The document discusses the combustion process in internal combustion engines, detailing types such as normal and abnormal combustion, and their effects on engine performance, efficiency, and emissions. It highlights factors influencing combustion in spark ignition (SI) engines and common issues like knocking, providing methods for controlling these problems. Additionally, the document compares combustion stages and challenges in both spark ignition and compression ignition engines.

1. COMBUSTION AND ITS EFFECTS ON
ENGINE CYCLE
PRESENTED BY
Uzair Aslam (2014-ME-313)
Muhammad Talha Aziz (2014-ME-311)
Anees Ahmad Khan (2014-ME-340)
Khurram Ahmad (2014-ME-329)

2. COMBUSTION
 In an internal combustion engine the ignition and combustion of the fuel occurs within the engine
itself. The engine then partially converts the energy from the combustion to work
 Combustion controls
 Engine power
 Efficiency
 Emissions
Fig 1: Combustion process in I.C Engine

3. TYPES OF COMBUSTION
 Normal combustion:
• Initiated solely by a timed spark
• in which the flame front moves completely across the combustion chamber in a uniform manner at a normal
velocity.
 Abnormal combustion:
• flame front is started by hot combustion-chamber surfaces
• either prior to or after spark ignition

4. COMBUSTION IN S.I ENGINES
 Ignition & Flame development
 Flame propagation
 Flame termination
Mass Burn Fractions
0-5% 5-95% 95-100%
Mass Burn Fractions
0-5% 5-95% 95-100%
Fig 2: Stages of combustion in S.I
Engines

5. FACTORS AFFECTING COMBUSTION IN S.I ENGINES
 Rate of flame propagation affects the combustion process which further depends on
1. Air to Fuel ratio
2. Compression ratio
3. Engine load
4. Turbulence and engine speed

6. ABNORMAL COMBUSTION IN S.I ENGINES
 Knocking
 Surface ignition
 Partial burning

7. KNOCK
 Knock is the name given to the noise that results from the auto ignition of a portion of the
fuel, air, residual gas mixture ahead of the advancing flame.
 It creates local hot spots (Hot spots causes pre-ignition)
 It makes noisy, overheated, and inefficient engine, and perhaps eventual mechanical failure.
 The cylinder pressure rises beyond its design limits and if allowed to persist, it will damage or
destroy engine parts.

8. HOW TO CONTROL KNOCK IN ENGINE
 Fuel with higher octane rating.
 Increasing the amount of fuel injected
 Retardation of spark plug ignition
 Colder heat range spark plug (where the spark plug insulator has become a source of pre-
ignition leading to knock)
 Reduction of charge temperatures

9. KNOCKING EFFECTS ON ENGINE CYCLE
Fig 3: Pressure rise in knocking

10. DAMAGE CAUSED BY SEVERE KNOCK
PISTON
DETONATION
Cylinder Head
Damage

11. STEPS OF COMBUSTION IN C.I ENGINES
 After injection the fuel must go through a series of events to assure the proper combustion
process:
 Atomization
 Vaporization Mixing
 Self-Ignition
 Combustion

12. COMBUSTION IN C.I ENGINES
 It contains the following stages:
• Ignition delay
• Premixed combustion phase
• Mixed controlled combustion phase
• Late combustion phase.
Fig 4: Stages of combustion in C.I
engines

13. KNOCKING
 A low compression ratio permitting only a marginal self ignition temperature to be reached.
 A low combustion pressure due to worn out piston, rings and bad valves
 Low cetane number of fuel
 Poorly atomized fuel spray preventing early combustion

14. COMPARISON
Fig 5: Comparison of pressure rise in knocking for
S.I and C.I engines