V-1710 supercharger development potential

[BiffF15]

Greg,
Thanks for posting! What is the reason for the fork and blade rod set up? Does the Allisin have cylinders that are directly across from each other?
Cheers,
Biff

 

[gjs238]

Greg,
Thanks for posting! What is the reason for the fork and blade rod set up? Does the Allisin have cylinders that are directly across from each other?
Cheers,
Biff

From Wikipedia:
https://en.wikipedia.org/wiki/Connecting_rod

Compound rods

Many-cylinder multi-bank engines such as a V12 layout have little space available for many connecting rod journals on a limited length of crankshaft. This is a difficult compromise to solve and its consequence has often led to engines being regarded as failures (Sunbeam Arab, Rolls-Royce Vulture).
The simplest solution, almost universal in road car engines, is to use simple rods where cylinders from both banks share a journal. This requires the rod bearings to be narrower, increasing bearing load and the risk of failure in a high-performance engine. This also means the opposing cylinders are not exactly in line with each other.
In certain engine types, master/slave rods are used rather than the simple type shown in the picture above. The master rod carries one or more ring pins to which are bolted the much smaller big ends of slave rods on other cylinders. Certain designs of V engines use a master/slave rod for each pair of opposite cylinders. A drawback of this is that the stroke of the subsidiary rod is slightly shorter than the master, which increases vibration in a vee engine, catastrophically so for the Sunbeam Arab.

Radial engines typically have a master rod for one cylinder and multiple slave rods for all the other cylinders in the same bank.

The usual solution for high-performance aero-engines is a "forked" connecting rod. One rod is split in two at the big end and the other is thinned to fit into this fork. The journal is still shared between cylinders. The Rolls-Royce Merlin used this "fork-and-blade" style. A common arrangement for forked rods is for the fork rod to have a single wide bearing sleeve that spans the whole width of the rod, including the central gap. The blade rod then runs, not directly on the crankpin, but on the outside of this sleeve. The two rods do not rotate relative to each other, merely oscillate back and forth, so this bearing is relatively lightly loaded and runs as a much lower surface speed. However the bearing movement also becomes reciprocating rather than continuously rotating, which is a more difficult problem for lubrication.
A likely candidate for an extreme example of compound articulated rod design could be the complex German 24-cylinder Junkers Jumo 222 aviation engine, meant to have — unlike an X-engine layout with 24 cylinders, possessing six cylinders per bank — only four cylinders per bank, and six banks of cylinders, all liquid-cooled with five "slave" rods pinned to one master rod, for each "layer" of cylinders in its design. After building nearly 300 test examples in several different displacements, the Junkers firm's complex Jumo 222 engine turned out to be a production failure for the more advanced combat aircraft of the Third Reich's Luftwaffe which required aviation powerplants of over 1,500 kW (2,000 PS) output apiece.

 

[wuzak]

Greg,
Thanks for posting! What is the reason for the fork and blade rod set up? Does the Allisin have cylinders that are directly across from each other?
Cheers,
Biff

Fork and blade was common for WW2 era "in-lines". All th emajor engines had them - DB 6, Merlin, Allison, Jumo 211, 213, Sabre.

The reason is to save some length, as the banks do not need to be staggered.

There may also have been some bending moment considerations for the crank pin.

 

[GregP]

They work very well and are easy to balance. Joe typically gets his pistons within .5 - 1.0 grams of each other. He does the same for rods and wrist pins, but the wrist pins are MUCH closer in weight ... on the order of .05 grams. You don't balance valves ... you make them have perfect edges and ensure perfect seats, and that is their contribution to power ... perfect sealing.

Joe has helped develop some racing cams for the Allison .. .for the tractor crowd, but uses stock cams for aero engines, as required by the Allison overhaul books. He is the only person I know of who actually has and uses an Allison factory crankshaft bearing sizing gauge. When you torque the mains to spec, you MUST be able to tunr the gauge by hand. If you can't, it's too tight. He checks initial sizing with plastigage and rarely has to hone bearings after inital preparation. You might expect that after years of overhauliing nothing but ALlisons.

 

[kool kitty89]

I believe the story may have it's roots in the fact that they used a Vulture impeller in the first test rig. It was handy, it was about the right size (desired airflow) and gave them something to go on. I haven't read anything about using a Peregrine impeller though. I would guess it was too small. You have to deal with both pressure and volume.

