ENGINE air INTAKE
To see how nitrous oxide works, let's go back to the two basic rules of power production.
1. Power is produced solely by burning fuel.
2. The amount of fuel that can be burned
depends mostly on the amount of oxygen that is available to burn the fuel.
It's easy to get raw fuel into the engine. Pull the jets out of the carburettor,
or better still, just run the fuel pump hose straight into the manifold
without the carburettor! It won't work will it? The engine will drown in
it's own fuel, because there just isn't enough oxygen to burn it. What
we have to do is supply as much oxygen as possible to the engine and meter
the amount of fuel to match the available oxygen.
It's the oxygen that's hard to get into the engine, not the fuel.
It takes up a lot more space, so we have things like trick cylinder heads
and camshafts to maximise the flow of oxygen into the engine. For maximum
power, we supply a little more fuel than is needed, just to make sure that
every last molecule of oxygen gets used.
Some of you may wonder why I am using the term "oxygen" instead of "air",
when I talk about burning the fuel. The reason is that it's only
the oxygen part of the air that does the burning and the oxygen accounts
for only about 23% of the air by weight. This consideration
is important when we get into the details on nitrous. So the
remainder of the air, which is mostly nitrogen, just goes along for
the ride. We put cold nitrogen in; we get hot nitrogen out.
What this nitrogen does, however, is soak up some of the heat
from combustion and lower the combustion temperatures.
ADDING THE NITROUS OXIDE
Nitrous oxide is made up of two nitrogen atoms and one oxygen atom, which form one molecule of nitrous oxide. Exposing this molecule to extremely high temperatures causes it to break down into nitrogen and oxygen. The sort of temperature required for this breakdown is present in the combustion chamber of an engine and hence feeding nitrous oxide into an engine produces oxygen. So there, in a nutshell, we have the secret of power production by nitrous oxide - extra oxygen.
Before we get into more detail about how it all comes together, let's just assume for the moment that nitrous does provide a certain amount of extra oxygen to the engine. But power is not created by extra oxygen - the extra oxygen allows us to add extra fuel and this is where the power comes from. Remember the rule above - power depends on the amount of fuel being burned. Nitrous by itself will not increase the power output; we must provide the extra fuel to go along with it. Because only a limited amount of nitrous oxide can be carried in the vehicle, its use is limited to bursts of power rather than continuous use. Drag racing is an obvious use, but an extra burst of power can be very helpful in most forms of racing.
THE HISTORY OF NITROUS
None of the books on nitrous oxide seem to be
able to name the person who came up with the idea of adding nitrous to
the intake of an engine. What we do know is that both the Allies
and the Germans were fooling around with the system on their aircraft,
both fighters and bombers, during the second world war. The idea was not
so much to increase the power output, but to allow the planes to fly
higher where the oxygen was thin. This would naturally give
them an advantage in a dogfight, allowing a burst of nitrous to send their
plane higher than the opponent.
Nitrous oxide was used secretly for a long time
in automotive competition engines, particularly in stock
car racing, where bonus money was available for fast qualifying
laps. Bursts of power could be used very effectively at the right moment
to grab the lead. The product was not exactly banned at the time because
its very existence was not widely known or advertised. These systems were
very crude, both because the system was hidden and could not be set up
very scientifically, but also because the technicalities of nitrous oxide
were at a very early stage of development. Once the governing bodies discovered
the existence of nitrous, it was very quickly outlawed. Tech inspectors
would go to great lengths to search for hidden systems in the race cars.
The gas was often hidden inside the roll cage tubing from where it was
secretly ducted to somewhere over the top of the carburettor, or even piped
in from underneath the manifold.
THE PROPERTIES OF NITROUS OXIDE
Nitrous oxide is a colourless, non-flammable
gas at normal room temperature and pressure. In small concentrations it
can cause laughing and giggling fits and so from there it gets its name
"laughing gas". The medical fraternity, particularly dentists, makes use
of nitrous oxide as a mild anaesthetic. For this usage it must be in high
concentrations, but always mixed with oxygen since pure nitrous can cause
death by suffocation or drowning.
Under a pressure of 760 psi, the nitrous oxide gas becomes a liquid. Storing it in the bottle as a liquid means that a far larger quantity can be stored than if it were in a gaseous form. Without this compression of nitrous to a liquid, not enough nitrous could be carried in a car to be used for more than a couple of seconds.
