Kestrel Engine Cooling

CD, PM RSVP, WT, P&P.:eagerness:

It is a genuine Battle-of-Britain relic from a shot-down Hurricane (well, as sure as I can be) so I would be pretty reluctant to part with it; how much block do you need to do materials testing and the like? Does it need to be Merlin II or will any Merlin, or Packard stuff do?

CD, you are a very handsome man. Clever too ! Um, would you part with that chunk of Merlin II ? Did I ever say how much I admire your work ?

I can really appreciate the development of the ‘power egg’ principal!

I cannot imagine what it would be like working outside up on the wing of a Lancaster on some desolate Lincolnshire airfield in the dead of winter; I am staggered at the dedication of the ground crews and the reliability they managed to maintain in the engines they serviced. I wonder if there are any books written by RAF ground crew; I don’t suppose they had enough ‘glamour’ for the publishers at the time the classic Bomber / Fighter Command books were being published?

Gents, fascinating – thank you. For those with iPads, the split screen function (press and hold double page icon, top right) enables you to read text on one window, and view pictures on the other, simletaneously.

No, you don’t. Engines built in the factory have the rods fitted before the blocks, to check fit. Merlin block changes were routinely done on the wing, rods in place. In some installations it would be achieved with an engine change. Always engine out with the German inverted V’s. Cheers

V

Thinking about it I bet you never fit a cylinder-block like that; I suppose you put the pistons in the liners first and then fit the block to the crankcase!

Well, that’s how I’d do it anyway, but it would mean ‘engine out’ in most cases.

I have a copy of ‘A Designer Remembers’ somewhere, actually two copies I think, but three house moves in 2014-2015 means that it is still languishing in a storage box somewhere! A book that deserves to be in A3 format rather than A5; the original Rols-Royce drawings and sketches are priceless!

I’m surprised that the first sketch puts so much emphasis on these clamping-blocks ‘pulling the cylinder-liner tight…..ensuring gas-tight joint’! Frankly, they are a terrible arrangement for applying a clamping-force onto a cylinder; there need to be three (evenly spaced), not just two, for starters, and how the hell would you torque-up those brass nuts through that tiny portal? Not that I could come-up with any arrangement on the pre-existing Merlin design where it was possible to get three evenly-spaced clamping-blocks in you understand, but this has to be categorised as a classic British ‘bodge’ from Rolls-Royce if it really is applying any serious clamping-force!

I think anybody who has ever fitted a piston into the engine of their car or truck will appreciate the nightmare that fitting a cylinder-block onto a Merlin crankcase must be; six pistons to be fitted, all at the same time (at different positions on their stroke), five fragile cast-iron piston-rings per piston, a heavy cylinder-block on a chain-hoist, and how the hell do you get the piston-rings compressed (or the piston-ring compressor out after?)…

…the last thing you need is one of the (unrestrained?) sleeves being jiggled enough for the sealing gasket-ring to become unseated!

You wouldn’t even know you’d unseated it, or damaged it, probably, it would be impossible to see (except with an endoscope through the spar-plug holes; at least there is one each side!), and the clamp-down force from the cylinder-head (cylinder-block) nuts would easily crush any gasket-ring that wasn’t seated correctly? And all this on a wartime production-line with ‘semi-skilled’ workers producing hundreds of Merlin engines a day?

No, I think the real reason for these clamping-blocks is to hold the cylinder-liners securely in place, and the gasket-rings correctly seated, while the whole cylinder-block and cylinder-liners go through the traumatic experience of being assembled onto the crankcase; and in that I’m sure they do help ensure a gas-tight seal!

But that’s just my educated guess…..and I’m happy to be corrected, especially by anybody who has actually built a Merlin engine!

Ooooooooooooooooooooowwwww CD, yes please ! That is one doorstop I would like to fondle….

You are correct pandp, the materials mix was usually a compromise to meet a need. Rolls Royce learned a lot about cooling between the start and end of Merlin development. Corrosion inhibitors came into that equation. However, I have seen one-piece blocks today that have obviously not had corrosion inhibited coolant in and so, full of corrosion muck. Even, so bad that the alloy top joint is corroded away to cause internal leakage- not what they probably suffered in the 1940’s when I think the leakage was possibly caused more by the loss of clamping of the liner against the alloy top joint.
Your words ” The ultimate answer to stopping coolant leaks is probably to “do as they did”, once you know what that was!” are interesting. Alec Harvey-Bailey explains that “progressive improvements were made to the design” of the top joint, including the fully shrouded joint ring Mod 155. However, one piece cylinder blocks retained a weakness in the top joint until the two piece block was introduced that effectively removed the coolant/gas top joint. That, was the “ultimate answer to stopping coolant leaks”, eliminate the top joint. Cheers

