Building "Thor"
Discussion
At 2500rpm, a V12 engine using only 1 coil pack for 12 cylinders is going to be limited to a theoretical maximum dwell time of only 4ms, and that's assuming the coil could be 100% rated (which it won't be). I'd suggest that your going to need a really "good" coil (low resistance, excellent thermal performance, and low inductance) to get that to work well.
Max_Torque said:
At 2500rpm, a V12 engine using only 1 coil pack for 12 cylinders is going to be limited to a theoretical maximum dwell time of only 4ms, and that's assuming the coil could be 100% rated (which it won't be). I'd suggest that your going to need a really "good" coil (low resistance, excellent thermal performance, and low inductance) to get that to work well.
Not suggesting for a second you are wrong but Jaguar managed fine with a single coil / 12 way distributor, although they did AFAICS improve it with a twin-coil/twin level 12 way distributor (ie 6+6).It's rather a moot point though as having looked at the manual the merlin (and I presume the Meteor) don't have a distributor in the classic sense anyway.
Edit to add:
http://miravim.org/avimlibrary/Manuals/Powerplant%...
Pages 54-56
Not quite a haynes manual
Edited by Fastdruid on Wednesday 27th August 00:58
cribbed from here ..
http://www.mapleleafup.net/forums/showthread.php?t...
Correctly, it was identified that magnetos don't make sparks until their speed is sufficient to generate enough energy, even though the mags (but not rotor arms) do rotate at 1.5 x engine speed, and the Cent mags are not impulse types. The engine is of course ex-aircraft being a 27 litre RR Merlin but with no supercharger and softened cams and CR. This is of course the land based Meteor engine.
It has a boost coil and twin-finger rotor arms in the magnetos, in the aircraft application the boost coil is operated by a separate button thus allowing the engine to be turned and primed, owing to the very lengthy inlet manifold and huge blower casing volume, without any possibility of ignition and backfire.
In the tank version with simple updraught carbs in the V, the boost coil is activated with the single starter button and the system is now of an age when things fail. In essence the boost system is nothing more than a contact trembler activated by the core of the ignition boost coil making a continual shower of sparks, it should be audibly buzzing whan cranking a Centurion at some 500Hz on a 24V supply.
These sparks are fed to one mag only, the right hand, B bank - exhaust side, and then to the boost rotor arm finger that is 35º behind the the main rotor arm contact. The left hand magneto has this HT input blanked and in fact runs the opposite way round.
Thus the engine is started well retarded as each cylinder is fired when the sparks finally jump to each cap segment a little less than 35º ATDC, this means there can be no kick-backs on the direct starter drive and although the power realised when it fires is well below what should be the case, it is quite enough to accelerate the engine from a slow cranking speed, about 30 rpm, to the point where the mags start firing both sets of plugs on time and the start is secured with idle at 450 rpm. With a well primed and unmodified Spitfire you can see this happen as the exhaust stubs produce lazy flames of burning Ki-Gas prime fuel and the engine gathers pace against the prop load.
It is indeed the case that the engine stop is a ground connection to the magnetos that shorts the LT side of the magneto, but like an aircraft, the Cent also has two mag test buttons that can disable each mag in turn for the classic mag-drop test and in an emergency be used to stop the engine also.
Cents are so old now that all this wiring, point gaps and all HT cabling may well be faulty in some respect.
also:
Left hand mag (clockwise rotation) Simms FST 12 RG/3C
Right hand mag (anti-clock rotation) Simms FST 12 RG/3AC
LHS timed 5º BTDC Mk.4 engine or 0º-3º BTDC Mk.4A/B engine.
RHS timed 10ºBTDC Mk.4 engine or 5º-8º BTDC Mk.4A/B engine.
Timing mark "A1" is engraved on front mag case.
and from here
http://www.thunderboats.org/history/history0324.ht...
an interesting approach to starting ..
