AskDefine | Define dynamometer

Dictionary Definition

dynamometer n : measuring instrument designed to measure power [syn: ergometer]

User Contributed Dictionary



  1. Any of various devices used to measure mechanical power, especially those that measure the torque of a rotating shaft


  • Italian: dinamometro
  • Russian: динамометр

Derived terms

Extensive Definition

confuse dynameter
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 or passive Dynamometer. A dynamometer that can either drive or absorb is called a universal or active dynamometer.
In addition to being used to determine the torque or power characteristics of a machine under test (MUT), Dynamometers are employed in a number of other roles. In standard emissions testing cycles such as those defined by the US Environmental Protection Agency (US EPA), dynamometers are used to provide simulated road loading of either the engine (using an engine dynamometer) or full powertrain (using a chassis dynamometer). In fact, beyond simple power and torque measurements, dynamometers can be used as part of a testbed for a variety of engine development activities such as the calibration of engine management controllers, detailed investigations into combustion behavior and tribology.
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.

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:
\mathrm =
Where K is determined by the units of measure used as can be seen below:
To calculate power in horsepower (hp) use:
\mathrm =
Torque is in pound-feet (lbf·ft)
Rotational speed is in revolutions per minute (rpm)
To calculate power in kilowatts use:
\mathrm =
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).

Detailed dynamometer description

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.

General testing methods with types of dynamometer systems

A Brake dynamometer applies variable load on the engine and measures the engine's ability to move or hold the rpm as related to the "braking force" applied. It is usually connected to a computer which records the applied braking torque and calculates the power output of the engine based on information from a "load cell" or "strain gauge" and rpm (speed sensor).
An Inertia dynamometer provides a fixed inertial mass load and calculates the power required to accelerate that fixed, known mass and uses a computer to record rpm and acc. rate to calculate torque.
The engine is generally tested from somewhat above idle to its maximum rpm and the output is measured and plotted on a graph.
There are essentially only 2 types of dynamometer test procedures:
  1. Steady State (only on brake dynamometers), where the engine is held at a specified rpm (or series of usually sequential rpms) for 3-5 seconds by the variable brake loading as provided by the PAU (power absorber unit).
  2. Sweep Test (inertia or brake dynamometers), where the engine is tested under a load (inertia or brake loading), but allowed to "sweep" up in rpm in a continuous fashion, from a specified lower "starting" rpm to a specified "end" rpm.
Types of Sweep Tests:
  1. Inertia Sweep: An inertia dyno system that provides a fixed inertial mass flywheel and computes the power required to accelerate the flywheel (load) from the starting to the ending rpm. The actual rotational mass of the engine or engine and vehicle in the case of a chassis dyno is not known and the variability of even tire mass will skew power results. The inertia value of the flywheel is "fixed", so low power engines are under load for a much longer time and internal engine temperatures are usually too high by the end of the test, skewing optimal "dyno" tuning settings away from the outside world's optimal tuning settings. Conversely, high powered engines, commonly complete a common "4th gear sweep" test in less than 10 seconds, which is not a reliable load condition as compared to operation in the outside world. By not providing enough time under load, internal combustion chamber temps are unrealistically low and power readings, especially past the power peak, are skewed low.
  2. Loaded Sweep Tests (brake dyno type)consist of 2 types:
    1. Simple fixed Load Sweep Test: A fixed load, of somewhat less than the engine's output, is applied during the test. The engine is allowed to accelerate from its starting rpm to its ending rpm, varying in its acceleration rate, depending on power output at any particular rpm point Power is calculated using torque * rpm / 5252 + the power required to accelerate the dyno and engine's / vehicle's rotating mass.
    2. Controlled Acceleration Sweep Test: Similar in basic usage as the above Simple fixed Load Sweep Test, but with the addition of active load control that targets a specific rate of acceleration. Commonly, 20fps/ps is used.
The advantage of controlled acc. rate is that the acc. rate used is relatively common from low power to high power engines and unnatural overextension and contraction of "test duration duration" is avoided, providing more accurate and repeatable test and tuning results.
There is still the remaining issue of potential power reading error due to the variable engine / dyno / vehicle's total rotating mass. Most modern computer controlled brake dyno systems are capable of deriving that "inertial mass" value to eliminate the error.
The advantage of controlled acc. rate is that the acc. rate used is relatively common from low power to high power engines and unnatural overextension and contraction of "test duration duration" is avoided, providing more accurate and repeatable test and tuning results.
Interestingly, A "sweep test" will always be suspect, as many "sweep" users ignore the inertial mass factor and prefer to use a blanket "factor" on every test, on every engine or vehicle. Inertia dyne systems aren't capable of deriving "inertial mass" and are forced to use the same inertial mass.
Using Steady State testing eliminates the inertial mass error, as there is no acceleration during a test.

