Bristol Jupiter engine

That’s what I was thinking; something to do with the temperature / density of the charge and the burn-rate. What plays against that is how much ‘swirl’ there would be in the charge especially considering how advanced the spark is timed; 45 BTDC seems quite advanced to me (but I suppose it isn’t really).

As you say this timing may just have been arrived at by experimentation or it may be a method of ensuring consistency; if the two sparks were synchronised any slight variation could mean inlet or exhaust sparking first. At least with a five degree difference the exhaust plug is always going to spark first?

Morning gentleman,

Thinking a little more about the timing on the Lion engine and others, as one magneto supplies the spark for the inlet side and the other supplies the exhaust side of the cylinders, maybe there is a slight difference in the charge density from on side to the other and this changes the burn time to end up with a central expansion ?

This is a section of an early Lion engine.

Andy

Burning question

On full advance the Lion Va fire’s at 40 BTDC on the exhaust side magneto and 45 BTDC on the inlet side magneto, the Magneto’s are BTH AV12’s. Andy

CD, having being alerted to this assymetrical burn, a few other engines seem to have this too, after pouring myself a sherry and retiring to the library. I guess that a more complete burn was required on the side of the charge evacuated via the exhaust port first. All these aeroplane engine designs had ‘crossflow heads’ in 1925 before this concept was introduced to me via gutting old Datsuns in 1986. One of those tweaks for taking the most out of the fuel charge, if you had to have a redundant magneto there for safety anyway. This could be set by the vernier coupling of the magneto drive, so you could try all sorts of combinations, which no doubt the boffins did, and probably found an extra 5 HP on a dyno with these ignition settings.

I can’t understand why one side would fire earlier than another…

Two spark-plugs per cylinder; one in each cylinder firing from one magneto, and one from the other magneto…

…but (thinking about it) I’m sure you understood that part didn’t you! 🙂

Maybe the temperature of the charge near the exhaust valves is higher and requires less advance than on the inlet side of the cylinder?

How mad is that ! Looks like Gilman Bearings ! Just like RR Kestrel. If the gunmetal bearing was worn the Mickey Mouse shaped steel backed bearing would be thrown away and articulating main and two slaves reattached to new Mickey. I am sure it has a more respectable name like “shouldered bearing shell”. It is simply fantastic to see this photo, better if drunk ! What a great engine to play with !:love-struck:

Afternoon P&P,

The Lion is cooled using a radiator in the front of the cart and still uses the engines water pump, the two magneto timings is something I don’t really know, I can only imagine it somehow gives a more central gas burn and expansion, if there is a suitably educated person out there with the answer please post.

This should balance your sugar/cereal ratio,

Andy

Mornin P&P,

On full advance the Lion Va fire’s at 40 BTDC on the exhaust side magneto and 45 BTDC on the inlet side magneto, the Magneto’s are BTH AV12’s.

https://www.youtube.com/watch?v=-FyHOXJp7YQ&index=24&list=PLDRCGAMcnKEgf4AV_Cm0lEC4vXsQEoz8f

There is plenty of starter magneto spark capability as on a couple of occasion’s I have started the engine with the main mag’s switched off and as long as I carry on turning the starting handle shaft to drive the starter mag the engine runs well.

Andy

Thank you Andy.
I can imagine you getting to 10 cranks and starting to wonder about the maggie switch ! Of course, AV 12s ! I do like the removable hand crank and the compact cart. Is there some form of radiator/cooling under the skirts of the cart ?
Because I haven’t had enough sugar in my cereal I can’t understand why one side would fire earlier than another but then my mind has yet to assimilate broad arrow engine configuration. Is this something to do with broad arrow ? Do you have a main conrod with two slave conrods outside the main on the same crankshaft journal ?

Mornin P&P,

On full advance the Lion Va fire’s at 40 BTDC on the exhaust side magneto and 45 BTDC on the inlet side magneto, the Magneto’s are BTH AV12’s.

https://www.youtube.com/watch?v=-FyHOXJp7YQ&index=24&list=PLDRCGAMcnKEgf4AV_Cm0lEC4vXsQEoz8f

There is plenty of starter magneto spark capability as on a couple of occasion’s I have started the engine with the main mag’s switched off and as long as I carry on turning the starting handle shaft to drive the starter mag the engine runs well.

Andy

Wow

Evenin P&P and all,

starting these old aero engines by hand is not difficult, as seen here,

https://www.youtube.com/watch?v=Ihb8_STa3GE&list=PLDRCGAMcnKEgf4AV_Cm0lEC4vXsQEoz8f&index=1

The Lion starting handle takes 12 revs to 1 crankshaft and the engine usually starts after about 8 or 9 rotations, I don’t remember the sprocket ratio I used but it must be about 4 to 1, it seems the starter mag must be turning fast enough at that.

