Alternate Bristol Engine Development (Mercury & Pegasus)

[loftonhenderson]

I'm considering Kaiserreich-esque Alt Hist timeline where Canada + a bunch of British exile engineers have to build aero engines -- for speed of development, reliability, and simplicity they just develop the Bristol Mercury/Pegasus, avoid sleeve valves and stick with poppet valves, and also go back to 2-valves per cylinder. I'm mainly interested in what those hypothetical radial engines look like (as specifically as possible) assuming further development of the Mercury and Pegasus past their OTL capabilities.

I've scoured the web for AH engine development threads and what seems to keep coming up in posts by people with far more expertise than I'll ever have is:

• The 9-cylinder Mercury tops out at a bit less than 1000 HP
• The 9-cylinder Pegasus tops out around 1100 HP
• "Twinning" those engines offers a lot of additional potential for development -- most seem to think a 14-cylinder Mercury and 18-cylinder "Twin Pegasus" hold the most promise
• There are some other ideas about an 18-cylinder Twin Mercury, a 10-cylinder Mercury, and an over-bored 9-cylinder Pegasus (asterisk because they weren't the prevailing ideas)

I'm looking for as many specifics as possible from all the engine experts and retired engineers hanging around here. All the specs I've posted below are other peoples' estimates. They mainly seem to agree with each other thankfully so I expect they're on the right track.
Beyond all the unknown unknowns, I could really use some help filling in the "?" specs listed below (hoping for reasonable displacement, HP, diameter, and bore x stroke estimates for each). Additionally:

• Would a Pegasus or Twin Pegasus be too big for most fighters, especially '30s fighters?
• What is the benefit of a Twin Pegasus over a Twin Mercury (or vice versa)?
• How would 2-valves per cylinder instead of 4 affect things? Would that change specs like diameter...etc.? Would sticking to 4vpc actually be better?
• What are the benefits of a reduced stroke Mercury or over-bored Pegasus? How does that work in practice?

ALT Bristol Mercury
Mercury (OTL 9-cylinder redesigned with 2-valves per cylinder)
24.9L
900+ HP (995 HP on 100 oct fuel)
51.5" diameter
5.75" x 6.5" bore and stroke

Mercury (14-cylinder)
38.7L or 38.9L
1400-1600 HP
51.5" diameter
? bore and stroke
Alternative to Hercules

*Twin Mercury (18-cylinder)
50L or 49.2L
~1800-2100 HP (1800+ HP on 87 oct, 1900 HP on early 100 oct fuel, 2000+ HP should be available in low S/C gear)
? diameter (since it's twinned, diameter would be the same as a single Mercury right?)
? bore and stroke

*Mercury (14-cylinder with stroke reduced to 5" or 5.5")
(with a 5.5" stroke the cylinder dimensions and displacement are identical to the R-2000 Twin Wasp)
29.5-32.8L
? HP
? diameter
? x 5" or 5.5" bore and stroke
Alternative to Taurus but lighter and with slightly more power

*Mercury (10-cylinder)
???

ALT Bristol Pegasus
Pegasus (OTL 9 cylinder redesigned with 2-valves per cylinder)
28.7L
1100 HP max
55.3" diameter
5.75" x 7.5" bore and stroke

Twin Pegasus (18-cylinder)
2000-2200+ HP (other estimate are ">2000 HP" and "1,800-2,000 HP with more in later versions")
? diameter (since it's twinned, diameter would be the same as a single Pegasus right?)
? bore and stroke
Alternative to Centaurus
flying by 1939 or earlier

*Pegasus (9-cylinder over-bored by 1/10")
? liters
1000-1100 HP on 87 oct, 1200-1300 HP on early 100 oct fuel
? diameter (expect slight increase in diameter over normal-bore Pegasus)
? bore and stroke (bore would be 1/10" increased presumably?)


