If pure oxygen is used in a petrol engine instead of air (which is a mixture consisting of 76.8 percent, nitrogen and 23.2 percent oxygen by weight), the explosion will be almost instantaneous and very violent; This extremely rapid explosive effect is termed DETONATION and this is the biggest combustion problem in I.C Engines.
Many theoretical explanations have been given for the phenomenon of detonation, it is only more recently that experimental investigations have established that detonation occurs when the rate of inflammation or burning of the mixture exceeds a certain critical value; the latter depends upon several factors including the nature of the fuel used, compression ratio, combustion chamber design, ignition timing, etc.
It is now believed that the earlier stages of burning of the charge after the spark occurs normal in character, but that towards the end of the combustion process in the combustion chamber, a sudden burning effect of the remaining part of the charge, or “end gas” occurs which gives rise to a very rapid rate of pressure rise which produces impact effects out the combustion chamber walls, usually termed detonation or knock.
In recent years a considerable amount of investigation work has been carried out on combustion process and detonation. The following practical aspects have been established.
1. In certain overhead valve engines there is greater tendency towards detonation at lower engine speeds. This is believed to be due to increased temperature of the mixture.
2. Detonation occurs at maximum power output, so that the anti-knock fuel is only needed at larger throttle openings; a lower grade of fuel could be used for cruising conditions.
3. To obtain the greatest benefit from a given antiknock fuel an engine should be run with a higher compression ratio that that giving no detonation, but the ignition should be retarded at full throttle.
4. Detonation is more likely to occur with weaker than with rich mixtures, within certain limits. Detonation is a minimum for any rich mixtures, namely 9 to 11.1 (air fuel ratio) and a maximum for rich mixtures of 12 to 14:1.
5. Combustion chamber design has a very important influence on detonation so that while some engines, using a given fuel, mixture strength and compression ratio will detonate.
Overhead valve engines on account of their higher volumetric efficiencies and higher mean pressure tend to detonate more than side valve engines under similar conditions of mixture, load, speed, etc.
Pre-Ignition another cause of engine knocking, which is quite distinct from detonation effects, since it always occurs before the ignition spark, whereas the latter always occur afterwards, is that known as pre-ignition. It is often due to the inefficiency of the engine cooing arrangement, so that all of the surplus heat cannot be disposed of, with the result that the temperature within the combustion chamber increases progressively until some projecting part, such as the electrodes of the sparking plug or a protruding piece of carbon deposit, becomes so hot that it ignites the charge before the spark occurs at the sparking plug. The piston is actually ascending when this happens and it therefore receives a retarding force. Generally, once pre-ignition commences, its effects become more and more enhanced with the result that the engine loses power and eventually stops.
The chief causes of pre-ignition may be summarised, as follows:
1. carbon deposits;
2. unsuitable types of sparking plug, i.e., one that runs too hot or has a too long a reach:
3. overhead exhaust valve head edge;
4. ignition too far retorted; and
5. mixture much too weak or rich, giving too slow a burning rate
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