Manifold Pressure and Detonation/Over-Squaring
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Manifold Pressure and Detonation/Over-Squaring
I've heard from a lot of people and a lot of books that I read that over-squaring an engine - i.e -> MP higher than RPM can cause engine damage and possibly lead to detonation at high MP.
I'm curious as to how high MP and low RPM can lead to high engine temps and thus detonation?. So lets use for example this scenario...... 25 " MP, 2500RPM. Now I reduce the pitch so that the RPM indicates 2000RPM but leave the MP at 25".
Apparently this is bad becuase there is a pressure buildup but I don't see this straight away. This is how I see it........
The throttle butterfly is left in the same position as it was originally set, now the RPM is reduced, pistons travel up and down at a slower rate so the amount of fuel/air mix getting sucked into the engine is thus reduced as its the pistons that suck the air from the throttle chamber right, so thus if the pistons are going up and down slower wouldn't this mean that less fuel/air is getting drawn in which would ultimately result in less pressure being drawn in??
I could understand how all this would work if fuel/air was pumped into the system but the amount of fuel/air taken is somewhat related to the speed of the crankshaft. Could some expert please fill me in here on my mis-understanding.
Thanks in advance.............
Kev
I'm curious as to how high MP and low RPM can lead to high engine temps and thus detonation?. So lets use for example this scenario...... 25 " MP, 2500RPM. Now I reduce the pitch so that the RPM indicates 2000RPM but leave the MP at 25".
Apparently this is bad becuase there is a pressure buildup but I don't see this straight away. This is how I see it........
The throttle butterfly is left in the same position as it was originally set, now the RPM is reduced, pistons travel up and down at a slower rate so the amount of fuel/air mix getting sucked into the engine is thus reduced as its the pistons that suck the air from the throttle chamber right, so thus if the pistons are going up and down slower wouldn't this mean that less fuel/air is getting drawn in which would ultimately result in less pressure being drawn in??
I could understand how all this would work if fuel/air was pumped into the system but the amount of fuel/air taken is somewhat related to the speed of the crankshaft. Could some expert please fill me in here on my mis-understanding.
Thanks in advance.............
Kev
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Big Kev,
This was covered extensively about a month or so ago.
http://www.pprune.org/forums/showthr...hreadid=117251
Have a read through this thread, and see whether your answer is there.
DIVOSH!
This was covered extensively about a month or so ago.
http://www.pprune.org/forums/showthr...hreadid=117251
Have a read through this thread, and see whether your answer is there.
DIVOSH!
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Gday Kev
A very interesting subject, I think you may be able find the answer to this by experimenting by yourself.
Firstly you need a manual motorcar, if yours is auto borrow a friends.
You then need to find a reasonably steep uphill gradient and also you will need to simulate the wearing of a headset so I suggest earplugs.
You will then start the said car moving towards the uphill gradient and have the vehicle in top gear at about 20 km per hour as you start up the grade, and remember the engine is only revving very slowly.
Now heres the fun bit, apply full throttle and ignore the vibration and because of the earplugs there’s no noise and keep the accelerator flat to the floor.
If you can maintain configuration for about a minute I bet the head will depart the engine if the pistons don’t disintegrate or a conrod doesn’t punch a hole in crankcase.
If you decide to over square a naturally aspirated aircraft engine against the manufacturers recommendations please put it in the MR so we innocents don’t have to contend with a dead stick forced landing.
Now lets look at turbo and supercharged aero engines they are over squared I hear you say, Yep but they are designed at a lower compression ratio to withstand the increased pressures.
Sorry Kev too many reds again tonight.
Cheers Q
A very interesting subject, I think you may be able find the answer to this by experimenting by yourself.
Firstly you need a manual motorcar, if yours is auto borrow a friends.
You then need to find a reasonably steep uphill gradient and also you will need to simulate the wearing of a headset so I suggest earplugs.
You will then start the said car moving towards the uphill gradient and have the vehicle in top gear at about 20 km per hour as you start up the grade, and remember the engine is only revving very slowly.
Now heres the fun bit, apply full throttle and ignore the vibration and because of the earplugs there’s no noise and keep the accelerator flat to the floor.
If you can maintain configuration for about a minute I bet the head will depart the engine if the pistons don’t disintegrate or a conrod doesn’t punch a hole in crankcase.
If you decide to over square a naturally aspirated aircraft engine against the manufacturers recommendations please put it in the MR so we innocents don’t have to contend with a dead stick forced landing.
