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8 - Plug Head advice
Moderators: KeithZ1R, chrisu, paul doran, Taffus
ooo .....Rich tell us more.
So the flame front as I understand it is the leading edge of the explosion that originates from the plug.
I thought this had to travel as fast as possible and to do this the compressed mixture was held as close to detonation as possible- and then the spark triggered a very rapid burn.
However if the mixture exceeded this detonation point then more than one explosion happens and not at exactly the same time (detonation is always early)- thus the noise we hear it the collision of the two explosions and also the associated mechanical stress of the pre-ignition firing the cylinder too far before TDC.
This is just what I understood- not even sure where I picked this up from. Is it close? :-)
So the flame front as I understand it is the leading edge of the explosion that originates from the plug.
I thought this had to travel as fast as possible and to do this the compressed mixture was held as close to detonation as possible- and then the spark triggered a very rapid burn.
However if the mixture exceeded this detonation point then more than one explosion happens and not at exactly the same time (detonation is always early)- thus the noise we hear it the collision of the two explosions and also the associated mechanical stress of the pre-ignition firing the cylinder too far before TDC.
This is just what I understood- not even sure where I picked this up from. Is it close? :-)
z650/1400 bonneville hybrid.
Pre-ignition has always been a bit of a "weird one" in my book. My Z1000A2 has done it for yonks, I run Soopa Unleaded (makes sod all difference), have tried loads of ignition settings and plug grades, and now just go easy on the throttle when it occurs!
I understand what it is & why it happens.... but the cure by retarding ignition never makes sense.
To reduce pre-ignition you wanna force the burn, so get the spark to fire sooner...
If you retard the ignition it'll be even later - so surely you need to advance it
My brain hurts
I understand what it is & why it happens.... but the cure by retarding ignition never makes sense.
To reduce pre-ignition you wanna force the burn, so get the spark to fire sooner...
If you retard the ignition it'll be even later - so surely you need to advance it
My brain hurts
And on the 7th day... Zeds were created!
Pre-ignition is different from detonation.
Pre-ignition is caused by hot spots and overheating (other sources).
Detonation is maximum cylinder pressure before the optimum point of maximum efficiency.
Detonation is a super-sonic event which is why you can hear it. It is faster than the speed of sound. Normal ignition is a sub-sonic event which is why you cant hear it.
Optimum is a fast but controlled burn. Detonation is an explosion. If the ignition is too advanced you can get detonation from the mix being first burnt but as maximum cylinder pressure builds the residual mix will explode. The two curves for each cross over at about 10 degrees before top dead centre.
Ignition at 40 degrees before top dead centre is the norm but because of the sub-sonic burn time maximum cylinder pressure reaches a peak at up to about 10 degrees before top dead to about 10 degrees after TDC centre because of the speed of travel of the piston. Many hundreds of feet per second. The mechanical advantage of the moment of the crank means that max cylinder pressure can be reached before it goes over the top without any negative consequences ( vibration stresses and mechanical damage) because the assembly is lined up in its strongest configuration.
If you look at the crowns of Wiseco high comp pistons with a single spark plug you will see that a low proportion of the crown is subject to burn. flame or detonation.
The reason is partly due to swirl but mainly because the distance across the crown is considerably greater and the piston is well on its way back down before it torches the entire piston surface.
The answer is twin plugs and two complimentary flame fronts.
Two fronts meeting each other cancel out and lead to considerably higher fuel efficiency, fuel economy, volumetric efficiency and power generation.
Power here just being the release of heat energy. Not detonation and its not a super sonic event when they cross over.
Retarding the ignition will help reduce detonation because it reduces maximum cylinder pressure. Delays it untill TDC is passed.
Retarding the ignition will only help pre-ignition by reducing the amount of heat released if that reduction in heat prevents the alternate heat source from generating an early ignition source.
Wiseco pistons of some types are very prone because of the pocketed valves and the sharp edges left on the cut.
This should make the process a bit more visible.
http://www.youtube.com/watch?v=sEf8va1S7Sw
AL
This is a good read for a general understanding it was written by the former tech director of Piper Cams
http://www.bgideas.demon.co.uk/tmanual/
CH 1 Power
CH2 Breathing
CH3 Power stroke
CH4 Exhaust system
CH5 Cams
CH6 Ignition
CH7 Turbo or SC
CH7 Reliability
Pre-ignition is caused by hot spots and overheating (other sources).
Detonation is maximum cylinder pressure before the optimum point of maximum efficiency.
Detonation is a super-sonic event which is why you can hear it. It is faster than the speed of sound. Normal ignition is a sub-sonic event which is why you cant hear it.
Optimum is a fast but controlled burn. Detonation is an explosion. If the ignition is too advanced you can get detonation from the mix being first burnt but as maximum cylinder pressure builds the residual mix will explode. The two curves for each cross over at about 10 degrees before top dead centre.
