If you are interested in supercharging, you probably
do not need a basic course in this subject, but we could recap a little. We
want more power out of our engine, and one very effective way to get that
is to put more air into it. With more air goes more fuel, of course, and so
there will be more energy released at each bang. This means that the temperature
in the combustion chamber will go up - and as a result of that, also the pressure.
It is that pressure increase in the cylinder we are after, because this is
what makes the engine put out more power. In principle, this goes for all
kinds of engines, but we will only discuss four-stroke cycle engines here.
When we say “more air” we mean a larger mass of air (in
pounds or ounces). As the volume in the cylinder is fixed, the air taken in
at the inlet stroke has to have an increased density, which we can get by
increasing the pressure and lowering the temperature of the charge.
To be able to create that increased pressure (“boost
pressure”) before the inlet valve, we need some sort of pump or compressor. In
this case we will only deal with the types that are mechanically driven, mostly
directly from the engine, and of the “positive displacement type”, meaning that
they displace a certain volume in a positive way. (To be absolutely “positive”,
the process should be without internal leakage, but we are not absolutely
strict with that.) Thus we will not
discuss the “turbo” here, which is a “dynamic” type of compressor, driven by a
turbine that gets its energy from the pressure in the exhaust pipe. It is
called “dynamic” because it is the aerodynamic forces inside the machine that
are doing the job,
By “pump” in this context we mean a rotary device that
is moving a given volume from one place to another, something like an oil pump
of the gear type. In the supercharging field it is often called a “blower”.
There is no “internal compression”, which means that the transported volume of
air suddenly is exposed to the pressure in the discharge line when the outlet
port opens. We could say that the
pressure is created after the pump in this case – and it happens when the
engine cannot swallow the volume of air that the pump is delivering.
A type that is used very often is the “Roots Blower”.
Two rotors with identical shape are sealing (well, not quite) against each
other and against the inside surfaces of the housing. For each revolution
they are pumping a certain volume of air from the inlet side to the discharge
side. The principle is very simple, but the basic form of the rotors is critical,
as well as the clearances between the working parts.
By “compressor” we normally mean a machine that really
is compressing the medium during the working cycle. Among the rotary types, we
have different kinds of vane compressors, screw compressors and scroll
compressors. These types have a better efficiency at higher pressure ratios,
thanks to the internal compression. This also means that the discharge
temperature will be lower than with a blower – a plus in several respects. They
are, however, more complicated, and more difficult to make. They would not be
the first choice for a hobbyist.
We could also think a little more about what happens
in the engine when we supercharge it. If we follow the path of the air through
it, we might get an idea. For this discussion we assume that we have a Roots
blower that is sucking a mixture of air and fuel from the carburetor, and is
pushing it into a single cylinder engine.
First we note, that we have
got a pretty steady sucking by the blower, instead of the quick gulp during
the intake stroke. This means that the capacity of the standard carburetor
most likely will be quite adequate, even if with supercharging we will be
passing a lot more air through it in total. The steady stream of air and fuel
will be well mixed by the blower, something that will be beneficial for the
combustion in the cylinder. (In a multi-cylinder engine, it will also be beneficial
for the fuel distribution between the cylinders.)
In the blower we are putting some power into that
air-fuel mixture. The result is that it warms up quite a bit, something we do
not like. But the fuel that goes through the blower has a cooling effect. If we
are running on methanol, this effect will be extra good. We could also put a
little oil in the fuel - unless there is a catalytic converter in the line. The
oil would help to lubricate the blower, and improve the sealing between the
rotors and between the rotors and the housing.
The downside of having the carburetor before the blower
is that we now are creating a pressurized, inflammable mixture that can go
off with a big bang, if the fire gets out of the cylinder through a leaky
inlet valve, or because the mixture burns too slowly. Thus we must think of
that: the chamber between the blower and the engine must be regarded as a
pressure vessel, and as such it has to be reasonably strong. There should
also be something like a safety valve or burst plate on it, which blows off
if a backfire occurs.
With a single-cylinder engine, we can only take in the
compressed air-fuel mixture during approximately 180 degrees of crankshaft
rotation, while the blower is pumping steadily during the total cycle of 720
degrees – so we understand that we have to have something like a buffer between
the blower and the engine. As a rule of thumb, the volume
of that buffer (plenum chamber) should be at least twice the size of the displacement of the cylinder.
(With more cylinders, the volume could be smaller.) We also understand
that if we can lower the temperature of the charge before it enters the cylinder,
we will be able to pack a larger mass of air and fuel into the engine. If
we go with an aftercooler, that volume will be included in the
buffer volume.
Now we come to the inlet valve. In this case, with the
pressurized charge before it, there will be a force from that pressure that will
tend to open it. That is a good reason for increasing the strength of the inlet
valve spring.
When the inlet valve opens, the pressurized charge is
rushing into the cylinder. In a naturally aspirated engine there is often
a fairly long overlap between the opening of the inlet valve and the closing
of the exhaust valve. In our supercharged case the full pressure is constantly
acting on the inlet valve - and if both valves are open at the same time,
some of our precious mixture is lost, out through the exhaust pipe. Thus we
should have less overlap in a supercharged engine. A little bit could be OK,
as that would help to clean out the cylinder and cool the piston and the exhaust
valve. Thus, ideally, the timing should be different in a supercharged engine,
but in the practical world it is possible to get quite good results just with
the standard valve timing.
We are filling the cylinder much quicker now than when
the engine had to suck in the charge. We are also creating a “positive” force
on the piston during the down-stroke, meaning that we get something back in
terms of power already here, from the power that is put into the blower to
drive it. We also understand that the filling will be more complete, and with
a fuel/air mixture of higher density than in the case of a naturally aspirated
engine. When the intake stroke is completed, the inlet valve should close.
If it stays open much longer, there will be an extra braking force created
– and we certainly do not want that.
When the compression stroke begins, we have a higher
pressure than in the standard case. If we have not changed the compression
ratio in the engine, the air-fuel mixture will have a higher pressure and
temperature at the end of the stroke. This can easily lead to knocking: the
whole charge goes off at once. Not good! The remedy is to lower the compression
ratio, increase the fuel-to-air ratio, and possibly retard the ignition a
bit. Under all circumstances, the peak pressure will be higher than before,
so there is a good reason to look after the seal between the cylinder and
the head. Hotter spark plugs might also be needed.
The extra fuel we filled the cylinder with now shows
up as more force on the piston during the power stroke. It also means that the
pressure is higher than before when the exhaust valve is about to open. The
direct result of this is that the valve has to open against a force that is
trying to keep it shut. This, in turn, means that the forces in the valve
mechanism will go up. We do not need a stronger spring on this side, but
possibly stronger components operating the valve.
We can also think about the exhaust situation in
such a way, that we now have to get rid of a larger mass of gas than before.
That means, that ideally the discharge valve should be larger (and the inlet
valve smaller). If not, the valve should open earlier. In any case, the heat load on the exhaust
valve will be increased, so a good material and good cooling of the same will
be important.
So, as always, it’s a give and take. We give away a
bit of fuel economy, we have to get rid of more heat, we have to live with
higher stresses in the engine – but some of us think that this is well worth
it.