Power correction factors

To give a correlation to power found on a dynamometer test bench, to the conditions that would prevail on the road, it is necessary apply correction factors to the measured power.

They are determined by severalstandardisation institutes, including, for example:

• DIN Deutsches Institut für Normung
• SAE Society of Automotive Engineers
• ISO International Organisation for Standardisation
• JIS Japanese Industrial Standard
• ABNT Brazilian Association of Technical Standards

EFFICIENCY OF ENDOTHERMIC ENGINES

The power produced by an engine is directly related to the mass and quality of the air injected in the cylinders, in the unit of time.

The latter is in turn related to atmospheric pressure and temperature values.

The release of energy in a combustion process depends on the presence of oxygen alone.

Therefore, also the change in humidity or presence of different gases, can lead to variations in developed power that, in an engine, is always directly proportional to dry air pressure.

E.g., the drop in power that occurs in the mountains, as you go up, it is due to the fact that the density of the air progressively decreases as you go up in altitude.

Each litre of air sucked in by the engine will contain fewer oxygen molecules, thus causing a decrease in the amount of fuel that can be burned and thus the amount of thermal energy that is released by combustion (= less engine power).

Or think of a engine test room without an adequate air recirculation and cleaning system; the combustion gases from the engine could flow back into the cycle, adversely affecting the validity of the tests performed.

It is therefore clear that environmental conditions must be carefully measured at the time of the test, then modifying the measured power using a special 'Correction Factor', thus bringing it back to the value that would be obtained under 'standard' conditions, i.e. temperature, reference pressure and humidity (although the general conditions of measurement are very different)