They are stable and take measurements without requiring the removal of the engine. These machines are also called water brake dynamometers because they use water or oil in the load cell.
They can create different loads on the engine and maintain a steady RPM rate while testing. Because of this, they are often used for trouble shooting tuning problems in the engine and determining if a part is not performing.
The working compartment of the hydraulic dynamometer consists of special semi-circular shaped vanes cast into stainless steel rotor and stators. Water flowing in a torodial vortex pattern around these vanes creates a torque reaction through the dynamometer casing which is measured by a precision load cell.
The hydraulic dynamometer fill level and consequently load is controlled by a water outlet valve, operated by a closed loop electro-hydraulic dynamometer fill level and consequently load is controlled by a water outlet valve, operated by a closed loop electro-hydraulic servo system.
The power absorbed by the dynamometer is carried away by the water in the form of heat. Applied torque by the dynamometer is measured by a load cell installed on the arm of the dynamometer.
The load cell signal is transmitted to a load cell amplifier. Control command is supplied to the outlet analog water valve, through an electronic/pneumatic transducer, which converts the electric signal to proportional air quantity as supply to the valve.
Hydraulic dynamometers are used as loading units in engine test rigs. They cover a wide range of dynamometer power and torque values. They are well suited for testing of passenger car and commercial vehicle diesel engines, testing of large railway- and marine engines, engine testing in tandem configuration with asynchronous motor, testing in research and development testing in production and quality assurance.
They are however more complicated than systems that use rolling barrels because of the way the dynamometer is attached. Also, the data requires more effort and knowledge to decipher because most hydraulic dynamometers are analog.
These machines are used in automotive, aircraft, aerospace, marine, and industrial processes to measure chain or belt drives, gear boxes, fluid power systems, gas or diesel systems or transmissions in vehicles and motorcycles.