Ignition: Ignition is by two high-tension rotating magnet magnetos mounted on each side of the wheelcase. The "A" bank magneto serves the intake spark plugs and the "B" side magneto serves the exhaust spark plugs. The magneto has an integral coil and breaker points operating off a four pole cam, each magneto is driven by a serrated coupling shaft turning at 1.5x engine speed. A distributor turning at 1/3 the magneto speed provides one complete cycle of 12 sparks every 2 revolutions of the engine. Provisions exist for an engine control mechanism to move the breaker base through 25 degrees of advance. In addition, the distributor features a two contacts on its rotor -- the leading finger is energized by the magneto and the trailing finger by an external terminal which can be connected to a booster coil to facilitate starting. The booster coils are essentially high tension coils with vibrating points which provide continuous spark -- when applied to the retarded finger they provide a high energy retarded spark which can greatly facilitate starting, especially with cold, slow turning (thick oil) engines which don't provide much speed for the magnetos. Merlins in aircraft only boosted the "A" bank, but I've seen boats with no boost, with booster coils on both banks, and both combinations in between.
it certainly does seem a bit complicated , and a ms running the distributors firing a couple of msd 7al3's would be simpler.
although touted 'for racing only ' the msd 7 has been used on my tvr for many years with no problems .
http://www.mapleleafup.net/forums/showthread.php?t...
Correctly, it was identified that magnetos don't make sparks until their speed is sufficient to generate enough energy, even though the mags (but not rotor arms) do rotate at 1.5 x engine speed, and the Cent mags are not impulse types. The engine is of course ex-aircraft being a 27 litre RR Merlin but with no supercharger and softened cams and CR. This is of course the land based Meteor engine.
It has a boost coil and twin-finger rotor arms in the magnetos, in the aircraft application the boost coil is operated by a separate button thus allowing the engine to be turned and primed, owing to the very lengthy inlet manifold and huge blower casing volume, without any possibility of ignition and backfire.
In the tank version with simple updraught carbs in the V, the boost coil is activated with the single starter button and the system is now of an age when things fail. In essence the boost system is nothing more than a contact trembler activated by the core of the ignition boost coil making a continual shower of sparks, it should be audibly buzzing whan cranking a Centurion at some 500Hz on a 24V supply.
These sparks are fed to one mag only, the right hand, B bank - exhaust side, and then to the boost rotor arm finger that is 35º behind the the main rotor arm contact. The left hand magneto has this HT input blanked and in fact runs the opposite way round.
Thus the engine is started well retarded as each cylinder is fired when the sparks finally jump to each cap segment a little less than 35º ATDC, this means there can be no kick-backs on the direct starter drive and although the power realised when it fires is well below what should be the case, it is quite enough to accelerate the engine from a slow cranking speed, about 30 rpm, to the point where the mags start firing both sets of plugs on time and the start is secured with idle at 450 rpm. With a well primed and unmodified Spitfire you can see this happen as the exhaust stubs produce lazy flames of burning Ki-Gas prime fuel and the engine gathers pace against the prop load.
It is indeed the case that the engine stop is a ground connection to the magnetos that shorts the LT side of the magneto, but like an aircraft, the Cent also has two mag test buttons that can disable each mag in turn for the classic mag-drop test and in an emergency be used to stop the engine also.
Cents are so old now that all this wiring, point gaps and all HT cabling may well be faulty in some respect.
also:
Left hand mag (clockwise rotation) Simms FST 12 RG/3C
Right hand mag (anti-clock rotation) Simms FST 12 RG/3AC
LHS timed 5º BTDC Mk.4 engine or 0º-3º BTDC Mk.4A/B engine.
RHS timed 10ºBTDC Mk.4 engine or 5º-8º BTDC Mk.4A/B engine.
Timing mark "A1" is engraved on front mag case.
and from here
http://www.thunderboats.org/history/history0324.ht...
an interesting approach to starting ..
