Vehicle and engine manufacturers are increasingly turning to improving exhaust gas turbocharging so as to comply with the future requirements of exhaust gas standards and emission values as well as fleet consumption. MAHLE is also heavily involved in this sector. Special test facilities are needed, including so-called hot gas test benches, to test the highly sensitive components both during and after the development phase. MAHLE operates 2 of these at its Stuttgart site.
As a development partner for the automotive industry and expert for engine parts and components in and on combustion engines, MAHLE is involved in the development of new engine concepts from the very beginning and performs extensive test series on numerous types of combustion engines – in some cases as a service paid for by car manufacturers. The Stuttgart site alone has 32 flexible engine test benches for car, commercial vehicle and small engines, some of which are special test benches for friction testing, for example, a cold cell and an anechoic test chamber for acoustic tests. There are also 2 hot gas test benches in operation where the turbochargers are first tested as components before being run with the engine on the engine test benches.
But what exactly happens on a hot gas test bench? Dr. Reiner Künzel, head of MAHLE engine tests, provides an overview of the extensive test series. “These can first be divided up into 3 areas: thermodynamic design, endurance testing and safety tests.”
THE THERMODYNAMIC TESTS
The main job of these tests is to achieve the optimum efficiency of compressor and turbine and to adapt the turbocharger to the engine’s full load curve. The structural design of the compressor and turbine wheel are tested, amongst others. The compressor wheel is normally made of an aluminium alloy and has to prove its dimensional stability – at the temperatures and speeds defined in the specifications. The turbine wheel, usually made of a steel alloy, works in the hot exhaust gas flow and should therefore withstand temperatures of up to around 1,050°C depending on the application. Thermodynamic tests also include tests of the flow rate where the volume of exhaust gas and fresh air handled by the turbocharger is compared to the set operating map.
ENDURANCE TESTS
The mechanical developments have to prove themselves in endurance tests – in other words the component strength of the turbocharger and its resistance to wear. To this end, a turbocharger may also be subjected to a thermal shock test that accelerates the ageing process of the material and components – high-speed wear, so to speak.
SAFETY TESTS
Oil can escape from a damaged compressor or turbine casing that may then be ignited by the hot temperatures of the exhaust manifold in a matter of seconds, thus triggering an engine fire. This means that the overspeed test first has to be passed to answer the question: “At what speed do the compressor and turbine wheel break?” In a second series of tests, the containment test, the wheels are then deliberately weakened to make them break – and then smash against the casing wall with a high energy. In this way it is possible to determine the structural strength of the casing.
Only when the turbocharger has passed the tough, but relatively static load tests on the hot gas test bench is it ready for the series of dynamic tests on the engine test bench which emulate the later operating conditions in the car – after all, the turbocharger has to cope with the vibrations and temperatures on the engine too. Only then is it possible to say whether the turbocharger and engine actually work together. Or, as Reiner Künzel puts it: “They have to breathe together.”
IMPORTANT: THE ADJUSTMENT OF THE ENGINE AND TURBOCHARGER
The operating map application is needed before any exact harmonisation of the engine and turbocharger is possible – in other words, the engine data saved in the control unit is adapted to the parts and components. Thermodynamic adjustments and endurance tests are then carried out (at full load and in combination with cold-hot tests). So-called approval runs are then performed according to specific requirements such as exhaust gas and consumption values or the fracture strength of the casing.
If all of these tests are passed, the turbocharger continues on to the field tests, in other words trial runs in the vehicle. Only then is the new type of turbocharger ready for the SoP, or “Start of Production“.