Over Square?
Join Date: May 2003
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Wow!
That has to be by far one of the most thorough and well layed out explanations of the engine instrument indications and their meanings/relationships I've ever read. Thanks be to Chimbu chuckles for that - I'm sure I'm not the only one who managed to understand it a little better.
Perhaps you can also tell me why in some aircraft (eg some C206 that I've flown) there is a small sticky label on the panel which appears to have been there since new that reads "Do not exceed 22" MAP when below 2000RPM" or something like that...(?)
Cheers,
520.
Perhaps you can also tell me why in some aircraft (eg some C206 that I've flown) there is a small sticky label on the panel which appears to have been there since new that reads "Do not exceed 22" MAP when below 2000RPM" or something like that...(?)
Cheers,
520.
Join Date: Sep 2001
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Probably a vibrational or detonation limitation on the airframe/engine combination. That could be as a result of testing, or a result of lack of testing. There are other airframe/engine combinations out there that have similar limitations, not always placarded. That's why it pays to RTFM.
Grandpa Aerotart
Conti,
Once again a label like that doesn't really give enough information to decide why it's there without some knowledge of what's going on inside the combustion chamber during a combustion event.
I agree it's probably to do with detonation margins for the following reasons.
All piston engines on aircraft have fixed timing, via the magneto.
The spark is timed so that it goes off at a specific point in the compression stroke BEFORE the piston reaches TODC. This point is based on a very narrow range of RPMs so that the peak power pulse (maximum pressure in the chamber) occurs after TODC to apply the maximum rotational leverage on the piston. ie it pushes as efficiently as possible on the piston to convert as much 'work' into rotation of the crankshaft as is possible.
The fuel air mix doesn't just explode BANG and all burn instantly it takes time...albeit not much time
The spark might happen 16 degrees before TODC and the maximum 'push' happens 12 odd degrees past TODC. At the point where the fuel/air mix is burning the hottest and applying the maximum force the piston is moving away at an accelerating rate so pressure is quickly dropping in the combustion chamber....hence combustion peak temps, for a fraction of a second might be 3000F but it (very) quickly cools to the 1650 odd snap shot that the egt probe reads as the exhaust valve opens.
Now the RPM range that keeps this relationship right is around max RPM to gve the widest possible detonation margins on takeoff.
Any reduction of RPM means that the piston will be closer to TODC when the combustion event is at max, the combustion chamber is smaller and the piston is not moving away as quickly due to the leverages involved with the piston rods/crankshaft geometry required to change linear motion (piston up/down) to rotational (crankshaft round and round).
So LOTS more heat and LOTS more pressure.
With MP over 22in (in this hypothetical) there is lots of air avail for combustion and if ROP lots of fuel too...LOTS & LOTS of heat/pressure.
ROP means reasonable amounts of unburnt fuel gumming up the system with carbon. Carbon deposits can be heated up by the extra heat/pressure and cause pre-ignition.
Pre-ignition is when the fuel/air mix ignites prematurely (maybe 20 degrees before TODC causing more of the above dramas with pressure/heat which can, probably will, lead to detonation...which may, probably will, blow the engine to bits....quickly.
The pre-ignition/detonation could also be caused by a bit of broken helicoil poking into the combustion chamber being heated up to very high temps by the above high pressures/temps...igniting the fuel/air mix early with the same results.
But we need heat to cause this.
If LOP the extra fuel is not there...the excess air is cooling things...the lean mixture burns slower so the piston is further past TODC at max pressure...no carbon buildups to get hot...less chance of preignition from either carbon/broken bits...lower temps/press all around mean detonation is almost impossible...certainly dramatically less likely.
So once again if we are not talking about the mixture we are not armed with enough information....that limitation, and I've not seen placards like that, ceases to be a problem 1/. LOP or 2/. when armed with knowledge.
Chuck.
Edit to clarify pre-ignition/detonation.
When the mixture is ignited early by pre-ignition it burns a lot quicker than the controlled flame front resulting from normal ignition (it truly explodes rather than burning very quickly)....I guess hotspots/overheated helicoils are like having multiple sparkplugs firing off all over the cylinder walls. Clearly that makes the situation reference piston position vs TODC even worse driving pressures/temp quickly through the roof which then causes detonation where the fuel/air mixtures just explodes (as opposed to burning) as soon as it enters the combustion chamber....within seconds it becomes self perpetuating, followed sooner rather than later to departure of effected cylinder heads.
Once again a label like that doesn't really give enough information to decide why it's there without some knowledge of what's going on inside the combustion chamber during a combustion event.
I agree it's probably to do with detonation margins for the following reasons.
All piston engines on aircraft have fixed timing, via the magneto.