Ignition: Ignition is by two high-tension rotating magnet magnetos mounted on each side of the wheelcase. The "A" bank magneto serves the intake spark plugs and the "B" side magneto serves the exhaust spark plugs. The magneto has an integral coil and breaker points operating off a four pole cam, each magneto is driven by a serrated coupling shaft turning at 1.5x engine speed. A distributor turning at 1/3 the magneto speed provides one complete cycle of 12 sparks every 2 revolutions of the engine. Provisions exist for an engine control mechanism to move the breaker base through 25 degrees of advance. In addition, the distributor features a two contacts on its rotor -- the leading finger is energized by the magneto and the trailing finger by an external terminal which can be connected to a booster coil to facilitate starting. The booster coils are essentially high tension coils with vibrating points which provide continuous spark -- when applied to the retarded finger they provide a high energy retarded spark which can greatly facilitate starting, especially with cold, slow turning (thick oil) engines which don't provide much speed for the magnetos. Merlins in aircraft only boosted the "A" bank, but I've seen boats with no boost, with booster coils on both banks, and both combinations in between.
it certainly does seem a bit complicated , and a ms running the distributors firing a couple of msd 7al3's would be simpler.
although touted 'for racing only ' the msd 7 has been used on my tvr for many years with no problems .
Storer said:
I am glad to see that this thread is generating some exercise for other peoples grey cells. Part of the reason for building this 'special' is to give others pleasure/a sense of involvement, so please keep the comment coming.
As far as I am aware the differential between the two plugs firing stays the same through the rev range. I am sure the twin plug arrangement is primarily a left over from the aero design but it does help achieve a complete burn (remember, each cylinder is 2.25 litres so a long way between plugs which are opposite each other). The planes didn't rev much higher though.
I like the 'theatre' of twin plugs and I will probably use the two distributors to feed the plugs but then use technology to manage when they fire.
Starting is an issue due to the low 'energy' in the spark from the magnetos at low revs and this is aided by a switchable booster coil which fires a 'boost' to all spark plugs on one of the distributors. Modern plugs will help but may not be sufficient to avoid this issue.
As you may already have gathered I am not very tech savvy. I need to be able to see how something works to be able to understand it so I tend to take the proven route from people with experience when it comes to this new fangled electricity stuff. The suck, squeeze and blow bits I can sort. It's the 'making it bang' bit I will probably use others experience for!
Keep the comments rolling.
Paul
As far as I am aware the differential between the two plugs firing stays the same through the rev range. I am sure the twin plug arrangement is primarily a left over from the aero design but it does help achieve a complete burn (remember, each cylinder is 2.25 litres so a long way between plugs which are opposite each other). The planes didn't rev much higher though.
I like the 'theatre' of twin plugs and I will probably use the two distributors to feed the plugs but then use technology to manage when they fire.
Starting is an issue due to the low 'energy' in the spark from the magnetos at low revs and this is aided by a switchable booster coil which fires a 'boost' to all spark plugs on one of the distributors. Modern plugs will help but may not be sufficient to avoid this issue.
As you may already have gathered I am not very tech savvy. I need to be able to see how something works to be able to understand it so I tend to take the proven route from people with experience when it comes to this new fangled electricity stuff. The suck, squeeze and blow bits I can sort. It's the 'making it bang' bit I will probably use others experience for!
Keep the comments rolling.
Paul
Edited by ivanhoew on Wednesday 27th August 06:58
For what it's worth, Jaguar, for whatever reason, used two six-cylinder distributors in the original pre-crashed XJ13. Whether this was due to voltage drops or simply because there wasn't a 12-cylinder distributor available except from Magnetti Marelli, who I would have thought would have had an exclusive deal with Ferrari at the time, I don't know. Can I ask why you aren't simply keeping the throttles, ignition etc standard and period-correct?
It is clear that some of you are doing a bit of research, which is nice....
Robert.
