04-02-2013, 12:16 PM
Turbo Tech 101 ( Basic )
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How a Turbo System Works
Engine power is proportional to the amount of air and fuel that can get into the cylinders.
All things being equal, larger engines flow more air and as such will produce more power.
If we want our small engine to perform like a big engine, or simply make our bigger
engine produce more power, our ultimate objective is to draw more air into the cylinder.
By installing a Garrett turbocharger, the power and performance of an engine can be
dramatically increased.
So how does a turbocharger get more air into the engine? Let us first look at the
schematic below:
• The air filter (not shown) through which ambient air passes before entering the
compressor (1)
• The air is then compressed which raises the air’s density (mass / unit volume) (2)
• Many turbocharged engines have a charge air cooler (aka intercooler) (3) that
cools the compressed air to further increase its density and to increase resistance to
detonation
• After passing through the intake manifold (4), the air enters the engine’s
cylinders, which contain a fixed volume. Since the air is at elevated density, each
cylinder can draw in an increased mass flow rate of air. Higher air mass flow rate
allows a higher fuel flow rate (with similar air/fuel ratio). Combusting more fuel results
in more power being produced for a given size or displacement
Other Components
Blow-Off (Bypass) Valves
The Blow-Off valve (BOV) is a pressure relief device on the intake tract to prevent the
turbo’s compressor from going into surge. The BOV should be installed between the
compressor discharge and the throttle body, preferably downstream of the charge air
cooler (if equipped). When the throttle is closed rapidly, the airflow is quickly reduced,
causing flow instability and pressure fluctuations. These rapidly cycling pressure
fluctuations are the audible evidence of surge. Surge can eventually lead to thrust
bearing failure due to the high loads associated with it.
Blow-Off valves use a combination of manifold pressure signal and spring force to detect
when the throttle is closed. When the throttle is closed rapidly, the BOV vents boost in
the intake tract to atmosphere to relieve the pressure; helping to eliminate the
phenomenon of surge.
Wastegates
On the exhaust side, a Wastegates provides us a means to control the boost pressure of
the engine. Some commercial diesel applications do not use a Wastegates at all. This
type of system is called a free-floating turbocharger.
However, the vast majority of gasoline performance applications require a Wastegates.
There are two (2) configurations of Wastegates, internal or external. Both internal and
external Wastegates provide a means to bypass exhaust flow from the turbine wheel.
Bypassing this energy (e.g. exhaust flow) reduces the power driving the turbine wheel to
match the power required for a given boost level. Similar to the BOV, the Wastegates
uses boost pressure and spring force to regulate the flow bypassing the turbine.