- Practice · 5 marks
Differentiate SI and CI 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.
Differentiate between spark-ignition and compression-ignition engines.
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 | NO 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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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