The turbocharger basically consists of two impellers fitted to the same shaft. It turns on bushes that are lubricated by a branch of the engine lubrication circuit.This oil also dissipates part of the great quantity of heat yielded up by the exhaust gas to the turbine.An impeller known as a turbine connected to the exhaust manifold is caused to rotate by residual energy in the gas directed over it. The turbine also causes the other impeller (known as a compressor) that is connected to the intake manifold to move (at the same speed).Due to its speed of rotation and the specific shape of its vanes, this compressor is able to taken in outside air and compress it in the intake manifold and hence the engine cylinders.If engine rpm rises, the turbine and compressor also turn faster to increase the amount of air supplied to the engine.The engine develops more power but the flow of exhaust gas increases and makes the turbine turn even faster. This cycle continues until one of the engine or turbocharger components breaks.It is therefore vital to have a supercharging pressure regulation system.This system consists of limiting the rotation speed of the turbine when the desired pressure is reached, introducing some of the exhaust gases directly into the exhaust manifold, avoiding the turbine.In this way the regulation speed of the turbine is reduced.
REGULATION VALVE (waste gate valve)
Description
The adjustment is achieved through the mushroom valve (5). This valve is located upstream of the turbine and consists of a diaphragm and a spring (6) calibrated at the maximum supercharging level. When service conditions create a higher-than-permitted turbo pressure, the spring compresses to open valve (5). Only a proportion of the exhaust gases pass through the turbine while the remainder emerges from the valve and flows directly to the exhaust.
VARIABLE GEOMETRY TURBOCHARGER
Description
The variable geometry turbocharger consists of a compressor and a turbine (1) equipped with a series of moving vanes (2) which regulate the flow of gases directed against the impeller (2) and constrained to pass through.Thanks to this solution it is possible to keep the speed of the gases and, consequently, the turbine, high even when the engine speed is low. In effect, making the gases pass through small sections means that they flow faster so that the turbine also turns faster.As the moving vanes are mechanically fastened to the rotary ring (3), they are in the maximum closed condition at low engine speeds.The rotation of the ring (3) and therefore the orientation of the vanes (2) is determined by the rod (4) and the tooth (5) mechanically controlled by a pneumatic type actuator (6) dependent on the compressor operating pressure.As the pressure is linked to the engine speed, the orientation of the vanes and consequently the variation in the sections the exhaust gases pass through also depend on the engine speed.At high engine speeds the pneumatic device intervenes, increasing the section of the passage and allowing the gases arriving to flow without causing the impeller to rotate too fast.The actuator (6) therefore, together with the moving vanes (2), carries out the function of regulating the maximum turbocharger speed.
Operation at high rotation speeds.
As the rotation speed of the engine increases, there is a gradual increase in the kinetic energy of the exhaust gases.As a result, the speed of the turbine (1) and, consequently the supercharging pressure, which also acts on the actuator (3), increases.The actuator (3), controlled by the solenoid valove, opens the moving vanes (2) by means of a rod, until the maximum opening position is reached.The increase in the section produces a slowing down of the flow of the exhaust gases passing through the turbine (1) at speeds which are the same as or less than the low speed condition.The speed of the turbine (1) decreases and settles down at a suitable level for the correct operation of the engine at high speeds.
Operation at low rotation speeds.
When the engine is operating at low rotation speeds, the exhaust gases possess little kinetic energy: under these circumstances a conventional turbine would rotate slowly, providing limited supercharging pressure.In the variable geometry turbine (1) on the other hand, the moving vanes (2) are in the maximum closed position and the small sections between the vanes increase the speed (C) of the intake gases.Greater intake speeds result in greater peripheral speeds (U) for the turbine and, consequently, the compressor.The speed of the gases inside the impeller is indicated by the vector (W).
The vectors acting on the compressor and the turbine interact as follows: the output speed C of the exhaust gases form the moving vanes is also the intake speed at the impeller. It forms an angle with the direction of the impeller peripheral speed U. This angle depends on the position of the moving vanes. Therefore input C absolute speed conditions being equal, the peripheral speed U decreases as W increases. This variation is determined by the relationship:U = C x cos W;remembering that:if W = 0° cos W = 1;if W = 90° cos W = 0;the absence of a waste-gate valve "cutting off" the flow of gas to the turbine, beyond a certain level, is due to the fact that as the supercharging pressure increases, the moving vanes reach an opening angle which decreases and stabilizes the peripheral speed U of the impeller and, consequently, the actual pressure.
