March 19, 2010

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



In this the final 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 the concept of Donkey power.


Chapter 4



Q13: Is it common that steady state performance measurements can be confused with measurements during acceleration?

A13: YES! Steady State measurements are generally not comparable with measurements during acceleration conditions! Measurements of Powertrain Performance during acceleration or dynamic conditions will always be affected by the powertrain inertia due to the energy consumed (stored) in the rotational inertia of the powertrain components, such as the engine flywheel.

Performance measurements during acceleration with varying acceleration rates are not comparable in any way, results from road dynos, garage grade rolling roads and hub dynos and engine dynamometers with no accurate speed control for constant acceleration rates. There are also, in many cases, a lack of correction methods for minor fluctuations in the acceleration rate.

NOTE! Acceleration speeds less than 100 Engine [rpm/second] (an increase of 1000 to 7000 engine rpm in a minute) are approximately comparable to steady state measurements.


Q14: What about performance measurements at steady state?

A14: Steady State is the standard test condition used by the automotive manufacturers. The Powertrain Performance™ at Steady State is measured at different constant engine speeds. Unless otherwise stated, all tests are conducted at Steady State, i.e. at a fixed engine speed, and the engine is kept at full load (wide open throttle, WOT) until certain conditions are met when the measurements are taken. The engine speed is then changed to the next engine speed usually about 500 rpm apart and/or closer at the expected peak power and torque. The tests conducted by RRI follow the same principal procedure with one important difference — the test equipment on which the tests are conducted. While the engine manufacturer states peak power and peak torque for the engine (at the flywheel) this requires the engine to be tested separately in an engine dynamometer. RRI uses a chassis dynamometer from Rototest that measures the Powertrain Performance™ produced at the wheel hubs. The Rototest chassis dynamometer is very similar to an engine dynamometer with the only difference that it is meant to measure Powertrain Performance instead of engine performance.

NOTE! Test point times at Steady State less than approx. 3 seconds are not appropriate due to normal engine output variations. A longer test point time gives more information about engine cooling capacity. The measurement points are joined by a line only for display purposes. There is no information about Steady State Powertrain Performance between the measurement points. At Steady State there is no performance influence due to the inertia of the powertrain (e.g. engine flywheel).


Q15: What about performance measurements at steady rate (constant acceleration)?

A15: Measurement during acceleration will always lose some power due to the acceleration of the powertrain inertia (the engine flywheel, gear wheels etc.). On the other hand, the engine control software is able to give you different engine control parameters during acceleration. In this way some engines are able to produce more power during acceleration in spite of the power losses due to acceleration of the powertrain inertia. A common example is over boost during short times. To be comparable the acceleration rate, Steady Rate must be the same for example 300, 500, 700, 1000 rpm/s.

NOTE! The acceleration rates must be performed within close tolerances. For Best Known Practice, BKP the acceleration rate must be present at the Certificate of Powertrain Performance, CPP.


Q16: What is Donkey power [Dp]?

A16: For Horse power [Hp] and kilo Watt [kW] there are standard definitions. However for Donkey power [Dp] there is no standard (ISO/SAE) definition.
The symbol for Donkey Power! Donkey power [Dp] is characterized by:

Unspecified "Power" (engine power or Powertrain Performance) with no standard support for definition and control and due to that a total lack of scientific value.

Power results from measurement equipment (dynamometer) with none or a defective calibration procedure.

Power measured during changing acceleration rates.

Power measured during constant acceleration without declaration of used acceleration rate [rpm/s].

Power results fabricated by calculations or "so called measurements" from a non-professional dynamometer.

Engine power measured by a chassis dynamometer.

Performance graphs without background information such as: atmospheric pressure [mbar], inlet temperature [°C], acceleration rate [rpm/s], used correction method (standard) and test company information.

Power claims in general generated from non-professional measurements.

Warning signs for Donkey power [Dp] claims:

Usage of test results that cannot be verified by others.

Performance graphs without background information.

The power and torque graphs are not corresponding.

Attempt to compare power and torque graphs from Steady State with Steady Rate™ measurements or worse, comparing power and torque graphs from measurements during changing acceleration rates (cheap rolling roads, hub dynos and/or so called road dynos).



Sven Andersson



Download Donkey power definition with picture! Donkey-power-SA2010-01.pdf is free to download for non-commercial purposes.

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The article, The Game of Deception, Horse Power [Hp] or Donkey power [Dp] that's the question! chapter 1-4, by Sven Andersson, March 2010 is free to download for non-commercial purposes.

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