Ignition at 40 degrees before top dead centre is the norm but because of the sub-sonic burn time maximum cylinder pressure reaches a peak at up to about 10 degrees before top dead to about 10 degrees after TDC centre because of the speed of travel of the piston. Many hundreds of feet per second. The mechanical advantage of the moment of the crank means that max cylinder pressure can be reached before it goes over the top without any negative consequences ( vibration stresses and mechanical damage) because the assembly is lined up in its strongest configuration.
If you look at the crowns of Wiseco high comp pistons with a single spark plug you will see that a low proportion of the crown is subject to burn. flame or detonation.
The reason is partly due to swirl but mainly because the distance across the crown is considerably greater and the piston is well on its way back down before it torches the entire piston surface.
The answer is twin plugs and two complimentary flame fronts.
Two fronts meeting each other cancel out and lead to considerably higher fuel efficiency, fuel economy, volumetric efficiency and power generation.
Power here just being the release of heat energy. Not detonation and its not a super sonic event when they cross over.
Retarding the ignition will help reduce detonation because it reduces maximum cylinder pressure. Delays it untill TDC is passed.
Retarding the ignition will only help pre-ignition by reducing the amount of heat released if that reduction in heat prevents the alternate heat source from generating an early ignition source.
Wiseco pistons of some types are very prone because of the pocketed valves and the sharp edges left on the cut.
This should make the process a bit more visible.
http://www.youtube.com/watch?v=sEf8va1S7Sw
AL
This is a good read for a general understanding it was written by the former tech director of Piper Cams
http://www.bgideas.demon.co.uk/tmanual/
CH 1 Power
CH2 Breathing
CH3 Power stroke
CH4 Exhaust system
CH5 Cams
CH6 Ignition
CH7 Turbo or SC
CH7 Reliability
1981 J1
1. FLAME FRONT = LEADING EDGE OF IGNITION INITIATED BURN ( BURN RATE IS CONSTANT REGARDLESS OF RPM'S HENCE TIMING NEEDS TO BE ADVANCED AS RPM'S RISE TO ALLOW MORE TIME FOR FUEL TO FULLY BURN BEFORE EXHAUST CYCLE STARTS).
TWIN PLUG SPEEDS UP BURN TIME AND ALLOWS FOR FULL IGNITION ADVANCE TO BE RETARDED USUALLY TO AROUND 28 TO 34 DEG DEPENDING ON APPLICATION.
2. PRE - IGNITION = UNCONTROLLED AND UNTIMED IGNITION OF FUEL CHARGE BY LOCALISED HOT SPOT ( GLOWING CARBON DEPOSIT ETC )
3. DETONATION = SPONTANEOUS COMBUSTION/EXPLOSION OF FUEL CHARGE CAUSED BY HIGH CYLINDER TEMP OR LOW OCTANE FUEL WITH POOR ANTI KNOCK PROPERTIES ( HENCE HIGH OCTANE FUEL REQUIRED FOR HIGHER COMPRESSION SITUATIONS)
TYPICAL SIGN OF DETONATION IS PEPPERED/PITTED APPEARENCE TO PISTON CROWN ESPECIALLY AROUND (BUT NOT LIMITED TO ) THE CIRCUMFERENCE OF THE PISTON CROWN AND ERROSION OF PISTON RING LANDS IN MORE SEVERE CASES.
SIMPLE.
TWIN PLUG SPEEDS UP BURN TIME AND ALLOWS FOR FULL IGNITION ADVANCE TO BE RETARDED USUALLY TO AROUND 28 TO 34 DEG DEPENDING ON APPLICATION.
2. PRE - IGNITION = UNCONTROLLED AND UNTIMED IGNITION OF FUEL CHARGE BY LOCALISED HOT SPOT ( GLOWING CARBON DEPOSIT ETC )
3. DETONATION = SPONTANEOUS COMBUSTION/EXPLOSION OF FUEL CHARGE CAUSED BY HIGH CYLINDER TEMP OR LOW OCTANE FUEL WITH POOR ANTI KNOCK PROPERTIES ( HENCE HIGH OCTANE FUEL REQUIRED FOR HIGHER COMPRESSION SITUATIONS)
TYPICAL SIGN OF DETONATION IS PEPPERED/PITTED APPEARENCE TO PISTON CROWN ESPECIALLY AROUND (BUT NOT LIMITED TO ) THE CIRCUMFERENCE OF THE PISTON CROWN AND ERROSION OF PISTON RING LANDS IN MORE SEVERE CASES.
SIMPLE.
Last edited by zed1015 on Wed Jan 04, 2012 12:46 am, edited 2 times in total.
Pigford wrote:Pre-ignition has always been a bit of a "weird one" in my book. My Z1000A2 has done it for yonks, I run Soopa Unleaded (makes sod all difference), have tried loads of ignition settings and plug grades, and now just go easy on the throttle when it occurs!
Might not be detonation.
I had a motor that consistantly tinkled/knocked under acceleration no matter what i changed.
Finally got pi**ed off and pulled the top end to find no visible evidence but did find one little end eye slightly looser than the rest ( always seems to be number one rod that wears ).
Just out of curiosity i changed the crank and hey presto problem solved.
Well I've learnt that what I knew ....wasn't quite right.
