Important Engine Combustion and Fuel 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:

Compression ratio
Engine Speed
Output
Atomization of fuel and duration of injection
Injection timing
Quality of the fuel
Intake temperature

Volatility:

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 Alternative 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