How Do Dynamometers Work?
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Background
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Dynamometers, usually called dynos, may sound like something that belongs at NASA, but they can be found at many garages and are common on the racing circuit. Dynos are devices used to measure the power output that an engine generates under different loads. There are three types of dynos: water brake dynos, eddy-current dynos and alternating current dynos. Eddy-current and alternating current dynos are more accurate than water brake dynos, but they are quite expensive which has limited their use. The water brake dyno is the most common type in use and the discussion will be limited to them
Preparation
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The primary purpose of an engine dyno is to artificially create a load on the engine in a controlled setting, such as a garage. In most cases, dyno tests are run on new engines or newly rebuilt engines. The first stage in the process is preparation. Every shop operates a little differently, which means the specifics of the test and necessary equipment need to be established in advance. Most shops will provide a detailed list of the equipment that needs to accompany the engine for a successful test. It is standard for the lubrication system to be pre-lubed or primed prior to testing.
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Pre-testing
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Once everything is on hand, the engine will usually be mounted onto a frame and attached to the dyno system. It is not universal, but many dyno systems are equipped to provide fuel, cooling and exhaust for the engine. The engine will be test fired, run through a break-in period and then checked for problems. If everything still looks good, the engine is then put through a tuning run, where the timing and fuel/air ratio are adjusted. Assuming the engine is still performing properly, the testing phase can proceed.
Testing
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During testing the dyno applies a range of load to the engine. This is accomplished through the use of an absorber that is attached to the crank. The basic principle is that the engine is run wide-open or full throttle. As the amount of water increases, the load on the engine increases. The load is increased on the crank until the RPM (revolutions per minute) reach the bottom of the established test range. In modern dyno systems, the load is adjusted and monitored through a computer that controls an electronic servo. The torque is measured with electric strain gauges. The horsepower can be calculated with just the rpm and the torque, but computer based dynos collect and report this data.
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