Cheers all!
Lofton

 

[Deleted member 68059]

I'm considering Kaiserreich-esque Alt Hist timeline where Canada + a bunch of British exile engineers have to build aero engines -- for speed of development, reliability, and simplicity they just develop the Bristol Mercury/Pegasus, avoid sleeve valves and stick with poppet valves, and also go back to 2-valves per cylinder. I'm mainly interested in what those hypothetical radial engines look like (as specifically as possible) assuming further development of the Mercury and Pegasus past their OTL capabilities.

I've scoured the web for AH engine development threads and what seems to keep coming up in posts by people with far more expertise than I'll ever have is:

• The 9-cylinder Mercury tops out at a bit less than 1000 HP
• The 9-cylinder Pegasus tops out around 1100 HP
• "Twinning" those engines offers a lot of additional potential for development -- most seem to think a 14-cylinder Mercury and 18-cylinder "Twin Pegasus" hold the most promise
• There are some other ideas about an 18-cylinder Twin Mercury, a 10-cylinder Mercury, and an over-bored 9-cylinder Pegasus (asterisk because they weren't the prevailing ideas)

I'm looking for as many specifics as possible from all the engine experts and retired engineers hanging around here. All the specs I've posted below are other peoples' estimates. They mainly seem to agree with each other thankfully so I expect they're on the right track.
Beyond all the unknown unknowns, I could really use some help filling in the "?" specs listed below (hoping for reasonable displacement, HP, diameter, and bore x stroke estimates for each). Additionally:

• Would a Pegasus or Twin Pegasus be too big for most fighters, especially '30s fighters?
• What is the benefit of a Twin Pegasus over a Twin Mercury (or vice versa)?
• How would 2-valves per cylinder instead of 4 affect things? Would that change specs like diameter...etc.? Would sticking to 4vpc actually be better?
• What are the benefits of a reduced stroke Mercury or over-bored Pegasus? How does that work in practice?

ALT Bristol Mercury
Mercury (OTL 9-cylinder redesigned with 2-valves per cylinder)
24.9L
900+ HP (995 HP on 100 oct fuel)
51.5" diameter
5.75" x 6.5" bore and stroke

Mercury (14-cylinder)
38.7L or 38.9L
1400-1600 HP
51.5" diameter
? bore and stroke
Alternative to Hercules

*Twin Mercury (18-cylinder)
50L or 49.2L
~1800-2100 HP (1800+ HP on 87 oct, 1900 HP on early 100 oct fuel, 2000+ HP should be available in low S/C gear)
? diameter (since it's twinned, diameter would be the same as a single Mercury right?)
? bore and stroke

*Mercury (14-cylinder with stroke reduced to 5" or 5.5")
(with a 5.5" stroke the cylinder dimensions and displacement are identical to the R-2000 Twin Wasp)
29.5-32.8L
? HP
? diameter
? x 5" or 5.5" bore and stroke
Alternative to Taurus but lighter and with slightly more power

*Mercury (10-cylinder)
???

ALT Bristol Pegasus
Pegasus (OTL 9 cylinder redesigned with 2-valves per cylinder)
28.7L
1100 HP max
55.3" diameter
5.75" x 7.5" bore and stroke

Twin Pegasus (18-cylinder)
2000-2200+ HP (other estimate are ">2000 HP" and "1,800-2,000 HP with more in later versions")
? diameter (since it's twinned, diameter would be the same as a single Pegasus right?)
? bore and stroke
Alternative to Centaurus
flying by 1939 or earlier

*Pegasus (9-cylinder over-bored by 1/10")
? liters
1000-1100 HP on 87 oct, 1200-1300 HP on early 100 oct fuel
? diameter (expect slight increase in diameter over normal-bore Pegasus)
? bore and stroke (bore would be 1/10" increased presumably?)


Cheers all!
Lofton

Well firstly you`d never revert to 2-valves per cylinder...why make such a stipulation ?

