In this article we will discuss about:- 1. Properties of Fuel 2. Quality of Fuel 3. Combustion.
Properties of Fuel:
Fuel is a substance consumed by the engine to produce energy.
The common fuel for internal combustion engines are:
1. Petrol
2. Power kerosene
3. High speed diesel oil
4. Light diesel oil
The important properties of these fuels are given in the following table:
Quality of Fuel:
The quality of the fuel mainly depends upon the following properties:
1. Volatility of fuel
2. Calorific value of fuel
3. Ignition quality of fuel
1. Volatility of Fuel:
Volatility of fuel has considerable effect on the performance of the engine by affecting the following:
(a) Ease of starting the engine.
(b) Degree of crankcase oil dilution.
(c) Formation of vapour lock in the fuel system.
(d) Accelerating characteristics of the engine,
(e) Distribution of fuel in multi cylinder engine.
In I.C. engine, all the liquid fuel must be converted into vapour fuel before burning. High speed diesel oil is most difficult to vapourise. Vapourising temperature of high speed diesel oil is higher than that of the petrol, hence the petrol vapourises quicker than diesel oil in the engine cylinder.
This helps in easy starting of petrol engines. If all the liquid fuel is not well vapourised, it would do crankcase oil dilution. A fuel having more volatility has got more ignition chances, so there is chances of vapour lock in the fuel system. Quickness in vapourising the fuel helps in producing more power swiftly which accelerates the engine.
The oil that vapourises quickly can be distributed well in different cylinders of the engine hence distribution of fuel in different cylinders is better in petrol engine than that of diesel engine.
2. Calorific Value of Fuel:
The heat liberated by combustion of a fuel is known as Calorific value or Heat value of the fuel. It is expressed in kcal/kg of the fuel. The heat value of a fuel is an important measure of its worth, since this is the heat which enables the engine to do the work.
3. Ignition Quality of Fuel:
Ignition quality refers to ease of burning the oil in the combustion chamber. Octane number and Cetane number are the measures of ignition quality of the fuel.
(a) Octane Number:
It is a measure of knock characteristics of a fuel. The percentage of iso-octane (C8H18) in the reference fuel consisting of a mixture of iso-octane and normal heptane (C7H16), when it produces the same knocking effect as the fuel under test, is called Octane number of the fuel. Iso-octane has excellent antiknock qualities and is given a rating of 100. Normal heptane would knock excessively and hence it is assigned a value of zero.
(b) Cetane Number:
The percentage of cetane in a mixture of cetane (C16H34) and alpha-methyl naphthalene (CnH16) that produces the same knocking effect as the fuel under test is called Cetane number of the fuel. Diesel fuels are rated according to cetane number which is the indication of ignition quality of the fuel. The higher the cetane number the better the ignition quality of the diesel fuel. The commercial diesel fuels have got cetane rating varying from 30 to 60.
Detonation (Knocking):
Detonation or engine knocking refers to violent noises, heard in an engine, giving a pinging sound during the process of combustion. It occurs during the process of combustion of the mixture within the cylinder after the ignition has taken place.
It is an undesirable combustion and results in sudden rise in pressure, a loss of power and overheating of the engine. It is caused by improper combustion chamber, high compression pressure, early ignition timing, improper fuel and inadequate cooling arrangement.
Pre-Ignition:
Burning of air-fuel mixture in the combustion chamber before the piston has reached the top dead centre is called pre-ignition. Pre-ignition occurs when the charge is fired too far ahead of the top dead centre of the piston due to excessive spark advance or excessive heat in the cylinder.
Qualities of Diesel Fuel:
The diesel fuel is of two types:
1. High speed diesel oil (HSD)
2. Light diesel oil (LDO)
High speed diesel oil is used for high speed diesel engines. It is lighter than LDO. Light diesel oil is the main source of fuel for slow speed engines.
(i) Diesel fuel should be free from acid or any foreign matter, dirt and moisture.
(ii) It must be able to lubricate the fuel pumps and fuel injection nozzles. Light fuels lack sufficient lubricating qualities.
(iii) Diesel fuels are rated according to the cetane number, which is the indication of ignition quality of the fuel. The higher the cetane number, the better the ignition quality of the fuel.
Fuel Combustion:
The process of fuel combustion converts the chemical energy of the fuel into heat energy.
Fuel combustion of I.C. engine takes place in two ways:
(i) A mixture of air and fuel is compressed and ignited by a spark. This is called spark ignition engine or carburetor type engine.
(ii) Air alone is compressed and fuel is injected near the end of the compression stroke. Fuel is ignited by the heat of compression. This is called diesel engine or compression ignition engine.
For getting maximum power from the carburetor engine, it is essential:
(i) To maintain proper proportion of fuel and air.
(ii) To ignite the mixture at the proper time.
Best result is obtained when carburetor is so adjusted that air-fuel ratio is about 15:1. Such mixture is called chemically correct mixture.
This relation is explained in the following example:
Example:
Find air-fuel ratio for complete combustion of fuel in a carburetor engine, using petrol which approximates to hexane C6H14.
Solution:
Air contains 23% oxygen by weight.
Petrol when mixed with oxygen forms carbon dioxide (CO2) and water.
C6H14 + O2 → CO2 + H2O
After balancing the above equation, it is obtained as:
2(C6H14) + 19(O2) = 12 (CO2) + 14 (H2O)
or (2 x 86) + (19 x 32) = (12 x 44) + (14 x 18) or 780 = 780
Oxygen per kg of fuel = 608/172 = 3.54kg.
Correct air supply per kg of fuel = 3.54 x 100/23
= 15.36 kg.
Correct air-fuel ratio = 15.37:1.