Editing Dynamometer (section)

==Types of dynamometers==
In addition to classification as absorption, motoring, or universal, as described above, dynamometers can also be classified in other ways.

A dyno that is [[coupling|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 directly from the drive wheel or wheels 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.

===Types of absorption units===
* [[eddy current brake|Eddy current]] (absorption only)
* Magnetic powder brake (absorption only)
* [[Hysteresis]] brake (absorption only)
* [[Electric motor]]/[[Electric generator|generator]] (absorb or drive)
* Fan brake (absorption only)
* [[Hydraulic]] brake (absorption only)
* Force lubricated, oil shear friction brake (absorption only)
* [[Water brake]] (absorption only)
* Compound dyno (usually an absorption dyno in tandem with an electric/motoring dyno)

===Eddy current type absorber===
Eddy current (EC) dynamometers are currently the most common absorbers used in modern chassis dynos. The EC absorbers provide a quick load change rate for rapid load settling. Most are air cooled, but some are designed to require external water cooling systems.

Eddy current dynamometers require an electrically conductive core, shaft, or disc moving across a magnetic field to produce resistance to movement. Iron is a common material, but copper, aluminum, and other conductive materials are also usable.

In current (2009) applications, most EC brakes use cast iron discs similar to vehicle disc brake rotors, and use variable electromagnets to change the magnetic field strength to control the amount of braking.

The electromagnet voltage is usually controlled by a computer, using changes in the magnetic field to match the power output being applied.

Sophisticated EC systems allow steady state and controlled acceleration rate operation.

===Powder dynamometer===
A powder dynamometer is similar to an eddy current dynamometer, but a fine magnetic powder is placed in the air gap between the rotor and the coil. The resulting flux lines create "chains" of metal particulate that are constantly built and broken apart during rotation, creating great torque. Powder dynamometers are typically limited to lower RPM due to heat dissipation problems.

===Hysteresis dynamometers===
Hysteresis dynamometers use a magnetic rotor, sometimes of AlNiCo alloy, that is moved through flux lines generated between magnetic pole pieces. The magnetisation of the rotor is thus cycled around its B-H characteristic, dissipating energy proportional to the area between the lines of that graph as it does so.

Unlike eddy current brakes, which develop no torque at standstill, the hysteresis brake develops largely constant torque, proportional to its magnetising current (or magnet strength in the case of permanent magnet units) over its entire speed range.<ref>{{cite web |url=http://www.magtrol.com/manuals/hbmanual.pdf |title=Hysteresis Brakes and Clutches |work=Magtrol Inc. |location=US |date=October 2019 |access-date=2023-01-02}}</ref> Units often incorporate ventilation slots, though some have provision for forced air cooling from an external supply.

Hysteresis and Eddy Current dynamometers are two of the most useful technologies in small ({{convert|200|hp|abbr=on}} and less) dynamometers.

===Electric motor/generator dynamometer===
[[Electric motor]]/[[electric generator|generator]] dynamometers are a specialized type of [[adjustable-speed drive]]. 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 that is driven by the unit under test or a motor that 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]], while the control unit for a DC motor is a [[Adjustable-speed drive#DC drives|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 via [[net metering]].

In engine testing, universal dynamometers can not only absorb the power of the engine, but can 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.

===Fan brake===
A fan is used to blow air to provide engine load. The torque absorbed by a fan brake may be adjusted by changing the gearing or the fan itself, or by restricting the airflow through the fan. Due to the low [[viscosity]] of air, this variety of dynamometer is inherently limited in the amount of torque that it can absorb.

===Force lubricated oil shear brake===
An oil shear brake has a series of friction discs and steel plates similar to the clutches in an automobile automatic transmission. The shaft carrying the friction discs is attached to the load through a coupling. A piston pushes the stack of friction discs and steel plates together creating shear in the oil between the discs and plates applying a torque. Torque can be controlled pneumatically or hydraulically. Force lubrication maintains a film of oil between the surfaces to eliminate wear. Reaction is smooth down to zero RPM without stick-slip. Loads up to hundreds of thermal horsepower can be absorbed through the required force lubrication and cooling unit.  Most often, the brake is kinetically grounded through a torque arm anchored by a strain gauge which produces a current under load fed to the dynamometer control. Proportional or servo control valves are generally used to allow the dynamometer control to apply pressure to provide the program torque load with feedback from the strain gauge closing the loop. As torque requirements go up there are speed limitations.<ref name=shear>{{cite web |url=http://www.ien.com/article/slashing-test-time/187285 |title=Slashing Test Time with Oil Shear Brake |work=Industrial Equipment News |access-date=22 July 2015 |archive-url=https://web.archive.org/web/20150924032918/http://www.ien.com/article/slashing-test-time/187285 |archive-date=24 September 2015 |url-status=dead }}</ref>