Andy

How good is that ! Thank you for sharing the starting info and video. Seems like these ratios were the ‘accepted wisdom.’ Looks like BTH SC12 magnetos so the trailing contact for the starter magneto on the rotor, 30 odd degrees, would be the same. When does the Lion fire BTDC ?

Now you have to build a Supermarine Schneider Trophy Racer…..

Evenin P&P and all,

starting these old aero engines by hand is not difficult, as seen here,

https://www.youtube.com/watch?v=Ihb8_STa3GE&list=PLDRCGAMcnKEgf4AV_Cm0lEC4vXsQEoz8f&index=1

The Lion starting handle takes 12 revs to 1 crankshaft and the engine usually starts after about 8 or 9 rotations, I don’t remember the sprocket ratio I used but it must be about 4 to 1, it seems the starter mag must be turning fast enough at that.

Andy

More spark

One last thing is the application of the hand starting spark in the cylinder. Both the Jupiter and the Mercury fired at 29 degrees BTDC and the Kestrel at 40 degrees BTDC. The hand starting magneto spark traveled to the main magneto and was distributed via a trailing contact on the rotor to all the cylinders. When you take off the distributor cap from a 12 cylinder

[ATTACH=CONFIG]235990[/ATTACH]

[ATTACH=CONFIG]235991[/ATTACH]

or 9 cylinder BTH magneto

[ATTACH=CONFIG]235992[/ATTACH]

[ATTACH=CONFIG]235993[/ATTACH]

you can see the trailing contact at 30 odd degrees behind the main contact in the direction of travel. So as the cold engine was hand cranked into life, the first standard spark would try and suggest the fuel to ignite, then 30 degrees later the biggest, continent splitting stream of sparks would explode into the cylinder, smashing the fuel out of existence. I should make a movie, “ The Brief and Savage Life of the Hand Starting Magneto Spark.”

Of course once the ground crew or pilot stopped hand cranking the hand starter magneto it would fizz into silence, brooding for its next explosion.

Put some spark in your life!

If you take engine magneto drive at 9/8 crankshaft speed or 1.125 crankshaft speed it all falls into place, with a serendipitous 9 sparks coming from the four lobed cam driving the breaker points ! See the whiteboard which lays out the full cycles below. Finding this exquisite organization of affairs always reinforces the fact that the old timers always went for absolute perfection with their engineering, there was never any fudging. Understanding this relationship, it is possible to understand how the 10 cylinder Armstrong Siddely Serval would have been arranged out of the same magneto body, if it had a four lobe cam driving the points. Say 10 sparks are needed per 2 revolutions of the crankshaft (full cycle) and one revolution of the rotor (full cycle). Therefore 10 sparks/4 lobes equals 2.5 revolutions of the engine to magneto drive gear during two revolutions (full cycle) of the crankshaft, or 1.25 revolutions per single revolution of the crankshaft, therefore a 1:1.25 gear running off the crankshaft. :applause:

Then the rotor, running off the 2.5 revolutions of the engine to magneto drive gear, would need to be slowed down to 1 revolution of rotor, so we gear down 2.5:1. Say we use the same rotor cog as the v12 at 75 teeth, we would need a 30 tooth drive.:applause:

I could not let this go without trying to figure out how the hand starting magneto worked within this system ! The default starting method seemed to be hand cranking in the 30’s. RAAF Bulldogs and Demons had Hucks starter clutches on the prop but I have never seen photographic evidence of Huck’s starters in Australian use. There is a lot of photographic evidence of hand cranking.

I was never sure where the hand starting magneto was on Australian Demons. No Demon or Hart family aircraft have hand starting magnetos in the cockpit. APs refer to a chain driven hand starting magneto in the engine bay, but photographic evidence of this eluded me until now. This photo of RAAF Demon A1-4 from the RAAF, via the South Australian Aviation Museum, finally showed its position of the starboard side clearly.

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Frustratingly, I have found no technical details on the chain gearing of this arrangement, until ebay offered this BTH AS magento with sprocket.

[ATTACH=CONFIG]235988[/ATTACH]

It has twelve teeth, and by reference to the photo of A1-4, 48 teeth are counted on the drive socket linked to the hand cranking shaft. So the starting magneto was driven 4:1 from the engine hand cranking shaft. BTH seemed to be ubiquitous in service aircraft hand starting magnetos. Not only the RAAF Demon but the Bristol Bulldog featured them. In the Bulldog the hand starting magneto was fixed in the cockpit panel, with a handle protruding for the pilot to spin as the engine was started.

