vibrations

Hi guys...looks like some might get a kick out of a primer on Knock and how it affects the engine.  There is a lot of misinformation on the internet but I finally found a good video explanation.  Just ignore the high performance things they are talking about, such as water/meth injectors, and look at the way knock works.  

The outcome is simple.  Good fuel will reduce the risk for preignition and the onset of knock.  High compression and turbos are much more susceptible to getting damaged.  High MAP pressures put you at risk.

Let me finish with this note.  Within the FAA/EASA rules we have a problem in that control of power can not be given to the computer.  It is up to the pilot to reduce power for safety reasons.  A small reduction in power might be a safety concern so we see in our ECU controls that we only read throttle position but we can't have the computer control it.  For the Carb engines the risk is really with the high compression engines like the 912iS and 912S/ULS and obviously the turbo versions.  The areas of risk are near peak torque where we have the most pressure in the cylinder.  As RPM can climb after peak torque it is under pressure reduction as the power stroke is absorbed into moving the piston downward.  Indeed in the 915 and 916 engines the compression ratio is significantly reduced from 11 to 1 down to 8.5 to 1 because of the turbo system.  The 916 is actually slightly lower than the 915 in that is has a different supplier and it runs higher boost on takeoff power. 

Just my view.

Cheers

Hi Mike, the two charts, “old” and “new”, describe operational parameters of the same engine, which gives us some confidence that they cannot completely contradict each other. Personally I like the old chart a little better because it does not seem to require any calculation - a matter of personal preference, not more.

However, I just stumbled over one particular note which must not be overlooked when interpreting the new chart. And to be honest, I did overlook it. Here you go: the yellow line is based on WOT for all given revs! I does not show a certain rev set by throttle position. It assumes WOT, full stop. And that raises the question who in this world would set WOT and then force the engine down to 2500 or 3000 or 4000 revs? 

My conclusion: over-pitching a prop is less bad for an Rotax 912 when RON98 is burned. I’ll keep that in mind when I touch the lever for the in-flight-adjustable prop next time?

regards

Peter

Peter

Yes the yellow line is a reference line, not a condition for use. In theory WOT is the most efficient mode of operation for an engine at any rpm, but in reality you have to take account of other factors too.

Similarly the RON98 line shows MAPs that must have been blown, if it's real test results not simulation. This is purely a study of pre-ignition thresholds.

I agree completely with your overall takeaway - much better to use 98!