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By C.J. Baker
Suppose you have a 300-cubic-inch
gasoline four-cycle engine. Most of you know how an engine works, but as a simple review,
a four-cycle engine has an intake stroke to draw the air/fuel mixture into the cylinder
as the piston moves down the cylinder bore, followed by a compression stroke during
the following upward movement of the piston. These first two strokes occur during one
revolution of the crankshaft (see Fig.1). On the next revolution of the crankshaft,
the power stroke occurs as the air/fuel mixture burns pushing the piston down. The following
upward movement of the piston is the exhaust stroke. Two revolutions of the crankshaft,
four distinct cycles – it’s the basic Otto-cycle piston engine.
By its very design, this means our 300-cubic-inch engine takes in 300 cubic inches of
air every two revolutions of the crankshaft. Now here’s the interesting part. It
does this whether the throttle is open or closed. But wait, you say. The engine takes
in more air when the throttle is open. And while it is true that more air mass
flows into the engine when the throttle opens, the engine’s size, or displacement,
never changes, so the only actual difference is the density of the air that fills that
displacement.
When the throttle is closed, very little air mass flows into the engine, so that small
amount has to expand to fill our 300 cubic inches. Thus, the air will be of very low
density. As the throttle opens, more air mass can flow in to fill the engine, and the
density will increase. This is often called the “charge density”.
Let’s think of charge density as the amount of oxygen available to support the
combustion of the fuel. The more oxygen (air) that flows into the engine, the more fuel
that can be burned, and the more power the engine can make. To say it another way, assuming
you mix the correct amount of fuel with the air, how much power an engine can make is
dependent on airflow. For normal cruising operation, a gasoline engine operates at around
a 14.7:l air-to-fuel ratio, so it would need roughly 14.7 pounds of air to mix with
every pound of fuel. To make maximum power that ratio would fall to approximately 12.5:l.
Besides throttle position, many things effect airflow, such as restrictions in either
the intake or exhaust paths (which can become de facto throttles in themselves), or
the design of the camshaft to control valve openings and closings (see Fig.2). Even
the temperature of the incoming air will affect its density (see "Cool
Air Equals Power" elsewhere on this site). But most of all, the biggest factor
is the pressure of the air available to flow into the engine on its intake stroke. For
a non-supercharged engine, that’s simply atmospheric pressure, or about 14.7 pounds
per square inch, measured at sea level (see Fig.3). If we use some sort of compressor
to increase the pressure above atmospheric pressure, that’s called “supercharging”
the engine. If that compressor is driven by a mechanical linkage to the engine, such
as a belt drive or gear drive, the compressor is simply called a supercharger. However,
if the compressor is driven by a turbine placed in the exhaust system of the engine,
that combination of turbine and compressor is called a turbocharger (see Fig.4).
Supercharging and turbocharging are very potent ways to increase an engine’s power
output. Doubling the charge density of an engine more than doubles its power output,
provided an optimal air/fuel ratio is maintained. Why does the power output more than
double? The answer is that the parasitic losses of the engine, such as friction and
pump drives, remains relatively constant, as does relative heat loss to the surrounding
air and coolant, so additional power provided by increased charge density is almost
totally available to do work.
Going back to our opening description of a four-cycle engine, it should be noted that
only one of the four cycles produces power. The other three cycles consume power. Anything
that increases the induction pressure of an engine reduces the pumping losses for that
engine on the intake stroke (see Fig.5). But there is no “free lunch”. It
takes power to drive the compressor that creates this increased induction pressure.
For a supercharger, that power comes directly off the engine’s crankshaft. In the
case of a turbocharger, the turbine creates a restriction in the exhaust path, thus
building exhaust backpressure between the cylinder and the turbine, increasing pumping
losses on the exhaust cycle.
More importantly, if the exhaust backpressure rises higher than the induction pressure
generated by the compressor, some exhaust gases will remain in the cylinder after the
exhaust stroke to dilute and reduce the incoming air/fuel charge, and in worst-case
scenarios, exhaust gases will actually flow backward into the induction system during
the “overlap” period inherent in most camshaft designs where both the intake
and exhaust valves are open simultaneously (see Fig.6). In this last case, such backflow
is very detrimental since it raises the temperature of the incoming air/fuel charge
and promotes damaging detonation.