The spark is timed so that it goes off at a specific point in the compression stroke BEFORE the piston reaches TODC. This point is based on a very narrow range of RPMs so that the peak power pulse (maximum pressure in the chamber) occurs after TODC to apply the maximum rotational leverage on the piston. ie it pushes as efficiently as possible on the piston to convert as much 'work' into rotation of the crankshaft as is possible.
The fuel air mix doesn't just explode BANG and all burn instantly it takes time...albeit not much time
The spark might happen 16 degrees before TODC and the maximum 'push' happens 12 odd degrees past TODC. At the point where the fuel/air mix is burning the hottest and applying the maximum force the piston is moving away at an accelerating rate so pressure is quickly dropping in the combustion chamber....hence combustion peak temps, for a fraction of a second might be 3000F but it (very) quickly cools to the 1650 odd snap shot that the egt probe reads as the exhaust valve opens.
Now the RPM range that keeps this relationship right is around max RPM to gve the widest possible detonation margins on takeoff.
Any reduction of RPM means that the piston will be closer to TODC when the combustion event is at max, the combustion chamber is smaller and the piston is not moving away as quickly due to the leverages involved with the piston rods/crankshaft geometry required to change linear motion (piston up/down) to rotational (crankshaft round and round).
So LOTS more heat and LOTS more pressure.
With MP over 22in (in this hypothetical) there is lots of air avail for combustion and if ROP lots of fuel too...LOTS & LOTS of heat/pressure.
ROP means reasonable amounts of unburnt fuel gumming up the system with carbon. Carbon deposits can be heated up by the extra heat/pressure and cause pre-ignition.
Pre-ignition is when the fuel/air mix ignites prematurely (maybe 20 degrees before TODC causing more of the above dramas with pressure/heat which can, probably will, lead to detonation...which may, probably will, blow the engine to bits....quickly.
The pre-ignition/detonation could also be caused by a bit of broken helicoil poking into the combustion chamber being heated up to very high temps by the above high pressures/temps...igniting the fuel/air mix early with the same results.
But we need heat to cause this.
If LOP the extra fuel is not there...the excess air is cooling things...the lean mixture burns slower so the piston is further past TODC at max pressure...no carbon buildups to get hot...less chance of preignition from either carbon/broken bits...lower temps/press all around mean detonation is almost impossible...certainly dramatically less likely.
So once again if we are not talking about the mixture we are not armed with enough information....that limitation, and I've not seen placards like that, ceases to be a problem 1/. LOP or 2/. when armed with knowledge.
Chuck.
Edit to clarify pre-ignition/detonation.
When the mixture is ignited early by pre-ignition it burns a lot quicker than the controlled flame front resulting from normal ignition (it truly explodes rather than burning very quickly)....I guess hotspots/overheated helicoils are like having multiple sparkplugs firing off all over the cylinder walls. Clearly that makes the situation reference piston position vs TODC even worse driving pressures/temp quickly through the roof which then causes detonation where the fuel/air mixtures just explodes (as opposed to burning) as soon as it enters the combustion chamber....within seconds it becomes self perpetuating, followed sooner rather than later to departure of effected cylinder heads.
Last edited by Chimbu chuckles; 10th Mar 2004 at 16:30.
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Chuck - I wus just gunna say the same thing........
......How's the 'big mutha' goin - don't need none of those fancy JPI thingies for engines what rotate instead of going backward & forward hey?
......How's the 'big mutha' goin - don't need none of those fancy JPI thingies for engines what rotate instead of going backward & forward hey?
Grandpa Aerotart
re the big mutha.
If it aint Boeing, I aint going!!
I have only two 'complaints' about the 767 so far.
1/. You can't slide the seat away from the control column far enough(before it moves outboard) to comfortably have your food tray on your lap...if you're a tall b-u-g-g-e-r such as we.
2/. A major design flaw in the fuel system...too much freaking fuel (72000+kg).
Makes for some very long sectors 
Lovely machine to handfly...not that we get much op for that. It does really nice aileron rolls at very low level down the runway (in the level 5 sim) A couple of the checkies managed a lovely stall turn the other day....but ripped the wings off recovering from the verticle exit dive 
Got those wingovers down pat in the Truck yet?
Chuck.
If it aint Boeing, I aint going!!
I have only two 'complaints' about the 767 so far.
1/. You can't slide the seat away from the control column far enough(before it moves outboard) to comfortably have your food tray on your lap...if you're a tall b-u-g-g-e-r such as we.
2/. A major design flaw in the fuel system...too much freaking fuel (72000+kg).
Makes for some very long sectors Lovely machine to handfly...not that we get much op for that. It does really nice aileron rolls at very low level down the runway (in the level 5 sim
) A couple of the checkies managed a lovely stall turn the other day....but ripped the wings off recovering from the verticle exit dive Got those wingovers down pat in the Truck yet?
Chuck.
