Turbocharger (Audi A4 B5)

Basically, a turbocharger consists of two blade wheels located on one shaft. Through one of them, the exhaust gases of the engine are removed, depending on the engine speed, this causes the shaft to rotate more or less quickly. The second blade wheel connected to it, of course, rotates with it and sucks air into the combustion chambers, but now under pressure.

With more air flow, the supply of gasoline also increases to ensure the correct composition of the working mixture.



On the left in the engine compartment of the 110 kW turbo engine you can see the charge air cooler (1) and the thick air hose (2) from the turbocharger to the charge air cooler.

Model of a turbocharger in section

• 1 - impeller on the exhaust side;
• 2 - turbocharger shaft;
• 3 - blade wheel on the suction side;
• 4 - turbocharger regulator vacuum box.

Boost pressure adjustment

The selected design data of the turbocharger have the pleasant property of supplying boost air to the cylinders even at relatively low engine speeds. It follows that at high speeds too much air can be supplied. The pressure in the boost system could rise to a level that would be detrimental to the longevity of the engine.



Preventing this is the job of the boost pressure regulator. By opening the valve, it ensures that only part of the exhaust gases pass through the supercharger impeller. As a result, the boost power is reduced and the pressure cannot exceed the maximum value.

In this case, a rigid adjustment is not allowed, since in some driving situations more or less air is needed. Therefore, the Motronic control unit influences the adjustment of the boost pressure. An electro-pneumatic valve, the "boost pressure limiting solenoid valve", controls the boost bypass channel via a vacuum box (boost pressure control valve). When the bypass channel opens, the boost pressure decreases.

The height sensor is connected to the Motronic control unit (installation location: electronic unit in the engine compartment). Thanks to this, the boost pressure regulation is able to compensate for changes in air density when driving in mountainous areas.

Thinner air with the same power offer (i.e. with the same amount of injected fuel) would cause oversaturation of the fuel-air mixture. This is prevented by increasing the boost pressure and, therefore, increasing the air supply.

Therefore, the engine power and exhaust gas composition with a turbo engine remain constant under all driving conditions.

[Content source: the specified website: AUDIMANUAL]