Magneto Checks
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I always thought the "pinking" noise you hear in car engines is due to the ignition timing being so far advanced that it causes the mixture to burn while one of the valves is still partially open....
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Join Date: Nov 2000
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This has just about all been said by instalments but:
RAF's magneto check procedure advice in my time instructing on pistons was as follows:
Immediately after start and at 850 RPM IDLE:
"Drop, no Stop" (no2 off).
Both ON (RPM recovers)
"Drop, no stop" (no1 off).
Both ON (RPM recovers)
Drop AND stop (both OFF).
Both ON (RPM recovers)
This proves that both mags are working. It also proves the single, combined rotary switch grounds out correctly in ALL positions. I recall at least one case where such a rotary mag switch became worn and left at least one mag live at "OFF".
At idle the amount of mixture going through the engine is very small, so that a "pop" in the exhaust was seen as a lesser evil than breaking someone's arm or worse because of a permanently live mag. Logically, the procedure is only about as stressful to the engine as what happens during normal engine start.
This check was repeated after flight, before finally shutting down on the mixture control, which makes it even safer to park, as all remaining mixture is burned rather than lurking in the manifold.
This was a separate check from the Run-up / power checks just prior to take off. During those checks, (@2200 RPM, max 150 drop per mag) if switching a mag off results in the engine cutting completely, leave the switch alone and let the engine stop. Do NOT switch the mag back on or you risk blowing the exhaust off or damaging the crankshaft through reverse twisting stresses because of the very sudden driving / driven / driving effect.
RAF's magneto check procedure advice in my time instructing on pistons was as follows:
Immediately after start and at 850 RPM IDLE:
"Drop, no Stop" (no2 off).
Both ON (RPM recovers)
"Drop, no stop" (no1 off).
Both ON (RPM recovers)
Drop AND stop (both OFF).
Both ON (RPM recovers)
This proves that both mags are working. It also proves the single, combined rotary switch grounds out correctly in ALL positions. I recall at least one case where such a rotary mag switch became worn and left at least one mag live at "OFF".
At idle the amount of mixture going through the engine is very small, so that a "pop" in the exhaust was seen as a lesser evil than breaking someone's arm or worse because of a permanently live mag. Logically, the procedure is only about as stressful to the engine as what happens during normal engine start.
This check was repeated after flight, before finally shutting down on the mixture control, which makes it even safer to park, as all remaining mixture is burned rather than lurking in the manifold.
This was a separate check from the Run-up / power checks just prior to take off. During those checks, (@2200 RPM, max 150 drop per mag) if switching a mag off results in the engine cutting completely, leave the switch alone and let the engine stop. Do NOT switch the mag back on or you risk blowing the exhaust off or damaging the crankshaft through reverse twisting stresses because of the very sudden driving / driven / driving effect.
Join Date: Feb 2002
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I remember this too. So when I started civvy flying they thought I was nuts when I wondered why we dont do a proper "dead cut cx" (mags off then on again).
The bulldog has a lycom engine like many others and I am not aware of any damage ever being caused this way in the 20 years plus the RAF operated these aircraft.
The bulldog has a lycom engine like many others and I am not aware of any damage ever being caused this way in the 20 years plus the RAF operated these aircraft.
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On the subject of detonation:
My understanding of the phenomena is that it occurs because peak pressure and temperature in the cylinder overcome the resistance to detonation of the air / fuel (octane rating) mixture. Detonation occurs AFTER the normal ignition by spark has taken place. In the event, the unburned mixture ahead of the flame front explodes rather than burns. At low RPM the peak pressure is likely to overcome the knock resistance of the fuel more readily simply because the piston is moving slower and therefore the combustion chamber volume is expanding less rapidly.
On the other hand, Pre-ignition is caused by a hot spot and is independent of the normal spark ignition. It may cause pinking in the same way as over-advanced ignition (that's all it is), or detonation does.
