SPARK IGNITION (SI) ENGINES COMBUSTION

1. Introduction

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.

2. Air–Fuel Mixture in 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 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 SI Engine 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 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