posted September 04, 2001 10:44 AM
CERTAIN fuels are more sensitive to pre-ignition than others and this is due to a function known as the Auto-ignition temperature characteristic, which in fact means that once above the flash point of the fuel under consideration, there is a temperature point at which this fuel will ignite, this being the particular temperature known as the Auto-ignition temperature of that fuel.
Since this point varies from fuel to fuel, it does mean by the choice of the fuel selected for use, plus if required the use of additives, the optimum fuel mixture can be selected to reduce pre-ignition sensitivity by the act of raising the Auto-ignition temperature point of the total fuel mixture.
In order therefore to take advantage of this information it is necessary to establish the actual temperature at which the fuel it is proposed to use suffers from this Auto-ignition characteristic.
Once this information is obtained it then becomes necessary to make quite sure that the temperature inside the combustion chamber does not under any circumstances exceed this figure, as otherwise we run straight into trouble.
If for example it does, due to the use of a very high compression ratio, we will, to avoid trouble, either have to reduce this ratio, or use another type of fuel, or yet again with the existing fuel, use an additive to increase the temperature point, bearing in mind that the higher the compression ratio used the greater the heat produced in the charge, as previously explained when looking at our simple heat engine.
In connection with this statement it must be appreciated that apart from the heat produced by the actual compression of the fuel, there is to this added the residual heat from the engine internals, which in themselves may be below the critical point, but when added to, or combined with the other, exceed the vital figures
A further point that should be considered is that the temperature of the fuel under compression is related to the actual time taken to compress it, so that as an example, a high revving engine may well pre-ignite at a certain point and not do so at lower revolutions.
This explains why in the Start area, or on the line, all may seem well, but once the power is turned on and the engine speed increased troubles commence.
The Octane rating of the fuel in use indicates the detonation sensitivity of the fuel and relates directly to the maximum possible compression ratio that may be used with that particular fuel.
Again the use of additives will allow that ratio to be a altered.
Naturally every effort must be made to eliminate detonation and on the smallest indication of it taking place, prompt action taken at once to correct the existing conditions.
With street vehicles it is possible to get the well known audible indications of "pinking", but with the competition engine, due to the high noise level this may not be so.
Also on multi-cylinder engines you may well have trouble in one or more cylinders, the rest of the engine then masking the trouble, and in fact running the faulty cylinders into destruction, the overall noise quite dominating the "pinking" to the point of it being inaudible.
The only real way to check on detonation taking place is by examination of the cylinder head and piston, or what remains of the latter if the trouble has been severe, which is often the case.
Yet once again you will appreciate how vital it is to make sure you are getting enough mixture to the engine, the cost of the fuel, even if most of it is blown out of the exhaust system being just so much less than that of mechanical failure and the resultant expense putting it right.
While on the matter of getting fuel to the engine we would say without hesitation that on competitive engines over three litres in capacity, the use of normal carburetors fed by means of float chambers is suspect when using fuel other than petrol, and if supercharged, the capacity figure will be even lower.
It is for this reason that fuel injection is so popular on large engines used on special fuels to produce high power outputs, and in the case of the very large engines it becomes the only practical way, the use of carburetors being abortive.
Since in general for competition work you are not too concerned with fuel economy, the simpler forms of fuel injection are quite satisfactory, eliminating the expensive and elaborate but effective systems of holding the optimum fuel to air ratio over the operating range of the engine, and in general we see used continuous feed types, such as for example the Hilborn, Enderle, etc.
Power is always difficult to obtain and you cannot take out more than put in, as many have found out, and in fact you cannot get as much in practice, so if you propose the use of certain horsepower, you must provide fuel in quantity enough to release the necessary energy to provide that amount of power, after taking into consideration the losses in the system of actually converting the energy from the input form to the output form.
We are now almost at the end of the road. With the information we now have, it only remains for us to use certain additives to mix with the fuel in order to obtain a mixture that will enable us to extract more power out of existing engines, without stressing them mechanically to destruction.