Dynamometer

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For the dynamometer used in railroading, see dynamometer car.

A dynamometer, or "dyno" for short, is a machine used to measure torque and rotational speed (rpm) from which power produced by an engine, motor or other rotating prime mover can be calculated.

A dynamometer can also be used to determine the torque and power required to operate a driven machine such as a pump. In that case, a motoring or driving dynamometer is used. A dynamometer that is designed to be driven is called an absorption dynamometer. A dynamometer that can either drive or absorb is called a universal dynamometer.

In the medical realm, hand dynamometers are used for routine screening of grip strength and initial and ongoing evaluation of patients with hand trauma and dysfunction.

[edit] Principles of operation

An absorbing dynamometer acts as a load that is driven by the prime mover that is under test. The dyno must be able to operate at any speed, and load the prime mover to any level of torque that the test requires. A dynamometer is usually equipped with some means of measuring the operating torque and speed.

The dynamometer must absorb the power developed by the prime mover. The power absorbed by the dynamometer must generally be dissipated to the ambient air or transferred to cooling water. Regenerative dynamometers transfer the power to electrical power lines.

Dynamometers can be equipped with a variety of control systems. If the dynamometer has a torque regulator, it operates at a set torque while the prime mover operates at whatever speed it can attain while developing the torque that has been set. If the dynamometer has a speed regulator, it develops whatever torque is necessary to force the prime mover to operate at the set speed.

A motoring dynamometer acts as a motor that drives the equipment under test. It must be able to drive the equipment at any speed and develop any level of torque that the test requires.

Only torque and speed can be measured; Power must be calculated from the torque and speed figures according to the formula:

Where K is determined by the units of measure used as can be seen below:

To calculate power in horsepower (hp) use:

where:

Torque is in pound-feet (lbf·ft)

Rotational speed is in revolutions per minute (rpm)

To calculate power in kilowatts use:

where:

Torque is in newton-metres (N·m)

Rotational speed is in revolutions per minute (rpm)

(On graphs of torque vs. rpm the numerical values of torque and power are always equal when the rpm value is equal to the constant, K. The numerical values of horsepower and lbf·ft of torque are always equal at 5252 rpm because 5252 rpm in the numerator cancels out the constant, 5252 in the denominator leaving only the torque figure equal to the power figure.)

See also internal combustion engine (performance section).

[edit] Detailed dynamometer description

Electrical dynamometer setup showing engine, torque measurement arrangement and tachometer

A dynamometer consists of an absorption (or absorber/driver) unit, and usually includes a means for measuring torque and rotational speed. An absorption unit consists of some type of rotor in a housing. The rotor is coupled to the engine or other equipment under test and is free to rotate at whatever speed is required for the test. Some means is provided to develop a braking torque between dynamometer's rotor and housing. The means for developing torque can be frictional, hydraulic, electromagnetic etc. according to the type of absorption/driver unit.

One means for measuring torque is to mount the dynamometer housing so that it is free to turn except that it is restrained by a torque arm. The housing can be made free to rotate by using trunnions connected to each end of the housing to support the dyno in pedestal mounted trunnion bearings. The torque arm is connected to the dyno housing and a weighing scales is positioned so that it measures the force exerted by the dyno housing in attempting to rotate. The torque is the force indicated by the scales multiplied by the length of the torque arm measured from the center of the dynamometer. A load cell transducer can be substituted for the scales in order to provide an electrical signal that is proportional to torque.

Another means for measuring torque is to connect the engine to the dynamometer through a torque sensing coupling or torque transducer. A torque transducer provides an electrical signal that is proportional to torque.

With electrical absorption units, it is possible to determine torque by measuring the current drawn (or generated) by the absorber/driver. This is generally a less accurate method, but it may be adequate for some purposes.

A wide variety of tachometers are available for measuring speed. Some types can provide an electrical signal that is proportional to speed.

When torque and speed signals are available, test data can be transmitted to a data acquisition system rather than being recorded manually. Speed and torque signals can also be recorded by a chart recorder or plotter.

[edit] Types of dynamometers

In addition to classification as absorption, motoring or universal as described above, dynamometers can be classified in other ways. A dyno that is coupled directly to an engine is known as an engine dyno. A dyno that can measure torque and power delivered by the power train of a vehicle without removing the engine from the frame of the vehicle, is known as a chassis dyno.

Dynamometers can also be classified by the type of absorption unit or absorber/driver that they use. Some units that are capable of absorption only can be combined with a motor to construct an absorber/driver or universal dynamometer. The following types of absorption/driver units have been used:

[edit] Types of absorption/driver units

• Water brake (absorption only)
• Fan brake (absorption only)
• Electric motor/generator (absorb or drive)
• Mechanical friction brake or Prony brake (absorption only)
• Hydraulic brake (absorption only)
• Eddy current or electromagnetic brake (absorption only)

[edit] Water brake dynamometer

The water brake dynamometers are very popular, due to their high power capability, controllability, and relatively low cost compared to other types. The schematic shows the most common type of water brake, the variable level type. Water is added until the engine is held at a steady rpm against the load. Water is then kept at that level and replaced by constant draining and refilling, which is needed to carry away the heat created by absorbing the horsepower (which in itself is a measure of power output of the engine). The housing attempts to rotate in response to the torque produced but is restrained by the scale or torque metering cell which measures the torque.