Having already built my engine running rig I had to find out what you have there. I purchased a new (old stock) booster coil but wired it to a separate button which I activate once I have the engine cranking. With fully charged very large batteries (2 x 12V) the standard starter, that has a dog clutch, cranks at about 40 rpm.
The earthing of the mags to stop the engine had us scratching our heads on the day we tried to fire her up for the first time but we got there in the end.
Rover
I want to use this car. UK and Europe, so I want it to be reliable and reasonably 'tractable' and if at all possible, get more than 5mpg. The system in the Centurion tank was not designed to be used as you would in a car. In the tank the engine is either ticking over or pulling hard to move 52 tonnes over muddy ground.
2.5 tonnes of car is not quite the same. If you could see how much fuel is 'dumped' out of the manifold if she doesn't fire up straight away, it would make you weep.
Paul
Robert.
Having already built my engine running rig I had to find out what you have there. I purchased a new (old stock) booster coil but wired it to a separate button which I activate once I have the engine cranking. With fully charged very large batteries (2 x 12V) the standard starter, that has a dog clutch, cranks at about 40 rpm.
The earthing of the mags to stop the engine had us scratching our heads on the day we tried to fire her up for the first time but we got there in the end.
Rover
I want to use this car. UK and Europe, so I want it to be reliable and reasonably 'tractable' and if at all possible, get more than 5mpg. The system in the Centurion tank was not designed to be used as you would in a car. In the tank the engine is either ticking over or pulling hard to move 52 tonnes over muddy ground.
2.5 tonnes of car is not quite the same. If you could see how much fuel is 'dumped' out of the manifold if she doesn't fire up straight away, it would make you weep.
Paul
this^^^^^ (driveability) is why you want a basic plenum system and well mapped ignition. Run with just 12 plugs to simplify things, get some nice big injectors in the ports close to the intake valves to minimise wall wetting and transient fuelling issues and bingo, should drive like a brought one!! ;-)
Hi Max
I know you are an engineer but where are you getting the information from to be able to base your advice on. Rolls Royce could have deleted the second plug (at a time when components were in short supply) if they didn't think it necessary.
I have one large throttle body on the Ultima but it would look better with 8! It may not function any better.......but it would look better.
The design of the head on the Meteor would allow the throttles to 'share' their mixture to some extent if I don't blank them off (I must take a photo to illustrate) and I could keep the balance pipe connected as with the standard M3 E46 setup.
With my planned setup the injector will fire the fuel almost directly into the cylinder via the two inlet valves.
Paul
I know you are an engineer but where are you getting the information from to be able to base your advice on. Rolls Royce could have deleted the second plug (at a time when components were in short supply) if they didn't think it necessary.
I have one large throttle body on the Ultima but it would look better with 8! It may not function any better.......but it would look better.
The design of the head on the Meteor would allow the throttles to 'share' their mixture to some extent if I don't blank them off (I must take a photo to illustrate) and I could keep the balance pipe connected as with the standard M3 E46 setup.
With my planned setup the injector will fire the fuel almost directly into the cylinder via the two inlet valves.
Paul
No strong opinion on the dual or single plugs discussion, but I do think a plenum has the best chance of giving you a tractable engine over using throttle bodies.
I'm not saying they wont work, but its likey to take quite a bit of trial and error with sizes and a whole heap of setup / testing - possibly resulting in having to bin it and start again.
Trying to think of a good analogy but cant right now.
I'm not saying they wont work, but its likey to take quite a bit of trial and error with sizes and a whole heap of setup / testing - possibly resulting in having to bin it and start again.
Trying to think of a good analogy but cant right now.
Storer said:
Having already built my engine running rig I had to find out what you have there. I purchased a new (old stock) booster coil but wired it to a separate button which I activate once I have the engine cranking. With fully charged very large batteries (2 x 12V) the standard starter, that has a dog clutch, cranks at about 40 rpm.