Another phenomena, Auto-ignition is akin to how a diesel fires off, from residual heat in the cylinder combined with the extra heat of compression.
The pinking (pinging if you're American) noise we may hear as a result of any of the above is worse on a worn engine because the con-rod bearings get well and truly rattled. To put the extra stresses caused by detonation in perspective, imagine the difference between being pushed hard in the ribs by a little bloke - it might not hurt much and just cause you to be moved out of his way, whereas a swift kick from the same chap definitely would hurt and might even cause a fractured rib or two.
On extremely highly boosted engines (dragsters etc) with turbo-charging and nitrous oxide injection, it is by no means unheard of for conrods to fail in compression, they twist into an S shape, causing the piston to get clouted from below by the crankshaft. This spectacularly results in the innards becoming the outards.....
My understanding of the phenomena is that it occurs because peak pressure and temperature in the cylinder overcome the resistance to detonation of the air / fuel (octane rating) mixture. Detonation occurs AFTER the normal ignition by spark has taken place. In the event, the unburned mixture ahead of the flame front explodes rather than burns. At low RPM the peak pressure is likely to overcome the knock resistance of the fuel more readily simply because the piston is moving slower and therefore the combustion chamber volume is expanding less rapidly.
On the other hand, Pre-ignition is caused by a hot spot and is independent of the normal spark ignition. It may cause pinking in the same way as over-advanced ignition (that's all it is), or detonation does.
Another phenomena, Auto-ignition is akin to how a diesel fires off, from residual heat in the cylinder combined with the extra heat of compression.
The pinking (pinging if you're American) noise we may hear as a result of any of the above is worse on a worn engine because the con-rod bearings get well and truly rattled. To put the extra stresses caused by detonation in perspective, imagine the difference between being pushed hard in the ribs by a little bloke - it might not hurt much and just cause you to be moved out of his way, whereas a swift kick from the same chap definitely would hurt and might even cause a fractured rib or two.
On extremely highly boosted engines (dragsters etc) with turbo-charging and nitrous oxide injection, it is by no means unheard of for conrods to fail in compression, they twist into an S shape, causing the piston to get clouted from below by the crankshaft. This spectacularly results in the innards becoming the outards.....
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S.A.S.
I am sorry but I can't agree with some of your statements.
"The higher cylinder temps caused by running at a leaner mixture are an aid to detonation compared to the much lower temps you would get with an excessively rich mixture. A mixture of around 13.5:1 has been proven to be the ratio most likely to cause detonation. If the mixture is leaned too far, then the slower speed of the flame front can cause all sorts of issues, especially as less heat is converted to mechanical energy and the cylinder temps will rise rapidly."
I suppose you are only considering half of the engine temperature curves: from rich to peak EGT; once past peak EGT both CHT and EGT fall quite rapidly.
The fastest burning mixture is found at about 50° ROP which is also the place to find the hottest CHT.
At high power both very rich (100°ROP +) or very lean (40-90°LOP) mixtures can be used to prevent detonation.
Since both mixtures are slower burning, more of the combustion event will occur later in the downward cycle of the piston, where pressures are dropping rapidly and thus lower temperatures.
The pinging heard on car engines but not on aircraft engines because of the noise is actually the shockwave of the detonation bouncing between the cylinder walls.
From the pilot's point of view, the definition of detonation is a rise of CHT of about 1°F per second with a simultaneous drop in EGT for that cylinder. I have seen this on my friends engine at full power after take-off. The only remedy was to switch on the fuel boost pump (not required in POH for this airplane). The extra fuel immediately stopped the rise and even brought down the temps.
I am sorry but I can't agree with some of your statements.
"The higher cylinder temps caused by running at a leaner mixture are an aid to detonation compared to the much lower temps you would get with an excessively rich mixture. A mixture of around 13.5:1 has been proven to be the ratio most likely to cause detonation. If the mixture is leaned too far, then the slower speed of the flame front can cause all sorts of issues, especially as less heat is converted to mechanical energy and the cylinder temps will rise rapidly."
