Combustion Stages and Working of SI and CI Engines

Posted on Jul 21, 2026 in Civil Engineering

Stages of Combustion in SI and CI Engines

The P-θ Diagram plots pressure versus crank angle (θ) around Top Dead Center (TDC), illustrating the distinct stages of combustion for both engine types.

Combustion Stages in SI Engines

The Spark Ignition (SI) engine combustion process consists of three main stages:

• Ignition Lag (A-B): Also called ignition delay, this is the time between the spark occurring (A) and the start of flame propagation (B). No visible pressure rise occurs during this phase. It depends on fuel type, temperature, pressure, and the air-fuel (A/F) ratio.
• Flame Propagation Phase (B-C): The flame spreads rapidly from the spark plug through the combustion chamber. Pressure rises sharply, and the slope of curve B-C indicates the rate of pressure rise.
• After-burning (C-D): Combustion continues even after peak pressure (point C) into the expansion stroke as some unburned fuel continues to burn. The flame velocity gradually decreases until point D.

Combustion Stages in CI Engines

The Compression Ignition (CI) engine combustion process is divided into four stages:

• Ignition Delay (A-B):
  • Physical delay: The time required for injected fuel to atomize, vaporize, and mix with air.
  • Chemical delay: The time for auto-ignition reactions to start after the physical delay.
• Uncontrolled or Rapid Combustion (B-C): Fuel accumulated during the delay period burns almost instantly at multiple locations, leading to a very high rate of pressure rise, which can cause diesel knock.
• Controlled Combustion (C-D): The rate of burning is controlled by the rate of fuel injection as additional fuel continues to enter the chamber.
• After-burning (D-E): Remaining unburned fuel continues to burn during the expansion stroke due to heterogeneous mixing.

Differences Between SI and CI Engines

The following table compares the construction and working principles of SI and CI engines:

SI Engine Construction and Working Principles

Main Components of SI Engines

• Cylinder block: Houses the cylinder, crankshaft, camshaft, gears, and pumps.
• Cylinder head: Fitted with the spark plug, intake valve, exhaust valve, and combustion chamber.
• Piston: Transmits gas force to the connecting rod; designed to be lightweight with high thermal conductivity.
• Connecting rod & Crankshaft: Converts the reciprocating motion of the piston into rotary motion.
• Carburetor or Fuel injector: Supplies the correct air-fuel mixture to the cylinder.
• Spark plug: Produces the spark to ignite the compressed air-fuel mixture.
• Valves (intake & exhaust): Control the entry of fresh charge and the exit of exhaust gases.
• Compression ratio: Typically low, ranging from 6–10.

The Four-Stroke SI Engine Cycle

• Suction Stroke: The piston moves from TDC to Bottom Dead Center (BDC), the intake valve opens, and the air-fuel mixture is drawn into the cylinder.
• Compression Stroke: Both valves are closed; the piston moves from BDC to TDC, compressing the mixture and raising its pressure and temperature.
• Power Stroke: Just before TDC, the spark plug ignites the mixture. Combustion pressure pushes the piston from TDC to BDC, producing power.
• Exhaust Stroke: The exhaust valve opens, and the piston moves from BDC to TDC, pushing burned gases out of the cylinder.

CI Engine Construction and Working Principles

Main Components of CI Engines

• Cylinder block & head: Similar to SI engines but stronger and heavier to withstand higher pressures.
• Fuel injector: Replaces the spark plug; injects fuel directly into the cylinder at high pressure.
• Fuel pump: Supplies high-pressure fuel to the injector at the correct timing.
• Piston: Heavier and designed to withstand higher compression loads.
• Note: There is no carburetor or spark plug; only air is drawn in and compressed.
• Compression ratio: High, ranging from 16–20.

The Four-Stroke CI Engine Cycle

• Suction Stroke: Only pure air is drawn into the cylinder as the piston moves from TDC to BDC with the intake valve open.
• Compression Stroke: Air alone is compressed to a very high pressure and temperature as the piston moves from BDC to TDC.
• Power Stroke: Just before TDC, fuel is injected as a fine spray. It self-ignites due to the high air temperature. Combustion pushes the piston down, producing power.
• Exhaust Stroke: The exhaust valve opens, and the piston moves from BDC to TDC, pushing out burned gases.

Detonation and Knocking in SI Engines

Mechanism and Causes of Detonation

Definition: Detonation is an abnormal combustion phenomenon where the end portion of the unburned air-fuel mixture (end charge) explodes spontaneously instead of burning smoothly, producing shockwaves and knocking sounds.

Mechanism: Normally, the flame spreads smoothly from the spark plug. However, if the temperature and pressure of the unburned end-gas rise too high before the flame reaches it, the end charge auto-ignites suddenly. This causes a sudden pressure spike seen as a jagged peak on a P-θ or P-V Diagram.

Causes:

• Low octane number fuel
• High compression ratio
• Advanced ignition timing
• High engine load and temperature

Effects on Engine:

• Loss of power output and engine overheating
• Increased mechanical stress and potential piston/cylinder damage
• Knocking noise and vibration
• Long-term damage if not corrected

Factors affecting detonation: Compression ratio (higher CR → higher chance), ignition timing (advanced timing → more chance), engine load and temperature, fuel quality (octane rating), and air-fuel mixture ratio.

Factors Affecting Engine Knocking

Knocking is the metallic sound produced by detonation. When the end charge auto-ignites, high-frequency pressure waves strike the cylinder walls and piston, creating an audible sound. Knocking indicates that detonation is occurring.

• Compression ratio: Higher CR increases temperature and pressure, raising knock tendency.
• Ignition timing: Advanced timing increases the likelihood of knock.
• Engine load & temperature: Higher loads and temperatures increase knock tendency.
• Fuel quality: Low octane fuel knocks easily; higher octane resists it.
• Air-fuel ratio: Affects flame speed and end-gas conditions.