Forced Induction
"Forced Induction" is the process of using a mechanical system to "force" more air
into an engine. This includes Superchargers, Turbochargers, Nitrious systems, and other
mechanical systems, but not hood scoops which just direct outside air into the engine.
Both Superchargers and Turbos use a compressor to "force" air into the engine, making
it more dense (i.e. more oxygen). When a proportionately larger amount of fuel is added
to the denser air charge, the mixture creates a much larger explosion and thus more power
from the engine. Nitrous Oxide Systems ("NOS") works by directly injecting Nitrous Oxide
(NO2) into the engine. The higher concentration of oxygen found in NO2 (compared to air)
and more fuel leads to the same effect as Superchargers and Turbos. Although the basic
concepts are similar, each differs in their design and execution.
Superchargers
A supercharger is a compressor driven by a belt attached to the engine's crankshaft. Since they
are powered directly by the crankshaft, they are actually a parasitic loss and actually rob
horsepower to drive the compressor. However, as they are directly attached to the crankshaft,
the extra power is available at all RPMs and there is no "lag" in the power delivery. There are
two basic types of Superchargers, Positive Displacement and Centrifugal. Positive Displacement
Superchargers fill a chamber of a fixed volume with air at atmospheric pressure nd move that air
to the high pressure side. These superchargers generally provide a flat torque curve boost
throughout the RPM range but are generally less efficient and thus produce less peak power.
The most common Positive Displacement Supercharger is the "Roots" Supercharger and is used
by most factory setups and Jackson Racing. A Roots Supercharger has two rotating, intermeshing
lobes that pump air simply by trapping it on one side of the supercharger between the lobes
and the supercharger housing and moving it to the other side of the supercharger. The main
advantage of a Roots Supercharger is that there is immediate and proportional response to
the throttle, because the blower is always spinning and its pumping ability increases
proportionately and linearly with engine speed. Therefore, a Roots Supercharger makes the
most low RPM power of any Forced Induction system.
A Centrifugal Supercharger (such as Vortech) is basically the compressor section of a turbocharger,
but driven by the crankshaft through a step-up gearbox. Centrifugal superchargers often work
best over a very small RPM range but provide higher peak power than Roots superchargers.
Unfortunately, the power delivery of the boost increases non-linearly with engine speed
meaning that they generate tremendous peak outputs, but at unuseably high RPMs. Nevertheless,
they are popular with muscle car owners, who have plenty of low end Torque but need high RPM
horsepower.
>Superchargers can be placed between the throttle body
of the carburetor or fuel injection system and the manifold; or at the
air inlet before the throttle body. Racing cars usually have it located
between the throttle body and the manifold. This design has the advantage
that the fuel can be supplied through the throttle body without modification
to any part of the system. If the supercharger is placed in front of the
throttle body, fuel must be supplied under sufficient pressure to overcome
the added air pressure created by the supercharger. The advantage of a
supercharger over a turbocharger is that there is no lag time of boost;
the moment the accelerator pedal is depressed, the boost is increased.
Advantages: Instant power delivery
Disadvantages: Parasitic power loss to system (less peak power)
Applications: Recommended for low RPM, high torque applications (i.e. V8s)
Turbochargers
A turbocharger uses a turbine that is driven by the hot, expanding exhaust system stream to power
a centrifugal compressor which forces air into the engine. Because a turbo is actually driven
by wasted energy, there is no loss from the engine to power the turbo. However, because the
system is run off the exhaust, it may take some time before the exhaust level builds up to a
sufficient level to run the turbo charger. This delay in power delivery is called "Turbo Lag" and
can lead to a significant pause between stepping on the gas and the feel of additional power.
Furthermore, turbos often require cool down periods after use (meaning you have to let the car
idle for a few minutes (!) everytime you want to shut the engine off, or else reduce the life
of your turbo-charger, which only last 7-10 years anyway. More modern turbo charger designs have
allowed turbos to slowly spin down even after the engine has been shut off and are more durable than
older designs.
Most turbos use intercoolers, wastegates and compressor bypass valves (blow off valves) to
help provide more useable power. An intercooler is a heat exchanger that is used to cool the
air heated by the turbocharger's compressor. Therefore, the intercooler can significantly boost
power because the cooler air it provides is more dense than the hot air straight out of the
turbo. An intercooler also takes thermal load off of the engine by cooling the intake air.
A wastegate is designed to regulate boost pressure, and is simply a valve in the exhaust
system that allows some of the exhaust to bypass the the turbo when it is not needed. The
Bypass valve is a valve between the throttle and the turbo that vents extra boost pressure.
When this valve vents to the outside air, it is called a blow off valve, and when it vents
back into the inlet of the turbo, it is called a compressor bypass valve.
A turbocharged engine's compression ratio must be lowered by using a lower compression piston,
since an excessive amount of pressure will wear on the piston, connecting rods, and crankshaft,
and destroy the engine. All of these parts then, as well as the transmission, must be
strengthened on a turbocharged engine or it will be torn apart by the increased horsepower.
Advantages: Greater peak power
Disadvantages: Lag in power delivery, long-term durability, cold-start emissions
Applications: Recommended for high RPM applications (i.e. I4s)
