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Turbo system, overview

Turbo system, overview

Supercharging, general

Unlike a conventional aspirating engine the supercharged engine provides improved filling during the induction stroke, which results in more efficient combustion, increased efficiency and a higher torque, i e a performance which is comparable to that of a larger engine but retains the advantages of a small engine as regards fuel economy, space requirements, weight etc.

The supercharging is brought about by a turbocharger, which means that the engine's exhaust gases are used as the propellant. The exhaust gases are led through an exhaust gas turbine and this makes the turbine blade wheel rotate. The turbine's blade wheel is fitted to the same shaft as a compressor blade wheel and rotates at the same speed. The compressor wheel is situated in the intake system and causes an increase in the pressure there, which results in improved filling of the combustion chamber.

The turbocharger is dimensioned in such a way that it starts to operate at fairly low speeds and this gives a high torque at the speeds which are utilized during normal driving conditions.

A. Lubrication

The turbo shaft, which rotates at a very high speed, is carefully balanced and journalled in so called floating plain-bearing bushings. This means that there is a considerable flow of oil through the bearings with the result that while it is rotating the shaft floats on a film of oil. The lubrication oil is led from the engine's lubrication system through a special oil pipe which comes from the oil filter adapter housing. The oil returns to the engine's oil pan. The seal between the shaft and the bearing housing consists of rings (of the piston ring type), which are situated in grooves in the shaft.

B. Cooling

The turbo compressor is equipped with a fluid cooler, which lowers the temperature of the bearing housing approx. 100°C (212°F). By lowering the temperature the risk of overheating and related damage is reduced. The coolant is led in a pipe out of a channel in the engine block. Once the fluid has passed the bearing housing it is lead through a pipe to the thermostat housing. When the engine is switched off and the coolant pump stops, the coolant system will self-circulate through the thermosiphon effect.

Adjustment of turbo pressure

The turbo pressure in the intake manifold depends primarily on the engine's speed and load. At higher loads the turbo pressure is, however, limited by the wastegate.

The wastegate is a flap valve which opens or closes the bypass channel at the side of the turbo wheel. The flap valve is operated by a lever from a diaphragm unit which is located by the compressor housing. The diaphragm is operated by the pressure in the compressor and a spring in the diaphragm unit has the function of closing the flap.

During low and normal loads the wastegate is closed. When the load increases and the turbo pressure comes close to the maximum permitted, the force of the spring is overcome and the wastegate opens the bypass channel, which reduces the pressure.

The pressure is continuously monitored by a manifold absolute pressure (MAP) sensor, which is connected to the intake manifold by a hose. A gauge in the main instrument shows the turbo pressure: it receives information on current pressure from the MAP-sensor.