 

[loftonhenderson]

Well firstly you`d never revert to 2-valves per cylinder...why make such a stipulation ?

The consensus in a lot of threads was that 4vpc was unnecessarily complicated and didn't give a lot of benefit. And other (foreign) Bristol Titan (or Mercury) derivatives reverted to 2vpc successfully apparently, which made twinning easier from what I understand.

I made that stipulation since apparently 2vpc is cheaper/simpler and facilitated better development. It's not critical though, I was just going off what I've read elsewhere.

 

[MiTasol]

Exactly - that reduces the airflow through the engine and therefore the horsepower as well

 

[Bretoal2]

Two valves in Jupiter's childs, your men are building Gnome-Rhône 9K and 14 K engines... and their descendants...

 

[loftonhenderson]

Two valves in Jupiter's childs, your men are building Gnome-Rhône 9K and 14 K engines... and their descendants...

Well, yes sort of. Or an Alfa 135/136. But also not quite.

Mainly I'm looking for spec estimates, e.g. anyone have a guess what the bore and stroke of a 14-cylinder Mercury would be or the displacement of a twin Pegasus?

 

[Deleted member 68059]

In my view a better alternative history is:

A) what happens if Bristol redo the failed Hydra to have a middle crank bearing instead of giving up and going to sleeves,

B) ...and then stick 4 valve heads on it (it already had double overhead cams, which Alfa Romeo and Japanese users of Bristol engines showed could be done). You then basically have a very early, reliable Centaurus-like engine without any of the sleeve valve issues and shave five years of development work off the Bristol timeline which went down the drain on sleeve metallurgy.

See the Barnwell lecture by Fedden & pg 25 of some book or other.

 

[loftonhenderson]

In my view a better alternative history is:

A) what happens if Bristol redo the failed Hydra to have a middle crank bearing instead of giving up and going to sleeves,

B) ...and then stick 4 valve heads on it (it already had double overhead cams, which Alfa Romeo and Japanese users of Bristol engines showed could be done). You then basically have a very early, reliable Centaurus-like engine without any of the sleeve valve issues and shave five years of development work off the Bristol timeline which went down the drain on sleeve metallurgy.

See the Barnwell lecture by Fedden & pg 25 of some book or other.

View attachment 736400

Very interesting! Never even heard of the Hydra
25.7 L
850 HP
46.5" diameter
5" x 5" bore and stroke

If development had continued what sort of HP do you reckon it could've gotten up to?

 

[Bretoal2]

The lethal vibration problem encountered with the Hydra was maybe in relation with the lack of center bearing (same errors for Armstrong-Siddeley Jaguar and G&R 14K / 14N) , but I think it was mainly caused by the very strange "octagon" disposition inducing a complex firing order : 1F, 2F, 7R, 4F, 1R, 6F, 3R, 8F, 5R, 6R, 3F, 8R, 5F, 2R, 7F, and 4R (!!!).

Maybe the solution was a 14 cyl. Hydra, with center bearing and "traditional" firing order..... But maybe with some cooling issues owing to the non-staggered cylinders barrels and heads, as in the A/S Deerhound.

 

[MiTasol]

In my view a better alternative history is:

A) what happens if Bristol redo the failed Hydra to have a middle crank bearing instead of giving up and going to sleeves,

B) ...and then stick 4 valve heads on it (it already had double overhead cams, which Alfa Romeo and Japanese users of Bristol engines showed could be done). You then basically have a very early, reliable Centaurus-like engine without any of the sleeve valve issues and shave five years of development work off the Bristol timeline which went down the drain on sleeve metallurgy.

See the Barnwell lecture by Fedden & pg 25 of some book or other.

View attachment 736400

Those Brian Lumsden photos are very interesting but the article at Bristol Hydra 16-Cylinder Radial Aircraft Engine is more informative

 

[MiTasol]

While looking at something different I found that Roy Fedden designed a 14 cylinder two row radial in WWI prior to Cosmos being absorbed by Bristol.

