Before the new engines were allowed onto the roads for practical endurance trials, they had already absolved a series of tortures on the engine test benches of the test facility in Untertürkheim. 24 of the very latest engine test benches are installed on each floor of this imposing three-storey building. These 72 test benches are in operation by day and night, on 365 days of the year. Overall, all of the petrol and diesel engines for the A-Class were subjected to some 256,290 hours on the test benches – that corresponds to 57.3 test bench years (based on 4500 operating hours per year). Tests for both transmissions (six-speed and 7G-DCT), meanwhile, clocked up some 14,979,237 kilometres.
The test-benches can be used to simulate a wide variety of road and load cycle situations to reflect every conceivable operating profiles, e.g. hot and cold-starting, stop-and-go traffic and long-distance operation under a wide variety of conditions. Even steep mountain gradients can be simulated in the laboratory: a pivoting test rig is e.g. used to tilt the engine by up to 40 degrees to examine the effects on the oil circuit.
At various development stages the engines are subjected to accelerated stress tests. “This simulates stresses that no customer can achieve,” says Thomas Uhr, who is responsible for the workshops at the Mercedes-Benz development centre and for powertrain testing. The test-bench programmes have a duration of 500 to 2400 hours, with large proportions under full load and partial load. Thermal characteristics and component durability are tested under full load conditions, however particularly low loads can also be a real torture for an engine in the form of e.g. engine oil sludging. Other tests include heavy stresses such as cooling the coolant from 110°C to approx. 25°C within a max. of 60 seconds, and running the engine up to its rated speed under full load when the coolant is cold. Thomas Uhr: “We test our engines more intensively than any other manufacturer.”
The new OM 607 diesel engine absolved more than 25,000 hours on the test-benches, for example, before it received approval for use in the A-Class. A total of more than 2700 test engines making up the various engine models series were built for testing, along with 2200 transmissions.
The energy generated by the test engines is recovered as fully as possible. The engine testing facility in Untertürkheim is certified as a combined heating and power plant: only active equipment is used to monitor and measure engine power. This power is not used to propel a vehicle as in normal use, but rather converted into electrical energy and fed into the plant’s power network.
In addition to long-term durability, fuel consumption, emissions and driveability in conjunction with the transmission are the major development goals. This requires enormously painstaking dynamometer tests followed by practical trials on the roads. These include test drives at -30°C in northern Sweden, or at over 40°C and an altitude of 2500 metres in Spain. On top of this are diverse test benches trials involving the climate tunnel and altitude chamber, where altitudes of up to 4000 metres can be simulated. As part of a separate programme there is also the endurance test, something of a service life compression test, at which a customer mileage load of 250,000 km is simulated on the high-speed oval.