NITROUS FUEL MIXTURES
As mentioned above, the extra power in a nitrous
system comes from the extra fuel added, which the nitrous makes it possible
to burn. Along with the extra power comes high combustion chamber temperatures
and pressures - conditions that are conducive to detonation. Because the
engine is now running on the edge, any leaning out in the fuel mixture
can cause disastrous results. Running the nitrous system slightly fuel
rich can give a percentage of safety for the mixture. Over and above this,
the very use nitrous oxide in the fuel causes higher combustion temperatures
because of the displacement of the nitrogen in the intake gas. We mentioned
earlier that the nitrogen from the air passing through the combustion chamber
reduces the temperature. When nitrous oxide is used, the percentage
of nitrogen in the combustion chamber is reduced and temperatures
increase accordingly. Ail this goes to show that an over-rich
mixture is the way to go. The extra fuel cools the charge through its "latent
heat of vaporisation". The latent heat is the amount of heat used to turn
the liquid fuel to gaseous fuel and this is quite high, as those of you
who have done physics will know. The chemically correct mixture for nitrous
oxide to gasoline is 9.65:1. This means that approximately 10 1bs of nitrous
is needed for every lb of gasoline or petrol. As we said above, a richer
mixture than the chemically correct one is needed to keep the charge cool.
POWER POTENTIAL
Nitrous oxide is undoubtedly the easiest, quickest
and cheapest way to increase power in an engine. No other changes
are needed for this simple, bolt-on application. The extra power that can
be added is almost without limit - right up to the point where the engine
simply explodes because of the massive grunt. The secret to actually achieving
the power increase is to use a correctly designed system. The
nitrous is normally fed to an engine at a constant flow rate, independent
of the engine speed or load. Systems are naturally only required
for full power situations and hence are wired so they will not
operate unless the engine is at full throttle. Because the system
delivers a fixed amount of gas per minute, accompanied by the correct amount
of fuel, it will supply a certain amount of horsepower, no matter what
the engine speed
is or even the engine size. This is very different to power
increases by the normal means. Normally an increase in power will change
the shape of the power curve, the rpm where maximum power occurs. With
nitrous oxide the power curve just moves upwards, showing the same power
increase right through the range. Because of this simple method used to
increase power, the
more nitrous (and fuel) is added, the more the power will increase. This
is controlled by the jetting on the system and often the nitrous system
will contain various jets for different levels of power increase. A typical
street system will increase power by about 100 - 200 horsepower. Competition
kits will regularly pump 300 - 400 extra horsepower into the engine, with
even larger gains being possible from custom race systems. When the
nitrous gas decomposes, approximately 36% of its weight becomes
oxygen, compared to the 23% of the air that is oxygen. So if
we could replace all the air with nitrous oxide gas, we could substantially
increase the amount of oxygen going into the engine and so increase the
power. There is benefit another in the equation, the fact that at the same
temperature, the nitrous is 50% denser than air.
Add this all together and the nitrous could increase the power by a theoretical
230% if it replaced all the air. In practice this increase is not only
achieved, but bettered.
LIQUID VERSUS GASEOUS NITROUS
The difference between the volume of nitrous as
a gas compared to a liquid varies with temperature, but in general it is
about
100:1. This means that if liquid nitrous oxide
were injected into the airflow instead of gaseous nitrous, it would displace
very little of the normal airflow. The potential oxygen increase from liquid
nitrous oxide would be almost limitless, as would be the
accompanying power increase. A 1000 horsepower increase in output would
be just a matter of injecting the required amount
of nitrous oxide plus the extra fuel to go with
it. The liquid nitrous oxide entering the combustion chamber
would also draw heat from the combustion chamber because of its latent
heat of vaporisation and this would help reduce detonation. In reality,
the limit on added power output from the nitrous system is caused by the
physical strength of the engine to handle the cylinder pressures associated
with the extra power. The strength of the cylinder block, the pistons,
rings, rods and crankshaft are all tested when the power goes sky high.
For this reason, big block engines are the most popular when serious nitrous
kits are fitted. The extra bulk and strength of the fat block engines can
handle the bigger pressures and power boosts.
Detonation is the other big problem brought about by the high cylinder
pressures. Detonation can turn an Engine to pieces of junk in a very short
time if the pressures and temperatures get too high. Adding nitrous is
like bumping the compression ratio up. There is only
so much nitrous and fuel that can be added before the detonation limit
is reached.
NITROS ENGINE PERFORMANCE
As a result of adding a properly
designed nitrous system, vehicle performance will increase
dramatically When power is increased by fitting bigger
camshafts, bigger valves with enlarged ports, bigger manifolds and
carburettors, power increase goes hand in hand with a narrow power band
and the need to feed more rpm's to the engine. The story with nitrous is
different, with the power band staying the same width and the extra power
being added right through the range. The average power and torque through
the rpm range will be way up and the vehicle's performance will be far
better than with the traditional modification route. A nitrous
equipped car with 500 horsepower will be much quicker than the cammed,
headed and carburetted car with the same maximum 500 horsepower. In the
next issue, we'll look at the actual hardware used to inject the nitrous
and its extra fuel into the engine and the sort of parts used in the engine
itself for serious nitrous applications.