Dynamometers: Part two

AFTER covering elementary engine dynamometers last month, we now move on to the more advanced types.

These can be classified as either engine or chassis units, depending on whether the dyno is coupled directly to the engine or activated by rollers driven by the vehicle's drive wheels.

ENGINE DYNOS

Engine dynos are used extensively by engine manufacturers to measure engine output, or to conduct endurance runs of sometimes up to 1 000 hours to ensure that new designs are strong enough to endure many years of average use.

They have the major advantage that the dyno-cell can be equipped to control all the parameters that affect an engine's output. This is something that cannot be done on a chassis dyno.

Engine dynos are usually of the absorption type, meaning that the dyno is capable of absorbing an engine's output in some way that usually changes the energy into heat.

These dynos are able to apply an adjustable load to the engine so that the engine's capability at various speeds can be measured.

They also have the advantage that a steady load can be applied for as long as it takes to either make adjustments or subject the engine to endurance testing.

Mechanical dynos are usually of the mechanic type, as discussed in last month's article. They have the disadvantage that a steady flow of water at a certain minimum pressure is needed at all times.

If municipal water is used, there is always the risk that the pressure may suddenly drop, resulting in a loss of load and drastic engine over-speeding. This may result in a blown engine.

Bigger hydraulic dynos usually have a closedloop water supply, allowing the water to be circulated via an outside cooling tower. Most modern dynamometers utilise some electrical phenomenon.

Eddy current dynamometers make use of a metal disc, coupled to the engine, that rotates inside a coil supplying a variable electromagnetic field. This supplies the load, and any change in voltage can be controlled and measured by a computer.

An eddy current dyno is sometimes modified to become a powder dynamometer. This is done by introducing fine magnetic powder into the air gaps between the rotor and the coil.

The magnetism causes the powder to cling together and separate again with the result that this kind of dyno can absorb more torque than an eddy current dyno of the same size.

If an eddy current dyno's electromagnetic coils are enclosed in a ribbed and ventilated cylinder, and this cylinder is rotated by the engine while the disc is kept stationary, the resulting dyno is called a hysteresis dynamometer.

Finally, an electric motor/generator, if fitted with an adjustable speed drive, can be used as a dynamometer.

These units, in AC or DC form, are usually more complex and more expensive than other electric dynos, but have the advantage that they can be used to measure output and monitor internal friction by rotating the engine. Some of these electric dynos are water-cooled.

CHASSIS DYNOS

Any dyno can be coupled to a set of rollers that are installed at ground level, allowing it to be driven by a car's drive wheels. In practice, electrical dynos are preferred because they're quick-acting.

These units are either of the absorption or inertia type. The latter utilises the rotational inertia of the rollers to measure the torque delivered by the driven wheels to the rollers. This is done by measuring the time taken to accelerate the total inertia of the system that consists of the drums plus the car's rotating parts. This means that there is a fixed load, so that steady-speed running is not possible.

These dynos can only measure the torque delivered by the wheels to the rollers, so the output is between 20 to 30 per cent less than would be measured at the flywheel by an engine dyno. The difference in output represents the frictional losses in the vehicle's drivetrain as well as the frictional loss between the tyres and the drums. Some manufacturers use chassis dynos for endurance or development work. In this type of application, the dyno has to have a huge fan in front of the vehicle to prevent overheating.

The fan's air-speed must at least be as high as the vehicle's maximum speed, because it has to match the conditions the car would face when delivering maximum power on the road.

Some workshops are equipped with chassis dynos, ostensibly to assist in tuning, but I often get letters from readers who complain about being ripped off.

They've paid for a tune-up and have been given a set of before-and-after power and torque curves that show a worthwhile improvement, but cannot feel any difference on the road.

These dynos are also favoured by gadget-merchants who peddle magnets and magic powders claiming to improve power output and fuel consumption.

There are two reasons for this state of affairs. The obvious one is that most dynos are operated by people who have not had sufficient scientific training to appreciate how easy it is to get false readings.

This follows from the fact that most chassis dynos require the engine to be operated at full throttle in a particular gear for the time it takes to get to the maximum allowed revs.

The readings obtained are highly sensitive to the initial oil and water temperatures, so it becomes very difficult to achieve repeatability. If there is any significant ambient temperature and/or pressure difference between the before-andafter tests, this also has to be taken into account.

Some dynos are equipped to measure these values, but older types have to be fed with the information from separate instruments. Another reason is the ease with which the results can be modified by wilful tampering.

Top of the list would be playing around with correction factors by giving the computers false temperatures and pressures.

Tyre pressures are also important: an easy way to get better results for the "after" run would be to let the tires down slightly for the "before" run and set them at the correct reading for the "after" run.

Not so long ago a reader sent us a set of curves that could only be explained by assuming that the computer was set to show kilowatt on the "before" run and horsepower on the "after" run.

The difference in readings was exactly equal to the difference in units used!