Виж това може да те убеди:Turbocharger - The function of the turbocharger is to increase the amount of air which enters the engine, thus allowing the power and torque to be increased tremendously compared to an engine without a turbocharger. It works rather like a miniature jet engine. A turbine is located in the exhaust from the diesel engine, and converts the pressure of the exhaust gases to mechanical work on a shaft. A compressor wheel is driven by this shaft, and draws air from the air intake, compresses it, and pressurizes the intake manifold of the diesel engine. The shaft of the turbine and compressor is mechanically independent of any connection to the rest of the engine - it spins freely on its own. The bearing on this shaft is lubricated by engine oil, fed by a separate oil line from the vicinity of the oil filter, and the oil drains into the crankcase through a drain pipe. The oil also serves to cool the turbine; there is no connection to the engine coolant.
Conventional turbochargers have the advantage of recovering part of the exhaust energy to increase the power of the main engine, but the disadvantage that they only operate under optimum conditions at a single engine speed - which, in the case of the 1.9 4-cylinder, is about 2000 rpm. Below that speed, there is not enough energy contained in the exhaust gases to allow the design level of "boost" to be achieved. Above that speed, there is too much energy in the exhaust, and the turbine tends to overspeed and generate too much boost. There are various ways of dealing with this - some models use a so-called "wastegate" which is the original design, and other models use a set of vanes in the turbine section to vary the angle and speed that the exhaust gases strike the turbine.


The A3 90hp engines use a conventional Garrett GT15 turbocharger with an internal wastegate. When the engine speed is below the optimum design speed for the turbo, the turbo does whatever it can, and the engine gets less boost than design ... nothing can be done about it. As engine speed increases, the boost level also increases because the extra exhaust energy spins the turbine faster. When the boost level starts to exceed a certain amount, the engine controls supply pressure to the wastegate actuator, which then bypasses part of the exhaust around the turbine. This causes less energy to be delivered to the turbine, so the boost level drops, and the engine controls operate so as to keep the boost level at the design level.
To improve performance in "off-design" conditions, the A4 engines (and the A3 110hp engines) use a variable-geometry Garret VNT15 turbocharger. There is no wastegate, and all of the engine exhaust passes through the turbine all the time. When the engine is running slowly, a set of vanes inside the turbine housing move, to direct the exhaust gases through the turbine at a shallower angle but a higher speed; this causes the turbine to spin faster, so that the turbo compressor can reach the design boost level sooner. As the engine runs faster and the amount of exhaust gases increase, the boost level starts going up; this is detected by the engine controls which act to move the vanes toward a more open angle. This allows less energy to be transmitted to the turbine wheel, so the boost level is reduced. Compared with the previous design, this arrangement allows the design boost level to be developed faster and at lower engine speeds, and allows more efficient operation with reduced exhaust back pressure at higher engine speeds.
Another design change with the new A4 engines is that the turbine housing is built into the exhaust manifold instead of being a separate piece, thus eliminating a potential leakage point.
For the 90hp engines, typical boost level at full load is 15 to 16 psi intermittently and 12 to 13 psi continuously.Back to Top
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