WI Exe used Pennine dimensions?

[wuzak]

The Rolls-Royce Exe was designed in the mid '30s as an air-cooled X24 sleeve valve engine with a bore of 4.2" and a stroke of 4". Thus gave 1150hp @ 4200rpm from 1330cu.in/21.8l (Rolls-Royce Exe - Wikipedia, the free encyclopedia says 1348cu.in/22.1l), for a BMEP of 1125kPa/163psi.

The size of the engine limited its development somewhat. The Exe development was suspended in 1939 and cancelled in 1941.

So what if the Exe had been designed using the bore and stroke of the later Pennine - ie 5.4" bore x 5.08" stroke, giving 2792cu.in/45.8l?

Would the Exe program have been attractive enough to recommence development in 1941/1942?

Also, according to Rubbra (IIRC), a liquid cooled version of the Exe was built up (with fabricated blocks). If this also occurs for the 2800cu.in WI version, would that merit transferring the resources being spent on the Vulture to the liquid cooled Exe?

My calcs show that using the historic bmep of the Exe for the 2800cu.in version would give 2012hp @ 3500rpm (design maximum speed) and 2300hp @ 3500rpm using Vulture V (rated 1955hp) bmep.

Historically the Exe exhibited few problems, other than being rather weighty for its capacity and having high oil consumption. Would the larger version have more issues, such as the extra power causing problems with the master rod arrangement, as per Vulture?

How much trouble would it be for RR to get a sleeve valve engine into production? Historically it took Bristol some time to get the right materials of block and sleeve, but no so Rolls-Royce. It took Bristol a lot of time to get the production processes right - how long would it for RR? Napier also took a long time to get it sorted, but that ended up due to assistance from Bristol.

 

[Shortround6]

Would Rolls have wound up in the same situation as Bristol? Bristol didn't have much trouble with prototype engines, ( I am not saying they didn't have any) or very small production runs, it was when they tried for for mass production that things really went wrong. Excessive oil consumption was one of the main problems which was also a complaint of the prototype Exe.

While 3500rpm may have been feasible in 1944/45 it might have been a bit much for 1940. Piston Speed of the Merlin was 3000fps, The Vulture at 3000rpm was 2750fpm, the Exe was 2800fpm while the Pennine at 3500rpm would have been 2963fpm. I have not adjusted for piston size. Piston speed is an indicator of stress on crank and main bearings more than a problem with piston rings. Under square cylinders have a slightly lower value than a 1:1 value while over square cylinders have a slightly higher value.

 

[wuzak]

I use m/s for piston speed. Pennine at 3500rpm gives 15.0m/s, Vulture @ 3200 14.9, Merlin @ 3000 is 15.24. The Bristol Hercules was 15.1m/s @ 2750rpm and the Taurus 15.5 @ 3250rpm.

The Exe had a maximum speed of 4200rpm, which is a piston speed of 14.2m/s. That equates to 3300rpm in a Pennine and around 1855hp using the Exe's bmep.

It could be that Rolls-Royce had similar issues to Bristol with regards to productionising the sleeve valves, though. (It has to be said that Napiers had few problems with prototypes also.)

 

[wuzak]

How do you adjust for piston size? I have a method, but I am not sure it is correct.

Corrected piston speed = mean piston speed / sqrt(stroke/bore)


I made an error before...with the mean piston speed and bmep of the Exe the Pennine would give 1900hp @ 3307rpm.

On corrected piston speed. Merlin 14.5m/s @ 3000rpm, Exe 14.6m/s @ 4200rpm. Plugging this back into the calcs gives 1890hp @ 3287rpm.

The Pennine at 3500rpm has a corrected piston speed of 15.52m/s.

 

[Shortround6]

The formula for corrected piston speed is to take the mean piston speed (which you already have) and divide it by the square root of the stroke/bore ratio. Repeat and not a typo stroke/bore ratio.

This takes into account the larger mass of big bore pistons and the smaller mass of small bore engines. In one of the most extreme examples the Bristol Pegasus has an uncorrected piston speed of 3250fpm. Corrected it is 2846fpm so it falls from being one of the highest piston speeds of all aircraft engines to just being in the middle of the pack so to speak.

edit> we crossed posts. Your formula is correct.

 

[wuzak]

Around 1900hp in 1939 for an air-cooled engine of 2800ci would have been quite competitive, I would think.

