Performance Measurement

bk Simmerath

After the torque has been measured for all accessible revs on the performance test stand, subsequently, the performance can be calculated from the data. In contrast to former test stands, today's measuring methods can also determine the torque delivered to the flywheel. Furthermore, it is possible to include all four wheels in the testing, e.g., constant four-wheel driven vehicles, on special test benches (see figure 2) with two roll sets.

Although the measuring station almost looks like a brake roller test stand, nevertheless, more preparations are needed. The vehicle should be parked with its driving wheels straight on the rolls and tied down in both directions (see figure 1). In addition to the roll sets a powerful electric motor and a proportionate blower are necessary. The exhaust gas withdrawal system should not be of a standard workshop size, but larger, because considerably more exhaust gases are produced at full load than in idle speed.

Two rolls are needed for every wheel and in between lifting equipment to lift the wheels out of the stand at the end. The front rolls of each pair are provided with a friction surface and mechanically connected to an eddy current engine. In order to explain the principle, imagine that the vehicle engine is running, causing the wheels of the driven axle to turn at a certain speed. This in turn affects the rolls, and subsequently the rotor (armature) of the electric motor. If it is switched on, the electric motor builds up torque opposite to the present rotation direction. If it slowly receives more current, it uses the energy to slow down the wheels of the driven axle. The full load check implies that the slowing down process is performed thus that with fully slammed accelerator pedal different engine speeds are accomplished. Taking the current that is needed for slowing down the wheels and the roll revs, it is possible to determine the torque of the engine at the driven axle at the specific revs.

Practically, it suffices to switch on the test stand, put the accelerator pedal down to the maximum in a not too small gear at low revs. The smaller the gear the more accurate the measurement results; the higher the gear, the less liable to wear is the test stand and engine drive. The test stand regulates the opposing forces in such a way, as if the vehicle would accelerate normally. At the same time it measures the torque. Slightly above the nominal rotation speed and before reaching the maximum rev you should stop. At this point the specific feature of today's test stands comes forward. You should engage the clutch after easing off the accelerator pedal, witnessing a progressive decrease in indicated driving speed on the speedometer. During this action the test software determines for different revs the necessary drag force from the wheels to the clutch and thus allows conclusions on the engine torque.

Previously, you had to adjust the counter torque of the test stand manually for each rev/speed. Also, the system was limited to the torque at the wheel. The losses of the drive train were disregarded and had to be estimated. In addition, it was necessary to control regularly that the electric motor would not overheat, especially at low wheel speeds in combination with high torque developments. However, in spite of efficient blowers you should even today not expand full load-test runs for too long. Is it dangerous? The mass of the turning parts as compared to the mass of the motionless vehicle is relatively low. Nevertheless, you should tie down the vehicle thoroughly, but, indeed, with a certain freedom of movement.

Getting out reliable figures when all four wheels are permanently driven is more challenging.For example, a complete second roll set is necessary. In addition, both roll sets cannot be countersunk in the floor, because at least one of them must be arranged adjustable according to the wheelbase of the vehicle. This distance must be constantly corrected depending on the test.