Given both the vulture and peregrine used pre-Hooker supercharger designs, using one (or both) would have made sense for convenience of preliminary testing, but I'd think all the production merlins from the XX onward used Hooker based designs. (be it the impeller, diffusor, manifold, or other ducting)


I also hadn't realized the Napier Sabre used a double-sided supercharger impeller akin to Whittle's turbojet designs. That's an interesting way to go about things.

http://4.bp.blogspot.com/-T4uySZSSv0g/TqG3YrIFitI/AAAAAAAAAj4/IqOZAPTxsO0/s1600/napier_sabre.jpg

Everyone we spoke with at Joe Yancey's said they were running them at 75 inches in late production models. By "everyone" I mean former pilots, since usually only owners, former pilots, and former crew chiefs came by the shop when we were breaking in newly-overhauled Allisons. Joe usually had anywhere from zero to 4 visitor during engine runs, and we usually ran them for anywhere from 2 - 6 hours on the test stand before the rings seated sifficiently for Joe to be happy with it going flying.

Late model P-38s seemed to handle 75" (2000 HP WEP) fairly well, were those comments regarding P-82 experience, P-38, or both?

 

[wuzak]

I also hadn't realized the Napier Sabre used a double-sided supercharger impeller akin to Whittle's turbojet designs. That's an interesting way to go about things.

Yes, until it was replaced by a single sided impeller in the Sabre V.

The Rolls-Royce R also had a double sided impeller.

Cutaway from Lyndon Jones.

 

[Shortround6]

The Merlin XX and 45 used the same impeller and diffuser as earlier Merlins, they just changed the intake elbow and front cover. Just is actually not a good word as the intake elbow and front cover and carb mount were all a one piece casting which made it rather hard to do experiments. If you wanted to change anything you needed a new and complicated casting.

 

[tomo pauk]

Interestingly the V-3420 used a 10.0" diameter supercharger impeller for single stage models. Only 1/2" larger than the V-1710, despite being twice the capacity.

The big US engines used fairly small superchargers/impellers. The R-2600 and R-2800 were mostly at 11 in diameter, the C series of the R-2800 went to 11,5 in. Engine stage S/C of the R-2800-21 (used on many of P-47s) was only 10.5 in. Contrary to that, Hercules and BMW 801 used a 13 in impeller, though some Hercules versions used the 12 in impeller (result was a bit better low alt performance). The R-3350 started at 12 in, later versions were at 13 in.

Aux stage was at 12.18 in for the V-3420, and 13 in for the R-2800.

 

[Shortround6]

Impeller diameter helps tell you the tip speed and may have something to do with pressure. Impeller thickness is not mentioned and that may have something to do with the total mass airflow. Large diameter shallow impeller vs smaller diameter but thicker impeller?
An R-2800 is going to need around 45-48% more pounds of air per minute than an Allison at the same pressure?

 

[gjs238]

And what about impeller speed?

 

[GregP]

Most of the comments we heard were regarding the P-38J and L. Joe only spoke to about 2 - 3 people who had anythning to do with the P-82 while I was there, and none of them mentioned MAP specifically. They also didn't say anything bad about the engines except to say they had not experienced any of the issues they had heard about from other guys. That's not a very big cross section, so I usually don't make much of it one way or the other.

When Joe built up a G-6, he ran it at 65" on the test stand and it ran just fine. We had to stop there because it was pulling the test stand (A Ford F-350) across the pavement backwards and was threatening to pull the front tires off the ground. Joe mounts the Allion so the prop hangs off the back of the bed and he uses a 55-gallon drum of coolant for the radiator. Usually we limit test runs to about 30 - 40 minutes or so each out of nothing else but getting tired of standing 10 feet from a howling Allison in the propwash.

 

[tomo pauk]

And what about impeller speed?

I reckon it you mean 'tip speed'? Most of designers seem to want to stay at tip speed that is equal or just a bit lower than speed of sound. Going faster than speed of sound 'kills' impeller efficiency (impeller sucks too much of engine power for the pressure ratio achieved), going too slow means that S/C is not used as much as possible. Higher impeller speeds will consume more power than lower impeller speeds, hence the 2-speed or infinite-number-of-speeds supercharger gearing.
The V-1650-1 have had impeller tip speed of 1273 fps* (maximum) in high gear (9.49:1 S/C gearing), the V-1710 with 8.80:1 gearing was at 1094 fps, the V-1710 with 9.60:1 was at 1194 fps. The later V-1710 have usually had 2500 ft greater rated height, the rated powers being usually 1125 HP @ 14500 ft for the 9.60:1 S/C versions, and 1150 HP @ 12000 ft for the 8.80:1 S/C versions.
The cost was that 'higher' V-1710s have had less power down low than 'lower' types.

Merlin II/III were at 1151 fps (10.25 in impeller - same as with Merlin XX/45/V-1650-1, geared 8.58:1, when crankshaft speed is 3000 rpm).

* yep, faster than speed of sound, 1,127 fps

 

[Shortround6]

Speed of sound inside the supercharger may be different. Depends on the speed of sound in the higher pressure/higher temperature air.