 

[Edgar Brooks]

The Pennine was an enlarged version of the Exe, and got as far as a test run, but was cancelled in 1945. The Exe was given to a R-R employee, to keep him occupied after his return from a long illness, rather than chuck him back into the general works. It was eventually cancelled, to concentrate on the Merlin, so any continuation of effort in that direction, would have had a detrimental effect on the Merlin.
Edgar

 

[Shortround6]

You may consider my cynical or suspicious but I have a hard time seeing the Pennine as a serious project or as a true 1944-45 development. I certainly don't know what was going on in R-R at the time but developing/funding two totally different 24 cylinder sleeve valve engines of about 2800cu in, one air cooled and one water cooled (Eagle) certainly looks strange. Was the Pennine actually drawn up pre war and drawings dusted off in 1944 as a starting point or to provide a spur to the Eagle project?

 

[wuzak]

The Pennine was very much intended for the post war transport market, from what I understand. The Egale was definitely intended as a military engine.

The Exe was designed by Rowledge during his time away from Rolls-Royce due to illness.

Work on the Exe was suspended in 1939, but not cancelled until 1941. Thus it had no effect on the Merlin development in those critical years - where the Merlin went from single stage single speed with one piece blocks through two speed single stage versions and just starting with the two speed two stage engine with two piece blocks.

I can't see a larger Exe being continued during those years, but 1942 onwards a 2800ci Exe would have been more attractive than the 1330ci Exe was. By then the Exe was too small and its power potential too limited.

 

[Shortround6]

I still can't see an single engine maker trying to develop two different engines of about the same size for different markets. If that is what they were really planning some should have been taken out behind the woodshed for a sever thrashing. Even in 1944-46 it could cost millions of dollars/pounds to develop a new high powered aircraft engines and then even more money to tool up for it. Doubling your cost for the same amount of sales has got to be something covered in the most basic of business courses. With Bristol already tooled up and making 2360 cu in Hercules and 3270 cu in Centaurus air cooled engines the chances of sliding a British 2800 cu in air cooled engine in between them was about zero.

 

[wuzak]

The Pennine and Eagle 22 shared bore and (near enough) stroke dimensions. I wonder if components such as pistons, sleeves, sleeve drives, etc, were shared.

In terms of fitting in between the Hercules and the Centaurus, the Pennine had the potential to match the latter for power, for slightly more weight.

 

[wuzak]

Oh, and the Eagle 22 was more powerful again.

 

[Shortround6]

The Pennine and Eagle 22 shared bore and (near enough) stroke dimensions. I wonder if components such as pistons, sleeves, sleeve drives, etc, were shared.

Possible but not likely, One is an "X" engine with a single crankshaft whiel the other is an "H" with two crankshafts geared together. Trying for too much commonality might have been more trouble than it was worth. Rough forgings might be usable but trying to get finished parts with tolerances in hundreds of an inch if not thousands to work in two different applications might have required a lot more development work that different parts.

In terms of fitting in between the Hercules and the Centaurus, the Pennine had the potential to match the latter for power, for slightly more weight.

By Early 1946 Bristol was getting 2800hp for take off using ADI and was planning/hoping to get to 3500hp in the not too distant future.
It is also not just peak/max power for bomber/transport engines. A Napair Sabre VII was rated at 3000hp (ADI) for take off but 1730hp at 8,500ft for "cruise" and 1570hp at 17,000ft. The Bristol Centaurus 57 (2800hp T.O.) was rated at 1665hp at 10,750ft cruise in low gear and 1535hp at 21,250ft.

The bigger engine may offer a more 'relaxed' cruise. it also needs 12 fewer spark plugs at plug change time

 

[wuzak]

The Pennine was already rated at 2800hp for takeoff and maximum without ADI. Don't know about cruise.

 

[Edgar Brooks]

The trouble with talking incessantly about power outputs, etc., is that the basic premise is being ignored; are there any potential customers? You can make the best engine in the world, but, if there isn't an airframe in which to install it, it's just wasted effort.
America had the capacity to look beyond the war, and plan for a commercial future, hence the Stratocruiser, Constellation, etc; no such capacity existed in the U.K., so "airliners" were converted bombers or flying boats. The Brabazon and Princess turned into massive white elephants, with only the Viscount turboprop being a post-war financial success.
Rolls-Royce had the foresight to see that concentrating on piston power was not the way of the future, so moved towards the jet engine; even that didn't stop them going bust, and needing a government bail-out/takeover around 1970 (can't remember the exact year.)
Edgar

 

[Shortround6]

You are right but I believe the going bust had to do with the R211 engine?

Developing an engine that makes 1 HP for every 3-4 cubic inches isn't that expensive. Developing one that makes 1hp for every 1 cubic in is a lot more expensive but still cheaper than developing a jet that gives you 3lbs of thrust for every lb of engine weight and gives poor fuel economy. Developing 40-50,000lb thrust engines with power to weight ratios of 4-5:1 and 50% better fuel economy than first Generation jets gets very expensive.

