Why Do Diesel Engines Use Turbochargers?
A turbocharger is an exhaust-driven supercharger (fan or blower) that forces air into the engine under pressure. Turbochargers are frequently used on diesel engines to increase power output.
Turbochargers are well suited to operation in diesel engines because the diesel engine is immune to detonation as the fuel is not injected until the moment of combustion.
By harnessing engine exhaust energy, a turbocharger can also improve engine efficiency (fuel economy and emissions levels).
The 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.
A turbocharger consists of a turbine and a compressor. They are both mounted on the same shaft. The turbine is a heat engine. It converts exhaust heat and pressure to rotation. This rotation is used to turn the compressor. The compressor takes in the draws in the outside air. It squeezes or compresses the air. It then sends the air to the engine. Because the air pressure has been increased, more air and fuel may be put into the cylinders. This is sometimes called boost pressure.
Theoretically, the turbocharger should be located as close to the engine manifold as possible. Because then the 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.
Turbocharger 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.
Turbocharger 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.
Turbocharger 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.
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.
Turbocharger units weigh very little but can provide significant power, torque, and efficiency improvements. 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.
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.
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.
Turbocharger 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.
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 intercoolers.
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 intercooler, 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.
Turbocharger 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.