From my research it now seems that pinking occurs after the spark has sparked- and NOT as I previously stated before (that is pre-ignition and different).
SORRY FOR THE CONFUSION.
When the sparkplug fires and combustion begins there is an increase in cylinder pressure (obviously) and this can precipitate the self ignition of as yet unburnt parts of the fuel air mix. This self ignition has particular characteristics unlike the spark induced ignition. Its an explosion not a burn.
This self ignition creates very short sharp shock waves (can be measured as high pressure spikes in the combustion chamber during the combustion process). Some maintain that the sound of pinking is the ringing of the engine structure as if hit by a hammer- in response to this short sharp shock wave. And NOT as I previously suggested the colliding wave fronts of the burning gas.
Given that our engines have 40 degrees of advance there is a lot of potential for pressures to rise in the cylinder and for the pinking to take place.
With a twin plug head set up the burn should be faster as it starts in 2 places. This means that because pressures rise faster the ignition can be retarded from 40 deg so that peak cylinder pressure still occurs at 14deg atdc (apparently this is the perfect number).
Thus twin plug heads are just better.
I found this work easy to read and explanitory.
http://www.streetrodstuff.com/Articles/ ... etonation/
"Engine Basics: Detonation and Pre-Ignition
by Allen W. Cline"
From my research it now seems that pinking occurs after the spark has sparked- and NOT as I previously stated before (that is pre-ignition and different).
SORRY FOR THE CONFUSION.
When the sparkplug fires and combustion begins there is an increase in cylinder pressure (obviously) and this can precipitate the self ignition of as yet unburnt parts of the fuel air mix. This self ignition has particular characteristics unlike the spark induced ignition. Its an explosion not a burn.
This self ignition creates very short sharp shock waves (can be measured as high pressure spikes in the combustion chamber during the combustion process). Some maintain that the sound of pinking is the ringing of the engine structure as if hit by a hammer- in response to this short sharp shock wave. And NOT as I previously suggested the colliding wave fronts of the burning gas.
Given that our engines have 40 degrees of advance there is a lot of potential for pressures to rise in the cylinder and for the pinking to take place.
With a twin plug head set up the burn should be faster as it starts in 2 places. This means that because pressures rise faster the ignition can be retarded from 40 deg so that peak cylinder pressure still occurs at 14deg atdc (apparently this is the perfect number).
Thus twin plug heads are just better.
I found this work easy to read and explanitory.
http://www.streetrodstuff.com/Articles/ ... etonation/
"Engine Basics: Detonation and Pre-Ignition
by Allen W. Cline"
z650/1400 bonneville hybrid.
This all has made me think- if we assume two things- 1) max power is made when peak cylinder pressure is at 14 degrees atdc, and 2) flame speed is constant at all rpms ....then
a) when the ignition is timed to give max power at say 8000 rpm surely we could calculate the starting point for igntion timing at any other rpm.
b) we could compare the peak power ignition timing for an 8 plug head and a 4 plug head and get a good idea of the difference in the speed in the rise of cylinder pressure.
I know this is going to vary from engine set-up to engine set-up, but it is going to be better than just adopting the standard bike advance curve without question.
Is this worth doing?
a) when the ignition is timed to give max power at say 8000 rpm surely we could calculate the starting point for igntion timing at any other rpm.
b) we could compare the peak power ignition timing for an 8 plug head and a 4 plug head and get a good idea of the difference in the speed in the rise of cylinder pressure.
I know this is going to vary from engine set-up to engine set-up, but it is going to be better than just adopting the standard bike advance curve without question.
Is this worth doing?
z650/1400 bonneville hybrid.
A quick calculation suggests that if using an 8 plug head you retard the ignition from 40 deg btdc to 34-28 deg btdc for max power then this suggests that cylinder pressure builds faster with an 8 plug head-
In the simple calc... between 11 and 22% shorter time to max pressure.
method-
I took the time it takes at 40 deg advance (btdc) for the engine to turn to 14 deg after tdc (max power timing point) and compared this to the time taken from 34 and then 28 deg btdc.
In the simple calc... between 11 and 22% shorter time to max pressure.
method-
I took the time it takes at 40 deg advance (btdc) for the engine to turn to 14 deg after tdc (max power timing point) and compared this to the time taken from 34 and then 28 deg btdc.
z650/1400 bonneville hybrid.
OK... so back to my assumptions... WRONG!
Flame speed does not change much in the gas/air mixture.... BUT the distance that the flame front needs to travel does change significantly- as a result of turbulence. So as revs increase the burn happens faster because of the extra swirl speed of the gasses in the combustion chamber.
My graph for the theoretical flame speed is therefore not accurate.
Flame speed does not change much in the gas/air mixture.... BUT the distance that the flame front needs to travel does change significantly- as a result of turbulence. So as revs increase the burn happens faster because of the extra swirl speed of the gasses in the combustion chamber.
My graph for the theoretical flame speed is therefore not accurate.
z650/1400 bonneville hybrid.
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davy thomson
- ZedHead
- Posts: 1000
- Joined: 24th Aug 2005
- Location: scotland [north]
seems like a lot of bother for no real gain.
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