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-Series
  • 01-alternate fuels - liquified petroeum gas - Compressed natural gas - liquified natural gas - LPG, CNG, LNG

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