March 5, 2010

The Game of Deception - Horse power [Hp] or Donkey power [Dp] that's the question!



In this the second of a total of four issues, formed as four different chapters, the RRI Column's Sven Andersson will try to give you answers for most of the common concerns about performance measurements. This particular issue deals with measurement quality.


Chapter 2



Q4: Is use of inaccurate measurement tools a method of performance deception?

A4: YES! A dynamometer has to be a professional measurement device not only named as such. For example the ROTOTEST VPA-RX chassis dynamometers are designed to meet industrial demands on measurement accuracy, according to Best Known Practice, BKP. A performance customer has always the right to demand high quality and safety for both the car and tyres when purchasing performance measurements.

WARNING! Most chassis dynamometers (garage grade rolling roads and/or garage grade hub dynos) have as a best practice a total measurement tolerance of ±7% on the absolute measured value (for the most uncomplicated method, the Steady State measurement). This means 93 to 107 hp for a 100 hp engine, or 279 to 321 hp for a 300 hp engine. In most cases this implies less than accurate measurements. For a race car Powertrain Performance measurements with such low level of accuracy chances are small to perform a winning engine calibration and it is not possible to carry out a professional investigation on why the car is not fast enough.

Another method used to claim false measurement accuracy is to show relative measurements during a short period. Often is a test car used as an example, head lights on and off for example. This type of demonstrations only shows that the test equipment has a good sensitivity for small power outputs. It tells nothing about calibrated absolute measurement accuracy. For calibration please see Q5 below.

NOTE! A clock that stands still, shows you the absolute right time twice, every 24 hours!


Q5: Is dynamometer calibration a critical quality issue?

A5: YES! Always ask for the calibration procedure for the used dynamometer and with what accuracy the total measurement change is performed. Any calibration of only the torque- or “pressure” sensor will not tell the whole story, it must be a calibration of the total measurement chain, with tyres and parasitic losses (rolling roads). Second hand measurements (hydraulic pressure instead of torque measurements for hydraulic garage grade dynamometers) are not good enough for professional usage. If your questions are not answered properly, do not waste your money, use another test supplier.


Q6: Is use of inertia engine and inertia chassis dynos with measurements during changing acceleration rates good enough?

A6: NO! Uncontrolled acceleration engine or/and chassis dynos will always give measurement that are not possible to compare - so called Donkey power [Dp] (see A16) due to non-constant acceleration rates [rpm/sec]. Measurement results will change with the acceleration rate, when using a known inertia [kgm2] (a drum, flywheel etc.) as a braking device. The acceleration rate will change with the engine power output:
More power = more braking, less power = less braking!?

The inertia influences: The larger inertia (flywheel), the larger “mechanical filtration” of the test results (The power variations will be hidden due to the physics of a flywheel).

NOTE! Flywheels are used in combustion engines to smooth out torque variations.

WARNING! Inertia engine and chassis dynamometers are a low cost unprofessional dynamometer solution! Performance graphs from these types of garage grade dynos do not in any way give correct test result for comparing of performance.

NOTE! Test results are not comparable in any way (due to acceleration rate fluctuations), even between test runs at the same "acceleration garage grade dyno".


Q7: What is the worst case dyno concept?

A7: The "Road Dyno" is a computer box, measuring the engine speed. This concept uses the same physical background as the inertia rolling roads or engine dynos. The base concept is useless due to performance measurements during non constant acceleration rates [rpm/sec]. A "Road Dyno" has all of the worst drawbacks of the rolling road. Non constant acceleration rates instead of Steady Rate™ [rpm/sec], its dependence on powertrain inertia, the tyre-road traction, used gear, plus a couple of other variation factors as wind speed, road slope angle, weight of the vehicle, aero dynamics of the vehicle and dragging brakes.

The many uncertainties with this type of attempt to measure performance are why racing engineers (with advanced data acquisition systems) cannot claim missing engine performance, despite well grounded suspicions.

NOTE! "Road Dynos" could be used as a toy, just for fun, but absolutely not in professional performance measurements.



Sven Andersson



The article, The Game of Deception, Horse Power [Hp] or Donkey power [Dp] that's the question! chapter 1-2, by Sven Andersson, March 2010 is free to download for non-commercial purposes.

[ Download ] (7 pages, 179 kb)