I suppose you are only considering half of the engine temperature curves: from rich to peak EGT; once past peak EGT both CHT and EGT fall quite rapidly.
The fastest burning mixture is found at about 50° ROP which is also the place to find the hottest CHT.
At high power both very rich (100°ROP +) or very lean (40-90°LOP) mixtures can be used to prevent detonation.
Since both mixtures are slower burning, more of the combustion event will occur later in the downward cycle of the piston, where pressures are dropping rapidly and thus lower temperatures.
The pinging heard on car engines but not on aircraft engines because of the noise is actually the shockwave of the detonation bouncing between the cylinder walls.
From the pilot's point of view, the definition of detonation is a rise of CHT of about 1°F per second with a simultaneous drop in EGT for that cylinder. I have seen this on my friends engine at full power after take-off. The only remedy was to switch on the fuel boost pump (not required in POH for this airplane). The extra fuel immediately stopped the rise and even brought down the temps.
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You are correct in what you are saying reference to the EGT, but the the bores and head do get warmer with a very lean mixture. This is almost going into a philosopical argument here, but the siting of CHT and EGT probes is generally very poor in a/c engines and often the results you get don't truly represent what is happening inside the bores.
Octane tests and knock values are usually done on a specific engine design. Automotive octane ratings are determined in a special single-cylinder engine with a variable compression ratio (CR 4:1 to 18:1) known as a Cooperative Fuels Research ( CFR ) engine. I only put this in as most research of this type has been done on a very different engine design to the ones we are talking about, so there may be differences.
When the mixture is leaned to a great extent, the speed of the flame front reduces alot and in some cases it can slow down enough for there to still be a flame present when the exhaust valve opens. Cue a dirty great backfire. The heating of the cylinder walls is greater since the energy transfer (chemical to mechanical) is reduced whereas the transfer to heat energy continues unabated.
We all agree that energy can be neither created or destroyed, when a charge goes into the bore, we have a finite amount of energy. This is all converted. Since the mechanical energy is reduced, the heat energy increases therefore we get more heat transfered.
If the probes were arranged across the engine, then we would see hotspots, but since we usually don't have this apart from in a lab we can only base our judgement on what we see.
For a real simple explanation, when we have excess oxygen any burn is hotter. For a lean mixture we have more oxygen, cue hotter temps.
This is real research stuff and goes into a level of complication that is unnecessary here for flying, but it does make for interesting discussions. (If you are as sad as me that is!)
The method for cooling an engine by dumping whacking great gobs of fuel in has been used many times. I have read numerous books written by WW2 pilots after getting hit in the cooling system. Keep pumping the fuel in and you may get home. Not good for efficency however. What caused the detonation in your friends machine? Was ant damage done?
Octane tests and knock values are usually done on a specific engine design. Automotive octane ratings are determined in a special single-cylinder engine with a variable compression ratio (CR 4:1 to 18:1) known as a Cooperative Fuels Research ( CFR ) engine. I only put this in as most research of this type has been done on a very different engine design to the ones we are talking about, so there may be differences.
When the mixture is leaned to a great extent, the speed of the flame front reduces alot and in some cases it can slow down enough for there to still be a flame present when the exhaust valve opens. Cue a dirty great backfire. The heating of the cylinder walls is greater since the energy transfer (chemical to mechanical) is reduced whereas the transfer to heat energy continues unabated.
We all agree that energy can be neither created or destroyed, when a charge goes into the bore, we have a finite amount of energy. This is all converted. Since the mechanical energy is reduced, the heat energy increases therefore we get more heat transfered.
If the probes were arranged across the engine, then we would see hotspots, but since we usually don't have this apart from in a lab we can only base our judgement on what we see.
For a real simple explanation, when we have excess oxygen any burn is hotter. For a lean mixture we have more oxygen, cue hotter temps.