===Hydraulic brake===
The hydraulic brake system consists of a hydraulic pump (usually a gear-type pump), a fluid reservoir, and piping between the two parts. Inserted in the piping is an adjustable valve, and between the pump and the valve is a gauge or other means of measuring hydraulic pressure. In simplest terms, the engine is brought up to the desired RPM and the valve is incrementally closed. As the pumps outlet is restricted, the load increases and the throttle is simply opened until at the desired throttle opening. Unlike most other systems, power is calculated by factoring flow volume (calculated from pump design specifications), hydraulic pressure, and RPM. Brake HP, whether figured with pressure, volume, and RPM, or with a different load cell-type brake dyno, should produce essentially identical power figures. Hydraulic dynos are renowned for having the quickest load change ability, just slightly surpassing eddy current absorbers. The downside is that they require large quantities of hot oil under high pressure and an oil reservoir.

===Water brake-type absorber===
[[File:Tech-Talk_Animation_on_How_Water-Brakes_Work.webm|thumb|A  4-minute ‘how-it-works video’ tutorial explaining how engine-dynamometer water-brake absorbers work.]]

The hydraulic dynamometer (also referred to as the [[water brake]] absorber)<ref>{{Cite web |date=2017-12-24 |title=Hydraulic Dynamometers - Froude |url=https://froudedyno.com/products/dynamometers/standard-hydraulic-dynamometers-f-type |access-date=2023-09-01 |website=Froude {{!}} Dynamometer Manufacturer |language=en-gb}}</ref> was invented by British engineer [[William Froude]] in 1877 in response to a request by the [[British Admiralty|Admiralty]] to produce a machine capable of absorbing and measuring the power of large naval engines.<ref>{{cite web |title=History {{!}} About Us |url=https://froudedyno.com/about-us/history |url-status= |archive-url= |archive-date= |access-date=1 September 2023 |publisher=Froude Hoffmann}}</ref> Water brake absorbers are relatively common today. They are noted for their high power capability, small size, light weight, and relatively low manufacturing costs as compared to other, quicker reacting, "power absorber" types.

Their drawbacks are that they can take a relatively long period of time to "stabilize" their load amount, and that they require a constant supply of water to the "water brake housing" for cooling. Environmental regulations may prohibit "flow through" water, in which case large water tanks are installed to prevent contaminated water from entering the environment.

The schematic shows the most common type of water brake, known as the "variable level" type. Water is added until the engine is held at a steady RPM against the load, with the water 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). The housing attempts to rotate in response to the torque produced, but is restrained by the scale or torque metering cell that measures the torque.

[[Image:Dyno schematic.svg|frame|center|This schematic shows a water brake, which is actually a fluid coupling with a housing restrained from rotating&mdash;similar to a water pump with no outlet.]]

===Compound dynamometers===
In most cases, motoring dynamometers are symmetrical; a 300&nbsp;kW AC dynamometer can absorb 300&nbsp;kW as well as motor at 300&nbsp;kW. This is an uncommon requirement in engine testing and development. Sometimes, a more cost-effective solution is to attach a larger absorption dynamometer with a smaller motoring dynamometer. Alternatively, a larger absorption dynamometer and a simple AC or DC motor may be used in a similar manner, with the electric motor only providing motoring power when required (and no absorption). The (cheaper) absorption dynamometer is sized for the maximum required absorption, whereas the motoring dynamometer is sized for motoring. A typical size ratio for common emission test cycles and most engine development is approximately 3:1. Torque measurement is somewhat complicated since there are two machines in tandem - an inline torque transducer is the preferred method of torque measurement in this case. An eddy-current or waterbrake dynamometer, with electronic control combined with a variable frequency drive and AC induction motor, is a commonly used configuration of this type. Disadvantages include requiring a second set of test cell services (electrical power and cooling), and a slightly more complicated control system. Attention must be paid to the transition between motoring and braking in terms of control stability.