Bristol Radial Engine Development

[kool kitty89]

Would it have been better for Bristol to continue to develop their conventional poppet valve engines and forgo development of their sleeve valve designs.

Developing the Pegasus a bit further in its own right (having a bit more potential than the smaller ercury) as well as new 14 and/or 18 cylinder versions. (perhaps 14 and/or 18 cylinder versions of the Mercury as well)


One particular note is that Bristol used 4 valves per cylinder, something almost no one else had on radial engines.

 

[red admiral]

I feel they would have got their engines into service a lot quicker by avoiding the long development time with the sleeve valves. A 14 cylinder Mercury would replace the Hercules with more power available early one but limited later. An 18 cylinder Pegasus would be powerful (>2000hp) very reliable and fairly light to boot. You've got something a bit more powerful than the R-2800 with slighter larger dimensions but around 100kg lighter. Cooling might start to be a problem but the Bristol engines didn't seem to particularly suffer from this (apart from in the Albemarle at high power and low speed).

 

[kool kitty89]

Do you think it would be more practical to just go with a 14 and 18 cylinder Pegasus and drop the Mercury?

Or would the characteristics (namely lower weight and smaller diameter) of a 14 cylinder Mercury based design be better suited for fighter designs in the early war period?



Also the 18 cylinder Pegasus would have offered some of the early war designs wth the RR Vulture a viable alternative.

 

[kool kitty89]

I've been looking around a little more on info on bristal engines and thinking. One funny thing I hadn't noticed is that the bore and stroke (146x165mm) of the Perseus/Hercules cylinders is the same as the Mercury's as well as the earlier Bristol Titan. (coincidentally, the Titan was the basis for many of Gnome Rhone's engines, and Italian designs based on GN's, so they share Bore and Stroke figures as well, albeit all those designs used only 2 valves per cylinder)


And the earlier 14 cylinder Mercury + 18 cylinder Pegasus makes perfect sense as alternatives for the Hercules and Centaurus. (and both would likely be available much earlier than their historical counterparts)

There would still probably be some use for an engine in the size/weight and power class as the 9-cylinder pegasus (ie as an alternative the Taurus), so development could be continued for it as well. The 9-cylinder Mercury would not be developed any further (than historically) as it just doesn't have the same room for development as the Pegasus.




Another thing though, besides historical replacements for the Hercules and Centaurus, there are still 2 other configurations of these engines that can be considered: an 18-cylinder Mercury, and a 14 cylinder Pegasus

Would either of these fit better than the 2 other configurations (18-peg and 14-merc) have been more useful than the other 2 proposed to replace the Hercules/Centaurus or should one (both seeming impractical) be developed along side the previous 2?

The 14 Cylinder Pegasus would seem comperable to the R-2600 or BMW 801. (although probably a bit lighter -moreso to the 801- Albeit it would probably be closer to the 2600 in diameter)

The 18-cylinder Pegasus would seem to compare with the R-2800.

 

[red admiral]

The tech between the Mercury and Pegasus is pretty much the same, its just a case of playing with sizes until you hit a sweet spot.

A 14 cylinder Mercury would be similar to the Hercules, probably around 1400-1600hp. An 18 cylinder Mercury giving around 1900-2100hp. Diameter is fairly small and suitable for fighters.

14 cylinder Pegasus around 1500-1700hp. 18 cylinder Pegasus around 2000-2200hp and more later. Engine diameter is a bit bigger, maybe too large for fighters (depends on which ones anyway)

Taurus replacement is difficult but theres limited applications. 9 cylinder Pegasus has similar power but much greater diameter. 14 cylinder Mercury is just too powerful and still a bit big.

I think I'd go with the 14 cylinder Mercury as a Hercules replacement and the 18 cylinder Pegasus to compete with the Sabre and Vulture. Given the greater reliability I'd imagine Typhoons with radial engines. There wouldn't be many problems in fitting it looking at the later Tempest Mk II. An 18 cylinder Mercury would give a little less power but a more closely cowled and less draggy machine.

