Adiabatic Efficiency
A 100% adiabatic efficiency means that there
is no gain or loss of heat during compression. Most turbochargers will have a
65-75% adiabatic efficiency. Some narrow range turbo's can get higher, these
types of turbo's work well in engines that operate over a narrow rpm range. In
general the wide range turbo's don't have as good peak efficiency, but have
better average efficiency and work better on engine that operate over a wide rpm
range.
Pressure Ratio
This is the inlet pressure compared to the
outlet pressure of the turbocharger's compressor. For single stage turbo's, the
inlet pressure will usually be atmospheric (14.7 psi) and the outlet will be
atmospheric + boost pressure. For staged turbo's the inlet pressure will be the
outlet pressure of the turbo before it + atmospheric, and the outlet will be
inlet pressure + additional boost from that turbo.
Density Ratio
Turbochargers compress the air to make it
more dense, this is what allows more oxygen in the engine and give the potential
to make more power. The density of the inlet air compared to the density of the
outlet air is the density ratio.
Turbine
The side of the turbocharger that converts
the energy of the exhaust into mechanical energy to turn the compressor.
Compressor
The side of the turbocharger that compresses
the incoming air charge and directs it to your engine.
Cartridge
This is the center section of your
turbocharger. It houses the bearings for your turbocharger, they have oil
passages to lubricate the bearings and some have water jackets for water
cooling.
Intercooler
When intake air is compressed by a
turbocharger it is also heated, even more so than when supercharging due to the
turbo being heated by the exhaust. Hot intake air is not good for power and will
increase the chance of detonation. An intercooler reduces the intake temperature
by pushing the air through a heat exchanger (much like a small radiator) that
absorbs some of the heat out of the charge. With less heat, you'll need less
boost pressure to get the desired power and decrease the chance of detonation.
Anything that reduces the intake temperature is a big plus in a supercharged
engine.
Turbo Lag
A turbocharger uses a centrifugal compressor,
which needs rpm to make boost, and it is driven off the exhaust pressure, so it
cannot make instant boost. It is especially hard to make boost at low rpm. The
turbo takes time to accelerate before full boost comes in, it is this delay that
is known as turbo lag. To limit lag, it is important to make the rotating parts
of the turbocharger as light as possible. Larger turbo's for high boost
applications will also have more lag that smaller turbo's, due to the increase
in centrifugal mass. Impeller design, and the whole engine combo also have a
large effect on the amount of lag. Turbo lag is often confused with the term
boost threshold, but they are not the same thing, lag is nothing more the the
delay from when the throttle is opened to the time noticeable boost is achieved.
Turbo Boost
Usually measured in pounds per square inch,
it is the pressure the turbocharger makes in the intake manifold. One of the
ways to increase airflow through a passage is to increase the pressure
differential across the passage. By boosting the intake manifold pressure,
airflow into the engine will increase, making more power potential. Boost is
also measured in Bar. One Bar equals 14.7 psi.
Boost Threshold
Unlike turbo lag, which is the delay of
boost, boost threshold is the lowest possible rpm at which there can be
noticeable boost. A low boost threshold is important when accelerating from very
low rpm, but at higher rpm, lag is the delay that you feel when you go from
light to hard throttle settings.
Wastegate
The wastegate is a valve that allows the
exhaust gasses to bypass the turbine. The waste gate relies on boost pressure to
open it. Spliced into the wastegate pressure feed there must be some form of
pressure bleed. By bleeding pressure to the wastegate, it is possible to control
the amount of boost by reducing the pressure at the wastegate.
Turbo Cool Down
A turbocharger is cooled by engine oil, and
in many cases, engine coolant as well. Turbo's get very hot when making boost,
when you shut the engine down the oil and coolant stop flowing. If you shut the
engine down when the turbo is hot, the oil can burn and build up in the unit
(known as "coking") and eventually cause it to leak oil (this is the most common
turbocharger problem). It is a good idea to let the engine idle for at least 2
minutes after any time you ran under boost. This will cool the turbo down and
help prevent coking
