Chassis dynamometer

Chassis dynamometer, sometimes called rolling road is device for measurement and testing developed to simulate the road in controlled environment, mainly inside building. Instead of the road there is roller. Vehicle is fixed to the building with restraint device and it is on roller either with driven only or all axles.

There are many types of chassis dynamometer according to the target application - for example, emissions measurement, miles accumulation chassis dynamometer (MACD), Noise-Vibration-Harshness (NVH or "Acoustic") Application, Electromagnetic Compatibility (EMC) testing, end of line (EOL) tests, performance measurement and tuning. Another basic division is by type of vehicle - motorcycles, cars, trucks, tractors or the size of the roller - mostly 25", 48", 72", but also any other. Modern dynamometers used for development are mostly one roller to the wheel construction and the vehicle wheel is placed the top of the roller. Older constructional solutions are two roller per wheel and vehicle is place between these rollers - this design solution is cheaper and simpler, however, due to the requirements for accuracy and strict limits is no longer used for the development of new vehicles, but only as a test dynamometer at the end of the line or to measure the performance of the engine without dismantling, or performance tuning in "garage" companies.

Directly measured variables are only force on the torque transducer (i.e. loadcell) and revolutions measured on the role encoder dynamometer. All other variables are calculated based on known design (i.e. roller radius and loadcell mounting).

Due to friction and mechanical losses in various parts of the power train is the measured power at the wheels by about 15 to 20 percent lower than the power measured directly at the output of engine crankshaft (measuring device with this purpose is called engine testbed).

Vehicle do not behave the same way on chassis dynamometer as on the road. For example, aerodynamic shape of the vehicle does not matter. Sum of all forces on the vehicle on a real road are simulated through tires on chassis dynamometer. Increasing air drag with the speed on the road manifests as increasing braking force of the vehicle wheels. The aim is to make the vehicle on the dynamometer accelerate and decelerate the same way as on a real road. First you need to know the parameters of the "behavior" of the vehicle on a real road. In order to get "road parameters", vehicle must be driving on ideal flat road with no wind from any direction, gear set to neutral and time needed to slow down without braking is measured in certain intervals i.e. 100–90 km/h, 90–80 km/h, 80–70 km/h 70–60 km/h etc. Slowing down from higher speed takes shorter time mainly due to air resistance. Those parameters are later set in dynamometer workstation, together with vehicle inertia. Vehicle is restrained and so called vehicle adaptation has to be performed. During vehicle adaptation dynamometer automatically slowing down from set speed, changing its own "dyno parameters" and trying to get same deceleration in given intervals as on real road. Those parameters are then valid for this vehicle type. Changing of set simulated inertia it is possible to simulate vehicle ability to accelerate if fully loaded, with setting gradient it is possible to simulate force if vehicle going downhill etc. Chassis dynamometers for climatic chamber does exists, where it is possible to change temperature in give range i.e. -40 to +50 °C or altitude chamber where it is possible to check fuel consumption with different temperatures or pressure and to simulate driving on mountain roads.

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Wikipedia