Now lets look at turbo and supercharged aero engines they are over squared I hear you say, Yep but they are designed at a lower compression ratio to withstand the increased pressures.
Sorry Kev too many reds again tonight.
Cheers Q
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Hey QNIM,
Hehe - thanks for the advice but I kinda get it now without having to do your wonderful experiment - I'm sure my mechanic would be most pleased if i did that experiment (NOT!).
I realise now as well that when the engine is turning over slower, that the inlet valves stay open longer as well and there is a higher pressure as there isn't as much suction when the engine is rotating slower and thus there is more available static air in the process which registers as increase in MP as you would expect.
Cheers
Have another red!
BK ;-)
Hehe - thanks for the advice but I kinda get it now without having to do your wonderful experiment - I'm sure my mechanic would be most pleased if i did that experiment (NOT!).
I realise now as well that when the engine is turning over slower, that the inlet valves stay open longer as well and there is a higher pressure as there isn't as much suction when the engine is rotating slower and thus there is more available static air in the process which registers as increase in MP as you would expect.
Cheers
Have another red!
BK ;-)
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Drivel
Hi Q,
In answer to your reply I do not think that is quite correct you are just repeating the same old bull from your instructors before you, esp. by using an extreme example.
Dont confuse Labouring, with pre ignition and detonation they are all different.
Please read John Deacons column in www.avweb.com, as he can explain it better with actual recorded examples from the test cell. Without the old wives tales of bad explanations.
Here is a question for you. Take off Manifold and RPM of an O-470-U in a C182 2400 rpm; SL approx MP 29 inches (good engine on a good day) on take off. Rated power is is available continously, from the AFM. Over square?
C172R approved for full throttle (about 29 inches) all day for 2400 hours, max RPM 2400. Yes hard to actually get it, but it is OK with the manufacturer. Over square?
What you explaned is the extreme which is outside the AFM.
Regards P
The flight manual is the legal document, follow it. If it allows over square it is OK (and all engines I have flown have). It does not harm the engine
Kev,
I think you should re read your post, it does not compute.
Regards P
Good to question everything though, stay sceptical until proven.
In answer to your reply I do not think that is quite correct you are just repeating the same old bull from your instructors before you, esp. by using an extreme example.
Dont confuse Labouring, with pre ignition and detonation they are all different.
Please read John Deacons column in www.avweb.com, as he can explain it better with actual recorded examples from the test cell. Without the old wives tales of bad explanations.
Here is a question for you. Take off Manifold and RPM of an O-470-U in a C182 2400 rpm; SL approx MP 29 inches (good engine on a good day) on take off. Rated power is is available continously, from the AFM. Over square?
C172R approved for full throttle (about 29 inches) all day for 2400 hours, max RPM 2400. Yes hard to actually get it, but it is OK with the manufacturer. Over square?
What you explaned is the extreme which is outside the AFM.
Regards P
The flight manual is the legal document, follow it. If it allows over square it is OK (and all engines I have flown have). It does not harm the engine
Kev,
I think you should re read your post, it does not compute.
Regards P
Good to question everything though, stay sceptical until proven.
Last edited by piontyendforward; 27th Apr 2004 at 23:10.
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Gday PEFD
If you had bothered to read my post thoroughly you would have realised I especially noted that,
If you decide to over square a naturally aspirated aircraft engine against the manufacturers recommendations please put it in the MR.
If the engine manufacturer says it’s ok then you may do it.
And please tell me, whats a Pionty?
Have a great day.
Cheers Q
If you had bothered to read my post thoroughly you would have realised I especially noted that,
If you decide to over square a naturally aspirated aircraft engine against the manufacturers recommendations please put it in the MR.
If the engine manufacturer says it’s ok then you may do it.
And please tell me, whats a Pionty?
Have a great day.
Cheers Q
Hi QNim,
If your definition of "over square" is an MAP in inches of mercury which is higher than RPM * 100, then I have never seen an engine, naturaly aspirated or supercharged, that could not be run "over square" according to the manufacters specs.
If the convention for MAP had been hectopascals instead of inches, this myth would never have been perpetuated.
If your definition of "over square" is an MAP in inches of mercury which is higher than RPM * 100, then I have never seen an engine, naturaly aspirated or supercharged, that could not be run "over square" according to the manufacters specs.
If the convention for MAP had been hectopascals instead of inches, this myth would never have been perpetuated.
