Chassis Dynamometer (rolling Road)
A chassis dynamometer, sometimes referred to as a rolling road, measures power delivered to the surface of the "drive roller" by the drive wheels. The vehicle is often parked on the roller or rollers, which the car then turns, and the output measured thereby.
Modern roller-type chassis dyno systems use the "Salvisberg roller", which improves traction and repeatability, as compared to the use of smooth or knurled drive rollers. Chassis dynamometers can be fixed or portable, and can do much more than display RPM, horsepower, and torque. With modern electronics and quick reacting, low inertia dyno systems, it is now possible to tune to best power and the smoothest runs in real time.
Other types of chassis dynamometers are available that eliminate the potential for wheel slippage on old style drive rollers, attaching directly to the vehicle hubs for direct torque measurement from the axle.
Some dyno systems can add vehicle diagnostic information to the dyno graph as well. This is done by gathering data directly from the vehicle using on-board diagnostics communication links.
Emissions development and homologation dynamometer test systems often integrate emissions sampling, measurement, engine speed and load control, data acquisition, and safety monitoring into a complete test cell system. These test systems usually include complex emissions sampling equipment (such as constant volume samplers and raw exhaust gas sample preparation systems) and analyzers. These analyzers are much more sensitive and much faster than a typical portable exhaust gas analyzer. Response times of well under one second are common, and are required by many transient test cycles. In retail settings it is also common to tune the air-fuel ratio using a wideband oxygen sensor that is graphed along with the RPM.
Integration of the dynamometer control system with automatic calibration tools for engine system calibration is often found in development test cell systems. In these systems, the dynamometer load and engine speed are varied to many engine operating points, while selected engine management parameters are varied and the results recorded automatically. Later analysis of this data may then be used to generate engine calibration data used by the engine management software.
Because of frictional and mechanical losses in the various drivetrain components, the measured rear wheel brake horsepower is generally 15-20 percent less than the brake horsepower measured at the crankshaft or flywheel on an engine dynamometer. Other sources, after researching several different "engine" dyno software packages, found that the engine dyno user can integrally add "frictional loss" channel factors of +10% to +15% to the flywheel power, raising the claim that 20% to 25% or even more power is actually lost at high power outputs. On a motorcycle, typical power loss at higher power levels (mostly through tire flex) is about 10%, while gearbox, chain/belt, and other power-transferring parts account for another 2% to 5%.
In drag racing a 1/4 mile prediction based on dynamometer measured horsepower figures are a strong predictor of, but do not guarantee, a specific 0-60 mph or 1/4 mile elapsed time (ET), or a certain 1/4 mile speed. Inexpensive "inertia dynamometers" commonly provide insufficient loading, and complete their "test" in less time than the real world 1/4 mile takes, causing high inherent power value errors, due to unrealistic internal engine temperatures. More sophisticated dyno systems are capable of "loaded testing", which can potentially recreate the same temperatures as on the dragstrip. An engine accelerating in a vehicle experiences different conditions than on a dyno. G forces and temperature variations, as well as different modes of vibration in a vehicle, can cause significant differences in true power output.
Read more about this topic: Dynamometer, Types of Dynamometer Test Procedures