Development, Matching and Testing
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
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
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.
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.
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.
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 Â°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.
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.