But that goes back to why I have trouble believing that R-R would build two almost identical sized engines (if not power) at the same time and split their sales (what ever they might be) between them. Especially if the only thing the Pennine could offer over the already in production Centaurus would be smaller frontal area, not a real consideration on a transport plane. Or shouldn't have been in 1945. The Brabazon committee did may some choices based way to much on prewar thinking.
Other companies tried going for different markets. Alvis tried going for the small airliner market that never panned out for anybody. Luckily their small radial found a home in helicopters.

 

[wuzak]

But that goes back to why I have trouble believing that R-R would build two almost identical sized engines (if not power) at the same time and split their sales (what ever they might be) between them. Especially if the only thing the Pennine could offer over the already in production Centaurus would be smaller frontal area, not a real consideration on a transport plane. Or shouldn't have been in 1945. The Brabazon committee did may some choices based way to much on prewar thinking.
Other companies tried going for different markets. Alvis tried going for the small airliner market that never panned out for anybody. Luckily their small radial found a home in helicopters.

The fact is that Rolls-Royce did develop both the Eagle 22 and Pennine in 1944/5.

Frontal area is, surely, a consideration for transport planes as it affects fuel economy. Also, if you look at the Brabazon - I'm not sure why the wings were so deep, but if it was so they could house the Centauruses then changing to the Pennine could save 16" in wing depth. They could be more closely positioned side by side, which means they wouldn't need the bevel gears in the gearbox - which would improve, even if only slightly, drive efficiency.

There were also going to be larger capacity versions of the Pennine - from a simple resizing of the cylinder, to 32 cylinder versions.

 

[wuzak]

The trouble with talking incessantly about power outputs, etc., is that the basic premise is being ignored; are there any potential customers? You can make the best engine in the world, but, if there isn't an airframe in which to install it, it's just wasted effort.

Allison wanted to develop the V-3420 and the turbocompound V-1710 post war but would not commit funds for it without firm orders. Thus they never sold any.

 

[Shortround6]

Frontal area is, surely, a consideration for transport planes as it affects fuel economy. Also, if you look at the Brabazon - I'm not sure why the wings were so deep, but if it was so they could house the Centauruses then changing to the Pennine could save 16" in wing depth. They could be more closely positioned side by side, which means they wouldn't need the bevel gears in the gearbox - which would improve, even if only slightly, drive efficiency.

In a Fighter with an inline engine the largest cross section of the fuselage is often the cockpit, you can only fold the pilot up so much. In a large transport with 4 abreast seating and 4 engine nacelles the difference between inlines and radials is going to be a few percent of the total drag. One reason the Brabazon had such a thick wing was they specified an insanely low landing speed, or short field capability. It was like they didn't believe long runways were here to stay or that major cities would enlarge their airports from prewar size. That is why they also spent so much time on big flying boats. They didn't need airfields. Once the world starting building (or just using left over bomber airfields) the so called advantages of the British planes went away. A smaller thinner wing would have done a lot more for drag reduction if they had accepted the higher stalling speed.

With high powered aircraft engines there is no such thing as " a simple resizing of the cylinder, to 32 cylinder versions". Changing the size of the cylinder affects all kinds of things. Bristol stayed with the same diameter of cylinder for every major engine they made except the Taurus. They played with the stoke a bit but that was it. mercury and Pegasus used the same bore, different strokes, Perseus used the same dimensions as the Mercury, Hercules used 14 Perseus cylinders, Centaurus used same bore and a stroke in between the Mercury and the Pegasus. The Mercury was actually a short stroke Jupiter and the Gnome-Rhone company used their licence built Jupiters and Mercury'sto branch off and Build their own 14 cylinder engine (with two valves) using Murcury dimensions, which the sold to the Russians which became the M-88.
Pratt and Whitney didn't like to play with cylinder dimensions much either. Strangely enough, the R-2000, R-2800, R-4350 and the post war R-2180 all use the same bore and that bore is the same size as Bristol used.
(we could get a good conspiracy theory going out of this . Three of those engines used the same stroke.

Sketching an X-32 is one thing, getting one to stay together is another.

There were also going to be larger capacity versions of the Pennine - from a simple resizing of the cylinder, to 32 cylinder versions.[/QUOTE]

 

[wuzak]

For the 32 cylinder versions the drive was taken off the centre of the crankshaft - so it should be posible.

As an aircooled engine, going from 24 to 32 cylinders was not a big deal - needed new crank, new sleeve drives, new crankcase, but the cylinders, sleeves, heads, rods should all be the same.

For the larger cylinders they planned to increase bore and stroke by 1in. So, no, that wouldn't have been a simple task.