This is real research stuff and goes into a level of complication that is unnecessary here for flying, but it does make for interesting discussions. (If you are as sad as me that is!)
The method for cooling an engine by dumping whacking great gobs of fuel in has been used many times. I have read numerous books written by WW2 pilots after getting hit in the cooling system. Keep pumping the fuel in and you may get home. Not good for efficency however. What caused the detonation in your friends machine? Was ant damage done?
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S.A.S.
What is really really important is the Internal Combustion Pressures, this takes into account temperatures, compression, type of fuel (leaded or unleaded), and they define the real limits of spark ignition combustion engines.
Running an engine at high power at peak EGT is NOT OK because the ICPs are very high (some 900 PSI), drop the power to 60% and we are talking about some 700 PSI. Being leaner than peak EGT (leaner than stoichiometric) the ICPs will drop, keeping MP and RPM constant.
If a mixture burns slower, then, at the same RPM, it must be cooler than a faster burning one, because it occurs later in the cycle, father from TDC, with the piston gong downwards at a greater rate, meaning less compression, less heat. On a M231 I was able to pull the mixture so far out that the EGT temperatures went UP again, this is because the charge was still burning with the exhaust valve open (like you said). Needless to say, this mixture was too lean for practical use and the engine was producing far too little power, the most efficient point of lowest BFSC is around 40°F LOP.
An engineer will speak about fuel/air ratios, but the pilot only has an EGT to navigate his way around the mixture curve.
In front of me sits the Basic Theory of Operation for the Wright R-3350 TC18 engine (turbocompound). It was written at the summit of piston engine operation by the airlines in October 1957. They had it all right, with a full time flight engineer watching the engines, leaning to 10% BMEP drop. This is an amazing little book, I can tell you where to get it if you want.
Unfortunately, all this wisdom has somehow bypassed GA until a couple years ago. We now have all-cylinder engine analysers with downloadable memory so we can play our own flight engineer.
I don't agree when you say that excess oxygen will mean higher temps, the hottest point is about 50%ROP , still a 'rich' mixture by any definition. Also a lean mixture does not cause burnt valves, lousy manufacturing QC control does, nothing the pilot does will help here.
Have a look at John Deakin's column on mixture:
http://www.avweb.com/news/columns/182084-1.html
You will find all the familiar graphs with explanation. Please read it more than once, you will understand my background.
Here is where the data is confirmed:
http://www.engineteststand.com/testdata.htm
They are now measuring ICP inside the cylinder in real time, sampling at 48kHz. Never before in the field of GA has this been done.
What is really really important is the Internal Combustion Pressures, this takes into account temperatures, compression, type of fuel (leaded or unleaded), and they define the real limits of spark ignition combustion engines.
Running an engine at high power at peak EGT is NOT OK because the ICPs are very high (some 900 PSI), drop the power to 60% and we are talking about some 700 PSI. Being leaner than peak EGT (leaner than stoichiometric) the ICPs will drop, keeping MP and RPM constant.
If a mixture burns slower, then, at the same RPM, it must be cooler than a faster burning one, because it occurs later in the cycle, father from TDC, with the piston gong downwards at a greater rate, meaning less compression, less heat. On a M231 I was able to pull the mixture so far out that the EGT temperatures went UP again, this is because the charge was still burning with the exhaust valve open (like you said). Needless to say, this mixture was too lean for practical use and the engine was producing far too little power, the most efficient point of lowest BFSC is around 40°F LOP.
An engineer will speak about fuel/air ratios, but the pilot only has an EGT to navigate his way around the mixture curve.
In front of me sits the Basic Theory of Operation for the Wright R-3350 TC18 engine (turbocompound). It was written at the summit of piston engine operation by the airlines in October 1957. They had it all right, with a full time flight engineer watching the engines, leaning to 10% BMEP drop. This is an amazing little book, I can tell you where to get it if you want.
