment:diesel-cycle-cooling-merits" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/diesel-cycle-cooling-merits site: BCT Engineering --- # Diesel cycle, cooling, advantages and disadvantages Power Plant Equipment · Chapter 5: Diesel Power Plant - Asked in: 2080 Baisakh, 8 marks - Video: [Diesel Power Plant || working and layout || Diesel Engine](https://www.youtube.com/watch?v=ly9CI9nXaC8) — Aeran Classes - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/diesel-cycle-cooling-merits - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-5 ## Question What is diesel cycle and how the system is cooled during operation? Discuss the advantages and disadvantages of diesel power plant. ## Short answer The diesel cycle is the ideal air-standard cycle of a compression-ignition engine with heat addition at constant pressure; processes are isentropic compression, constant-pressure heat addition, isentropic expansion, and constant-volume heat rejection. Cooling is by air, open water, or closed forced circulation with heat exchanger and cooling tower. Advantages include simple layout, quick starting, and high efficiency; disadvantages are high fuel and maintenance costs, limited size, and noise pollution. ## Answer #### 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: ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the air-standard diesel cycle, how is heat added and rejected? A. Added at constant volume, rejected at constant pressure B. Added at constant pressure, rejected at constant volume C. Both added and rejected at constant pressure D. Both added and rejected at constant volume Answer: B. Process 2-3 adds heat at constant pressure while fuel burns as the piston moves out, and 4-1 rejects heat at constant volume. (Key point: Heat addition in diesel cycle) 2. What is compressed in process 1-2 of the diesel cycle, and why? A. A fuel-air mixture, to keep the spark plug firing B. Fuel only, to raise its pressure C. Air alone, so its temperature rises above the fuel's self-ignition point D. Exhaust gas, to recover heat Answer: C. Only air is compressed (r = 14 to 22); the high temperature reached ignites the fuel when it is injected. (Key point: Compression process) 3. Which cooling method is usual in diesel power plants? A. Air cooling with fins and a fan B. Open once-through river water cooling C. Closed forced-circulation cooling with soft water and a heat exchanger D. Oil cooling only Answer: C. Soft water is pumped through the jacket and a heat exchanger cooled by raw water; a thermostat holds the temperature. (Key point: Usual cooling method) 4. Why must the engine be cooled? A. About 30% of fuel heat goes to the walls, which would damage the oil film and metal parts B. To increase the cut-off ratio C. To make the exhaust hotter D. Because cooling raises the compression ratio Answer: A. Cooling removes the wall heat so the lubricating oil film and metal parts stay safe. (Key point: Need for cooling) 5. Which is a disadvantage of a diesel power plant? A. Slow starting B. Large space requirement C. High fuel and operating cost with limited unit size D. Heavy water requirement Answer: C. Imported oil makes running cost high and unit size is limited to a few MW, with short life. 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