Realistic limits of the small 12 cyl engines, 1935-45

yulzari said:

I suppose de Havilland's Albatros would give us some idea of what he meant.

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Right ! Reverse flow cooling performed very well in this splendid aircrat !

Shortround6 said:

I disagree.
The Dagger was 'much to clever' and that is in a bad way.
It makes the French 700hp 14 cylinder radials look good.

24 cylinders and 1390lbs (630kg) is an expensive way to 1000hp at 2650 meters.
Just for engine design it is good in some way and not good in others. A Merlin at 3000rpm has about 81% of the piston ring drag/friction that the Dagger has at 4200rpm.
Piston/ring friction is about 80% of the total drag in many engines not counting the supercharger cost.
Dagger used a 7.5 compression ratio due to the small cylinders, even with 100 octane it may either hit it's limit or need lower compression.
Using more boost pressure puts you back in the heat problem.
View attachment 851654
Compared to a radial you have less room between the cylinders/heads and the cam boxes block some of the air flow across the cylinder heads.
All the radials that gained power increased their fin area and many in three ways. Making the fins long is one of them. In the inline engines you have problem,
You cannot make the fins much longer along the axis of the engine and you can't make them much wider (across the engines width) as you start getting uneven cooling around the circumference of the cylinder and piston.
A valiant effort but I would pass.

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The "little" engines, i.e. with small cylinders, need much less finning on each cylinder owing to their very low individual capacity.

Same for the piston/rings (and crank and pushrods bearings) friction , owing to the very low stroke.

In the '60, Honda drawn some racing engines whose revving was unbelievable (more than 20,000 rpm !) with 25 cc or 41 cc individual capacity. These engine were air-cooled and ran at relatively low speed, about half of that of a plane.

Well, in motor cycle terms, one cylinder from a Dagger is 700 ccs and is making just under 42hp at 4200rpm. Granted it has the aid of a supercharger to do it. Of course we have to figure out the power needed to run the supercharger.
Now run a 1939 motorcycle through the same type test as an aircraft engine.
Or

Continental O-170 engine. 2.8 liters, topped out a 80 hp but needed 80 octane to do it. Same displacement cylinders (a little more bore and a little stroke than the Dagger) and was running at 2700rpm. Note the relatively crude head fins on even a 20hp cylinder. Of course most planes it was in had trouble hitting 100mph so airflow is a little limited.

History is not on the side of fast running, supercharged small displacement engines. They look good on paper. Not so good when maintenance costs/problems are looked at.
During the 50s and early 60s both Lycoming and Continental built a series of engines (including 4 cylinders) where they increase the rpm limits and mounted superchargers on the engines, by the early 60s they were starting use turbos. In the mid 60S (?) they started building larger displacement, slower running engines and they got rid of the superchargers and got rid of the reduction gears and went back direct drive. The larger displacement engines weren't much different in weight because they were not carting around the reduction gears and the supercharger. They ran at lower rpm and would run on lower octane fuel. On some of the last 6 cylinder opposed engines the superchargers and reduction gears could almost double the cost of the overhaul and they needed overhauling more often.
All of the big American radials needed redesigning for every jump in power. The R-2800C needed 10% less airflow through cowling than the R-2800B at the same power levels. But the R-2800C was just about an entirely new engine with a lot more and deeper fins on the cylinders and heads. Not sure how much they tweaked the oil system.

The Dagger had the same number of valves as a Merlin (or other V-12s) because they only used two valves per cylinder, but they needed twice the spark plugs.

The engine had nowhere to go. You want 1000hp at 15-16,000ft you may need double the power to the supercharger (another 100hp in the cylinders) on an engine that has trouble cooling. Yes you can tweak the cooling system but you need more air. It was an interesting experiment but trying to improve/fix it took manpower the British did not have.

What can we get out of the 18.4 L air cooled de Havilland Gipsy Twelve? Original spec was low compression, 6:1, with a limited 5-min output of 425 hp at an unboosted and low 2,450 rpm.

don4331 said:

I was thinking about this in conjunction with what would be possible with modern material/modern analysis:

Sticking with RR Peregrine size: 5.0" bore/5.5" stroke = V-1295 (in American terms): 960 hp @ 3,000 rpm, ?9? psi boost:

Torque (hp*5252/rpm) = 1,680 ft.lbs works out to average BMEP of ~90 psi
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Merlin 71: 5.4" bore/6.0" stroke = V-1650: 1710 hp @ 3,000 rpm, 18 psi boost:

Torque = 2,990 ft.lbs works out to average BMEP of ~130 psi​

If RR had applied all the improvements to the Peregrine which were done to the Merlin

Back figuring the 130 psi BMEP = 1,460 hp...not too shabby.​

But wait, those boys at Junkers got the Jumo 213E up to 3,250 rpm; for the 213J to 3,700 rpm

If RR can hold the BMEP while adding 250/700 rpm respectively, the "hyper" Peregrine will have 1,590/1,810 hp
We will note the stroke of the Peregrine is ~9% shorter than Merlin, so 250 rpm increase (8%) - basically same max piston speed (there are vibration issues that need to be analyzed and addressed.)​

This version of the Peregrine isn't going to be substantially lighter than a Merlin - it needs basically the same supercharger/intercooler and same reduction gear, and it needs material in the block/heads/etc to survive that power. Now, where did the tooling for the Whirlwind go?

​

If you could go to the same piston speed as the Jumo 213J with its 165mm stroke, you could spin your Peregrine to 4,370 rpm! Then, same BMEP of the Merlin 130 25psi boost = 160 psi BMEP. And it would be making >3,100hp - taking names and kick @$$.

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I did some calculations based on BMEP and piston speeds for the Peregrine years ago.

