Combustion

COMBUSTION is any process (usually involving oxygen and some hydrocarbon arrangement, though sulphur and oxygen become involved with diesel fuels) where chemical energy, stored prior to the onset of the process, is converted into heat energy during the process.

It is normal practice to regard the substance, which is involved in the combustion process, with the oxygen as the fuel (or food, in the case of animal stomachs).

In engineering – combustion chambers, the fuel is usually some mixture of hydrocarbons on liquids (derived from crude petroleum) or in gas mixtures (natural or produced) or in solid substances (such as coal, wood, etc).

The ingredients, suitable for burning (i.e. combustion) must be initially raised to a particular temperature, called the IGNITION TEMPERATURE or FLASHPOINT TEMPERATURE, when the chemistry, of rearranging the molecular arrangement of the atoms – involved in the process - caused flame to be initiated.

Once flame has been started, by the spark at the spark plug in S.I. engines, OR spontaneous ignition in C.I. engines (or, as in PRE IGNITION or POST IGNITION in SI.I engines) heat energy results from the conversion of the fuels chemical energy. Such heat energy rapidly rises the air fuel mixture to much higher pressures and temperatures to that of the products of the combustion process, in spark or compression ignition engines.

It is the increased pressures, which are these, used to drive the engine piston and so produce output power to drive the vehicle.

i) SPARK IGNITION ENGINES

The compressed charge of all air and vapourized fuel has the initial stage of the combustion process, started in the spark plug gap by the discharge of a spark caused by the engine ignition system. The resulting flame rapidly moves throughout the combustion chamber clue to the swirling charge; caused by the flow conditions through by the inlet valve port during the cycle induction stoke. The very rapid rise in temperature and pressure can cause flame to be started with the compressed mixture once the flashpoint is reached prior to the arrival of the spreading flame. Such conditions are called POST IGNITION and are desirable providing that the burning is controlled (i.e. stable) by the way in which the various hydrocarbon (which comprise the fuel) react with the oxygen.

Circumstances, during the compression stroke can develop whereby overheating of the cylinder conditions can result in the cylinder charge reaching flash point prior to the spark arriving at the spark plug. Such conditions are called PRE IGNITION and should be avoided as a loss of engine power, overheating, valve burn out and engine run on (i.e. engine continues to run when the ignitions is switched off) can occur.

Air alone is delivered to the cylinder via the inlet valve part (unless hybrid arrangements are employed whereby a combustible gas such as methane or ethane are supplied with the air, as an extremely lean mixture). This enables much higher compression rations to be used (typical values being 17 up 25 to 1) than can be used by spark ignition engines (because of PRE IGNITION difficulties). Such compression rations cause very high charge temperatures to be developed at the end of the compressions stroke (600 to 700oC being a typical range).

An injector nozzle mounted in the cylinder head sprays fuel oil in the form of minute droplets, into the combustion chamber. The droplets are quickly heated to flash point by the air and an explosion results followed by torch burning of the follow-on fuel, delivered by the injector. Combustion only occurs by the action of the oxygen on the hydrocarbon droplets. Much of the resulting heat energy from combustion of these droplets as used to heat the surrounding air, which is not used for combustion. Such conditions enable very high air to fuel rations to be used – though the actual combustion occurs within the inflammable range (see section on Air to Fuel Rations). The limiting air to fuel ratio is a lean mixture that caused smoke to be just visible in the exhaust emissions for public road vehicles or tolerable smoke for off-public road vehicles, such as forestry or civil engineering sites.

For public road vehicles, a limiting SMOKE POINT air to fuel ratio is about 22 to 1 by mass with values of 100 to 1 being normal for light-throttle driving conditions.