February 26, 2010

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

In the following 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 deceptive use of performance statements.

There are a lot of questions and beliefs about engine and powertrain performance at almost any automotive web-site or forum, all over the Internet. The performance issue (speed [rpm], torque [Nm], power [Nm, Bhp] and acceleration rate [rpm/s]) is not easy to explain due to the abstract and complex physics involved.

Please consider that I will only talk about powertrain performance due to the fact that this is the only performance statement that is affordable to verify for most performance customers.

Chapter 1

Q0: What is the most important document for demanding performance customers?

A0: The Powertrain Performance Graph(s), supplied by professionals. The PPG document is an essential for all new or used cars, standard or modified, performance or racing cars. Without a Powertrain Performance Graph from the actual car, performance customers are vulnerable to inaccurate information.

Q1: Do all engines with the same specification perform at the same level?

A1: NO! Production tolerances are something all manufacturers struggle with. Some have too wide tolerances or defects in production for mechanical parts, (cylinder heads, pistons, cylinders, camshafts) sensors, fuel injectors etc. All components are slightly different (even within specification) and this will have an effect on the total performance. The goal for the manufacturer is to have a robust design that is not sensitive to tolerances. However, the difficulty and expense increases with performance and the complexity of the design. One way to reduce the effect of tolerances is to use engine adaptation, which will to some degree compensate for the variations.

Stated maximum torque and power are allowed to vary in the production within ±5% in EC / according to ISO standard. It is then up to the manufacturers to maintain as little variation as possible. Some try to maintain the he average variation as closely as possible, whereas others try to stay within the specific interval, and whilst others struggle to keep it above the lower limit.

It is important to remember that the specifications are certified in an engine dynamometer with more or less unlimited cooling. This also incorporates long stabilization times before the actual performance measurement. This opens possibilities, for instance, to use over-boost functions for shorter periods. Additional power consumers, i.e. air conditioning, servo pumps, cooling fans, etc may reduce the available performance from the engine.

The installation in the vehicle can affect the output. A very cramped engine installation may give local heating problems. Routing of inlet air can easily raise the temperature and reduce performance. There are also installation effects (defects) from exhaust systems- and engine cooling capacity problems, too high exhaust back pressure and too high water cooling temperature due to high demands for head wind cooling. The engine cooling system (radiator) may be designed for a maximum "surrounding temperature" of for example +30°C and above that temperature, the engine control system has to reduce the performance output.

The engine design may be sensitive to fuel quality and even to test conditions. An engine running close to the border of knocking may reach the knocking limit if the atmospheric pressure is high.

Q2: Is it possible that the stated engine power of my new and/or used car has been falsified?

A2: YES! To prove that a manufacturer is misleading is very difficult and that is because of the official test itself. It has to be made on an engine dyno and to remove the engine from a modern vehicle to get it running on the engine dynamometer is practically impossible. The whole car will be needed because the ECU (Engine Control Unit) checks for all the other onboard computers, sensors, etc in the car and if it cannot find them it will not start!

The manufacturer can be close to the lower limit of the allowed tolerance. Whether they choose it deliberately or not makes no difference to the performance customer.

NOTE! Automotive customers pay for what is claimed in the engine but they get what is available to the wheel hubs!

Performance may have also been adjusted at normal service work. It is usual that the service station updates the software and in the worst case this may include reductions of performance. The engine performance may be reduced when the engine is used in the car, due to the following; engine overheating or engine mechanical problems, exhaust emission problems, transmission over load, general reliability problems, all resulting in high warranty costs.

Some sports car / race engines may be specified at a low inlet temperature of +15°C instead of the common +25°C. This will boost the specification of maximum power with approx. 2% only based on the temperature, added to that you may have knocking problems at a higher inlet temperature.

Official tests are done Steady State, but in reality the performance at full throttle (WOT)accelerations are of greater interest. The only Steady State in real life will be at the top speed.

NOTE! Performance during accelerations may differ significantly from the Steady State results. Maximum stated power and torque may be outside the usable areas, i.e. automatic gearboxes may shift before maximum power is reached and maximum torque may be specified below the stall speed of the transmission converter.

Maximum power and torque only specifies two points. It might be two narrow peaks and not a good description of the engine performance as a whole. Always demand a Powertrain Performance Graph, PPG over the whole "engine" working area (idle to maximum engine speed).

The market situation may from time to time force some automotive manufacturers to state the "same" performance numbers as tough competitors. In the best case this will be reached during ideal conditions with unlimited cooling.

Q3: Is it possible to mislead when dealing with performance measurements?

A3: YES! Inaccurate measurement systems are common and this is the main reason for the spreading the Donkey power [Dp] statements (please see A16) instead of accurate measured [Bhp] or [kW] according to Best Known Practice, BKP.

Some of the dyno concepts, such as the rolling road, are simply not suitable for quality and scientific measurements and they clearly shows that there is a lack of underlying physical understanding and sense of professionalism.

A well-informed performance consumer could easily distinguish this type of garage grade dynos from a true measurement system tool and should always demand true powertrain performance according to Best Known Practice, BKP.

Sven Andersson

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

[ Download ] (4 pages, 168 kb)