The accurateness of dynamometers
At its simplest, a dynamometer is a means of measuring an engines power in Horsepower or Kilowatts.
Slightly more complex dynamometers can also give you a print out of your bike or cars BHP versus RPM and Torque versus RPM. This is achieved by running the back wheel (in the case of a bike) on a steel drum, at maximum power.
An engine dynamometer is a dyno attached directly to the engine. Engine dynamometers enable you to efficiently monitor performance parameters. They are used by big car manufacturers, race engine builders, or formula one cars constructors, and allow for total access, control of all parameters and can simulate race conditions and more! Engine dynamometers are usually braked dynos, and they are not always very complex.
Some people have built very simple engine dynos for things like model aircraft engines, and kart engines. These basically are just an engine driving a flywheel either directly or through a chain, or shaft.
A very rudimentary engine dynamometer works by calculating the flywheels mass (rotational mass) and then calculating the power curve using simple mathematics. All you need at this point is something to count and time the pulses from the flywheel. A single sensor or ignition pulse can be used.
Having cleared the basics regarding the way a dyno works, you should now understand the differences between a “brake” type dyno and an “inertia or inertial” dynamometer.
A brake type dynamometer measures power by “braking” the drum or roller (or the engine, in an engine dynamometer directly). Then, it uses a load cell or spring balance to measure the torque required to hold the engines RPM steady. This is calculated by using the following formula: Torque x RPM = Power.
An inertial dyno uses a heavy solid steel drum, but no brake! The drum is free to accelerate. A computer then logs the rate of acceleration, and as it knows the mass of the drum, it can plot the power required against either road speed or real time engine RPM.
Power loss is an important aspect which should be considered when using dynamometers.
A chassis dynamometer measures the power produced “at the wheel”. Some dynamometer systems allow you to see an “at the engine” or “flywheel” BHP figure.
There is always more power available at the engine than is available at the wheels! Gearboxes and axles, along with the tires all absorb some power. As the shaft speed increases, the amount of power absorbed increases too.
Tires absorb more power as the road speed increases, so there will be greater losses here in top gear than in say 2nd or 3rd gears.
A very important fact to remember is that on all chassis dynamometers, the “power at the engine” BHP figure can never be accurate! At best, the power measurements at the engine should be considered as a guide. It is important to remember to always account for the power losses in order to achieve a good performance diagnose. There are several ways of calculating or measuring the power losses, but these all depend on the manufacturer you choose for your dyno.
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