Theory
Calculation of power dissipation
POWER AT THE CRANKSHAFT
On a roller test bench, the shaft power is the only reference value, also because it is NOT influenced by variables in play including:
- Type of tyres (winter, summer..)
- Tyre pressure
- Inefficient transmission components due to wear or minor damage
It is also theonly reference value that can also be measured on the engine test bench, an instrument on which the natural course of development of any type of engine begins.
POWER TESTS ON CHASSIS DYNAMOMETERS
When it comes to dyno tests on roller test benches, the dyno test is therefore carried out according to certain principles, in order to correctly trace the power back to the crankshaft.
The test must be taken in two distinct phases:
- Step 1 - Calculation of POWER ON THE WHEEL
In the first phase of the test, the test bench measures power at the wheel, by the radial acceleration of the roller (its inertial component) and the force applied by the eddy current brake to the load cell.
This period usually coincides with the period of vehicle acceleration on the test stand, which normally lasts between 12 and 15 seconds. This time must correspond as closely as possible to the same time it would take (for the same acceleration) on the road.
- Second step - Calculation of POWER LOSSES
In the second phase, which begins at the end of the load test on the dyno (acceleration), the accelerator is released and the clutch is pulled (thus disconnecting the crankshaft from the gearbox output shaft).the vehicle is made to slow down naturally, by inertia, until it stops. This period is also referred to, in the regulatory sphere, Coast-Down.
During this natural deceleration of the vehicle, we are going to measure radial acceleration, this time negative.
This measurement is necessary to enable the same power balance to be applied to the vehicle placed on the rollers as we would have on the road, where, with a flat, straight road and no strong wind, the vehicle would be braked simply by internal leaks (friction of mechanical parts) and its resistance to air.
Using the velocity trend, measured over time, it is then possible to calculate the 'dissipated power'.
DEFINITION OF POWER DISSIPATION
We can therefore define power dissipation as the sum of all those friction forces in which the engine dissipates its energy, to allow the vehicle to accelerate.
These include all the mechanical organisms of the transmission, ranging from the drive shaft to the point of contact between tyre and roller.
CALCULATION OF POWER AT THE CRANKSHAFT
Once the two test phases have been completed, to obtain a precise measurement of power at the crankshaft, the power dissipation is then added to the power measured at the wheel.
Reading the graphical results of the power test, we can deduce this data from the three curves depicted.
- One for power at the wheel (normally in the centre of the graph)
- One for 'dissipated' (lower part of the graph)
- One for the drive shaft (top of diagram)
4wd test bench double roller series
INSIGHTS
- History of dyno test benches
- SINGLE-roller Vs. DOUBLE-roller benches, which one to choose
- Advantages of 4×4 dynos with linked axles
- Power correction factors DIN, CE, SAE..
- Homologation and C.O.P. test benches fields of application