| By :
Sarah Simmons
This article is the first in a series of three articles highlighting the basics of dynamometer testing. In this first article, "What is an Engine Dynamometer," we will look at the principle components of the water brake engine dynamometer (dyno) and how they work. An engine dyno is a service tool that allows the operator to safely place a controlled load on an engine. A loaded engine dynamometer test is the only method of verifying engine capability. With the use of a dyno, an engine can be properly operated throughout its power range without being placed into service. Assembly deficiencies may be detected before the engine is installed into a chassis and an actual evaluation of an engine's operating condition may be performed. The dynamometer is the final quality test before an engine is installed. Construction An engine dynamometer has two major components: the absorption unit and the torque indication system. A water brake dynamometer uses an absorption unit (absorber) to absorb power through momentum exchange; using water as the working fluid. A water brake absorber consists of one or more shaft mounted rotors and at least two stators (or end bells). The rotors spin freely inside the absorber housing in the absence of water. When water is introduced into the absorber of the dyno, the spinning rotor accelerates the water and "throws" it into the stators. If the stators weren't restrained, they would also begin to rotate, similar to a torque converter. But the dyno's stators are restrained using a torque arm that is connected to a load cell. The load cell measures the force with which the stators are trying to rotate. By knowing the distance from the axis of the absorber to the torque arm, torque can be measured by: Torque = force x distance If we measure speed, horsepower can then be found by the relationship: HP = (torque x rpm)/5252 The amount of load absorbed is proportional to the volume of water inside the absorber housing. The water is ultimately absorbing all of the horsepower in the form of heat; therefore the warm water must be exhausted and replenished with cool water to avoid boiling. By restricting the exhaust and controlling the flow of water through the dyno absorber, the volume of water inside, and therefore the load, can be precisely controlled. Operation An engine without a load can only produce speed. Maintaining a given RPM requires very little engine horsepower. The dynamometer is a means by which a controlled load can be added and monitored. With a water brake type of dynamometer, the horsepower of the prime mover is converted into heat of the dynamometer water. The rotors and stators accomplish this transfer of energy. Both the rotors and stators have pockets built into them. As water is brought in to the dynamometer by passages in the stator, it is discharged into the dynamometer near the center of rotation of the rotor assembly. This water entering the dynamometer will flow into the pockets of the rotor. The water is accelerated by the rotation of the rotor assembly, which is attached to the output shaft of the engine. As it is accelerated, it tends to fly out due to centrifugal force. As it does, it ends up in similar pockets in the stator plates. The water in the stator pocket tends to run out and is met again by the rotating rotor assembly. The water is again accelerated. This constant acceleration and deceleration of the water as it passes from rotor to stator to rotor, etc. requires power and converts this energy into frictional heating of the water. This thermal exchange of engine power to frictional heating of the water is based on pure laws of physics. The amount of water in the dynamometer at any given instant determines the amount of horsepower that it can absorb. The more water that is in the dynamometer, the more the dynamometer can absorb. What makes up an Engine Dynamometer System? The engine dyno itself is only one component of a complete engine dynamometer system. A typical dynamometer system consists of an engine dynamometer and the following components: drive shaft, drive shaft guard, flywheel adapter plates, engine cart, cooling column, and a data acquisition and control system. Drive Shaft and Guard Once the engine is in position, the flywheel is connected to a guarded universal joint drive shaft with just a few bolts. Time spent connecting the engine is reduced due to the practical design of the engine cart. Engine Cart and Adapter Plates An engine cart is used for supporting and transporting engines. Adapter plates are provided for quickly locating the rear of the engine to the engine cart in proper alignment with the dynamometer, while an adjustable support bears the weight of the engine from the front. Once the engine is mounted, the cart is simply rolled into place and pinned into position. Cooling Column The engine cooling column is utilized to maintain jacket water temperatures on liquid cooled engines. The engine cooling column provides pressure regulated, thermostatic control of the water temperature to guard against the engine overheating during testing. Data Acquisition and Control Systems A typical dynamometer controller contains the dyno system's pressure and temperature sensors. The sensors are housed in an industrial cabinet and supplied with quick disconnects. Information is collected from the sensors and in many cases an ECM and combined with torque, speed, and power measurements from the dynamometer and sent to the dyno system's computer. The computer in a data acquisition and dyno control system interfaces with the dynamometer controller and the dynamometer and performs all of the embedded control functions. It is also the location where new tests are run and reports are generated, printed, and stored.
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