Engine power?

The two IS engines are very good at using all the inputs to calculate an accurate amount of fuel to inject to keep the engine running at stochiometric ratios.

I cannot remember off the top of my head but there is a formula for fuel to power assuming an average volumetric efficiency and STP...but..

I have found that unlike with many other engines which have rich and lean areas the IS engines burn the right amount of fuel.

Therefore I have found that rather than looking at MAP, or RPM as a measure of "how much power am i using" I seem to default to fuel flow.

With my NA IS engine full power uses about 25L/hr, I cruise at 16 (16/25= 65%) and best fuel economy ie MPG in my plane is 14 or 56%.

As a scientist I would flinch at using such a rough measure as there are many other factors, but after 800 hr I find it the best and easiest way to do this.

I doubt that anything more sophisticated would get you a better answer

You can get fancy with calculations based on fuel MAP IT etc

The 915 OM has a lookup table with 2480 data points using the inputs of rpm, temps throttle and altitude then outputs power and fuel burn.

Take the Power in kw and fuel burn divide one by the other and graph vs the data point....that will tell you how close Fuel burn is to an accurate ....or not approximation.

P.S I tried a few random numbers, reasonably close at lower altitudes but a bit off over 20,000 ft !

However if you have the time and interest playing with the table will tell you for the 915 the effects of altitude and ambient temp and pressure, also boost pressure etc.

I am not inclined but a spreadsheet guru could produce some nice graphs

A few years ago, I attended a course called "The Advanced Pilots Seminar" taught by a group of guys who have impeccable aviation credentials.  Their course material included the fact that at the perfect stoichiometric ratio (fuel-to-air ratio for "perfect" combustion), any gasoline powered engine would produce 14.9 HP for each gallon of avgas consumed per hour.  

In other words, to determine the HP being produced by this theoretical perfect engine, you just have to multiply the GPH by 14.9.  Note that this is only true while operating at the perfect stoichiometric ratio, and "real world" numbers for engines running rich of peak EGT (basically all Rotax engines) will produce somewhat (and in some cases - significantly) lower HP / GPH.

This data validates Glenn's idea of using fuel flow as an approximation of engine power produced - especially for fuel-injected engines.  You just need to determine the "real world" fuel flow at a couple of power settings (preferably a fairly wide spread of power settings), then use the charts to determine the nominal HP produced at those "book" power settings.  Then divide power (in HP) by fuel flow (either GPH or LPH) to determine YOUR engine's fuel constant.

Given that Rotax is likely targeting to run just a bit richer than peak EGT (run cooler, avoid any possibility of detonation, etc.), the fuel consumption to power produced ration will likely be somewhat lower than the theoretical 14.9 HP per gallon...  But the calculated ratio should be roughly the same for both power settings.  And even if it's not "perfect" it's a 'good enough' way to determine the power output.

Once you have the "constant" for HP to GPH ratio for your engine, it's a simple math exercise to determine the power setting at any fuel flow...  You just multiply the fuel flow by the "constant" you determined above.  

For example, if you determined that your engine produces 14 HP for each gallon per hour of consumption, and you're consuming 5.2 GPH, your engine is putting out roughly 73 HP (5.2 x 14 = 72.8).

Obviously, the IS engines with their far more optimized fuel injection will produce more accurate results than you can get with the carbureted engines (which run significantly rich-of-peak EGT, thus using more fuel for the same power).  Still, it's a somewhat useful approximation...

Thanks guys,

I wanted to figure out the best way to look at the OM chart but it is a lot of plotting. The 14gphp works close enough until you leave ECO mode and the flow increases over 100%. I hear rumors of some ratio of power to rpm, yet cant find anything better than your contributions. I will start some extensive recordings at various altitudes and speeds to get a comfort level and compare it to the 14gphp.

Thanks again

I have a home built gyro plane with a 915 iS and an Emsis with Daqu engine instrument.  It gives me a wealth of information, including percent of power on one digital instrument.