Cosmos Mercury - Wikipedia

en.wikipedia.org

 

[Bretoal2]

While looking at something different I found that Roy Fedden designed a 14 cylinder two row radial in WWI prior to Cosmos being absorbed by Bristol.

Cosmos Mercury - Wikipedia

en.wikipedia.org

Yes. This strange engine had no master rod, but side-by-side connecting rods enducing a "spiral" cylinders arrengement.

 

[Bretoal2]

A very bad pic from an original blueprint showing Cosmos Mercury 's 7 identical connecting rods side by side (2 x).

Note the lack of center bearing ! In "the life of sir Roy Fedden", Bill Gunston says the Mercury rods had roller bearings. So we can suppose that cranshaft is not solid, but build-up... but this pics is much too bad to see really how all this work.

 

[Jugman]

The lethal vibration problem encountered with the Hydra was maybe in relation with the lack of center bearing (same errors for Armstrong-Siddeley Jaguar and G&R 14K / 14N) , but I think it was mainly caused by the very strange "octagon" disposition inducing a complex firing order : 1F, 2F, 7R, 4F, 1R, 6F, 3R, 8F, 5R, 6R, 3F, 8R, 5F, 2R, 7F, and 4R (!!!).

Maybe the solution was a 14 cyl. Hydra, with center bearing and "traditional" firing order..... But maybe with some cooling issues owing to the non-staggered cylinders barrels and heads, as in the A/S Deerhound.

The thing is a non-staggered odd numbered two-row radial has only 180° of crank phasing. Not the 360° needed for even firing. Basically it's like having a bunch of two cylinder inline engines coupled together. You could compensate for this by using an offset crank but that introduces its own problems. Even numbered inline two-row engines can work like several opposed cylinder engines coupled together. Two cylinders in each row will fire consecutively but they can have proper spacing.

 

[Bretoal2]

The thing is a non-staggered odd numbered two-row radial has only 180° of crank phasing. Not the 360° needed for even firing. Basically it's like having a bunch of two cylinder inline engines coupled together. You could compensate for this by using an offset crank but that introduces its own problems. Even numbered inline two-row engines can work like several opposed cylinder engines coupled together. Two cylinders in each row will fire consecutively but they can have proper spacing.

Yessss!

A "traditional firing order" for such an engine only works with a 360° crankshaft.

The French Salmson 18Ab, with 9 x 2 paired cylinders, had this configuration and worked very well - but gave barely more power than the Jupiter and was much more complex and heavy.

 

[MiTasol]

A very bad pic from an original blueprint showing Cosmos Mercury 's 7 identical connecting rods side by side (2 x).

Note the lack of center bearing ! In "the life of sir Roy Fedden", Bill Gunston says the Mercury rods had roller bearings. So we can suppose that cranshaft is not solid, but build-up... but this pics is much too bad to see really how all this work.

Thank you for that blueprint graphic. As you say pretty bad quality but it does still show a lot of detail. Any idea if that original still exists and where?

The Salmson 18Ab graphic you posted is excellent.

The Americans have digitized many of their old patents and put them on line. Maybe I am using the wrong search terms but I cannot find a British equivalent collection at Search for a patent though some files back to 2007 appear to be accessible.

This is the oldest expired Cosmos patent I can find

This is the oldest expired Fedden patent I can find. There are thousands of expired Bristol patents going back to 2007 but none are for the engine company.

From this page it appears digital records only go back to 2007

 

[Shortround6]

Two valves in Jupiter's childs, your men are building Gnome-Rhône 9K and 14 K engines... and their descendants...

This is a 1932 G-H9 K, the valve angle was shallow. The valve gear was not enclosed but I don't think anybody else was at that time. That would change by the mid 30s.

