Design and function of the turbocharger


The demands on our turbochargers are pretty tough

Speeds of well over 300,000 rpm, exhaust gas temperatures above 1,000°C.

The material has to endure quite a bit while achieving a high level of efficiency. To meet these requirements over a wide operating map area requires the highest level of development expertise and manufacturing precision. MAHLE has developed a new process that integrates development and manufacturing.

Cutting-edge simulations, tough tests, and computer-controlled production bring top-class turbochargers to the repair shop. The ultra-high quality of their castings and precise balancing make them unique. As one would expect from MAHLE.


Materials: for the safety of people, engines, and the environment—only the best

The turbocharger components are cast from proven aluminium or steel, and the turbines are made from high temperature-resistant materials.

Together with a refined design, this guarantees very good properties. But of course there is always room for improvement. A team of product and process engineers is constantly working on the ongoing development of manufacturing, assembly, and testing processes. Examples include high-speed machining, joining technology, balancing technology, and coating with liquid and solid materials.

Operative balancing

Operative balancing simulates various operating conditions and identifies dynamic imbalances.

Electron-beam welding

The shaft and turbine wheel are made from different materials, but MAHLE engineers use electron-beam welding to produce an extremely precise and secure connection.


Ready to go: MAHLE Original turbochargers

The performance of a combustion engine depends greatly on the air volume that is available for combustion. Turbochargers are used to increase this volume:

they make use of the energy in the exhaust gas to precompress the intake air and feed a greater mass of air—and therefore more oxygen—into the engine for more efficient combustion.

Thanks to exhaust gas turbocharging, it is possible to increase maximum torque and maximum power output (for a constant working volume) and to increase the brake mean effective pressure without sapping mechanical drive power from the engine, as would be the case for mechanical pressure charging using a compressor. This improvement can be used to deploy a more powerful engine with dimensions nearly identical to the original unit. Or—and this is the current trend—it opens the way for downsizing concepts, which reduce fuel consumption and CO2 emissions without sacrificing performance.

Ongoing development

Ongoing development of exhaust gas turbocharging: engineering in turbo mode.

Future exhaust gas turbochargers will exploit the potential to increase efficiency, responsiveness, and acoustic behaviour. This will further reduce fuel consumption and CO2 emissions. The proportion of turbocharged engines will increase steadily due to ongoing development of turbocharging technology. In the next decade, the internal combustion engine will retain its dominant position over alternative powertrains.

Design and function of the components

Boost pressure control

Exhaust gas turbochargers deliver great engine torque even at low engine speeds with a small exhaust gas mass flow (low-end torque).

Boost pressure is regulated to prevent the exhaust gas turbocharger from overcharging the engine at higher engine speeds.

Exhaust gas turbochargers also provide the option to “overboost”. This refers to a temporary excessive increase in the boost pressure, for example, when accelerating.

Wastegate—the pressure cell

At high engine speeds, the wastegate diverts part of the exhaust flow to the turbine. This reduces the exhaust flow through the turbine and decreases the exhaust back pressure. At low engine speeds, the wastegate closes and the entire exhaust flow actuates the turbine and thus the compressor.

The product portfolio includes wastegate boost pressure control for all diesel and petrol engines up to 560 kW power output. Wastegate boost pressure control is notable for its good functional durability.

VTG—variable turbine geometry

The mechanism of variable turbine geometry uses adjustable guide vanes to regulate the boost pressure independently of the engine speed. In order to produce high boost pressure at low engine speeds, the guide vanes are set to a tight inlet cross section, which increases the velocity of the exhaust gas flow. The greater kinetic energy of the exhaust gas is transferred to the turbine and raises its rotational speed.

At high engine speeds, the guide vanes expose a large inlet cross section and the exhaust gas flow impinges on the inner side of the turbine blades at a much lower speed.

Exhaust gas turbine

Exhaust gas turbine

The turbine wheel, together with the turbine housing, wastegate, and VTG make up the exhaust gas turbine.

The hot exhaust gas backs up ahead of the turbine and is converted into kinetic energy in the turbine, while the turbine accelerates to up to 300,000 revolutions per minute. The exhaust gas flows into the turbine wheel in the radial direction and out of it in the axial direction. The turbine geometry (turbine wheel & housing) is optimally and individually tuned, together with the wastegate or VTG, to meet the engine requirements. Commercial and in-house software tools can be used to quickly produce a graphical representation of a customised tuning.

Turbine housing

The right turbine housing for every application:

Standard monoflow turbine housing

  • Twin-flow turbine housing
  • Turbine housing with integrated exhaust manifold

High-strength, heat-resistant materials are available to meet the highest temperature requirements in petrol engines (>1,000°C).

Twin-flow turbine

housing In order to avoid mutual influence of exhaust gas flows during gas exchange, the exhaust gas is guided through two separate exhaust ducts from the cylinders to the turbine housing. The turbine can be positioned close to the exhaust valves in order to prevent loss of enthalpy in the exhaust gas. This produces good turbine efficiency and thus good engine responsiveness.



The compressor consists of the impeller, the diffuser, and the compressor housing.

As with the exhaust gas turbine, the compressor is tuned optimally and specifically to the engine. The radial compressor impeller transfers the majority of the kinetic energy provided by the turbine to the air flow.

This brings about the required pressure increase in a diffuser in the compressor housing. High-strength, precision-milled impellers made of special aluminium alloys are used. This guarantees high acceleration and improves low-end torque. The uniform geometry of the impeller greatly reduces pulsation noise.

For more demanding wear requirements, the impeller can be coated accordingly. This may be necessary, for example, when using low-pressure exhaust gas recirculation.

Compressor housing

The compressor housing is thoroughly optimised in terms of aerodynamics. Optional features are available to extend the usable operating range. To this end, recirculation systems have been developed to stabilise the compressor’s characteristic curves.