[ATTACH=CONFIG]235989[/ATTACH]

The BTH magneto manual of the day for hand starter magnetos describes “ the average speed the handle can be turned is 80 RPM”. This is reasonable, as I am sure you can open a window winder at 80 RPM too ! The hand starting spindle is geared to the armature 5: 1, so the armature, on this basis, is doing 400 RPM. In simple terms, it is sending out a constant stream of sparks. Many of these hand starting magnetos were preserved on the benches of gas welders to fire up oxy acetylene torches, while I am sure a few sophisticates may have used them to light cigars.
Back to the ground crew hand cranking the engine, and starter magneto, to life. How fast could they do this ? Delightfully, the following preserved films give a fair indication.
https://www.youtube.com/watch?v=wIuQyKhSys4

In the first, at 1:15 secs elapsed, a Hawker Hart Kestrel engine is brought to life in 6 turns in 3 secs, two turns per second, or 120 RPM for a briefly sustainable human effort.
If the hand crank runs at 120 RPM then the 4:1 chain gearing runs the hand starting magneto spindle at 480 RPM, more than adequate according to the manufacturers recommendations. The actual armature, at 5:1, will spin at the dizzying speed of 2,320 RPM, the same speed as a standard industrial electrical motor. So it would be sending out a plasma stream of electrical sparks which would remove all doubt about ignition. It amazes me how quickly the engine starts in the footage, and no doubt trial and error found out that such a cascade of sparking was essential for prompt and reliable starting, particularly in the worst case backstop of prop spinning.

Delightfully again, the following film of SAAF Gladiators with Bristol Mercury engines, at 1:20 secs elapsed, shows a similar ratio of 5 turns over 2-3 seconds, akin to 120 RPM.
https://www.youtube.com/watch?v=oG8wQicz6tc

I do not know whether, like the Bulldog, the hand starter magneto was hand cranked in the cockpit, or enjoyed a chain drive arrangement like the Hart family aircraft.
By reference to another earlier post, the Kestrel hand cranking ratio was 13 : 1 of the crankshaft. On the whiteboard the following information could be added to precede the 2 crankshaft revolutions of one full Otto cycle for Kestrel V12 :

[ATTACH=CONFIG]228269[/ATTACH]

Starting Handle – 26 revolutions
Hand starting Magneto at 4:1 – 104 revolutions
Hand staring magneto armature at 5: 1 – 520 revolutions
Crankshaft – 2 revolutions.

Considering that the ground crew started the Hart Kestrel with 6 cranks, the engine crankshaft would have only done 6/26 or 0.23 of a revolution. By referring back to the whiteboard, it can be seen that the distributor rotor would have traversed only 12 sparks X 0.23 = 2.76 sparks/distributor points/cylinders out of the 12, so the engine started from the firing of only 2 cylinders out of 12. For a brief 2-3 seconds of human effort, these ratios resulted in the engine crankshaft turning a fifth of a revolution, while the armature of the hand starting magneto spewed a plasma stream of over a thousand high tension sparks.

Sometimes you see today’s ground runners linked to electric starters that turn and turn and turn and you wonder if the old ground fitters would be turning the same way in their graves ! Getting a V12 to fire on 2 cylinders would mean a nice, tight, well loved and understood engine. It is interesting when you read about the sand of the Middle East campaigns getting into the engines and they started to loose compression, becoming “harder to start”. Certainly if you were hand cranking !

Fedden himself was Bristol ‘Old Family’ certainly if he curbed his relentless energy he could have “fitted in” more easily, but he made it hard. He is one man who refused Churchill’s request to coordinate aircraft production during the war, citing his work at Bristols as being more pressing. So he certainly made it easier for his enemies to push him aside.

Here, for the sake of a contemporary and influential take on the sacking of Sir Roy Fedden, is the editorial from Aircraft Engineering, December 1942 :

[ATTACH=CONFIG]235540[/ATTACH]

Not really familiar with the Bristol Draco; so it is fuel-injection, rather than direct fuel-injection?

What is the silver fitting between the valve-gear compensating rods; is that part of the standard gas starter system?

Gas starting

Are you sure you are not confusing parts of the Jupiter ‘starting system’ for direct fuel injection?

The Jupiter engine carried some fairly complex and exotic engine starting systems, particularly in civilian service where staffing levels were more critical, in the days before efficient batteries and electric starters. One system involved supplying a compressed air and fuel mixture directly into the cylinders although I forget the exact details; direct fuel injection, in British aircraft engines anyway, wouldn’t be seriously considered for decades after the Jupiter engine?