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe.html

speed of sound in 100C/212F air is 1287.4fps.

 

[kool kitty89]

Speed of sound inside the supercharger may be different. Depends on the speed of sound in the higher pressure/higher temperature air.

Speed of Sound

speed of sound in 100C/212F air is 1287.4fps.

Yep, though I'd thought it was mainly temperature and not pressure (or density) that affected speed of sound. (humidity is a factor too though, but that's actually changing the chemical composition of the air -more water in it)

This would also be one point to using aftercooling over intercooling (air stays hot until after the final stage), though also a point to using a larger diameter impeller for the first stage and/or runnin the final stage slower. (intercooler cooling air between stages means lower local speed of sound for the 2nd stage -or 3rd if there's one)

 

[gjs238]

The DB and Junkers superchargers appear to be "external" to the engine and driven from the engine accessory gearbox.
If so, that seems to offer greater modularity and ease of making changes (or indeed, mounting multiple superchargers) as opposed to the Allison, which appears to be, to a large extent, part of the engine structure itself.

 

[kool kitty89]

The DB and Junkers superchargers appear to be "external" to the engine and driven from the engine accessory gearbox.
If so, that seems to offer greater modularity and ease of making changes (or indeed, mounting multiple superchargers) as opposed to the Allison, which appears to be, to a large extent, part of the engine structure itself.

Allison's (and Pratt and Whitney's) auxiliary superchargers were configured more like those of Jumo and DB designs, also driven via hydraulic fluid coupling (akin to a torque converter on automatic transmissions) facilitating the modular arrangement.

This is the same reason I brought up the potential for side-mounting the Aux stage on the V-1710 and reducing the length. (potentially allowing it to fit in existing designs more easily)

Not only were the auxiliary stage superchargers late to the scene, but they added so much length to the engine that it made mounting impossible in the P-39 without heavy modification, while it added enough nose length to cause problems with center of gravity on the P-51 (possibly P-40 as well)


German engines also used direct fuel injection, not carburetors (or manifold fuel injection as in 'pressure carburetors' ) so it somewhat simplified supercharger arrangements as well. (the merlin and allison have carburetors injecting fuel into the supercharger intake or between the aux -or turbo- and integral stages)



Though it's also worth noting that, in spite of the supercharger arrangement, the german manufacturers were also rather late in introducing larger/more powerful superchargers (be it 2-stage like the Jumo 213 or large single stage as with the DB 605AS).

I'm not sure on the exact reasons for this on the German end of things, but for Allison it was more of a funding and overall engineering resource limits problem. They were a relatively small company compared to other major aero engine manufacturers and if the US government wasn't forthcoming with funding, they'd be very hard pressed to invest in independent developments. (the Army was uninterested in auxiliary supercharger development during the critical early period, leaving over a year in development lag time compared to the likes of Pratt and Whitney and Rolls Royce)

The Army did manage to push a ton of funding for a variety of other projects, including their Contenental Hyper engine pet project ... which the V-1710 itself eventually overtook in spite of less focused interest.

I wonder if Allison could have gained some funding from the Navy if they'd offered alternative water cooled variants early on (glycol safety issues being one of the major concerns of the USN -radiator vulnerability was a concern too, but overcoming the glycol issue might have been enough to at least make it worth investing in).

 

[GregP]

It doesn't look more "modular" to me. It just looks mounted 90° to the crankshaft. The Allison takes a special 5-sided accessory case, but yoiu have to mount the carb / supercharger / accessories somewhere. All you'd have to do is make a matching accessory case, and people have done so. The guys on the European tratcor-pull circuit run twin turbocharged Allisons with fuel injection, so we KNOW it can be done.

In the case of the Jumo above, if you want to change the S/C, you need another one engineered to fit. Same with an auxilliary ALlison accessory case ... just a bit smaller.

I'd like to see that Jumo running!

 

[kool kitty89]

Oh, perhaps worth noting is that Jumo 211 is also mounted upside down.

 

[Shortround6]

The better to see the fuel injection pump 😀

 

[tomo pauk]

The early example of V-1710 - the F11R, with a 2-speed S/C was the prototype featuring a 'mixed-flow' supercharger designed by TEC company, of Mr. Birmann. The S/C measured 10.25 in in diameter and was geared 8.80:1 in high speed, and made same power at 16000 ft as the other engines with same gearing at 12000 ft. It was also slightly better at altitude than V-1710s with 9.60:1 S/C gearing, but those went into production, not the F11R, that made more sense for bomber aircraft due to higher TO power.
Too bad the F11R's S/C was not geared 6.93:1 and 9.60:1, the power would've been at least on par with Merlin 20s, and much better more flexible than other 1-stage V-1710s.

(open the pic separately for high resolution; disregard the supposed aircraft)