The earthing of the mags to stop the engine had us scratching our heads on the day we tried to fire her up for the first time but we got there in the end.
Much the same as our Griffons are set up on the Shackleton. Mag switches, and then separate booster coil and starter switches.The earthing of the mags to stop the engine had us scratching our heads on the day we tried to fire her up for the first time but we got there in the end.
Just as an idea, it may be worth plumbing in a fuel cutoff fairly close to the engine, as if one of the mags decides not to earth properly the engine will keep happily running until its out of fuel. A bit more of a problem with a couple of 13 ft propellers on the end of it, but an expensive annoyance in a car no doubt.
(I was very happy that the tank fuel cocks and cut off on the Shack worked. We had 500 gallon of fuel in the tank feeding the engine...)
Regards,
Rich
Storer said:
Hi Max
I know you are an engineer but where are you getting the information from to be able to base your advice on. Rolls Royce could have deleted the second plug (at a time when components were in short supply) if they didn't think it necessary.
I'm getting "my information" from over 20 years designing, developing and calibrating engines, from F1 ones, to aero ones! ;-)I know you are an engineer but where are you getting the information from to be able to base your advice on. Rolls Royce could have deleted the second plug (at a time when components were in short supply) if they didn't think it necessary.
RR couldn't have "deleted" the second plugs back in the day because they relied on the "spark stagger" to start the engine. (ie the engine needs to have retarded ignition to start cleanly, and the second set of plugs is retarded 25deg, so by starting on just these, they get there ignition retard for starting. The very fact these second plugs run that much retard suggests to me they are there for pretty much starting and as an emergency "fall back", in most cases, by the time they fire, the vast majority of the charge will have already burnt i suspect. (i'd expect the 10-90 mass fraction burn angle to be around 45degCA for a chamber of this size/design)
For you, moving to computer controlled ignition, you can have any ignition angle (within reason, set by the distributor geometry if you use one) you like at any speed/load, and so don't need to use the second set of plugs necessarily.
The slower burn rate from a single set of plugs could be an issue with the higher performance Merlin, which being supercharged is heavily detonation limited. Running decent modern high octain fuel (95RON) and "mapped" ignition means you, with your much lower dynamic compression ratio Meteor, can precisely control your ignition angle to develop Pmax in the correct location wrt TDC, virtually no matter what the actual burn rate is.
Years ago Channel 4's Salvage Squad had a go at a Centurion Tank with Meteor engine. One of the issues with it was the ignition system. Because the tank was to be driven in displays and needed reliability it was fitted with a modified Jaguar V12 electronic ignition dizzy etc.
This is based on a memory of a TV program 12 years ago so details may be missing, and there may be a bit of TV bullpoo as wellbut it may be worth hunting out the DVD.
Just found it on YouTube http://youtu.be/0gIHaCu_1fs?t=8m30s
This is based on a memory of a TV program 12 years ago so details may be missing, and there may be a bit of TV bullpoo as well
Just found it on YouTube http://youtu.be/0gIHaCu_1fs?t=8m30s
Max_Torque said:
I'm getting "my information" from over 20 years designing, developing and calibrating engines, from F1 ones, to aero ones! ;-)
RR couldn't have "deleted" the second plugs back in the day because they relied on the "spark stagger" to start the engine. (ie the engine needs to have retarded ignition to start cleanly, and the second set of plugs is retarded 25deg, so by starting on just these, they get there ignition retard for starting. The very fact these second plugs run that much retard suggests to me they are there for pretty much starting and as an emergency "fall back", in most cases, by the time they fire, the vast majority of the charge will have already burnt i suspect. (i'd expect the 10-90 mass fraction burn angle to be around 45degCA for a chamber of this size/design)
For you, moving to computer controlled ignition, you can have any ignition angle (within reason, set by the distributor geometry if you use one) you like at any speed/load, and so don't need to use the second set of plugs necessarily.
