SOME BASIC FACTS
WELCOME to the world of racing fuels. Within the pages of this book we will
be discussing such exotic and volatile fuels and fuel additives as Methanol,
Nitromethane, Dinitropropane, Acetone, Propylene Oxide, Nitrobenzene and others.
However, in order to evaluate the advantages and disadvantages of these special
fuels for use in high performance engines it is necessary to have a clear
understanding of the laws of thermodynamics as applied to the internal combustion
engine.
HOT AIR ENGINE
For our purpose it is sufficient to state that such an engine is in simple
terms a hot air engine depending on the expansion of a quantity of air, heated
by the combustion of fuel, in a confined space, thus providing the driving
force on the piston the reciprocating motion of which is converted by the
crankshaft into a rotary motion, so driving the flywheel and thus the source
of power.
It follows, therefore, the more air, by weight, we can ignite in a given combustion
space and the greater we can increase the temperature, the greater will be
the expansion and the force applied to the piston.
COMPRESSION
This at once highlights a term used by many without quite understanding its
implications, the term being Compression Ratio.
This is the ratio between the piston at the top of its stroke leaving a space
in the combustion head and this volume added to that swept by the piston,
that is the bore and stroke volume. If, for example, this works out at ten
to one it means the mixture is compressed to one tenth of its normal volume
and then ignited.
If we assume the engine is not supercharged and at normal atmospheric pressure
14.7 Ibs. per sq. in. approximately, and 100 per cent volumetrically efficient,
the force on the piston would be 147 Ibs. per sq. in.
This, however, will not be the case in practice as the engine will not take
in a charge of mixture equal to the space left by the piston at bottom dead
center, in some cases the exhaust gases are not completely evacuated and the
heat of the cylinder walls, head and valves, all have their effect on heating
up the incoming charge, reaching, with petrol, some 700°F.
Thus it is obvious, owing to the heat conditions existing in the engine, that
there are definite limitations to the power output which can be maintained,
and these conditions, so far as petrol is concerned, are reached much earlier
than with alcohol.
LESS POWER
There is a common mistake in thinking that so called racing fuels contain
in themselves, quantity for quantity, more energy than petrol. This is not
so and in fact alcohol's have considerably less internal energy than petrol,
their respective calorific values being 19,000 British Thermal Units per-pound
weight for petrol and some 8100 for alcohols.
This means that, gallon for gallon, less power can be obtained from the alcohol's,
but the limiting conditions mentioned above eliminate this particular drawback.
The amount of air required by petrol to burn correctly is several times more
than that required by alcohol, so that in effect the advantage of alcohol
is that the amount of heat liberated per pound weight of air used is greater.
Since the engine is an air engine where air is the working medium, heated
by the ignition of the mixture, causing expansion, the fact that we can get
a cooler and therefore heavier charge into the engine means a higher power
output to be attained, fuel for fuel and weight for weight.
In other words the advantage of alcohol is its high latent heat evaporation
figure as compared with petrol, allowing it to act as a refrigerant.
You may have at some time or other noticed inlet pipes tending to show frost
when using alcohol.
The difference, for those interested in the heat values, is some 135 BTU for
petrol and 472 BTU for alcohol, and just to clear all that up, it might be
worth while saying that one British Thermal Unit is defined as the amount
of heat required to raise the temperature of one pound of pure water one degree
F.
OPTIMUM ON PETROL
Over the years the motor engine has been developed and adjusted by design
features to operate at its optimum on petrol, this being available in bulk
at a reasonable cost.
This fuel is a natural product of the earth, but, as we know, it has undergone
a number of changes and has had certain additives incorporated to produce
the required results when used with the modern combustion engine. Here then
is our reference level or datum line.
If you have an engine, in good condition and tune, running, shall we assume,
on top grade petrol and providing a known power output, it is possible by
change of fuel to obtain a higher output, and to do this you can go to an
alcohol based fuel.
To get maximum benefit from the new fuel you will have to adapt the engine
to run under the new conditions being applied, and you will at once find there
are advantages and disadvantages.
As we now have some understanding of the nature of the work the fuel is expected
to carry out, they can be considered, and the new fuel used to its maximum.
