What Is A Brake Dynamometer

Every once in a while, we find one of those technology related words that sounds vaguely familiar, but we still don’t know the meaning. Maybe you’ve seen the word just often enough to recognize the letters, but beyond that, the word may as well be meaningless. So, what’s a dynamo?

Specifically, what’s a brake dynamometer? A brake dynamometer is a device with a variable resistance in regards to physical motion. This device is often used to measure an engine’s power output to determine several things about the engine’s performance. They can also be used with motors.

That may sound like the most boring thing in the world, but, dynamometers are extremely useful. These devices are the reason that anything using an engine or electric motor can work so well. Problems can be caught before the consumer ever notices them.


What Is A Brake Dynamometer

Brake Dynamometers are used to measure the forces that an engine generates. For this reason, it can be considered one of the multiple types of engine dynamometers, also known as motor testers.

The name comes from their method of measuring a motor’s force: Apply a brake (an opposing force) to the motor and measure how much force is needed to slow and stop the motor.

The resulting measurement and any useful expressions of that measurement are the main purpose of the dynamometer. These are force measurement tools. A simple formula often used with dynamometers to understand the results is: Torque X Rotational Speed = Power.

One of the first recorded devices was the de Prony brake, which was invented in 1821 by Gaspard de Prony. This was a mechanical friction brake, just one of the many types of dynamometers.

These days, the results from a dynamometer test will be shown on a display built into the dynamometer – a far cry from the what de Prony built almost 200 years ago, but the uses and basic theory behind these devices still remain the same.

Measuring the output of an engine is extremely important for the development of almost any engine.

Imagine spending thousands of dollars designing and building an engine for a large truck, then moving straight to mass production without knowing if the engine is strong enough to actually move a heavy vehicle effectively.

Whenever we want to know that a tool is powerful enough to perform a certain task, we need a way to measure that power before we try to perform that task.

This makes the world of engines function much more smoothly, and prevents a company from being embarrassed in front of a client when – oops! – the huge, expensive motor you ordered isn’t going to work.

Different Types

Eddy Current (EC) Dynamometer – A conductive material moving or rotating through a magnetic field results in “eddy currents”, the product of resisting movement through the field. The electromagnet voltage can be, and often is, controlled by a computer that sees the changes in the magnetic field and tries to match it.

Many modern EC dynamos use cast iron discs, making their design slightly similar to brake rotors on vehicles.

Powder Dynamometer

This is quite similar to the EC dynamo, but between the rotor and the coil, a very fine magnetic powder is placed in the gap. When a voltage is applied to the device, the magnetic particles “line up” with the field, but are constantly broken by the rotating rotor.

This adds to the resistance, but these versions are less able to dissipate heat, and so are only used at lower RPM.

Hysteresis Dynamometer

These are also similar to EC dynamos, with one of the biggest differences being that these are able to develop a torque at standstill, while the EC version can’t. This version can actually develop constant torque proportional to either the permanent magnet used, or the magnetizing current.

Electric Motor/Generator Dynamometer

These may also be called universal dynamometers. These are often more complicated and expensive, but can absorb power from the attached engine and drive it for measuring various losses.

Fan brake

A fan is attached to the engine being measured. The air resistance of the fan can be altered by adjusting the fan blades, the gearing, or by changing the airflow to the fan.

This design, however, is more limited in the amount of torque that can be used due to the air’s low viscosity (Meaning, the air has very little “flowing resistance”).

Force Lubricated Oil Shear Brake

Multiple friction disks and steel plates are mounted on a shaft. The motor provides circular motion and a piston powered either hydraulically or pneumatically pushes these plates together.

Excess wear on the plates is avoided by forcing a film of oil into the gaps between the plates. This allows all motion to be smooth without having the plates stick, then suddenly slip repeatedly as they come to a stop.

Hydraulic Brake

Hydraulic fluid is pumped through some piping and an adjustable valve. As the engine runs, the valve is slowly closed to restrict the movement of the fluid while the power is calculated by measuring the flow volume, fluid pressure, and RPM of the engine.

Though this method of calculation is obviously different from the others, the result is still very accurate.

Water-Brake Absorber

This functions exactly as the name implies and was originally invented in 1877 to measure the power of large naval engines. These are fairly common and have several advantages:

  • ability to handle high power
  • small size
  • light weight
  • low cost of manufacturing

Compound Dynamometers

Put simply, there are occasionally situations where it makes more sense to combine two different dynamometers, rather than use just one. Both will be used simultaneously, which can often complicate the process of measuring, powering, cooling, and controlling.


Dynamometers are a fairly complex subject with several different forms and uses, as you’ve seen. It may be difficult to visualize all of the various benefits, so a simplified list is provided below. These devices allow us to:

  • Accurately understand what is and is not functioning correctly with and engine, motor or pump.
  • Use the gathered information to fine tune the equipment in question and prevent wasted fuel.
  • Use the previous two points to encourage and develop trust in your equipment and the systems it functions within.
  • Calibrate other equipment such as engine management controllers – which also assists in performing the previous three points.
  • Make a record of performance measurements, the collection of which can greatly assist other developments or modifications in the future.

These devices are also used outside of fields related to engine, pump, and motor testing. In some medical fields, Dynamometers are used to measure:

  • Hand and grip strength for patients with suspected nerve damage or other related dysfunction or trauma.
  • Strength of the back, arms, or legs of patients, athletes, and workers to determine ability, performance and demands of varying tasks.

Does The Average Person Need One?

Unless you have some engines to measure or patients to treat, you probably don’t need one! A brake dynamometer is a specialized tool that benefits engineers and companies developing products to be sold, but not the average person who wants to hook one up to the family minivan for some reason.

Remember, when we say “dynamometer”, this is specifically referring to measuring devices, not just any old electric motor used in reverse, which can be similar, but has different applications.

Consider yourself fortunate that you don’t need to pull out the family dynamometer to make sure the engine in your new car is strong enough to get you to work tomorrow!


I'm Arwood, but the grandkids call me Big Papa. After retiring from teaching automotive classes for 30+ years I decided to create a blog about all the questions I used to get about brakes!

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