4.3 Superchargers and Turbochargers

Supercharging and turbocharging are methods of increasing engine volumetric efficiency by forcing the air into the combustion chamber, rather than merely allowing the pistons to draw it naturally. Supercharging and turbocharging, in some cases, will push volumetric efficiencies over 100 percent.

Superchargers

A supercharger is an air pump that increases engine power by pushing a denser air charge into the combustion chamber. With more air and fuel, combustion produces more heat energy and pressure to push the piston down in the cylinder.

The term supercharger generally refers to a blower driven by a belt, chain, or gears. Superchargers are used on large diesel and racing engines.

The supercharger raises the air pressure in the engine intake manifold. When the intake valves open, more air-fuel mixture can flow into the cylinders. An intercooler is used between the supercharger outlet and the engine to cool the air and to increase power (cool charge of air carries more oxygen needed for combustion).

A supercharger will instantly produce increased pressure at low engine speed because it is mechanically linked to the engine crankshaft. This low-speed power and instant throttle response are desirable for passing and for entering interstate highways.

Centrifugal Supercharger

The centrifugal supercharger has an impeller equipped with curved vanes (Figure 4-16). As the engine drives the impeller, it draws air into its center and throws it off at its rim. The air then is pushed along the inside of the circular housing. The diameter of the housing gradually increases to the outlet where the air is pushed out.


Figure 4-16 Centrifugal supercharger.

Turbochargers

A turbocharger is an exhaust-driven supercharger (fan or blower) that forces air into the engine under pressure (Figure 4-17). 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).


Figure 4-17 Turbocharger.

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.

Components of a Turbocharger

The turbocharger consists of three major components: a radial inward flow turbine wheel and shaft, a centrifugal compressor wheel, and a center housing that supports the rotating assembly, bearings, seals, turbine housing, and compressor housing. The center housing also has connections for oil inlet and oil outlet fittings.

Turbine Wheel. The turbine wheel is located in the turbine housing and is mounted on one end of the turbine shaft. Exhaust gases enter the turbine housing and spin the turbine wheel.

Compressor Wheel.The compressor wheel is located on the turbine shaft on the opposite end of the turbine wheel. As the gases spin the turbine wheel, the turbine shaft spins the compressor wheel.

Turbine Housing. The turbine housing is made of a heat-resistant alloy casting that encloses the turbine wheel and provides a flanged exhaust gas inlet and an axially-located turbocharger exhaust gas outlet.

Operation

The 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.

Advantages

The turbocharger offers a distinct advantage for a diesel engine operating at higher altitudes. The turbocharger automatically compensates for the loss of air density. An increase in altitude also increases the pressure drop across the turbine. Inlet turbine pressure remains the same, but outlet pressure decreases as the altitude increases. Turbine speed also increases as the pressure differential increases.

Lubrication

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 allow 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.

Controls

While there are many types of turbocharger controls, they fall into two groups: those that limit turbocharger speed and those that limit compressor outlet pressure, or boost. Controls that limit turbocharger speed keep the turbocharger from destroying itself.

Those that limit boost keep the turbocharger from damaging the engine. Since the modern turbocharger can produce more pressure than the engine can use, most controls are designed to limit the amount of boost. One of the most common methods of limiting the boost is with a waste gate valve.

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.

Aftercooler

The use of a turbocharger increases the temperature of the intake air. This increase in temperature is because the turbocharger compresses the air. To help counteract this increase in temperature, an intercooler or aftercooler is installed. There are two types of aftercoolers being used today: coolant aftercoolers and air-to-air aftercoolers.

In coolant aftercoolers, engine coolant flows through the aftercooler core tubes. As the hot compressed air from the turbocharger passes around the tubes, it is dropped to the temperature of the coolant.

In air-to-air aftercoolers, the air is a 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 denser (contains more oxygen by volume). Cooling also reduces the tendency for engine detonation.

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.

Question 6. Which part does NOT drive a supercharger?

A. Belt
B. Exhaust
C. Chain
D. Gears