Combustion and Combustion Chambers | What does it mean When an Engine Detonates | What is Pre-ignition Knock | Types of Combustion Chamber

Three phases of combustion in S.I. Engines:

01-three phases of combustion in SI engines

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

01-regular combustion - combustion process steps


01- detonation in IC Engines - Knocking in SI Engines

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 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 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


01- Preignition in IC Engine - detonation in IC Engines - Knocking in SI Engines

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

01 - preignition - Spark plug igniting the air fuel mixture

01-preignition steps - combustion process steps - Failures in combustion processes

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

01- side valve type Combustion Chamber

Wedge type

01-wedge shape combustion chamberInverted bath tub type01- Inverted bath Tub type Combustion chamber for Diesel EnginesFlat head type01-flat head type combustion chamberHemispherical type01-hemi spherical type combustion chamberStratified charge type01-stratified charge type combustion chamberMulti – valve type01-multi valve type combustin chamberSplit level type

Twin spark plug type01-twin spark plug type combustion chamber

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

01-how knocking occurs in a Diesel engine steps

Combustion chambers for C.I. Engines:

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

Engine Combustion and Fuel Properties | Types of Auto Engine Fuels | Calorific Value of Fuels

Important properties of Engine fuel:

  • Knock rating
  • Volatility
  • Calorific value
  • Gum content
  • Sulphur content
  • Aromatic content

Requirements of an ideal Engine Fuel:

  • Knock resistant  –  Octane number
  • Readily mix with air – Volatility
  • Clean – Contamination
  • Non corrosive – Sulphur content
  • Not form gum – Gum content

Knock Rating:

Knock rating of a fuel is essentially a direct comparison of the intensity of the knock produced by it with that of a standard fuel. Chemical analysis or Bomb Explosions methods have been used to find out the knock ratings. Standard fuels used are heptane, isooctane.


There are several methods by which the knock rating of SI fuel could be rated:

  • Highest useful Compression ratio
  • Octane number
  • Sensitivity
  • Performance Number

Many design and operating factors affect the Knock ratings. The important properties are:

  1. Compression ratio
  2. Engine Speed
  3. Output
  4. Atomization of fuel and duration of injection
  5. Injection timing
  6. Quality of the fuel
  7. Intake temperature


Fuel volatility is the tendency to evaporate under a given set of conditions with which the liquid changes to vapour. Methods used to find the volatility are ASTM distillation test and Reid vapour test. Normally higher volatility reduces the HC emission and increases the NOx emissions

Important properties of fuel volatility are:

  • Hot and Cold start ability
  • Vapour lock
  • Short and long trip economy
  • Smooth running of the engine
  • Engine worm up period
  • Hot stalling
  • Carburettor icing
  • Acceleration
  • Power and Deposit formation
  • Crank case dilution
  • Spark plug Fouling
  • Evaporative losses
  • Varnish and Sludge deposits

Calorific Value or Heating value:

Calorific value is the heat released by the fuel when completely burnt and this value determined at constant volume or constant pressure and flue gas is cooled back to the initial ambient temperatures. Ultimate analysis testing method is used to find out the calorific value. Heating value is used for energy content evaluation. Liquid fuels cannot be burnt in liquid form, so it has to be convert to gaseous form. So liquid fuels has to be evaporated.


Gum content or Deposits:

The gum content is the non-volatile residue that is left after the evaporation of the sample under controlled conditions. Many fuels oxidize slowly during storage and the sediments that forms may be precipitate and clog filters. When the filtered fuel, containing soluble gum comes in contact with hot metal and it leave hard deposits that clog screens and narrow passages making diesel engines inoperable. Formation of both sediment and hard deposits results from oxidation of fuel. The amount of gum should be as low as possible since the use of fuels with high gum contents can lead to deposits in induction systems or cause intake valves and fuel injectors to stick. Jet Evaporation test procedure is used to determine the existing gum content.

Sulphur Content:

When fuel is burnt the sulphur combines with oxygen (SOx) to create emissions that contribute to decreased air quality and have negative environmental and health effects. High sulphur content decreases the catalytic conversion capacity of a system, thus increasing the emissions of Nitrous oxides (NOx), Carbon monoxide (CO), Hydrocarbons and Volatile organic compounds (VOC). Use of Sulphur free fuel will reduce environmental emissions of particulate matter from existing automobiles.

Aromatic content:


Hydrocarbons (HC) derived from crude oil has aromatic odour. For improving the octane rating of fuels aromatic materials such as Benzene, Toluene, Xylene are used as additives to gasoline.  Methods to find out the aromatics are: Fluorescent indicator absorption, Nuclear magnetic resonance method, Gas chromatography / Mass spectrometry

Types of Auto engine Fuels and Alternate Fuels:

01-types of fuels - alternate fuels

Conventional Fuels

  • Petrol
  • Diesel
  • Alternative Fuels

Compressed Natural Gas (CNG)

  • Liquified Natural Gas (LNG)
  • Liquified Petroleum Gas (LPG)
  • Alcohols, e.g., Methanol, Ethanol
  • Electricity
  • Hydrogen
  • Bio Diesel
  • P-Series01-alternate fuels - liquified petroeum gas - Compressed natural gas - liquified natural gas - LPG, CNG, LNG