By the late 30s the US makers were using pretty much hemi heads with around 90 degree included angle Valve angles which allowed for somewhat larger valves, not as good as 4 valves but better than the shallow valve angles.
Bristol had also trapped themselves. With the 4 valve head you needed 2 ports in back of the head for the inlets. and they used two ports out the front of the head for the exhausts.

Bristol early on had used a single inlet pipe with a Y near the top but later switched to a pair of long inlet pipes for each cylinder.
The companies that used two valve heads could, eventually, use one inlet on one side of the head for the inlet and a rear facing exhaust port on the other side of the head.
It may not have been routing the push rods only to the rear cylinder that gave Bristol trouble with two row poppet valve radials. Trying to route 28-36 intake pipes up through the engine and keep them away from 14-18 exhaust pipes AND get the rear push rods through all the piping might very well have been "untidy". Only using two valves also left room for large ports and more room for cooling fins.
There were a lot of trade-offs.

Good valve arrangements were often let down by less than optimum ports or intake manifolds or exhaust systems.

 

[Bretoal2]

Thank you for that blueprint graphic. As you say pretty bad quality but it does still show a lot of detail. Any idea if that original still exists and where?

The Salmson 18Ab graphic you posted is excellent.

The Americans have digitized many of their old patents and put them on line. Maybe I am using the wrong search terms but I cannot find a British equivalent collection at Search for a patent though some files back to 2007 appear to be accessible.

This is the oldest expired Cosmos patent I can find
View attachment 736611

This is the oldest expired Fedden patent I can find. There are thousands of expired Bristol patents going back to 2007 but none are for the engine company.
View attachment 736612

From this page it appears digital records only go back to 2007
View attachment 736613

Hey Mitasol,

Did you try Espacenet worldwide patents database ?

Espacenet - Advanced search

worldwide.espacenet.com

Regards

 

[MiTasol]

I will now but I had never heard of it until you told me.

Many thanks

 

[Bretoal2]

View attachment 736614
This is a 1932 G-H9 K, the valve angle was shallow. The valve gear was not enclosed but I don't think anybody else was at that time. That would change by the mid 30s.

By the late 30s the US makers were using pretty much hemi heads with around 90 degree included angle Valve angles which allowed for somewhat larger valves, not as good as 4 valves but better than the shallow valve angles.
Bristol had also trapped themselves. With the 4 valve head you needed 2 ports in back of the head for the inlets. and they used two ports out the front of the head for the exhausts.
View attachment 736626
Bristol early on had used a single inlet pipe with a Y near the top but later switched to a pair of long inlet pipes for each cylinder.
The companies that used two valve heads could, eventually, use one inlet on one side of the head for the inlet and a rear facing exhaust port on the other side of the head.
It may not have been routing the push rods only to the rear cylinder that gave Bristol trouble with two row poppet valve radials. Trying to route 28-36 intake pipes up through the engine and keep them away from 14-18 exhaust pipes AND get the rear push rods through all the piping might very well have been "untidy". Only using two valves also left room for large ports and more room for cooling fins.
There were a lot of trade-offs.

Good valve arrangements were often let down by less than optimum ports or intake manifolds or exhaust systems.

Hi Shortround

From the beginning, the G&R 9K engines had their valve gear entirely under enclosed and airtight hoods. But at first there was no oil circulation, only grease.

I agree about the valves narrow angle, the G&R cylinder head is quite opposite to US practice. I think this distinctive feature shows that the Kinsei engine was inspired by the French 14K engine, which Mitsubishi had licensed - Cylinder heads, valve gear an crankshaft in early models are clearly "G&R" in this japanese engine.

Bristol (or Fedden) obviously had problems fitting all its pipes, intake and exhaust, under the cowling ! But on the other hand, its 4-valve rod and rocker arm system was exceptionally clean !

And on the Pegasus, the final development of the Jupiter, induction system is quite special, with 9 compressor outlets each connected to two neighboring cylinder heads. In total 18 intake manifolds and as many valves. Maybe (I don't really know...) this gave a more prononouced sweep / scavenging.