This schematic shows a water brake which is actually a fluid coupling with the housing restrained from rotating. It is very similar to a water pump with no outlet.

[edit] Electric motor/generator dynamometer

Electric motor/generator dynamometers are a specialized type of adjustable-speed drives. The absorption/driver unit can be either an alternating current (AC) motor or a direct current (DC) motor. Either an AC motor or a DC motor can operate as a generator which is driven by the unit under test or a motor which drives the unit under test. When equipped with appropriate control units, electric motor/generator dynamometers can be configured as universal dynamometers. The control unit for an AC motor is a variable-frequency drive and the control unit for a DC motor is a DC drive. In both cases, regenerative control units can transfer power from the unit under test to the electric utility. Where permitted, the operator of the dynamometer can receive payment (or credit) from the utility for the returned power.

In engine testing, universal dynamometers can not only absorb the power of the engine but also, drive the engine for measuring friction, pumping losses and other factors.

Electric motor/generator dynamometers are generally more costly and complex than other types of dynamometers.

[edit] How dynamometers are used for engine testing

Dynamometers are useful in the development and refinement of modern day engine technology. The concept is to use a dyno to measure and compare power transfer at different points on a vehicle, thus allowing the engine or drivetrain to be modified to get more efficient power transfer. For example, if an engine dyno shows that a particular engine achieves 400 N·m (300 lbf·ft) of torque, and a chassis dynamo shows only 350 N·m (260 lbf·ft), one would know to look to the drivetrain for the major improvements. Dynamometers are typically very expensive pieces of equipment, reserved for certain fields that rely on them for a particular purpose.

[edit] General testing method

The dynamometer applies various loads on the engine and measures the engine's ability to move the load. The dynamometer may be connected to a computer which calculates the output of the engine. The engine is run from idle to its maximum rpm and the output is measured and plotted on a graph. Nearly all aspects of engine operation are measured during a dyno run.

[edit] Engine dynamometer

An engine dynamometer measures power and torque directly from the engine's crankshaft (or flywheel), when the engine is removed from the vehicle. These dynos do not account for power losses in the drivetrain, such as the gearbox, transmission or differential etc.

HORIBA engine dynamometer TITAN

[edit] Chassis dynamometer

A chassis dynamometer measures power from the engine through the wheels. The vehicle is parked on rollers which the car then turns and the output is measured. These dynos can be fixed or portable.

Because of frictional and mechanical losses in the various drivetrain components, the measured horsepower is generally 15-20 percent less than the brake horsepower measured at the crankshaft or flywheel on an engine dynamometer.^[1]

Saab 96 on chassis dynamometer

[edit] Common misconceptions about dynos

Horsepower and torque figures are a strong predictor but do not guarantee a specific 0-60 mph or 1/4 mile E.T. (elapsed time). An engine accelerating in a vehicle experiences different conditions than on a dyno. G forces and different temperatures as well as different modes of vibration in a vehicle can cause significant differences in power output.

Another common misconception about dynamometers is that they can damage engines. The dynamometer cannot make an engine work any harder than it is capable of, and this is no different to driving full throttle up a steep hill or towing a heavy trailer. Dynamometers cannot damage engines, but uncaring dynamometer operators can, just as uncaring drivers can on the road.

[edit] History

Gaspard de Prony invented the de Prony brake in 1821. The de Prony brake (or Prony brake) is considered to be one of the earliest dynamometers.

Froude Hofmann of Worcester, UK manufactures engine and vehicle dynamometers. They credit William Froude with the invention of the hydraulic dynamometer in 1877 and say that the first commercial dynamometers were produced in 1881 by their predecessor company, Heenan & Froude.

In 1928, the German company "Carl Schenck Eisengießerei & Waagenfabrik" built the first vehicle dynamometers for brake tests with the basic design of the today's vehicle test stands.

The eddy current dynamometer was invented by Martin and Anthony Winther in about 1931. At that time, DC Motor/generator dynamometers had been in use for many years. A company founded by the Winthers, Dynamatic Corporation, manufactured dynamometers in Kenosha, Wisconsin until 2002. Dynamatic was part of Eaton Corporation from 1946 to 1995. In 2002, Dyne Systems of Jackson, Wisconsin acquired the Dynamatic dynamometer product line. Starting in 1938, Heenan and Froude manufactured eddy current dynamometers for many years under license from Dynamatic and Eaton.^[2]

[edit] See also

• Dynamometer car for railroad usage
• Engine test stand dynamometer for engines, e.g. combustion engines

[edit] External links

• Froude Hofmann re history of hydraulic dynamometer
• Concept2 for a manufacturer of dynameters designed for measuring leg press, arm pull, and chest press of athletes.

[edit] References

[edit] Citations

1. ^ John Dinkel, "Chassis Dynamometer", Road and Track Illustrated Automotive Dictionary, (Bentley Publishers, 2000) p. 46.
2. ^ Winther, Martin P. (1976). Eddy Currents. Cleveland, Ohio: Eaton Corporation. 

[edit] General references

• Winther, J. B. (1975). Dynamometer Handbook of Basic Theory and Applications. Cleveland, Ohio: Eaton Corporation. 

• Plint, M.; Martyr, A (1999). Engine Testing - Theory and Practice, Second Edition, Oxford, UK: Butterworth-Heinemann. ISBN 0-7506-4021-9.