posted September 04, 2001 10:42 AM
TIPPING THE CAN
AS with the introduction of Methanol, the introduction of Nitromethane fuel means that ignition settings will have to be adjusted to take full advantage of this volatile fuel.
Starting from the position found most satisfactory for Methanol it will be found that as the amount of Nitromethane is increased so will the ignition point have to be advanced, due to the slower burning of the fuel.
The actual amount will depend on the engine design and will vary from engine to engine, but as a guide could be as much as 60 degrees, and since in fact the amount is not too critical some 40 degrees would be a good starting point.
Insufficient advance is usually made obvious by misfiring under load and at high engine speed, plus a general feeling of lack of power.
It is almost impossible to state a limit of advance as it varies so much engine to engine, but here again a falling off in power would indicate the limit point has been passed.
With very high Nitromethane content fuel the ignition point may well come back to a lower reading since owing to the large amount of oxygen being released the mixture becomes more sensitive, the flame pattern changing and the lower setting more effective.
Since we are talking about using Nitromethane in fair quantity, once again a warning to use a face mask for the driver when he is situated behind the exhausts and therefore in the fume area.
In certain cases it will be possible to extend the use of Nitromethane until the absolute figure of 98 percent is attained, usually regarded as 100 percent, at which stage you really have to pour it in to keep the mixture rich enough as the fuel itself generates its own supply of oxygen, also at a very high rate.
At 80 percent and above, the optimum air fuel ratio no longer exists and the power output becomes well related to the actual amount of fuel fed into the engine by weight.
In all the information given the engine has so far been regarded as a normally aspirated engine, that is unsupercharged, but in fact the use of Nitromethane is providing chemically similar results to the mechanically supercharged engine, but of course advantage can be taken of both methods together, provided certain precautions are taken, in particular that of using a suitable compression ratio.
If for example the normal compression ratio is 10 to 1, then if we now supercharge at some 15 pounds boost or approximately one atmosphere, the total compression ratio in effect is now doubled, or at 20 to 1 so far as the fuel is concerned, but in practice it would not be quite so high as this due to losses, but could well be some 16 or 17 to 1.
We are now at a stage where having started on Methanol and then progressed to the introduction of Nitromethane, we are starting to consider other possible additives to obtain high power at perhaps a lower cost since as yet pure Nitromethane is relatively expensive in this country.
Tetranitromethane which is very expensive and almost unobtainable can be used, but requires great care in handling as it has an explosive characteristic coupled with instability.
Dinitropropane which is solid at room temperature and again normally almost unobtainable, could be however a fairly safe additive and effective.
Isopropylnitrate is yet another very reactive substance, inclined to be unstable and unsafe in unskilled hands, and of course one may add, expensive.
Propylene Oxide has some handling problems in pure state but is quite safe when blended in other fuels.
Used in conjunction with Nitromethane it helps to increase power as it acts as an ignition accelerator, increasing the flame speed and up to 20 percent may be used.
When used with other fuels up to 5 percent, better starting and smoother running are the result.
In practice the usual amount used is some 10 percent as with more than this level it is necessary to introduce other components, such as for example, water or benzole to reduce detonation possibilities.
It is only fair to say that when you get to this stage of mixing up your own blends of fuel you are, to a great extent, more or less on your own and you become part chemist plus mechanical engineer.
Due to the high power levels involved great care must be taken and extreme cleanliness is essential.
Yet again do check that at all times your fuel lines
and pump capacity is more than adequate to cope with the heavy extra demand as you tend to get increased power output from these exciting fuels now available, and once again always tend to run on the rich side, the extra cost of the fuel being cheaper than a wrecked engine due to a weak mixture.
CHECK YOUR PLUGS
The only sensible way of increasing the amount of Nitromethane is in small progressive steps, and at each step checking the plugs which will indicate when the limit for that particular engine is being reached by the following signs:-
1 Chipping of bode insulator, similar to weak mixture.
2. Overheating of all metal parts of the plug, in extreme cases to the external and exposed body of the plug.
3. The center insulator ashen in color with grey streaks, not to be confused with the white grey color of weak mixture.
It being assumed at all times there is no question of the engine being on weak mixture and here again we repeat at all times work on the rich side