 

[kool kitty89]

Seeing as the only operational uses of the Taurus were with the Albacore and the Beaufort I don't think there would be too much trouble with using the Pegasus instead.

Though the alternate R-1830 Twin-Wasp (on the Beaufort) was much closer in size to the Taurus. Also note that on many US a/c the R-1830 and Wright R-1820 Cyclone-9 -with similar dimentions as the Pegasus- were used somewhat interchangably as well: most notably the C-47 and Wildcat used both engine types -as well as others like the P-36.

 

[Reluctant Poster]

The tech between the Mercury and Pegasus is pretty much the same, its just a case of playing with sizes until you hit a sweet spot.

A 14 cylinder Mercury would be similar to the Hercules, probably around 1400-1600hp. An 18 cylinder Mercury giving around 1900-2100hp. Diameter is fairly small and suitable for fighters.

14 cylinder Pegasus around 1500-1700hp. 18 cylinder Pegasus around 2000-2200hp and more later. Engine diameter is a bit bigger, maybe too large for fighters (depends on which ones anyway)

Taurus replacement is difficult but theres limited applications. 9 cylinder Pegasus has similar power but much greater diameter. 14 cylinder Mercury is just too powerful and still a bit big.

I think I'd go with the 14 cylinder Mercury as a Hercules replacement and the 18 cylinder Pegasus to compete with the Sabre and Vulture. Given the greater reliability I'd imagine Typhoons with radial engines. There wouldn't be many problems in fitting it looking at the later Tempest Mk II. An 18 cylinder Mercury would give a little less power but a more closely cowled and less draggy machine.

From Fedden by Bill Gunston:
"Fedden could see big problems ahead. He used two inlet and two exhaust valves on each cylinder, to give better breathing, and studies convinced him that the valve gear for a two row engine with four-valve cylinders would be a mechanical nightmare."

If the designer of the Pegasus didn't think he could make it into a two row engine it probably couldn't be done.

 

[kool kitty89]

Didn't the French Gnome Rhone 14R used 4 VPC? (incedentally a developmend originating to the Bristol Titan -with identical bore and stroke to the Mercury's cylinders-)

 

[red admiral]

If the designer of the Pegasus didn't think he could make it into a two row engine it probably couldn't be done.

It would be challenging, but no more so than developing sleeve valves from scratch. Theres also the simple fact that the double-row Pegasus was created by Alfa-Romeo in Italy as the model 135/136. They had some problems with the gearing, but they were mainly material related with the need for high strength steels because the parts were under a lot of stress. Maximum power for the series was 2400hp.

 

[Reluctant Poster]

It would be challenging, but no more so than developing sleeve valves from scratch. Theres also the simple fact that the double-row Pegasus was created by Alfa-Romeo in Italy as the model 135/136. They had some problems with the gearing, but they were mainly material related with the need for high strength steels because the parts were under a lot of stress. Maximum power for the series was 2400hp.

Again to quote Gunston on the Alfa Romeo (from the World Encyclopedia of Aircraft Engines):
"This was a Pegasus with only two valves per cylinder, and thus the company was able to do what eluded Fedden at Bristol: double up and produce an 18 cylinder two row version.
The Gnomes were also 2 valves per cylinder (same source).

 

[red admiral]

I don't think Gunston is right on this point as the pictures I have clearly show 4 valves per cylinder and Archivio Storico Alfa Romeo - Volume II. Torino, novembre 1998 gives 4 valves per cylinder as well.

 

[kool kitty89]

Are the pushrods oriented in the same fassion as on the Bristol engines? And where are the pushrods located for the second row of cylinders?

 

[kool kitty89]

In the case of most 2-row radial engines I've seen (with the usual 2-Valves), each row has a set of pushrods, with one set in front of the first row, and a second set behind the secons row. So oly one row's pushrods are clearly visible when the engine is viewed from in front or behind.