Unfortunately, all this wisdom has somehow bypassed GA until a couple years ago. We now have all-cylinder engine analysers with downloadable memory so we can play our own flight engineer.
I don't agree when you say that excess oxygen will mean higher temps, the hottest point is about 50%ROP , still a 'rich' mixture by any definition. Also a lean mixture does not cause burnt valves, lousy manufacturing QC control does, nothing the pilot does will help here.
Have a look at John Deakin's column on mixture:
http://www.avweb.com/news/columns/182084-1.html
You will find all the familiar graphs with explanation. Please read it more than once, you will understand my background.
Here is where the data is confirmed:
http://www.engineteststand.com/testdata.htm
They are now measuring ICP inside the cylinder in real time, sampling at 48kHz. Never before in the field of GA has this been done.
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My experience with engine design is not from an aviation standpoint, but comes from working on and designing 'race' engines of differing types, mainly in the car/bike world. The way things are done is vastly different, for example I was lucky enough to have a look at Lotus' clear engine. To actually see the combustion process happening in front of you rather than just modelling it is fascinating, especially with pneumatic valves!
I agree totally that much of this knowledge is useless to pilots. We only have very rudimentary information given to us and all engine management techniques need to be as idiot proof as possible. Which is why I still can't understand why this responsibility is given to pilots when much more accurate and efficient methods are available so easily now.
I would definately like to read the book you mention as I have limited knowledge about aviation piston lumps and I may be missing something, as your explanation does make sense when I think about it. The engines I'm used to run at much higher loads and with much tighter tolerances and less 'real world' useability so I could be using the wrong mindset about them.
Just a quick aside. the fact is, that EGT WILL drop when leaned too far, BUT there is a greater heat transfer due to the speed of the flame front. This does go against a lot of thinking. Less temp so how could we get hotter conditions? ICP obviously plays a role, but after ignition the pressure increases beacause of the gas expansion. Lean mixtures expand greatly, simply because they can! Nice and warm in there.
I have to say that I may have slightly shot myself in the foot by simply talking about a lean mixture and not clarifying exactly what I mean. I am certainly not talking about mixtures far below stoichiometric where the engine is only just running and I think we are talking around the same point. Getting the old grey matter working though, not often that happens on a Sunday.
The links you have put up are very interesting, 48k is a lot of data.
Looking at the EGT trace, I cannot believe the spread of temps. Who designed that? John Deakins comments about the gash design seem very true!
A quick Q, is AVGAS oxygenated? or is it a bog standard fuel?
I agree totally that much of this knowledge is useless to pilots. We only have very rudimentary information given to us and all engine management techniques need to be as idiot proof as possible. Which is why I still can't understand why this responsibility is given to pilots when much more accurate and efficient methods are available so easily now.
I would definately like to read the book you mention as I have limited knowledge about aviation piston lumps and I may be missing something, as your explanation does make sense when I think about it. The engines I'm used to run at much higher loads and with much tighter tolerances and less 'real world' useability so I could be using the wrong mindset about them.
Just a quick aside. the fact is, that EGT WILL drop when leaned too far, BUT there is a greater heat transfer due to the speed of the flame front. This does go against a lot of thinking. Less temp so how could we get hotter conditions? ICP obviously plays a role, but after ignition the pressure increases beacause of the gas expansion. Lean mixtures expand greatly, simply because they can! Nice and warm in there.
I have to say that I may have slightly shot myself in the foot by simply talking about a lean mixture and not clarifying exactly what I mean. I am certainly not talking about mixtures far below stoichiometric where the engine is only just running and I think we are talking around the same point. Getting the old grey matter working though, not often that happens on a Sunday.
The links you have put up are very interesting, 48k is a lot of data.
Looking at the EGT trace, I cannot believe the spread of temps. Who designed that? John Deakins comments about the gash design seem very true!
A quick Q, is AVGAS oxygenated? or is it a bog standard fuel?
