How Does a Water Brake Dynamometer Work? |
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Sarah Simmons
September 14, 2010
Sarah Simmons
http://www.siteurlvaries.com
The purpose of using a dynamometer is to test the load capability of an engine prior to putting it back in service. It allows for the break-in of a new or newly rebuilt engine in a controllable environment. Engine manufacturers, rebuilders and many fleets have proven this type of break-in procedure through many years of experience. Properly run-in engines last longer, run better and cost less to maintain. This article highlights the operation of the water brake engine dynamometer.
An engine without a load can only produce speed. Maintaining a given rate of revolutions per minute (RPM) takes a very small amount of engine horsepower. The dyno is a means by which a controlled load can be added and monitored. With a water brake dynamometer, the horsepower of the prime mover is converted into heat of the dynamometer water. The stators and rotors successfully achieve this transfer of energy. Both the stators and rotors have pockets built within them. As water is brought into the engine dyno or chassis dyno by passages in the stator, it is discharged into the dyno near the center of rotation of the rotor assembly. This water entering the dyno will flow into the pockets of the rotor. The water is then accelerated by the rotation of the rotor assembly, which is connected to the output shaft of the engine. As it speeds up (or accelerates), the water tends to fly out due to centrifugal force. And as the water flies out, it ends up in pockets in the stator plates. The water in these similar pockets in the stator plates tends to run out and is once again met by the rotating rotor assembly. The water is sped up (or accelerated) again, and with the constant acceleration and deceleration of the water, power is required which is converted into frictional heating of the water. The thermal conversion of engine power to frictional heating of the water is supported by pure laws of physics.
Horsepower can be defined in terms of heat. Heat loads are measured in terms of "BTUs" or British Thermal Units. A BTU of heat is the amount of heat that it would take to raise the temperature of one pound of pure water by one degree Fahrenheit. When dealing with water brake dynamometers, the heat loads in terms of BTUs are important.
Because we are raising the temperature of one pound of water by one degree Fahrenheit, we will need to know a few more definitions. There are 62.4 pounds of pure water in a cubic foot. There are 231 cubic inches in one gallon of pure water. Therefore, one gallon of pure water would weigh 8-1/3 pounds. One horsepower is equal to 2,545 BTUs per hour or about 45.5 BTUs per minute. With this information, we can now determine the amount of water that is necessary to absorb a given amount of horsepower. The amount of water in the dyno at any given instant determines the amount of horsepower that it can absorb. The more water that is in the dynamometer, the more the dyno can absorb. You cannot put more water into the dyno than the amount for the level of horsepower that you intend to test.
The dynamometer does not hold water. It is only a tool for converting the horsepower into heat of the water. Because of this, you need a given flow through the dyno at a given horsepower rating. The amount of flow varies with the amount of horsepower to be absorbed and is directly proportional. The larger the amount of load required, the larger the amount of water will need to be supplied.
The amount of water supplied to the dynamometer is controlled externally to the dynamometer. This can be accomplished either by a pair of manual shut off valves mounted in the water supply line, by the electric remote load control valve or by the servo-operated inlet manifold option. Whether the manual or electric valves are used, they both accomplish the same task. The wider the valve is opened, the more water it allows to flow to the dynamometer. This flow to the dyno is directly proportional to the amount of horsepower being absorbed.
An exhaust or outlet valve is also mounted on the dynamometer. The purpose of this valve is to conserve water while allowing the dynamometer to run on various temperatures of inlet water. As we are converting horsepower into heat of the water, the relationship of the inlet and outlet water temperatures becomes an important consideration. The exhaust valve is a controlled orifice. The farther it is opened, the greater the water consumption will be.
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For more than 30 years, Power Test has been an industry leader in the design, manufacture, and implementation of dynamometers and dyno control systems, including engine dyno systems. Power Test has provided dynos and data acquisition and control systems to manufacturers, rebuilding facilities, and distributors worldwide. Read the Dyno article here:
http://www.pwrtst.com/news-and-articles/how-does-a-water-brake-dynamometer-work.htm