Press Release

Combination of dynamometers for testing automotive drives

Introduction

Weinlich, a developper and manufacturer of controls, had their first contacts with the technology of control and measurement for automotive brake and power tests in the early 1970ies. Based on these experiences they have been developping and offering engine test beds (dynamometers) since 1980 with the following special features:

• no special foundation required
  
• dynamometer and engine support are separate units
  
• simplification of the mechanical design by arithmetic functions for processing measured values

The dynamometers are usually equipped with the MP Computer, a special microprocessor unit for control of the dynamometer and for processing measured values.

The most popular series of dynamometers is the series MP with air-cooled eddy-current brakes suitable for engines of passenger cars and commercial vehicles. Combined with the RWB mobile engine supports they allow tests and measurements of automotive combustion engines outside of the vehicle. Such a combination is shown on picture 1.

On occasion of Engine Expo 2001 in Stuttgart Weinlich introduced a new combination consisting of two MP dynamometers and one RWB engine support (picture 2) thus offering an extended application.

The tasks

Resulting from contacts with customers and the experiences during preparation of engines for tests at the dynamometer several reasons for this new combination arose.They can be summarized under the following headings:

1. It is getting more and more difficult to prepare and run engines outside of vehicles.

For example:

• It is increasingly impossible to flange damping couplings proven for tests at dynamometers to those flywheels which consist of more than simply a solid disc and thus require a guide by the input shaft of the gearbox.
  
• The complete units of engine, gearbox and distribution gear are based on only three supports, only one of them is at the engine.
  
• In rare cases the engine and the gears share a common lubrication circuit.

2. The measuring task concerns the complete drive system or only the transmissions driven by the engine.

For example:

• Measurement of sound emissions of the engine flanged to gearbox and distribution gear.
  
• Measurements at the engine can be falsified by replacing the gears by a dynamometer, because the interaction of vibrations within the complete drive system and thus the influence of the gears on the engine cannot be simulated by the dynamometer and the transmission parts between engine and dynamometer.

The solution

The obvious solution of these tasks and difficulties is to test the engine together with the complete gear system by replacing each drive wheel by a dynamometer.

Of course one might object, that the measurement of the engine's output is thus falsified by unknown losses during the transmission. However, in an increasing number of applications of a dynamometer the objective is not the determination of the engine's maximal power or fuel consumption, but the reproducibility of certain load states for doing other measurements.

Even if one is interested in the engine's exact power, the measurement at the wheel hub provides a value closer to real conditions in a vehicle, which is important e. g. for predicting the vehicle's fuel consumption.

But why shouldn't one simply use a chassis dynamometer which is much more advantageous with respect to the time needed for preparing an engine with or without drive system? Compared to the proposed solution the disadvantages of a chassis dynamometer are clear: limitation of the power by the tyres, if the diameter of the rollers is not extremely high; higher measuring uncertainty caused by the tyres; worse working conditions.

Although the idea of replacing each drive wheel by a dynamometer is not new, its realization with MP dynamometers and the RWB engine support is (see drawing).

In order to achieve this the standard MP series has been modified by

• clamps at the dynamometer's base frame for fixation of a connecting track
• the connecting track itself which guarantees the right positioning of the two dynamometers and the engine support
• plates with threads for fixation of various adaptors for adjusting and holding the wheel bearing units.

The flexibility of RWB engine supports allows mounting of an engine with drive system for this special application, i. e. turned by 90deg. to the standard position, as shown on picture 2 and picture 3.

The positioning of operating panel and gas lever permits an operation of the clutch-pedal and gear-lever in the usual manner.

The engine and the drive system flanged to it can and should be fixed with their original supports.

The connection between the drive and the dynamometer is done by the wheel bearing units with their flanges for fixation of the wheels.

The wheel bearing unit's fixation plate and threads for the brake calliper offer the best possiblities for alignment and fixation at the dynamometer's base frame.

The flange for fixation of the wheels and the dynamometer's cardan shaft are connected by an adapter disc.

The arrangement as shown on picture 3 has been tested with the two separate controls with MP Computer. Even though there was an interaction between the two independent controls by the connection via the differntial gear, no problems with the control arose.

It is intended to supply the combinations of dynamometers together with only one control based on the proven MP Computer. In addition to a common command speed value for both dynamometers it is possible to input a desired speed ratio of the dynamometers in order to be able to simulate not only drivng straight ahead, but also driving sharp or wide curves, and thus to load the differential gear in a defined way.

Conclusion

The combination of dynamometers as introduced above is available with air-cooled eddy-current brakes of different sizes. It is suitable for testing drives with internal combustion engines of passender cars, small commercial vehicles and the like. It should also be useful for hybrid drives with internal combustion engines.

Furthermore it offers a possibility to carry out comfortable tests at all-wheel-drives, because there is no reason to restrict the arrangement to two dynamometers for testing only one driving axle.

As can be seen from the drawing the isolated vehicle drive might be replaced by a complete vehicle, with the wheel hubs to be connected to the dynamometers.

In general the arrangement consisting of a combination of dynamometers and the RWB engine support is not restricted to braking dynamometers with air-cooled eddy-current brakes. However the application range of air-cooled eddy-current brakes is extended by this arrangement:

1. High-power tests are no longer restricted to short-time measurements.

Let us assume the following:

a simulated speed of 150 km/h, 0.6 m tyre diameter, and a combination of two MP 400S dynamometers;

in this case the load-carrying capacity of the dynamometers is 270 kW permanently.

2. The inertia of an air-cooled eddy-current brake is quite high compared to other types of dynamometers, but the effect of this inertia positioned at the place of the wheels is still less than the effect of the mass of a very light vehicle. Thus the simulation of the acceleration process is not limited by using the solid air-cooled eddy-current brakes.

Time will tell whether this kind of combination of dynamometers is also suitable for testing drives of heavy commercial vehicles.