Copyright by Goodheart-Willcox Co., Inc.
330 Auto Engine Repair
Oil
sealing
rings
Center housing
oil passages
Oil flow
Turbo
shaft
Turbo shaft
bearings
Figure 16-4. Lubrication for the turbocharger is provided by
the engine oil. Oil flow through a turbocharger is shown here.
(Chrysler)
Turbocharger Location
Turbochargers are normally bolted to the side, top, or
front of the engine. Exhaust tubing routes engine exhaust
gasses into the inlet of the turbine housing. Tubing or a
hose also connects the outlet of the compressor housing to
the engine intake manifold.
Theoretically, the turbocharger should be located as
close to the engine exhaust manifold as possible. Then, a
maximum amount of exhaust heat will enter the turbine
housing. When the hot gasses blow onto the spinning
turbine wheel, they are still burning and expanding to help
rotate the turbine.
Turbo Lag
When the accelerator pedal is pressed down for rapid
acceleration, the engine may lack power for a few seconds.
Turbo lag is a short delay before the turbo develops suf-
ficient boost. Boost is any pressure above atmospheric
pressure in the intake manifold. Turbo lag is caused by the
compressor and turbine wheels not spinning fast enough.
It takes time for the exhaust gasses to bring the turbo up to
operating speed.
Modern turbo systems suffer from very little turbo lag.
Their turbine and compressor wheels are very light so that
they can accelerate quickly. Some turbocharger impel-
lers (wheels) are made of carbon fiber–reinforced plastic.
This reduces impeller weight and the problem of turbo lag
considerably.
Turbocharger Lubrication
Adequate lubrication is needed to protect the turbo
shaft and bearings from damage. A turbocharger can
operate at speeds up to 100,000 rpm. For this reason, the
engine lubrication system forces engine oil into the turbo
shaft bearings.
Oil passages are provided in the turbo housing and
bearings, as shown in Figure 16-4. An oil supply line runs
from the engine to the turbo. A drain line runs from the
turbo to the engine. With the engine running, oil enters the
turbo under pressure. The turbo shaft rides on a thin film
of oil, avoiding metal-to-metal contact.
Sealing rings similar to piston rings are placed around
the turbo shaft at each end. They prevent oil leakage into
the compressor and turbine housings. A drain passage and
drain line allow oil to return to the engine oil pan after
passing through the turbo bearings.
Waste Gate
A waste gate limits the maximum boost pressure
developed by the turbocharger. Without a waste gate, the
turbo could produce too much pressure in the combus-
tion chambers. This could lead to detonation and engine
damage. A waste gate consists of:
Diaphragm. A flexible membrane that reacts to differ-
ent amounts of boost pressure.
Diaphragm spring. A coil spring that holds the waste
gate valve in the normally closed position.
Waste gate valve. A poppet, flap, or butterfly valve
that can open to bypass exhaust gasses away from the
turbine wheel.
Housing. An airtight, metal container that encloses
parts and has a pressure hose fitting.
Pressure line. The hose that connects the waste gate
with a source of intake manifold pressure.
The operation of a turbocharger waste gate is shown
in Figure 16-5. Under partial load, the waste gate is closed
by the diaphragm spring. All exhaust gasses are directed
against the turbine wheel blades. This ensures that there is
adequate boost to increase engine power. The waste gate
remains closed as long as boost pressure is not too high.
Under full load, boost increases as more exhaust gas-
ses are produced and turn the turbine. When boost reaches
a preset level (about 5 psi to 7 psi), pressure in the intake
manifold or compressor housing acts on the diaphragm in
the waste gate. It pushes the diaphragm down, which com-
presses the spring and forces the valve open. Some of the
exhaust gasses are diverted from the turbine wheel blades.
The turbo speed does not increase because less exhaust is
left to spin the turbine. In this way, boost is limited.
Turbocharger Intercooler
As air is compressed by the turbocharger, the heat of
compression increases the air temperature. By cooling the
air entering the engine, engine power is increased because
the air is more dense and contains more oxygen by volume.
Cooling also reduces the tendency for engine detonation.