The exhaust gases carry approximately one third of the energy released in the combustion process
(Bell, 1997). This energy could be used to power a compressor situated in the air intake system to push
more oxygen into the engine. The turbocharger consists of a turbine that is driven by the exhaust gases
from the engine, a compressor driven by the turbine, and a plain bearing for the connecting shaft.
The type of application determines what turbo to use and this is mostly dependent on the sizes of the
compressor and turbine. The main objective when choosing compressor size is where on the
revolution range one should place maximum efficiency. Several factors are to consider when doing
this; pressure ratio (how much percentage air pressure the turbo can provide), density ratio (density of
the air charge), airflow rate (volume over time) as well as compressor efficiency (depends on the
increase in air temperature due to compression). The A/R-ratio is important when choosing turbine
size and is a constant factor that describes the relationship between the cross sectional area and the
radius of the turbine inlet. This ratio controls the air flow on the turbine hence the rotational speed of
the turbine blades and the magnitude of the back pressure.
Two factors influencing the driveability when designing/choosing turbo charger are boost threshold
and lag. Boost threshold is the lowest engine speed where the turbo will produce boost. The engine
torque curve takes an upward swing at the boost threshold when having full throttle. The lag is the
delay between throttling and receiving boost pressure due to rotational inertia in the turbine. The lag
decreases as the engine speed increases. Once again, the vehicle application is important in
turbochargers selection. Increasing the speed of the turbocharger increases the pressure ratio, not the
flow (MacInnes, 1984).
Back pressure occurring from restricted flow of the exhaust gases through the turbine, creates small
power losses. The back pressure brakes the engine and raises temperature in the exhaust manifold and
turbo. The back pressure could be reduced through the use of a waste-gate or/and the use of a bigger
turbo. The waste-gate wastes or by-passes a portion of the exhaust energy, hence controlling the
turbine speed and thereby the boost. The variable area turbine nozzle turbocharger (VATN) can
change the area of the turbine inlet over time hence control turbine speed and boost through varying
the A/R-ratio. The VATN controller is the secret to the extreme benefit of the VATN concept (Bell,
1997).