Internal Combustion Engine Cycles and Combustion Stages
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Theoretical Engine Cycles
Suction Stroke
The engine cycle begins with this stroke. The inlet valve opens as the piston, currently at Top Dead Center (TDC), starts moving toward Bottom Dead Center (BDC). The air-fuel mixture (in petrol engines) or fresh air (in diesel engines) enters the cylinder until the piston reaches BDC.
Compression Stroke
After the suction stroke, the piston moves from BDC to TDC to compress the air-fuel mixture or fresh air. This raises the pressure inside the cylinder, which is essential for fuel combustion. The inlet valve closes during this operation to seal the chamber.
Expansion Stroke
Following compression, fuel combustion occurs, pushing the piston from TDC toward BDC. This releases the pressure developed by combustion, and useful work output is obtained.
Exhaust Stroke
After the expansion stroke, the piston moves from BDC toward TDC. The exhaust valve opens to remove combustion residuals and closes once the piston reaches TDC.
Stages of Combustion in SI Engines
- Ignition Delay / Preparation Phase (A–B)
- The time between the spark (point A) and the start of combustion (point B), also known as ignition lag.
- During this phase, the fuel-air mixture prepares for burning with no significant pressure rise.
- Depends on fuel properties, temperature, pressure, and air-fuel ratio.
- Flame Propagation Phase (B–C)
- The flame starts at point B and spreads rapidly.
- Pressure rises sharply to a maximum at point C.
- This is the main combustion phase; initially slow, then accelerating to high flame speeds.
- After Burning / Flame Termination Phase (C–D)
- Combustion continues even after peak pressure during the expansion stroke.
- Flame speed decreases gradually until combustion completes near point D.
Stages of Combustion in CI Engines
- Ignition Delay Period (A–C)
- The time between the start of fuel injection and the start of combustion.
- Fuel is atomized, vaporized, and mixed with hot compressed air.
- Consists of: Physical delay (A–B) for atomization/mixing and Chemical delay (B–C) for pre-flame reactions.
- Uncontrolled (Rapid) Combustion (C–D)
- Fuel accumulated during the delay burns suddenly, causing a sharp rise in pressure and temperature.
- Higher delay leads to a higher pressure rise.
- Controlled Combustion (D–E)
- Fuel burns as it is injected; the rate of combustion depends on the fuel injection rate.
- Pressure rise becomes gradual and controlled.
- After Burning (E–F)
- Remaining fuel burns slowly during the expansion stroke.
- Combustion completes a few degrees after TDC.
Engine Variable Comparison
| Factor | SI Engine | CI Engine |
|---|---|---|
| Compression Ratio | Low | High |
| Inlet Temperature | Low | High |
| Inlet Pressure | Low | High |
| Fuel Quality | High Octane | High Cetane |
| Delay Period | Long | Short |
| Engine Load | Low | High |
| Wall Temperature | Low | High |
| Speed | High | Low |
| Cylinder Size | Small | Large |
Combustion Chamber Types
The combustion chamber is the space in the engine cylinder where the fuel-air mixture burns to produce power.
- Side Valve Combustion Chamber: Both valves are fitted in the engine block; simple design but less efficient.
- F-Head Combustion Chamber: One valve is in the head, the other in the block; better than side valve designs.
- I-Head (Overhead Valve) Combustion Chamber: Both valves are in the cylinder head, providing better air-fuel mixing and higher performance.
