TURBOCHARGERS

A turbocharger is an exhaust-driven supercharger (fan or blower) that forces air into the engine under pressure. Turbochargers are frequently used on small gasoline and diesel engines to increase power output. By harnessing engine exhaust energy, a turbocharger can also improve engine efficiency (fuel economy and emissions levels).

The turbocharger (fig. 5-42) consists of three basic parts—a turbine wheel; an impeller or compressor; and housings that support the parts and direct the flow of exhaust gases and intake air. Basic operation of a turbocharger is as follows:

• When the engine is running, hot gases blow out the open exhaust valves and into the exhaust manifold. The exhaust manifold and connecting tubing route these gases into the turbine housing.
• As the gases pass through the turbine housing, they strike the fins or blades on the turbine wheel. When engine load is high enough, there is enough exhaust gas flow to spin the turbine wheel rapidly.
• Since the turbine wheel is connected to the impeller by the turbo shaft, the impeller rotates with the turbine. Impeller rotation pulls air into the compressor housing. Centrifugal force throws the spinning air outward. This causes air to flow out of the turbocharger and into the engine cylinder under pressure.

A turbocharger is located on one side of the engine. An exhaust pipe connects the exhaust manifold to the turbine housing. The exhaust system header pipe connects to the outlet of the turbine housing.

Theoretically, the turbocharger should be located as close to the engine manifold as possible. Then a maximum amount of exhaust heat will enter the turbine housing. When the hot gases move past the spinning turbine wheel, they are still expanding and help rotate the turbine.

Turbocharger lubrication is required 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 oil into the turbo shaft bearings. Oil passages are provided in the turbo housing and bearings and an oil supply line runs from the engine to the turbocharger. With the engine running, oil enters the turbocharger under pressure. A drain passage and drain line allows oil to return to the engine oil pan after passing through the turbo bearings.

Sealing rings (piston-type rings) are placed around the turbo shaft at each end of the turbo housing, preventing oil leakage into the compressor and turbine housings.

Turbochargers require little maintenance between overhauls if the air cleaners are serviced regularly according to the manufacturer’s recommendations. The turbocharger turbine requires periodic cleaning to remove carbon deposits that cause an unbalanced condition at the high relative speeds at which the turbine must rotate.

Turbocharging system problems usually show up as inadequate boost pressure (lack of engine power), leaking shaft seals (oil consumption), damaged turbine or impeller wheels (vibration and noise), or excess boost (detonation).

NOTE

Refer to a factory service manual for a detailed troubleshooting chart. It will list the common troubles for the particular turbocharging system.

There are several checks that can be made to determine turbocharging system conditions. These checks include the following:

• Check connection of all vacuum lines to the waste gate and oil lines to the turbocharger.
• Use regulated, low-pressure air to check for waste gate diaphragm leakage and operation.
• Use a dash gauge or a test gauge to measure boost pressure. If needed connect the pressure gauge to the intake manifold fitting. Compare to the manufacturer’s specifications.
• Use a stethoscope to listen for bad turbocharger bearings.

Turbo Lag
Turbo lag refers to a short delay before the turbocharger develops sufficient boost (pressure above atmospheric pressure).

As the accelerator pedal is pressed down for rapid acceleration, the engine may lack power for a few seconds. This is caused by the impeller and turbine wheels not spinning fast enough. It takes time for the exhaust gases to bring the turbocharger up to operating speed. To minimize turbo lag, the turbine and impeller wheels are made very light so they can accelerate up to rpm quickly.

Turbocharger Intercooler
A turbocharger intercooler is an air-to-air heat exchanger that cools the air entering the engine. It is a radiator-like device mounted at the pressure outlet of the turbocharger.

Outside air flows over and cools the fins and tubes of the intercooler. As the air flows through the intercooler, heat is removed. By cooling the air entering the engine, engine power is increased because the air is more dense (contains more oxygen by volume). Cooling also reduces the tendency for engine detonation.

Waste Gate
A waste gate limits the maximum amount of boost pressure developed by the turbocharger. It is a butterfly or poppet-type valve that allows exhaust to bypass the turbine wheel.

Without a waste gate, the turbocharger could produce too much pressure in the combustion chambers. This could lead to detonation (spontaneous combustion) and engine damage.

A diaphragm assembly operates the waste gate. Intake manifold pressure acts on the diaphragm to control waste gate valve action. The valve controls the opening and closing of a passage around the turbine wheel.

Under partial load, the system routes all of the exhaust gases through the turbine housing. The waste gate is closed by the diaphragm spring. This assures that there is adequate boost to increase power.

Under a full load, boost may become high enough to overcome spring pressure. Manifold pressure compresses the spring and opens the waste gate. This permits some of the exhaust gases to flow through the waste gate passage and into the exhaust system. Less exhaust is left to spin the turbine. Boost pressure is limited to a preset value.