Like so:

For some reason Gnome Rhone's 2-row engines had their pushrods in a rather crowded position all in front of the engine:

Additionally both the single and 2-row Gnome-Rhone engines have each pair of pushrods stagared, mounted in a V coming together at the base entering the crank-case. (as can be easily seen above) Opposed to usual configuration of 2 rods near the side of each cylinder extending out roughly allong the angle created by the cylinder's sides and each rod entereing sepearate at the base entering the crank-case.

This configuration leads bact to their original 5K Titan engine, but I'm not sure if this is a feature of the Bristol design, or a change made so it could be produced without paying royalties.




Now with the 4-VPC arrangement of the Jupiter, Mercury, and Pegasus the pushrod configuraton is considerably different than either of these layouts with the rods coming straight out at the middle of the cylinder in pairs, one directly in front of the other.

(looking superficially similar to the configuration of the WWI Gnome Rotary engines when the Bristol engines are fitted with covers over the pushrods)

This would obviously not allow the arrangement used on the Gnome-Rhone 2-row engines, but I don't see why it would be excessively difficut to use "normal" back-to-back 2-row configuration.

 

[red admiral]

Are the pushrods oriented in the same fassion as on the Bristol engines?

They are for the 125 series as you posted that image of the Mercury but I don't have a picture of the 135 series to hand to see what the arrangement was. I'd guess at it being a mirror image.

Alfa-Romeo also purchased the licence for the Mercury but only made a handful. A licence for the Hercules was also gained, but there seems to have been no production. There was a later project, the 1101 that used sleeve valves. It was a 28-cylinder liquid cooled radial of which around 20 pre-production examples were constructed. Work stopped after sabotage and attacks on the factory by partisans.

 

[kool kitty89]

They might have some trouble orienting the intake manifolds for both rows of cylinders with 2 manifold's leading into each cylinder. (as in the Pegasus) It wouldn't be excessively difficult, and they could always use a single manifold per cylinder that split off just before reaching the cylinder intakes. (as in the previousl configuration on the Jupiter)



Now, for an "engineering nightmare" of valve arrangement (not to mention manifold arrangement and cooling) there's the R-4360:

 

[PWR4360-59B]

4360's are beutiful. I have one, the above is the high tension model, with 7 magneto's. Mine has 4 low tension magnetos, with a coil for every spark plug.
And yes removing and reinstall of the spark plugs is a job.

 

[kool kitty89]

On another note, if you really wanted an engine comperable in size to the Taurus (or Twin Wasp): it might be possible to shorten the stroke of the 14 cylinder mercury to 5-5.5" which should give comperable diameter and weight to the Taurus, and a bit more power. (given a displacement of ~1800-2000 in2)

Also a 10-cylinder Mercury might be a consideration, but it would still be significantle larger in diameter than the Taurus or Twin Wasp.

Coincedentally, with a 5.5" stroke the cylinder dementions and displacement are identical to the R-2000 Twin Wasp.

 

[kool kitty89]

As I previously mentioned, using the Pegasus in place Taurus in the two production aircraft to use it (Beaufort and Albacore) would not have had significant disadvantages or difficulties resulting the Pegasus's larger diameter.


However having a smaller engine (as described above) around with suficient reliability and development support could be attractive for applications that would have abandoned the Taurus. Additionally designs like Glosters G.39 (F.9/37) and F.5/34 would have engines avialable to provide good performance without having to go all the way up to a Hercules sized engine.

 

[red admiral]

The Alfa-Romeo 135 definitely has four valves per cylinder. It looks like a mirrored 125 engine.

 

[merlin]

From Wilfrid freeman's biography:
"By 1931, however it was clear that two-row engines would be needed and Roy Fedden, the brilliant and dynamic head of the Bristol engine division, with the consent of the Bristol directors and the Air ministry, made a policy decision that all their new engines would have 'sleeve-valves' (SV's), and ceased to develop PV engines.
Fedden realised that greater power would require two-row radials, and was reluctant to revert to single inlet and exhaust poppet-valves on each cylinder. The difficulties of actuating four PVs per cylinder on a two-row radial would be so severe that the use of sleeve valves, long promoted by Ricardo, looked an attractive alternative."