Description
Turbocharger
OPERATION
The turbocharger is an exhaust-driven centrifugal air compressor which increases power output by supplying compressed air to the engine. The turbine wheel of the turbocharger uses the engine's exhaust gasses to drive the compressor wheel at speeds of up to 195,000 RPM. The compressor wheel draws in fresh air which is compressed and delivered through a charge air cooler to the engine cylinders.
By turbocharging the engine, the pressure and density of the air entering the cylinders is increased, and therefore so is the amount of oxygen. This enables a greater quantity of fuel to be injected, thus increasing the engine's power output, improving fuel consumption and the ability to maintain power at higher altitudes.
The internal components are oil and coolant cooled. Engine oil and coolant are circulated through the center housing which acts as a heat barrier between the "hot" turbine and the "cold" compressor. The bearing is a sleeve type and is lubricated by engine oil. Oil is circulated to the turbocharger center housing and returned to the sump through an oil drain to the cylinder block.
DESCRIPTION
Turbocharger
The turbocharger is attached to a sheet steel fabricated exhaust manifold with integral sheet steel turbine housing. The exhaust manifold is attached to the cylinder head with studs and nuts and sealed with a layered gasket. The manifold is fabricated in two layers which impedes heat dissipation.
The fabricated steel manifold has lighter weight than a cast manifold and also promotes faster warm-up of the catalytic converter, reducing emissions.
Manifold and Turbocharger Section
The manifold incorporates the turbocharger wastegate which is connected by a rod to the pressure actuator which is mounted on the turbocharger. The turbocharger turbine is also located within the manifold fabrication. The turbocharger housing is attached to the manifold with a two piece clamp which is secured with a bolt and nut.
The turbocharger compressor is located within the turbocharger housing. Pressure pipes are connected to the inlet air side of the turbocharger housing and are connected to a turbocharger wastegate control solenoid valve. When the solenoid is energized by a PWM (pulse width modulation) signal from the ECM (engine control module), the valve opens and pressure is used to operate the wastegate pressure actuator.
The wastegate will start to open at about 0.35 bar (5.07 lbf/in2), and will be fully open at about 0.55 bar (8.00 lbf/in2) applied pressure at the actuator. The opening is controlled by the ECM (engine control module), and may be modulated away from these values to provide optimum performance. The pressure to the actuator is modulated through the turbocharger wastegate control solenoid valve, mounted on the bracket on the compressor housing.
On a quick throttle closing, the compressor can be running at high speed with no air flow, which is an unstable operating condition. This creates to a sound, or surge, which is quite noticeable in vehicle. To counteract this, the turbocharger is fitted with a vacuum actuated recirculation valve. The valve is connected via a tube to the intake manifold. The valve addresses the sound by responding to intake manifold vacuum (created when the throttle is closed). The vacuum activates the valve in the turbocharger which opens up a channel from the compressor outlet back to the compressor inlet. This recirculation of compressor air minimizes the unstable surge condition and the related noise.
Turbocharger Cooling
The turbocharger receives an oil supply for lubrication and cooling purposes. In addition to the cooling provided by the engine lubrication system, the turbocharger also requires a supply from the engine coolant system to assist with the cooling due to the high loads on the turbocharger. A pipe connection from the cylinder block routes engine coolant through the body of the turbocharger bearing housing. Once the coolant has passed through the bearing housing it is returned to the cooling system via the water outlet on the cylinder head.
Turbocharger Lubrication
The rapid acceleration and deceleration response demands of the turbocharger rely greatly on a steady flow of clean oil. The oil supplied from the engine's lubrication system provides lubrication to the turbocharger's spindle and bearings, while also acting as a coolant for the turbocharger centre housing. A small oil filter is located in the cylinder block to prevent particulate matter entering the turbocharger via the engine oil supply pipe.
To maintain the life expectancy of the turbocharger, it is essential that the oil has a free-flow through the turbocharger and unrestricted return to the engines oil pan. It is therefore imperative that the engine oil is replenished at regular service intervals with the recommended quality and quantity of oil.
Charge Air Cooler
The charge air cooler is used to increase the density of air as it flows from the turbocharger compressor to the intake manifold. Compression of the charge air by the turbocharger raises the temperature of the air. This generation of heat further increases the charge air density and consequently less oxygen is able to enter the cylinders, reducing the engines power. To overcome this, the air is routed through the charge air cooler before it enters the engine; the temperature is reduced by transferring the heat to atmosphere. Cooling of the intake air also helps to reduce engine emissions by limiting nitrogen oxides (NOx) production.