Development, Matching and Testing

Development

As turbochargers have to meet different requirements with regard to map height, map width, efficiency characteristics, moment of inertia of the rotor and conditions of use, new compressor and turbine types are continually being developed for various engine applications. Furthermore, different regional legal emission regulations lead to different technical solutions.

The compressor and turbine wheels have the greatest influence on the turbocharger's operational characteristics. These wheels are designed by means of computer programs which allow a three-dimensional calculation of the air and exhaust gas flows. The wheel strength is simultaneously optimised by means of the finite-element method (FEM), and durability calculated on the basis of realistic driving cycles.

CAD-assembled model of a turbocharger CAD-assembled model of a turbocharger

Despite today's advanced computer technology and detailed calculation programs, it is testing which finally decides on the quality of the new aerodynamic components. The fine adjustment and checking of results is therefore carried out on turbocharger test stands.

Matching

The vital components of a turbocharger are the turbine and the compressor. Both are turbo-machines which, with the help of modelling laws, can be manufactured in various sizes with similar characteristics. Thus, by enlarging and reducing, the turbocharger range is established, allowing the optimal turbocharger frame size to be made available for various engine sizes. However, the transferability to other frame sizes is restricted, as not all characteristics can be scaled dimensionally. Furthermore, requirements vary in accordance with each engine size, so that it is not always possible to use the same wheel or housing geometries.

The model similarity and modular design principle, however, permit the development of turbochargers which are individually tailored to every engine. This starts with the selection of the appropriate compressor on the basis of the required boost pressure characteristic curve. Ideally, the full-load curve should be such that the compressor efficiency is at its maximum in the main operating range of the engine. The distance to the surge line should be sufficiently large.

The thermodynamic matching of the turbocharger is implemented by means of mass flow and energy balances. The air delivered by the compressor and the fuel fed to the engine constitute the turbine mass flow rate. In steady-state operation, the turbine and compressor power outputs are identical (free wheel condition). The matching calculaton is iterative, based on compressor and turbine maps, as well as the most important engine data.

The matching calculation can be very precise when using computer programs for the calculated engine and turbocharger simulation. Such programs include mass, energy and material balances for all cylinders and the connected pipework. The turbocharger enters into the calculation in the form of maps. Furthermore, such programs include a number of empirical equations to describe interrelationships which are difficult to express in an analytical way.

Testing

The turbocharger has to operate as reliably and for as long as the engine. Before a turbocharger is released for series production, it has to undergo a number of tests. This test programme includes tests of individual turbocharger components, tests on the turbocharger test stand and a test on the engine. Some tests from this complex testing programme are described below in detail.

Containment test

If a compressor or turbine wheel bursts, the remaining parts of the wheel must not penetrate the compressor or turbine housing. To achieve this, the shaft and turbine wheel assembly is accelerated to such a high speed that the respective wheel bursts. After bursting, the housing's containment safety is assessed. The burst speed is typically 50 % above the maximum permissible speed.

Low-Cycle Fatigue Test (LCF test)

The LCF test is a load test of the compressor or turbine wheel resulting in the component's destruction. It is used to determine the wheel material load limits. The compressor or turbine wheel is installed on an overspeed test stand. The wheel is accelerated by means of an electric motor until the specified tip speed is reached and then slowed down. On the basis of the results and the component's S/N curve, the expected lifetime can be calculated for every load cycle.

Rotor dynamic measurement

The rotational movement of the rotor is affected by the pulsating gas forces on the turbine. Through its own residual imbalance and through the mechanical vibrations of the engine, it is stimulated to vibrate. Large amplitudes may therefore occur within the bearing clearance and lead to instabilities, especially when the lubricating oil pressures are too low and the oil temperatures too high. At worst, this will result in metallic contact and abnormal mechanical wear.

The motion of the rotor is measured and recorded by contactless transducers located in the suction area of the compressor by means of the eddy current method. In all conditions and at all operating points, the rotor amplitudes should not exceed 80 % of maximum possible values. The motion of the rotor must not show any instability.

Start-stop test

The temperature drop in the turbocharger between the gases at the hot turbine side and at the cold compressor inlet can amount to as much as 1000 °C in a distance of only a few centimetres. During the engine's operation, the lubricating oil passing through the bearing cools the centre housing so that no critical component temperatures occur. After the engine has been shut down, especially from high loads, heat can accumulate in the centre housing, resulting in coking of the lubricating oil. It is therefore of vital importance to determine the maximum component temperatures at the critical points, to avoid the formation of lacquer and carbonised oil in the turbine-side bearing area and on the piston ring.

After the engine has been shut down at the full-load operating point, the turbocharger's heat build-up is measured. After a specified number of cycles, the turbocharger components are inspected. Only when the maximum permissible component temperatures are not exceeded and the carbonised oil quantities around the bearing are found to be low, is this test considered passed.

Cyclic endurance test

During engine operation, the waste gate is exposed to high thermal and mechanical loads. During the waste gate test, these loads are simulated on the test stand. During engine operation, the waste gate is exposed to high thermal and mechanical loads. During the waste gate test, these loads are simulated on the test stand.

The checking of all components and the determination of the rates of wear are included in the cycle test. In this test, the turbocharger is run on the engine for several hundred hours at varying load points. The rates of wear are determined by detailed measurements of the individual components, before and after the test.