The definition of an engine is apparently "a machine that converts power into motion". This may be true in its widest sense but when it comes to rotary outputs, I like things to be more precise. Far be it from me to contradict the Oxford English Dictionary, but in its basest form surely an engine is "a machine that converts torque into motion"?
I know I'm being slightly pedantic, but engines - at least the ones we are more familiar with - are designed to produce torque at a particular
rotational speed, the product of which is then power. Torque is described as the actual turning effort produced by the crankshaft, and power the rate at which that torque is produced.
Likewise, in response to the question, "How much power does it give?" I can be equally pedantic. Depending on the mood, in answering such a question, the rejoinder might well be, "It depends on what you want", and in all seriousness the point will have been made.
You see, the power of any engine burning a fossil fuel is proportional to the amount of air - or, more correctly, oxygen - consumed. So when the atmospheric air pressure is high there is more oxygen in it and so more power produced.
The same goes for the temperature of that oxygen but in reverse. In this case, the higher the intake charge temperature the less dense the air, and the power produced will consequently fall.
Taking this a step further, if the air has water vapour in it, this will contribute towards the overall atmospheric pressure and must therefore be compensated for in any calculation. So when stating the power of an engine, the conditions under which it was tested need to be carefully stated.
To help, or possibly even confuse the situation, various authoritative bodies around the world have come up with engine test codes designed to alleviate the problem. Thus we have SAE, ISO, ECE, JIS and even DIN standards which in one way or another attempt to set certain atmospheric references to which all engine output characteristics are to be corrected.
The SAE, ECE and JIS standards all now seem to be converging on the atmospheric references of 990 mb of pressure and 25 C of dry air, while the German DIN standard refers to 993 mb and 20 C With a higher pressure and lower temperature, the DIN standard generates slightly higher corrected values so one needs to be a bit cynical when hearing this reference.
The latest version of the SAE standard - J1340 - specifies 990 mb and 25 C but the older reference (J607), still favoured by some, uses 1000 mb and 15.5 C For the really brazen, however, we need to go to the old STP standard where power is corrected to 1013 mb and 15.5 C. Engine powers and torques when corrected using this approach will be as much as 4% higher. No wonder it is still used by some parts of the performance industry.
Using the correction factor C.F. = x for wide-open throttle performance of naturally aspirated spark-ignition units (where subscript 's' refers to the reference temperature and pressure, and 'o' denotes observed values), engine performance values can, in theory, be readily compared from day to day, week to week and even location to location. This of course only takes into account the indicated power and is adequate for most forms of engine development activities when the precise figure is not necessarily needed. Since friction power is not affected in the same way prefer as pressure and temperature, these standards - particularly J1349 - have been modified to assume a constant mechanical efficiency.
So if you are ever asked the question, "How much power does it produce?" just pause for a moment and reflect before retorting, "How much do you want?"
Fig. 1 - The engine dyno
Written by John Coxon