All Jupiters had two sparkplug openings and one opening in the barrel for the standard gas starting system. Compared to using a Hucks starter, hand cranking or swinging a prop gas starting was always a step above. In the Bristol Draco of 1935 the fuel was injected into the bifurcated induction elbow outside the barrel and it was a direct fuel injection experiment more commonly associated with WW2 German engines. The Bristol Phoenix of 1931 was also a 14:1 compression ratio diesel version of the Jupiter with fuel injection direct into the cylinder. They really tried a lot of things and pushed the envelope on concepts, driven by Fedden’s relentless drive to develop ideas. The Draco is a strange looking version, carrying both injection pumps typical of diesels and magnetos.

Creaking Door, exotic starting systems indeed , that will go some way to explain, that on the Wapiti, there are dope **** and dope control **** levers on the instrument panel.
I wonder where the dope priming tank was located?
Cheers Mike

…an experiment in direct fuel injection.

Are you sure you are not confusing parts of the Jupiter ‘starting system’ for direct fuel injection?

The Jupiter engine carried some fairly complex and exotic engine starting systems, particularly in civilian service where staffing levels were more critical, in the days before efficient batteries and electric starters. One system involved supplying a compressed air and fuel mixture directly into the cylinders although I forget the exact details; direct fuel injection, in British aircraft engines anyway, wouldn’t be seriously considered for decades after the Jupiter engine?

Ollie, thank you for matching the part number and the compression ration to Jupiter IX. Still, when I look in the IX manual, the finning is the ‘old type’. The only way this particular cylinder would make sense is as a test cylinder on what ultimately became the Pegasus development program. Certainly the histories show that Bristol Engines ran many trials on single cylinder test rigs.

Based on a Pegasus type cylinder, compression ration of 5.3:1, bifurcated inlet and nothing else making sense I now think this is a test cylinder or part of a Bristol Draco engine, an experiment in direct fuel injection.

Hi powerandpassion

Im pretty sure your cylinder comes from a Jupiter IX. The part number is within the right range and the key is the compression ratio (5.3-1) stamped further along from the FB part number. The Jupiter Mk.IX had a 5.3 – 1 compression ratio compare that to the 5 – 1 ratio of the Mk.XI or the 5.8 – 1 ratio of the Mk.VIII. With the finer cooling fins it fits in neatly with your believe that it is from a late Mk. Jupiter.

Ollie

Ollie, thank you for matching the part number and the compression ration to Jupiter IX. Still, when I look in the IX manual, the finning is the ‘old type’. The only way this particular cylinder would make sense is as a test cylinder on what ultimately became the Pegasus development program. Certainly the histories show that Bristol Engines ran many trials on single cylinder test rigs.

Balance and the effort to balance

p&p#29: managing “creatives”. The British way (=class system) segregated “hands” totally: Fedden was not the only innovator to be denied Board status: WEW Petter, Barnes Wallis, AE Hagg (by Sir Geo. DH), J.Lloyd (by JD Siddeley), Whittle and O.Short (Nationalised, as “too difficult”)…many auto designers. S.Camm and G.Edwards were made to wait. Board issues were on a higher plane.

Today in such as media/advertising, the suits and the creatives only thrive if they find a way to bring ideas down to earth…unlike, say this Wallis quote: ‘“practical shop element of personnel (was) bitterly opposed to (my) geodetics” S.Ritchie,Industry & Air Power, P83, who adds: ‘(He) was persuaded to accept design changes in the interests of cheapening manufacture’. Quite. See Napier for the consequences of untrammelled creatives.

alertken, when the balance is found wonders can happen. I figure an ultimate creative was Churchill, often rejected in usual times, but supreme in unusual times. How much he internalised the tension between creativity and discipline or had the wisdom to surround himself with strong personalities to balance his own energy may never be clearly delineated. Alanbrooke, CIGS of British High Command certainly fought him ferociously, in goodwill, to align the war effort to what was possible.

Fedden himself was Bristol ‘Old Family’ certainly if he curbed his relentless energy he could have “fitted in” more easily, but he made it hard. He is one man who refused Churchill’s request to coordinate aircraft production during the war, citing his work at Bristols as being more pressing. So he certainly made it easier for his enemies to push him aside.

Nevil Shute in “Slide Rule” reflects that it was the British aristocracy, prior to estate taxes, that combined the surplus capital and eccentricity to fund highly risky ventures in aviation, and to do this on a grand scale. Bristols certainly took great risks in entering into aviation. Perhaps the closest analogy today is the money being spent by Virgin on spaceflight. If only Fedden could have bit his lip and if only the Bristols board could have been more patient. What has happened, has happened. Still, after all this time, stripped of high emotion, Fedden was an incredible inventor and the Jupiter a remarkable engine.