View Full Version : Lies, damned lies, and engine performance data

24th Apr 2003, 20:22
I've been taking a look at this engine (http://www.simonini-flying.com/victor2plus_eng.htm) made in Italy. It quotes some remarkable performance figures, but when you read the small print the tests were done at 2C, 1048mb - or in other words somewhere in Italy there must be a dynamometer in a pressurised fridge.

Does anybody know of a reasonably reliable method to adjust IC engine performance figures like this to ISA/Sea-Level conditions so that I can try and reduce this to something meaningful (not to mention comparable with other engines that I know the behaviour of in a given aeroplane)?


oxford blue
24th Apr 2003, 21:58
2C and 1048 mb corresponds to an air density of 1327 grammes per cubic metre, as opposed to 1225 at ISA MSL. This is 108.3% of ISA MSL density. In other words, at ISA MSL, the density would be about 92.3% of the test conditions.

I'm not an engine performance specialist, but I would have thought that someone who is would know a correlation between intake air density and performance.

25th Apr 2003, 22:17
horsepower = mep x displacement x rpm / 792,000

Since all bar mep is constant then I suppose reducing to ISA & comparing ratios would work. Not really sure though...

28th Apr 2003, 14:03
The power output of a piston engine is proportional to the mass flow rate of air thru the engine (within typical operating conditions).

So Oxford Blue gave you the answer.


Dan Winterland
28th Apr 2003, 18:00
Are you sure it's an engine? It looks more like a fridge compressor to me!

That would explain the high density test conditions. :cool:

2nd May 2003, 19:32
There is a certain unique styling to it. Having been raised on the 1930s Flash Gordon movies (no I'm not that old, the BBC used to repeat them on BBC2 in the school holidays) I did wonder if Dr Zarkhof has retired to a life of Pasta and piston engine design.

ET, you are broadly correct, but only at I think constant mixture ratio. Mixture tends I think to vary with altitude with a venturi carb and is rarely stoichiometric I believe, it generally tends to be the rich side of that. Also, taking your statement as true...

- Is it linearly proportional, or some other function?
- Is the mass flow rate through a normally aspirated engine constant, a linear function of density, or some other function?


Still not sure, but we'll probably fly it anyway.