1. What is Spark Ignition (SI) Engines?
In Spark Ignition (SI) engines, combustion is initiated by an electric spark produced by a spark plug at the end of the compression stroke. The combustible air–fuel mixture is prepared either in a carburetor or by fuel injection and is ignited near top dead center (TDC).
Combustion in SI engines is a controlled flame propagation process, unlike the sudden auto-ignition seen in CI engines.
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2. Air–Fuel Mixture in Spark Ignition (SI) Engines
- The mixture is homogeneous
- Fuel is usually petrol (gasoline)
- Mixture ratio significantly affects combustion speed and efficiency
Typical air–fuel ratios:
- Rich mixture: 10–12 : 1
- Stoichiometric mixture: 14.7 : 1
- Lean mixture: 16–18 : 1
3. Ignition Process
- Spark occurs slightly before TDC
- Spark timing is advanced to compensate for:
- Finite flame speed
- Engine speed
- Load conditions
The spark creates a small flame kernel, which grows and propagates through the mixture.
4. Stages of Combustion in Spark Ignition (SI) Engines
Combustion in SI engines occurs in three distinct stages:
Stage 1: Ignition Delay Period
- Time between spark discharge and noticeable pressure rise
- Chemical reactions begin
- Flame kernel formation
- Short duration (10–15 crank angle degrees)
Stage 2: Flame Propagation (Rapid Combustion)
- Flame front travels across combustion chamber
- Major portion of fuel burns
- Rapid rise in pressure and temperature
- Accounts for about 70–80% of total combustion
Stage 3: After-Burning
- Burning of remaining fuel pockets
- Occurs during early expansion stroke
- Pressure rise slows down
5. Flame Propagation in SI Engines
- Flame travels in a spherical manner
- Propagation speed depends on:
- Air–fuel ratio
- Turbulence
- Engine speed
- Combustion chamber design
- Compression ratio
Typical flame speed: 20–30 m/s (turbulent)
6. Factors Affecting Spark Ignition (SI) Engines Combustion
a) Air–Fuel Ratio
- Maximum flame speed near stoichiometric mixture
- Rich mixture → higher power
- Lean mixture → better fuel economy
b) Turbulence
- Increases flame speed
- Achieved by swirl and squish motion
- Improves combustion efficiency
c) Spark Timing
- Too early → knocking
- Too late → loss of power and efficiency
d) Compression Ratio
- Higher compression → higher efficiency
- Limited by knocking tendency
7. Combustion Chamber Design
An ideal combustion chamber:
- Short flame travel distance
- Central spark plug location
- High turbulence
- Minimum heat loss
Common designs:
- Hemispherical chamber
- Pent-roof chamber
- Wedge-shaped chamber
8. Abnormal Combustion in Spark Ignition (SI) Engines
a) Knocking (Detonation)
- Auto-ignition of end-gas ahead of flame front
- Causes pressure waves
- Produces metallic knocking sound
- Leads to engine damage
Causes:
- High compression ratio
- Low octane fuel
- Advanced spark timing
- High engine temperature
b) Pre-Ignition
- Ignition before spark occurs
- Caused by hot spots (spark plug, carbon deposits)
- More dangerous than knocking
9. Methods to Control Knocking
- Use of high octane fuel
- Retarding spark timing
- Richening air–fuel mixture
- Improved cooling
- Proper combustion chamber design
10. Combustion Duration
- Measured in crank angle degrees
- Typical duration: 30–40° crank angle
- Decreases with increased engine speed due to turbulence
11. Advantages of SI Engine Combustion
- Smooth and controlled combustion
- Lower noise compared to CI engines
- Easier cold starting
- Lighter engine construction
12. Disadvantages
- Lower thermal efficiency
- Knocking limits compression ratio
- Higher fuel consumption
- Lower torque at low speeds
13. Applications
- Passenger cars
- Motorcycles
- Small power generators
- Light aircraft engines