COMBUSTION

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Combustion.

This is an exothermic reaction (one in which heat is liberated by the action) between a fuel and oxygen. Liquid fuels consist of carbon, & hydrogen, in the form of hydrocarbons, with small quantities of sulphur & traces of other metallic Impurities such as vanadium.

A typical fuel analysis, by mass would be:

C = 5%, H₂ = 12%, S = 3%, with a C.V. of 44000 KJ/Kg.                                                      

(19000 BTU/lb.)

The oxygen is obtained from the air, which can be considered to contain 77% nitrogen & 23% oxygen by mass.

The nitrogen plays no active part in the combustion process but it is necessary as it acts as a moderator. With pure oxygen, the combustion would be violent & difficult to control & it would produce very high temperatures, creating cooling, metallurgical & lubrication problems.

The reactions, which occur, are:

2H₂ + O₂ ----------- 2H₂O – liberating 142 MJ/kg. H₂.

C + O₂ -------------- CO₂ – liberating 33 MJ/kg. C.

S + O₂ --------------- SO₂ – liberating 9.25 MJ/kg. S.

2C + O₂ --------------2CO – liberating 10 MJ/kg. C.  

Combustion will only occur within limits in the air/fuel mixture. If too much air is supplied all the fuel will be burnt but the excess of oxygen & nitrogen will carry away heat. If too little air is supplied incomplete combustion will occur, when all the hydrogen will be burnt but only part of the carbon, with the remainder only burning to carbon monoxide or not burning at all. In diesel engine practice it is usual to supply between 100 & 200% excess air by mass, though 15% is sufficient for a steady flow combustion process (boiler).

This difference has two reasons:

1. As the combustion proceeds in the diesel engine, the fuel finds less & less air to combine with in a boiler air is constantly being fed in.
2. More air is needed in the diesel engine as it lowers the maximum temperature, allowing Cast iron to be used.

Combustion Process.

Fuel is injected into the clearance volume towards the end of the compression stroke, as a fine mist of very small droplets, which have a surface area many times that of the accumulated fuel charge. These droplets are rapidly heated by the hot compressed air, which has a temperature of between 550* to 650*C, causing vaporisation. The vapour mixes with air and when the mixture exceeds the spontaneous ignition temperature, (S.I.T.) combustion begins.

The process can be divided into four phases :

1.      Injection delay.

2.      Ignition delay.

3.      Constant volume combustion.

4.      Direct burning.

Injection delay:

A time lag of about 0.005 seconds occurs between trapping the fuel charge in the pump barrel and starting injection into the engine cylinder. This is due to:

a)      Elasticity of high-pressure fuel lines & system.

b)      Slight compressibility of the fuel charge.

c)      Leakage past the pump plunger & injector needle.

d)     Opening delay of the pump discharge valve & injector needle.

In a slow speed engine the lag period accounts for up to 5* of crank movement. In a high speed engine it may account for 20* or more and because of point (a) it is necessary to use fuel lines of similar length for all cylinders, when the fuel pumps are grouped together.

Ignition Delay.

Ignition delay is another short period of time delay, which is sufficient to account for several degrees of crank angle. Several factors are involved:

a)      Spreading and penetrating of the fuel in to the clearance volume space.

b)      Heating of the fuel to cause vaporization & then exceeding the fuels’ spontaneous ignition temperature.

c)      Mixing of the fuel & air in the clearance volume space before detonation.

Constant Volume Combustion.

Ignition occurs at T.D.C. when the fuel charge, which has entered during the ignition delay period, burns rapidly causing a sharp rise in cylinder pressure with little movement of the piston occurring. Modern four stroke engines may attain 100 bar; at this point where as a two stroke engines are likely to operate with pressures of 75 to 98 bar.

Direct Burning.

The remainder of the fuel burns as it enters the cylinder and mixes with air. The excess air and combustion gases prevent high temperatures and rapid combustion so the pressure remains about constant. Injection and combustion should cease simultaneously at the end of this period.

Factors Affecting Combustion.

In order to attain good combustion it is essential that:

a)      Sufficient air is supplied.

b)      Compression is high enough to give a temperature above the spontaneous ignition temperature.                                 

c)      Good mixing of the air and fuel is obtained.

