As suggested by the term, the turbo compressor is a mechanical
device which compresses the air by means of a turbine to exploit the power
and energy of exhaust gases.
The intake air is compressed by the rotation of the turbine under effect of
the thrust of exhaust gases, whereby increasing density. Consequently, a
greater amount of oxygen is let into the intake manifold making combustion
better and more powerful.
The air pushed by the turbine is heated by effect of compression; its density
consequently decreases. This is why an intercooler is used to cool the oxygen
which reacquires density before reaching the engine firing chambers.
The two most common types of turbo compressors fitting in diesel engine
vehicles are:
·
Turbo compressor with bypass valve (waste-gate)
·
Variable geometry turbo-compressor
A turbo compressor with bypass valve respects the principle
above with the sole addition of a bypass valve which limits the exhaust gas
pressure while maintaining engine output power at medium-to-high ratios. The
valve is controlled by the pressure of the air pushed into the manifolds.
Atmospheric pressure is used as the reference value. Consequently, the valves
is
CLOSED in the presence of atmospheric pressure only (i.e. exhaust gas
is exploited at medium and high ratios only)
OPEN in the presence of pressurised air in the manifold (limiting
boost to top ratios only).
N.B. (the diagram shown below is "generic"; in actual
fact, the bypass valve is controlled by another solenoid valve which balances
atmospheric pressure and is electrically controlled by the injection ECU;
this valve is also called a Pierburg valve)
·
A intake air input
·
B intercooler
·
C waste-gate or bypass valve (in open position)
·
D atmospheric pressure
This system (which is older) creates a power-engine output
compromise with a low "boost" perceived at all times at lower ratio
and provides a good progression at medium-to-high ratios.
A variable geometry turbo compressor also exploits the
mechanical principle described above, with the major difference that in this
case a supplementary mechanism (variable geometry) "meters" the
boost pressure at all engine ratios. The mechanism is shown in the following
figure, where:
·
A is the exhaust gas flow which reaches the vanes and the
impeller.
·
B is the direction of rotation of the impeller.
·
C is one of the vanes of the "variable geometry"
mechanism (located with the others in intermediate position).
·
D is the mechanism positioner which, as shown by the two
arrows, turns the vanes all together and is used to vary the position of
exhaust gas reaching the impeller.
The functional diagram of a variable geometry turbo compressor
and waste-gate vacuum valve is shown below
·
A intake air input
·
B intercooler
·
C valve input (atmospheric pressure side)
·
D valve output (vacuum side)
·
E exhaust gas output
·
F waste-gate valve
VANE POSITION:
In "flat" position, the vanes are arranged perpendicularly with
respect to the exhaust gas flow to decrease the gas emission section (figure A
).
In "open" position, the vanes are arranged to optimise
exhaust gas flow and increase the gas emission section (figure B ).
Finally, as shown in the following figure, the waste-gate vacuum
capsule (A) is controlled by a variable pressure/vacuum metered by a
solenoid valve (also called overpressure limiting valve). This valve is controlled
by the injection ECU according to engineering parameters and stored maps to
rapidly controlling the output air flow (B) with respect to the
variable input pressure (C) . The control process immediately reacts
to variations in driving conditions with the following advantages:
·
high power at medium ratios
·
less overpressure at top ratios (with less consumption)
·
fewer harmful emissions in the presence of metered, optimal
boosting pressure.
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DIESEL SYSTEMS AND COMPONENTS 