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.

Chassis dynamometer

A chassis dynamometer measures power delivered to the surface of the "drive roller" by the drive wheels. The vehicle is often parked on the roller or rollers, which the car then turns and the output is measured.
Modern roller type chassis dyne systems use the Salvisberg roller, which improved traction and repeatability over smooth or knurled drive rollers.
On a motorcycle, typical power loss at higher power levels, mostly through tire flex, is about 10% and gearbox chain and other power transferring parts are another 2% to 5% .
Other types of chassis dynamometers are available that eliminate the potential wheel slippage on old style drive rollers and attach directly to the vehicle's hubs for direct torque measurement from the axle. Hub mounted dynos include units made by Dynapack and Rototest.
Chassis dynos can be fixed or portable.
Modern chassis dynamometers can do much more than display RPM, Horsepower, and Torque. With modern electronics and quick reacting,low inertia dyne systems, it's now possible to tune to best power and the smoothest runs, in realtime.
It's also common to, on a retail level, with a wideband 02 Sensor, graphed along with RPM, to "tune to an air fuel ratio".
Some,like Dynojet and others, can also add vehicle diagnostic information to the dyno graph as well. This is done by gathering data directly from the vehicle's PCM via OBD communication.
Because of frictional and mechanical losses in the various drivetrain components, the measured rear wheel brake horsepower is generally 15-20 percent less than the brake horsepower measured at the crankshaft or flywheel on an engine dynamometer. Other sources, after researching several different "engine" dyno software packages, found that the engine dyno user can integrally add "frictional loss" channel factors of +10% to +15% to the flywheel power, raising the claim that 20% to 25% or even more power is actually lost between the crankshaft at high power outputs.

Common misconceptions about dynos

Drag racing: Horsepower and torque figures are a strong predictor but do not guarantee a specific 0-60 mph or 1/4 mile elapsed time (ET). 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. When attempting to crosscheck dynamometer power figures to drag strip performance, it is relatively consistent to compare improved brake hp figures to terminal MPH.
Engine damage: Can dyno testing damage engines? A brake dyno, in steady state mode only provides a load that is equal the amount of power that the engine is making at any specifically selected rpm point. If the engine makes 200 brake HP at 5000 rpm, the dynamometer's brake or power absorber will provide exactly 's worth of load against it, keeping the RPM at 5000 rpm. That's a realistic load, it's as if the engine was in a vehicle pulling a large trailer up a hill. Should be no problem on the dyno - if there's no problem on the road. However, the apprehension over dyno testing and engine damage does have solid roots in fact. Old style dynamometers commonly used an inexpensive water brake type of power absorber. Load was increased or decreased by filling and draining water in the housing to change the amount of internal water volume to change the load, all the while draining and refilling the water to keep the water from boiling - It would sometimes take quite some time for the operator or computer to get inflow and outflow rates stabilized and that is the problem. It's not the "amount" of load, it's the amount of "time" spent trying to stabilize the load at the desired rpm.
Water brakes are still commonly used in applications where their small size and light weight are important and engine torque curves are relatively straight, as in large automotive and boats.


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.
The first popular, true high speed, computer controlled, eddy current chassis dyne systems produced were motorcycle systems that were produced by Factory Pro Dynamometer of San Rafael,CA, USA in 1990.

See also



General references

  • Dynamometer Handbook of Basic Theory and Applications
  • Engine Testing - Theory and Practice

External links

dynamometer in Czech: Siloměr
dynamometer in Dutch: Dynamometer
dynamometer in Polish: Siłomierz
dynamometer in Turkish: Dinamometre
dynamometer in Ukrainian: Динамометр
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