--- title: "Differentiate SI and CI engines" subject: "Internal Combustion Engines" chapter: 2 key: "semester_6/internal_combustion_engines:si-versus-ci-engine-differences" source: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/si-versus-ci-engine-differences site: BCT Engineering --- # Differentiate SI and CI engines Internal Combustion Engines · Chapter 2: Engine construction and operation - Asked in: year not given, 5 marks - Video: [Difference Between Petrol (S.I) Engine And Diesel (C.I) Engine](https://www.youtube.com/watch?v=orcm9bfs88w) — BEST MECHANICAL ENGINEERING - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/si-versus-ci-engine-differences - Whole chapter: https://www.bctengineer.com/md/semester_6/internal_combustion_engines/chapter-2 ## Question Differentiate between spark-ignition and compression-ignition engines. ## Short answer SI engines use Otto cycle, petrol, spark ignition, premixed charge, compression ratio ~6-10, quantity governing; lighter, faster, less efficient (~25-30%). CI engines use Diesel cycle, fuel injected into hot compressed air, compression ratio 14-22, quality governing; heavier, slower, more efficient (30-40%) but produce more NOx and particulates. ## Answer Both engines are reciprocating internal combustion engines. An SI engine ignites a premixed fuel-air charge with a spark. A CI engine compresses air only and injects fuel into it near the end of compression, when it self-ignites. | Basis | SI engine | CI engine | |---|---|---| | Thermodynamic cycle | Otto | Diesel (or dual) | | Fuel | Petrol (high octane) | Diesel (high cetane) | | Charge during suction | Fuel-air mixture | Air only | | Fuel supply | Carburettor or injector in the manifold | Injection pump and injector in the cylinder | | Ignition | Spark plug, electrical system | Self-ignition by hot compressed air | | Compression ratio | 6-10 (limited by knock) | 14-22 | | Air-fuel ratio | Nearly constant (12-18) | Varies widely (18-80) | | Load control | Quantity governing: the throttle changes the quantity of mixture, its quality stays nearly constant | Quality governing: only the fuel quantity is changed, the air is unthrottled | | Thermal efficiency | 25-30% | 30-40% | | Engine weight and cost | Lighter, cheaper | Heavier, costlier (strong build) | | Speed | High (up to 6000 rpm and above) | Lower (up to 4000 rpm) | | Pollutants | CO, HC, NO$_x$ | NO$_x$ and particulates (smoke) | | Knock | Pre-ignition and end-gas knock | Diesel knock from ignition delay | | Fire risk | Fuel is volatile, higher risk | Safer fuel | Use: SI engines in cars and motorcycles; CI engines in trucks, buses, tractors, generators and ships. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What enters the cylinder during suction in each engine? A. SI: fuel-air mixture; CI: air only B. SI: air only; CI: mixture C. Both: mixture D. Both: air only Answer: A. A CI engine compresses only air and injects fuel near the end of compression. (Key point: Charge in suction) 2. Why is the SI compression ratio limited to about 6-10? A. Lack of a spark B. Knock of the premixed charge C. Diesel injection pressure D. Weight of the engine Answer: B. A premixed charge at high compression ratio auto-ignites (end-gas knock), whereas CI compresses air only. (Key point: Compression ratio limit) 3. How is load controlled in a CI engine? A. By a throttle changing mixture quantity B. By changing spark timing only C. By changing fuel quantity, with air unthrottled D. By changing compression ratio Answer: C. CI engines use quality governing; SI engines use a throttle to vary the quantity of mixture. (Key point: Load control) 4. How is the charge ignited in a CI engine? A. Spark plug B. Glow plug only C. Magneto D. Self-ignition by hot compressed air Answer: D. Compression heats the air above the fuel's ignition temperature, so the injected fuel self-ignites. (Key point: Ignition method) ## Related in Internal Combustion Engines - [Knocking in SI engines and its control](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aknocking-in-si-engines-control) — Internal Combustion Engines, chapter 5 - [Battery and magneto ignition systems](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Abattery-coil-magneto-ignition-systems) — Internal Combustion Engines, chapter 5 - [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 - [Four-stroke SI engine working and components](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Afour-stroke-si-engine-working-components) — Internal Combustion Engines, chapter 2 - [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 - [Cetane number and rating of CI fuels](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Acetane-number-diesel-fuel-rating) — Internal Combustion Engines, chapter 3 ## Related in other subjects - [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 - [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 - [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 - [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