--- title: "Phases of combustion in a CI engine" subject: "Automotive Technology" chapter: 3 key: "semester_5/automotive_technology:combustion-ci-engine-phases" source: https://www.bctengineer.com/q/semester_5/automotive_technology/combustion-ci-engine-phases site: BCT Engineering --- # Phases of combustion in a CI engine Automotive Technology · Chapter 3: Ignition and Combustion - Asked in: year not given, 6 marks - Video: [Stages of Combustion in CI (Diesel) Engines (Mechanical/Automobile Engg)-GATE/ESE/PSU](https://www.youtube.com/watch?v=FcW5QwPEZdQ) — Aneesh Kumar - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/automotive_technology/combustion-ci-engine-phases - Whole chapter: https://www.bctengineer.com/md/semester_5/automotive_technology/chapter-3 ## Question Describe the four stages of combustion in a compression ignition engine with a pressure-crank angle diagram. How does ignition delay affect diesel knock? ## Short answer The four stages are ignition delay (A-B, no pressure rise), rapid uncontrolled combustion (B-C, steep pressure rise from accumulated fuel), controlled combustion (C-D, mixing-controlled), and after-burning (after D, late burning). A long ignition delay causes a large fuel accumulation which burns suddenly, leading to high pressure rise rates and diesel knock. ## Answer In a CI engine, air is compressed to about 30 to 40 bar and 500 to 600 degrees C. Diesel is injected near TDC and burns by auto-ignition. Combustion occurs in four stages. ```text P | C | / \ D | /B \___ | A----/ . | inj starts TDC +------------------------ crank angle A-B delay B-C rapid C-D controlled after D: after-burning ``` 1. **Ignition delay (A-B):** from the start of injection to the first flame. It is made up of the physical delay (atomisation, vaporisation, mixing) and the chemical delay (pre-flame reactions). There is no pressure rise above the motoring curve. 2. **Rapid (uncontrolled) combustion (B-C):** the fuel injected during the delay burns almost at once, giving a steep pressure rise. This is the source of diesel noise. 3. **Controlled (mixing-controlled) combustion (C-D):** fuel injected now burns as it enters; the rate depends on injection and mixing. Peak temperature is reached in this stage. 4. **After-burning (after D):** the remaining fuel burns late in the expansion stroke. This is undesirable, since it raises the exhaust temperature and smoke. #### Diesel knock and ignition delay If the delay is long, a large amount of fuel accumulates and then burns suddenly. This causes a very high rate of pressure rise, shock waves and a metallic knocking sound called **diesel knock**. Remedies: use a high-cetane fuel, a higher compression ratio, higher intake temperature/pressure, pilot injection, a small injection quantity at the start (pintle nozzle), and keep the engine warm. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the correct order of combustion stages in a CI engine? A. Rapid, delay, controlled, after-burning B. Ignition delay, rapid combustion, controlled combustion, after-burning C. Controlled, rapid, delay, after-burning D. Delay, controlled, rapid, after-burning Answer: B. Combustion proceeds from ignition delay to rapid (uncontrolled), then controlled, then after-burning. (Key point: Four stages order) 2. What makes up the ignition delay? A. Physical delay (atomisation, vaporisation, mixing) plus chemical delay B. Only the spark timing C. Rapid burning plus after-burning D. Valve opening time Answer: A. Delay is the time from start of injection to first flame, with both physical and chemical parts. (Key point: Ignition delay) 3. Why does a long ignition delay cause diesel knock? A. Too little fuel is injected B. A large amount of fuel accumulates and burns suddenly, causing very high pressure rise C. The fuel burns too slowly D. The compression ratio is too high Answer: B. Fuel accumulated during the delay burns at once, giving shock waves and the metallic knock. (Key point: Cause of knock) 4. Which measure reduces diesel knock? A. Low cetane fuel B. Lower intake temperature C. High cetane fuel and higher compression ratio D. Large injection quantity at start Answer: C. These shorten the delay; a small initial injection quantity is also used, not a large one. (Key point: Remedies) ## Related in Automotive Technology - [Knock and pre-ignition in SI engines](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Aknock-and-pre-ignition-si-engine) — Automotive Technology, chapter 3 - [Engine displacement, compression ratio and power](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Aengine-displacement-bmep-numerical) — Automotive Technology, chapter 1 - [Requirements of SI and CI engine fuels](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Asi-engine-fuel-requirements) — Automotive Technology, chapter 2 - [Battery ignition system and spark timing calculation](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Abattery-ignition-spark-numerical) — Automotive Technology, chapter 3 ## Related in other subjects - [Stages of CI combustion and diesel knock](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Adiesel-combustion-stages-knock) — Internal Combustion Engines, chapter 5 - [Stages of SI combustion and flame propagation](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Astages-combustion-si-flame-propagation) — Internal Combustion Engines, chapter 5 - [Spark ignition vs compression ignition engine](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aspark-vs-compression-ignition) — Fundamental of Thermodynamics & Heat Transfer, chapter 6 - [Valve timing diagram of four-stroke engine](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Avalve-timing-diagram-four-stroke) — Internal Combustion Engines, chapter 4