1. Introduction
Compression Ignition (CI) engines, combustion is initiated by self-ignition of fuel due to the high temperature of compressed air inside the cylinder. Unlike SI engines, no spark plug is used. Fuel is injected directly into highly compressed hot air, where it ignites automatically.
Diesel engines are the most common example of CI engines.
2. Combustion Process in CI Engines
Combustion in CI engines is a heterogeneous process because:
- Air and fuel are not premixed
- Fuel is injected in liquid form
- Mixing and combustion occur simultaneously
Combustion depends heavily on:
- Fuel injection characteristics
- Air motion (swirl, turbulence)
- Ignition delay period
3. Fuel Injection in CI Engines
- Fuel is injected near the end of compression stroke
- Injection pressure is very high (200–2500 bar in modern engines)
- Injection systems:
- Common rail injection
- Unit injector
- Distributor pump
Proper atomization and penetration of fuel are essential for efficient combustion.
4. Stages of Combustion in CI Engines
Combustion in CI engines occurs in four distinct stages:
Stage 1: Ignition Delay Period
- Time between start of fuel injection and start of combustion
- Includes:
- Physical delay (atomization, vaporization, mixing)
- Chemical delay (pre-flame reactions)
Factors affecting ignition delay:
- Cetane number of fuel
- Compression ratio
- Intake air temperature and pressure
- Injection timing
A long ignition delay causes excessive fuel accumulation, leading to rough combustion.
Stage 2: Rapid or Uncontrolled Combustion
- Fuel accumulated during delay burns rapidly
- Sudden rise in pressure and temperature
- Responsible for diesel knock
- Occurs shortly after ignition
Stage 3: Controlled Combustion
- Fuel burns as it is injected
- Pressure rise is controlled
- Major portion of useful work is produced
- Combustion rate governed by fuel injection rate
Stage 4: After-Burning
- Burning of remaining fuel particles
- Occurs during early expansion stroke
- Caused by incomplete mixing
- Affects exhaust emissions
5. Pressure–Crank Angle Diagram
- Peak pressure occurs after TDC
- Pressure rise is smoother than in uncontrolled stage
- Combustion duration is longer than SI engines
6. Factors Affecting CI Engine Combustion
a) Cetane Number
- Indicates ignition quality of diesel fuel
- Higher cetane → shorter ignition delay
- Improves smoothness of combustion
b) Injection Timing
- Advanced injection → knocking
- Retarded injection → loss of power and efficiency
c) Air Motion
- Swirl and turbulence improve mixing
- Enhances combustion efficiency
d) Compression Ratio
- Higher compression → better ignition
- Typical range: 14:1 to 25:1
7. Combustion Chamber Design
Objectives:
- Efficient air–fuel mixing
- Controlled combustion
- Reduced emissions
Types:
- Open combustion chamber
- Pre-combustion chamber
- Swirl chamber
8. Abnormal Combustion in CI Engines
Diesel Knock
- Caused by sudden pressure rise during uncontrolled combustion
- Produces knocking sound
- Less severe than SI engine knock
Causes:
- Long ignition delay
- Low cetane fuel
- Advanced injection timing
9. Control of Diesel Knock
- Use of high cetane number fuel
- Proper injection timing
- High compression ratio
- Improved atomization
10. Comparison: SI vs CI Engine Combustion
| Aspect | SI Engine | CI Engine |
|---|---|---|
| Ignition | Spark plug | Self-ignition |
| Mixture | Homogeneous | Heterogeneous |
| Combustion | Flame propagation | Diffusion combustion |
| Fuel | Petrol | Diesel |
| Knock | Detonation | Diesel knock |
11. Advantages of CI Engine Combustion
- Higher thermal efficiency
- Better fuel economy
- Higher torque at low speeds
- No spark ignition system required
12. Disadvantages
- Noisy operation
- Higher emissions (NOx, particulates)
- Heavier engine construction
- Difficult cold starting
13. Applications
- Trucks and buses
- Agricultural machinery
- Marine engines
- Power generators
- Construction equipment