The slower burn rate from a single set of plugs could be an issue with the higher performance Merlin, which being supercharged is heavily detonation limited. Running decent modern high octain fuel (95RON) and "mapped" ignition means you, with your much lower dynamic compression ratio Meteor, can precisely control your ignition angle to develop Pmax in the correct location wrt TDC, virtually no matter what the actual burn rate is.
from the stuff I found max , I thought the big retard happened only on starting ,then the lagging row of plugs come back to a few degrees retarded .have I misunderstood ?RR couldn't have "deleted" the second plugs back in the day because they relied on the "spark stagger" to start the engine. (ie the engine needs to have retarded ignition to start cleanly, and the second set of plugs is retarded 25deg, so by starting on just these, they get there ignition retard for starting. The very fact these second plugs run that much retard suggests to me they are there for pretty much starting and as an emergency "fall back", in most cases, by the time they fire, the vast majority of the charge will have already burnt i suspect. (i'd expect the 10-90 mass fraction burn angle to be around 45degCA for a chamber of this size/design)
For you, moving to computer controlled ignition, you can have any ignition angle (within reason, set by the distributor geometry if you use one) you like at any speed/load, and so don't need to use the second set of plugs necessarily.
The slower burn rate from a single set of plugs could be an issue with the higher performance Merlin, which being supercharged is heavily detonation limited. Running decent modern high octain fuel (95RON) and "mapped" ignition means you, with your much lower dynamic compression ratio Meteor, can precisely control your ignition angle to develop Pmax in the correct location wrt TDC, virtually no matter what the actual burn rate is.
personally ,with such a big chamber and no squish , I would run the two plugs . it may well fire ok with the single plug option ,but the two plugs would give greater stability and consistency of burn , making it easier to set up ,and more predictable behaviour .
particularly in conditions where the burn speed changes ,eg colder weather ,varying petrol characteristics etc. I know air temp sensor can compensate ,and that it can all be adjusted out , but that all takes time and money ,the simpler it is to set up ,the better .
I don't know enough about how the system is set up to know if the running stagger is different from the starting stagger, it may well be (35degCA is a LOT of stagger!)
Of course, it's easy to see what the second set of plugs do, just run up the std engine and disabled one set by earthing the mag and see if the engine speed falls! (slow burn = less torque = lower engine speed)
Of course, it's easy to see what the second set of plugs do, just run up the std engine and disabled one set by earthing the mag and see if the engine speed falls! (slow burn = less torque = lower engine speed)
I meant you turn the secondary "retarded" plugs on and off, not the primary set (like you would for a Mag check in aircraft as mentioned) The difference of course is that the burn is still initiated by the primary set, at the normal crank angle, but if the secondary set are doing something, and increasing the burn rate, then you might get an rpm drop.
To do a mag check in an aircraft you test both sides. For the Shackleton (I've not been let loose with a Merlin yet...) set the engine to 2200rpm, drop one switch to off, the revs drop. Measure the drop, switch it back on, repeat process with the other switch. Briefly flick both off if one side showed no drop, to check for a live mag. There's negligible difference in using either magneto - if you switch either of them off there is a noticable - and measureable - drop in RPM.
Neither is listed as primary or secondary, and if you have a dual switch they are sometimes labelled 1 and 2. It varies from aircraft which is which, the only way to tell is to check the wiring diagram or the relevant AP manual.
Having said this - I am told by some of my REME friends that the magneto set up on the Meteor could be different to Merlin as the Army didn't want a tank engine to be quite as sensitive!
Neither is listed as primary or secondary, and if you have a dual switch they are sometimes labelled 1 and 2. It varies from aircraft which is which, the only way to tell is to check the wiring diagram or the relevant AP manual.
Having said this - I am told by some of my REME friends that the magneto set up on the Meteor could be different to Merlin as the Army didn't want a tank engine to be quite as sensitive!
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