V

CD, very interesting in respect of the materials used in the additional clamp arrangement in what I assume is a Merlin II. I wonder how much of the secret is in differential expansion of different materials. The aluminium block, like your aluminium cookware, absorbs heat and swells faster than steel and brass, so the aluminium is trying to move away from the steel liners, while the stainless and brass fight to keep the liner bedded down. These are horrible materials creating electrolytic corrosion in the block, so the only logic to do this might be harnessing differential expansion. I would like to get a chunk of Merlin II block and test all these clamping materials for differential expansion to satisfy this point. This factor is better understood in the use of carbon steel flanges on inconel exhaust stubs sealed with copper gaskets, a zoo of materials to stop high pressure exhaust leaks. High pressure glycol cooling leaks are a related problem. So much development work must have gone into selecting the right ‘balance’ of materials and all this stayed obscured behind clouds of pipe tobacco in the design conferences rather than being written down. It is interesting to look at the selection of materials that different engine designers selected for common elements like drivetrains, eg crankshaft-reduction gear-prop shaft, to figure that there were different schools of thought and experiences brought to bear. Aircraft Production from 1938 to 1940 is quite transparent around the materials used in British engines, but the Americans are utterly opaque about P&W and Cyclones. The Cheetah and the Kestrel and the Dagger are all quite different in their material recipes and I figure most of the secrets need to be unpacked today using forensic metallurgy to really understand what was going on. The ultimate answer to stopping coolant leaks is probably to ‘do as they did’, once you know what that was!

Overall, the wet liner was the best solution for efficient cooling. However, on the Kestrel/Merlin one-piece blocks, the top joint where coolant is only separated from the combustion space by a seal or joint, is a vulnerable point. Ultimately, the solution of the two piece block was able to avoid the joint and there was no point where the coolant was only separated from the combustion space by a seal. Daimler Benz stuck with dry liners in their cylinder blocks for the DB600/601/603/605 series. However, although they avoided many of the problems of leaks, they were left with a more complicated production process for the liner and, some heat dissipation and distortion issues.

V

Creaking Door I’m glad you added that last line…..it seems to me you’’ve either got an engineering mind or you haven’t. Being the latter, I struggle to visualise these posts but find it very interesting, especially as the subject is powerplants that I have seen and heard for so many years.

I’m not sure about the Kestrel but the sealing at the top of the cylinders on the Merlin seems to me to be designed so that the block retaining studs / nuts would squeeze each cylinder between the crankcase and the light alloy sealing gasket ring set into the cylinder-head (or cylinder-block); four sturdy studs with nice big brass nuts at the cylinder corners, easy access (well, sort of, with the camshafts removed?) and ideal for applying a measured even torque (and so, in theory, a consistent squeeze on the cylinders themselves). So far, so good; but it does require a great deal of consistency in the overall height of the cylinder-blocks and the cylinders themselves so that any variation is taken-up by the ‘crush’ of the light alloy sealing gasket rings as both the block and all six cylinders bottom-out together.

There are two other fasteners that clamp each cylinder to the cylinder-block; on either side of the cylinder a clamping-block applys tension through a step in the cylinder itself. These two clamping-blocks ride on a stainless steel stud, in the coolant passages of the block, and are retained by a brass nut that can be accessed through a blanking-plate in the cylinder-block wall. Quite a ‘fiddly’ arrangement it has to be said (an additional twenty-four blanking-plates with paper gaskets for water leaks for starters!) and not easy to correctly torque-up through the blanking-plate hole.

I’ve always assumed that these additional clamping-blocks were simply designed to retain the cylinders (and gasket rings) in the cylinder-blocks when the cylinder-blocks were fitted or removed but it would be possible, if difficult, to apply an additional clamping-load onto the cylinder itself; surely, when the main studs / nuts are tightened, these clamping-blocks must be (effectively) slackened-off?

I must try to add some photographs as this is going to make no sense to anybody reading this!

I think that some clarity about Kestrel and Merlin cylinder coolant leaks can be found in the descriptions given by Alec Harvey-Bailey in his book “The Merlin in perspective”. In this, he observes that the Merlin I was developed, with the flawed ramp head, and a two-piece cylinder assembly to “eliminate the internal coolant leaks to which the Kestrel type of one-piece block was subject”. After the failure of the ramp head was recognised, the Merlin was then quickly redesigned with a one-piece cylinder assembly, thereby accepting the consequent risk of internal coolant leaks, until the Merlin could be redesigned with the later two-piece blocks. Alec Harvey-Bailey gave coolant leaks from the top joints as probably being in the top five reasons for one-piece block engines having block changes in service. Also, the hard life of fighter engines and Mosquito installations were more likely to suffer top joint leaks in service. In the attached letter from Air Commodore Wheeler above, I feel he might have meant to refer to the later two-piece blocks as the ones which cured the internal coolant leak problems.

V

Memories of the 1960’s & 70’s when I had a succession of Hillman Imps & a Stiletto…………head gaskets letting go & radiators clogging up even using glycol…..

Yes, it is sometimes difficult to keep oil out of the coolant, and vise-versa, and both out of the cylinders. The gasket and the mating faces and the tightening procedure must be just right.

We frequently have the oil analysed in ours (3 Merlins and an Allison) to make sure nothing is creeping where it shouldn’t.

The Kestrel was quite an innovation in its day, and developed into a reasonably reliable engine, but there was a learning curve. If the coolant is allowed to boil, there could be warping, distortion and subsequent leakage.

Over-revving is also a problem, although it’s probably the mechanicals that are more likely to fail there.

And it’s a VERY expensive engine to overhaul in today’s world.

#6…Except behind the wheel……

Re ‘Running in please pass’ and bonnet release catch? Some Audi and Renault’s do not even have a manual oil level checking devise (A Dipstick ) :apologetic:

..shedding some light as to why drivers of new cars used to sign, ‘Runnning in, please pass’, and to the ignorance of some drivers as to where the bonnet release catch is located. Thanks, P&P for this new information. These old aircraft really are living history.