All of these give problems. The factors affecting combustion are:

1.      Atomisation.

2.      Penetration.

3.      Turbulence.

1. Atomisation.

The rate of heat absorption and burning depends upon the surface area of the fuel particles. As this must be rapid it follows that the surface area needs to be big & this is achieved by breaking up the fuel into small droplets. The amount of the fuel pressure, diameter of injector nozzle holes and the viscosity of the fuel, affect the process.

2. Penetration.

To use all the air in the combustion space it is necessary to give the fuel particles sufficient energy to enable them to penetrate to the extremes of the space. This is controlled by the fuel pressure, the size of the particle & the length to diameter ratio of the nozzle hole (From 2:1 to 5:1). The latter also controls the angle of spray.

3. Turbulence.

To aid mixing of fuel with air and atomisation, friction between the fuel & air is needed. Friction is a function of the relative velocity between the fuel particle and the air, and may be obtained by either of two methods.

a)      Fuel seeks air.

b)     Air seeks fuel.

a)      The air is static or slow moving and the mixing energy is obtained from the fuel particles. Injection pressures of 200 to around 1000 bars are needed from multi-holed nozzle injectors. Advantages are, simplicity, economy and easier for cold starting the engine. The latter because little air movement means reduced heat loss to the cold liner and piston crown (also assists in the burning of heavy fuel). Disadvantages are in producing and sealing high fuel pressures.

b)      The air is made to swirl rapidly at the end of the compression stroke by using a pre-designed combustion chamber. Single holed nozzles and lower fuel pressures are used, 70-100 bars. Advantages are simplicity of injection, equipment and rapid combustion (useful in high speed engines). Disadvantages are complicated combustion chambers and high rate of heat loss to surroundings. Causes difficulties in cold starting, sometimes needing cylinder combustion space heating system.

In practice, a combination is often used minimum fuel pressures being used with a small degree of swill produced by vaned inlet valves or tangentially cut scavenge ports.     Quantity of swirl causes half the liner circumference to be traversed during combustion.

Combustion Faults.

Detonation.

The combustion process is regarded as a controlled explosion with a flame front speed of about 25 m/s. However if combustion conditions are not correct double ignition may occur and a ‘detonation’ may result. The latter occurs when the mixture is rapidly compressed by an initial ignition and the remaining mixture is overheated and burns almost instantaneously (Flame speed 2000 m/s). The detonation can set up very high pressures, temperatures and causes vibration of the cylinder and piston. It also reduces the efficiency of the engine as energy is absorbed producing the vibration.

After burning.

This occurs when combustion extends into the expansion period after the injector has closed. It is caused by poor ignition qualities or very poor atomization and produces high exhaust pressures and temperatures.

Injection timing.

Early injection produces high firing pressures; late injection produces low firing pressures and high exhaust pressures. In both cases the engine power is reduced.

All these faults could be seen very clearly in indicator cards of each unit.

Ideal Combustion.

To obtain maximum thermal efficiency, the combustion process should be carried out as close to the Otto cycle as practically possible. This means, the rate of rise of pressure should be as rapid as possible, without exceeding the designed mechanical and thermal loading. To achieve maximum mean effective pressure the fuel remaining after the initial period of rapid rise, should be burned at a rate which will hold the cylinder pressure constant, at the maximum design value until the fuel is burned.

Some of those factors affecting the ideal combustion can be considered as follows.

Injection timing.

Using jerk injection system, it has been found that the shortest delay period occurs when it includes T.D.C.

1.      Early injection results in increased delay since the pressure and temperature are still rising, so auto injection energy has not been reached.                                                                                                                                     

2.      Late injection causes increased delay since the piston is accelerating away from the cylinder head and temperature and pressure fall rapidly.

In each case, the rate of pressure rise is increased due to the large quantity of the fuel in the combustion space before the chemical reaction is initiated. The reaction, which follows involves a massive amount of fuel and approximates to detonation.

This results in ‘Diesel knock’, the effects of which are determined objectionable. Many engines are timed later than that which gives maximum mean effective pressure to reduce the rate of pressure rise and the maximum pressure. This however involves some sacrifice in efficiency and power output.

Engine R.P.M.

Since the delay period is determined mainly by the fuel characteristics, it follows that delay tends to be independent of engine speed. The delay angle however will vary with engine speed and have considerable influence on the pressure / crank angle diagram.

In each case – 10 deg. BTDC & 20deg. BTDC the delay angle is increased with increase in speed.