Three phases of combustion in S.I. Engines:

Combustion of a fuel is the sequence of exothermic chemical reactions between fuel and oxidizer in an engine. The combustion process usually forms carbon dioxide and water, and releases heat, light, and various other forms of energy. It is responsible for providing large amounts of mechanical energy to power an engine. The release of heat can cause the expansion of the gases resulting in their subsequent motion against each other. If this gas contains sufficient kinetic energy (due to high temperature), it may propel the engine.

Ignition Lag or Delay Period:

It is the duration between the occurrence of the spark at the spark plug and the duration of the combustion curve from the motor curve. Factors influencing this phase are:

• Nature of fuel
• Mixture ratio
• Initial temperature
• Pressure
• Temperature of the flame between the spark plug electrodes

SOI – Start of Ignition; EOI – End of Ignition

Second phase or Rapid combustion chamber:

It starts when the combustion curve deviates from the motor curve and lasts till the maximum pressure is reached

Third phase or After Burning:

This phase occurs between points of maximum pressure and maximum temperature

Engine Detonation:

An uncontrolled explosion of the unburnt air – fuel mixture in the engine cylinder occurring after the regular combustion of some of the charge caused by the spark at the spark plug

Factors influencing Engine detonation:

Factors concerning engine

• Compression ratio
• Engine Dimensions
• Throttle opening
• Spark advance
• Spark plug location
• Combustion chamber design
• Cooling system
• Carburization
• Engine speed
• Valve timing
• Turbocharging

Factors Concerning Fuel

1. Paraffins
2. Naphthene’s
3. Aromatics

Factors concerning air

1. Temperature
2. Density
3. Humidity

Mixture strength

1. Effect of detonation:
2. Inefficient combustion
3. Loss of power
4. Local overheating
5. Mechanical engine failure

Prevention of Engine Detonation:

1. Anti – knock agents
2. Cooling of the charge
3. Reducing the time factor

Octane rating:

Octane number of any fuel is the percentage of is-octane by volume in the mixture of is-octane and normal heptane which gives the same anti-knock characteristics as the fuel under standard test conditions.

There are two methods of finding octane ratings:

1. CFR research method (RON)
2. CFR motor method (MON)

(RON – MON) is called Fuel sensitivity

Pre-ignition:

The phenomenon of a hot spot, such as  a glowing spark or deposit, igniting the air fuel mixture earlier than the spark plug

Terms relating to rate of combustion:

1. Squish
2. Quench area
3. Turbulence

Considerations affecting combustion chamber design:

1. Swirl – It is rotational flow of charge within the cylinder
2. Swirl ratio – It is the ratio of the angular rotational speed of air about the cylinder axis to the crank shaft rotational speed
3. Surface to volume ratio

Combustion chambers for S.I. Engines:

Side – Valve type

Wedge type

Inverted bath tub typeFlat head typeHemispherical typeStratified charge typeMulti – valve type

Split level type – Twin spark plug type

Desirable factors for combustion chambers in S.I. Engines:

1. Smallest possible ratio of surface area of chamber to its volume
2. Shortest travel distance for the flame front
3. Adequate swirl of the incoming mixture
4. Sufficient cooling of the exhaust valve
5. Provision for cooling of the spark plug by the incoming fresh air
6. Adequate sizes and numbers of inlet and exhaust valves

Four stages of combustion in C.I. Engines:

1. Ignition delay
2. Rapid or uncontrolled combustion
3. Controlled combustion
4. After burning

Cetane number:

Earlier it was defined as the percentage of cetane in a mixture of cetane and a-methylnaphthalene, which has the same diesel knock characteristics as  the fuel under test. Recently, alpha methylnaphthalene has been replaced by a more stable compound, heptamethylnonane has been replaced by a more stable compound, heptamethylnonane, which having a slightly better knock rating has given cetane number of 15. Most diesel fuels for high speed engines have cetane numbers from 45 to 55

Comparison between Detonation and Diesel knock:

1. To avoid detonation, auto – ignition of the end –gas has to be prevented; whereas to avoid diesel knock, earliest possible auto – ignition should occur
2. Fuels having higher octane rating have poor cetane rating and vice versa
3. Compression ratio has to be limited in case of SI engines, beyond which detonation would occur. However, in case of CI engines, higher the compression ratio, lesser possibility of Diesel knock occurring
4. Large cylinder size promotes detonation, whereas diesel knock is reduced with the same

Combustion chambers for C.I. Engines:

1. Direct Injection type or Open type
2. Turbulent or Swirl type
3. Pre-Chamber type