KO4 Turbo label diagram

1.Wastegate vacuum unit
2.Underlay strip
3.Turbine casing
4.Compressor housing
5.Recirculation dump valve
6.Charge pressure regulator
7.Air intake duct to compressor

Compressor
Turbocharger compressors are generally centrifugal compressors consisting of three essential components: compressor wheel, diffuser, and housing. With the rotational speed of the wheel, air is drawn in axially, accelerated to high velocity and then expelled in a radial direction.
The diffuser slows down the high-velocity air, largely without losses, so that both pressure and temperature rise. The diffuser is formed by the compressor backplate and a part of the volute housing, which in its turn collects the air and slows it down further before it reaches the compressor exit.


--------------------------------------------------------------------------------

Turbine
The turbocharger turbine, which consists of a turbine wheel and a turbine housing, converts the engine exhaust gas into mechanical energy to drive the compressor.
The gas, which is restricted by the turbine's flow cross-sectional area, results in a pressure and temperature drop between the inlet and outlet. This pressure drop is converted by the turbine into kinetic energy to drive the turbine wheel.

There are two main turbine types: axial and radial flow. In the axial-flow type, flow through the wheel is only in the axial direction. In radial-flow turbines, gas inflow is centripetal, i.e. in a radial direction from the outside in, and gas outflow in an axial direction.

Up to a wheel diameter of about 160 mm, only radial-flow turbines are used. This corresponds to an engine power of approximately 1000 kW per turbocharger. From 300 mm onwards, only axial-flow turbines are used. Between these two values, both variants are possible.

As the radial-flow turbine is the most popular type for automotive applications, the following description is limited to the design and function of this turbine type.
In the volute of such radial or centripetal turbines, exhaust gas pressure is converted into kinetic energy and the exhaust gas at the wheel circumference is directed at constant velocity to the turbine wheel. Energy transfer from kinetic energy into shaft power takes place in the turbine wheel, which is designed so that nearly all the kinetic energy is converted by the time the gas reaches the wheel outlet.


--------------------------------------------------------------------------------

Waste gate
The turbine-side bypass is the simplest form of boost pressure control. The turbine size is chosen so that torque characteristic requirements at low engine speeds can be met and good vehicle drive ability achieved. With this design, more exhaust gas than required to produce the necessary boost pressure is supplied to the turbine shortly before the maximum torque is reached. Therefore, once a specific boost pressure is achieved, part of the exhaust gas flow is fed around the turbine via a bypass.
Today, electronic boost pressure control systems are increasingly used in modern petrol engines. When compared with purely pneumatic control, which can only function as a full-load pressure limiter, a flexible boost pressure control allows an optimal part-load boost pressure setting. This operates in accordance with various parameters such as charge air temperature, degree of timing advance and fuel quality. The operation of the flap corresponds to that of the previously described actuator. The actuator diaphragm is subjected to a modulated control pressure instead of full boost pressure.
This control pressure is lower than the boost pressure and generated by a proportional valve. This ensures that the diaphragm is subjected to the boost pressure and the pressure at the compressor inlet in varying proportions. The proportional valve is controlled by the engine electronics.


--------------------------------------------------------------------------------

Bearing System


Radial bearing system
With a sleeve bearing, the shaft turns without friction on an oil film in the sleeve bearing bushing. For the turbocharger, the oil supply comes from the engine oil circuit. The bearing system is designed such that brass floating bushings, rotating at about half shaft speed, are situated between the stationary centre housing and the rotating shaft. This allows these high speed bearings to be adapted such that there is no metal contact between shaft and bearings at any of the operating points. Besides the lubricating function, the oil film in the bearing clearances also has a damping function, which contributes to the stability of the shaft and turbine wheel assembly. The hydrodynamic load-carrying capacity and the bearing damping characteristics are optimised by the clearances. The lubricating oil thickness for the inner clearances is therefore selected with respect to the bearing strength, whereas the outer clearances are designed with regard to the bearing damping. The bearing clearances are only a few hundredths of a millimetre.

The one-piece bearing system is a special form of a sleeve bearing system. The shaft turns within a stationary bushing, which is oil scavenged from the outside. The outer bearing clearance can be designed specifically for the bearing damping, as no rotation takes place.

Axial-thrust bearing system
Neither the fully floating bushing bearings nor the single-piece fixed floating bushing bearing system support forces in axial direction. As the gas forces acting on the compressor and turbine wheels in axial direction are of differing strengths, the shaft and turbine wheel assembly is displaced in an axial direction. The axial bearing, a sliding surface bearing with tapered lands, absorbs these forces. Two small discs fixed on the shaft serve as contact surfaces. The axial bearing is fixed in the centre housing. An oil-deflecting plate prevents the oil from entering the shaft sealing area.


--------------------------------------------------------------------------------

Oil drain
The lubricating oil flows into the turbocharger at a pressure of approximately 4 bar. As the oil drains off at low pressure, the oil drain pipe diameter must be much larger than the oil inlet pipe. The oil flow through the bearing should, whenever possible, be vertical from top to bottom. The oil drain pipe should be returned into the crankcase above the engine oil level. Any obstruction in the oil drain pipe will result in back pressure in the bearing system. The oil then passes through the sealing rings into the compressor and the turbine.


--------------------------------------------------------------------------------

Water cooling


Petrol engines, where the exhaust gas temperatures are 200 to 300 °C higher than in diesel engines, are generally equipped with water-cooled centre housings. During operation of the engine, the centre housing is integrated into the cooling circuit of the engine. After the engine's shutdown, the residual heat is carried away by means of a small cooling circuit, which is driven by a thermostatically controlled electric water pump.


--------------------------------------------------------------------------------

Recommendations for the Turbo's Servicing and Care
What's the best treatment for your turbocharger?
The turbocharger is designed so it will usually last as long as the engine (untuned). It does not require any special maintenance; and inspection is limited to a few periodic checks.

To ensure that the turbocharger's lifetime corresponds to that of the engine, the following engine manufacturer's service instructions should be strictly observed:

- Oil change intervals, the more frequent the better in our opinion (approx every 6-10 months with 5w/40 full synth)
- Oil filter system maintenance (@ every oil change)
- Oil pressure control
- Air filter system maintenance, replace filter, always ensure the filter provides proper particle filtration.
- Coolant system maintenance.

What's bad for your turbocharger?
90 % of all turbocharger failures are due to the following causes:

- Penetration of foreign bodies into the turbine or the compressor
- Dirt in the oil
- Inadequate oil supply (oil pressure/filter system)
- High exhaust gas temperatures (ignition system/injection system)
These failures can be avoided by regular maintenance. When maintaining the air filter system, for example, care should be taken that no tramped material gets into the turbocharger.


--------------------------------------------------------------------------------

Failure diagnosis
If the engine does not operate properly, you should not assume that the turbocharger is the cause of failure. It often happens that fully functioning turbochargers are replaced even though the failure does not lie with the turbo, but with the engine.

Before looking for faults in the turbocharger, the following points should be checked:

Power loss or black smoke:
- Is the air filter dirty?
- Is the engine compression too low?
- Is the injection system operating correctly?


Blue smoke:- Is the PCV (positive crankcase ventilation) system operating correctly?
- Are the compression ratios correct?
- Are the spark plugs in good condition?
- Is the oil drain free?

Only after all these points have been checked should you check the turbocharger for faults. Since the turbocharger components are manufactured on high-precision machines to close tolerances and the wheels rotate up to 300,000 rpm, turbochargers should be inspected by qualified specialists only.

zle.tco.uk