Detonation is uncontrolled combustion of the air/fuel mixture that generates excessive
cylinder pressure and temperature. Detonation will quickly break, burn, or melt internal
engine parts. Every fuel has detonation limits associated with pressure and temperature
where the fuel self-ignites and burns uncontrollably. Thus, the maximum amount of power
any spark-ignition engine can make is limited by the detonation resistance of the fuel,
which is expressed as the fuel’s octane number. Consequently, being able to both
control the induction pressure and the temperature of the incoming air/fuel charge is
critical to building reliable supercharged (or turbocharged engines). And in the case
of turbocharged engines, the turbine and compressor should be sized and matched to assure
that exhaust pressure between the turbine and the cylinder, which is called “turbine
inlet pressure”, doesn’t exceed the induction system pressure, which is usually
called “boost” (see Fig.7). In fact, optimizing boost pressure over turbine
inlet pressure is rarely discussed, yet it is one of the key elements to a successful
and reliable turbocharged application, especially for racing.
A great deal of attention is often given to throttle response of turbocharged engines,
which refers to the time between when the throttle is depressed and the engine responds.
Frequently small, highly-responsive turbines are mated with larger compressors to quicken
throttle response, but such small turbines quickly become restrictions in the exhaust
system and build excessive turbine inlet pressure, creating the backflow condition commonly
called “turbine choke”. Some such systems rely on today’s sophisticated
detonation sensors to retard ignition timing and enrich the air/fuel mixture to suppress
detonation under adverse conditions, but when these things are done, power output is
greatly diminished and fuel economy suffers to the point that the engine might actually
be making less power than if it was turbocharged to a lower boost level to keep it out
of detonation. A properly designed turbocharger system only relies on detonation sensors
as a failsafe for occasional bad fuel or a momentary overboost condition in normal operation.
As mentioned above, controlling peak induction system pressure and induction temperature
is key to preventing detonation. Lets look at the temperature issue first. Whenever
air is compressed, it is heated. And since heat is an undesirable that promotes detonation,
cooling the compressed air is desirable, even though such cooling will lower the induction
pressure. On the positive side, cooling will also increase the charge density of the
incoming air. The device used to cool the induction charge is properly called a “charge
air cooler”, although many people refer to it as an intercooler. Charge air coolers
are heat exchangers that can utilize an air-to-air configuration, or they can be of
the air-to-liquid variety. Both are effective, although the air-to-liquid variety then
requires yet another liquid-to-air heat exchanger to cool the liquid. Hence, the air-to-liquid
process is inherently less effective than the air-to-air concept. As a rule of thumb,
every 10-degree F. reduction in charge air temperature results in a 1% increase in charge
density that equates to approximately a 1% increase in power output. So charge air cooling
both helps prevent detonation and increases power output.
Controlling peak induction system pressure can be done three ways. The first way is
to install a pressure relief mechanism in the induction system. Such a device is often
called a “pop off” or “blow off” valve that simply opens at a pre-set
level. The second method is to utilize a device that bleeds off exhaust flow to the
turbine. Such a device is referred to as a “wastegate” (see "How
a Turbo Wastegate Works" elsewhere on this site). As with the pop-off, the
wastegate would be set to open at a pre-set boost level. The third way is to properly
match the turbine and compressor sizes to the application and to each other. When this
is done correctly, which is called a “floating match”, inherent flow restrictions
keep everything balanced.
All of the above is a simplified overview of engine science as it relates to gasoline
engines. Diesel engine applications are similar in many ways, although diesels have
no air throttle to vary charge density. Instead, diesels are throttled by regulating
the amount of fuel injected into the cylinders at precise times. In other words, they
are throttled by varying the air-to-fuel ratio. Normally this range is between 50:l
(at idle) to about 22:l at full power. Going beyond 22:l produces excessive temperature,
soot, smoke, and poor fuel economy. Supercharging and turbocharging do increase charge
density and the total power output for diesels, just as it does for gasoline engines,
and the same temperature and pressure controls apply to prevent detonation.
The basics of making power are simple. It begins with airflow, but it doesn’t end
there. That’s just the beginning. Then fuel must be properly metered to match the
airflow. And finally, precise controls need to be put in place to optimize related systems,
such as ignition (for gasoline engines), turbocharger boost, etc. When done correctly
as a system, not only is power increased, drivability, reliability, and economies are
also enhanced. Done incorrectly, the results can be destructive to the engine, pushing
it beyond the factory-specified safe operating limits. At Banks Power we’re experts
at doing it correctly.
Whatever power enhancement products you consider, use the engine science presented here
to evaluate how those products affect the basics of engine operation and how they achieve
their gains. Then you’ll be in a better position to make an informed purchase decision.
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