For the standard Peregrine I with 100 octane fuel:
885hp @ 3,000rpm, BMEP 1,244kPa/180psi, piston speed 13.97m/s.

For same BMEP and piston speed of Merlin (15.24m/s) I get 966hp.

For Merlin XX BMEP of 1,634kPa/237psi (1,480hp @ 3,000rpm) the Peregrine makes 1,163hp @ 3,273rpm.

The ultimate Merlin was the RM.17SM.

Maximum tested power was 2,620hp @ 3,150rpm, BMEP 2,755hp/400psi. Piston speed 16.0m/s.

Applying those parameters to the Peregrine gives 2,246hp @ 3,436rpm.

For Merlin 70 series power (1,710hp @ 3,000rpm) the BMEP is 1,563kPa/273psi.

With the same BMEP at 3,000rpm the Peregrine would give 1,112hp, and if the rpm is increased to match the Merlin's pistone speed it would be 1,213hp @ 3,273rpm.

Shortround6 said:

History is not on the side of fast running, supercharged small displacement engines. They look good on paper. Not so good when maintenance costs/problems are looked at.

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As a counter-point, the Rotax 900 series engines have been very successful. 4 cylinders, 1.2-1.35L displacement depending on model, so about 0.3L per cylinder. 5800 rpm max. 2000 hours TBO, including the turbocharged versions, which I believe about matches other widely used piston aviation engines today.

Granted, the stroke is very short at 61mm, so even at a blistering 5800 rpm the mean piston speed is a pedestrian 11.8 m/s.

Originally introduced in 1984, so let's say ~late 70'ies 'technology level', might not have been feasible in the 1940'ies.

Admiral Beez said:

What can we get out of the 18.4 L air cooled de Havilland Gipsy Twelve? Original spec was low compression, 6:1, with a limited 5-min output of 425 hp at an unboosted and low 2,450 rpm.

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With its supercharger at 2,600rpm at only 3.5 psi boost it produced 525bhp and did so for 5 hours. Presumably on civil aviation petrol of the time so there is still room for more boost if the cooling system is up to it. It's reverse flow system was very effective in normal use at 425bhp which it could maintain but normally only used long enough for take off and ran lower for cruise and climbing for fuel economy. I would not put a hard figure on what it might be made to do but I would hazard a guess possibly 750bhp for a limited time combat use.

yulzari said:

750bhp for a limited time combat use.

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Might be a good guess. Or it may depend on materials/manufacturing.
The Gypsy 12 was two banks of Gypsy 6 cylinders on a common crankshaft.
By some time in the 1950s they had gotten the Gypsy Queen up to 380hp using a supercharger with 7.5lbs of boost and 10/130 fuel but I have no idea of what was done to the cylinders
to improve cooling or what was done to the engine to use that power rating.

z42 said:

As a counter-point, the Rotax 900 series engines have been very successful. 4 cylinders, 1.2-1.35L displacement depending on model, so about 0.3L per cylinder. 5800 rpm max. 2000 hours TBO, including the turbocharged versions, which I believe about matches other widely used piston aviation engines today.

Granted, the stroke is very short at 61mm, so even at a blistering 5800 rpm the mean piston speed is a pedestrian 11.8 m/s.

Originally introduced in 1984, so let's say ~late 70'ies 'technology level', might not have been feasible in the 1940'ies.

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One thing we will note about the Rotax engines - while the sleeves are finned and air cooled, the heads are liquid cooled.

I would be interesting to see what could have been done with the deHavilland Gypsy King/Ranger V-770/Argus As 410 with similar mix of air and liquid cooling.

don4331 said:

One thing we will note about the Rotax engines - while the sleeves are finned and air cooled, the heads are liquid cooled.

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That's honestly one of the features I don't really understand the justification for on the Rotax. If you already have a water cooling system with pump and radiator etc, why not just have the water channel extend down around the cylinder barrels as well?

I can sort-of understand it if you start off with an air-cooled engine, and then at some point in the future you want to make a version generating more power while changing as little as possible; at that point adding liquid cooled heads might be a prudent decision. Particularly for something like an air cooled inline engine, where you'd otherwise might have issues with having to increase the bore spacing (at which point you're designing a mostly new engine anyway) in order to fit the necessary finning.

don4331 said:

One thing we will note about the Rotax engines - while the sleeves are finned and air cooled, the heads are liquid cooled.

I would be interesting to see what could have been done with the deHavilland Gypsy King/Ranger V-770/Argus As 410 with similar mix of air and liquid cooling.

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The Argus As-411 was an As-410 with enhanced features such as, among other things, an increased fin area and a special venturi-type system that used the exhaust flow to accelerate the cooling air.

Its final development was carried out by Renault/Snecma in the late 1940s (Snecma 12T). In this version, the engine produced 575 hp for takeoff at 3,300 rpm with 1,320 mm Hg boost, and 495 hp continuous at 2,400 m and 3,250 rpm (1,065 mm Hg). These performances were little different from those of the Argus As-411, but with a significantly greater reliability.

tomo pauk said:

... and by small 12 cyl engines in think of engines that are a good deal smaller then the small, but firmly mainstream V12 engine - the RR Merlin. So engines like the Kestrel/Peregrine, Jumo 210, small Fiat, HS and Curtiss engines (for the sake of discussion let's assume the siblings of these are still being developed in this time frame).

Yes, we will not be getting the equivalent of the 2-stage RR Griffon or the Jumo 213E here.

As-is, RR gotten the most from this, with Peregrine making around 1000 HP with the 100 oct fuel. Some of the engines, like the Jumo 210, will need to get to 3000, if not to 3200 rpm in order to compete. Being small, and looking at what Jumo were doing with the 213 series, even 3300 rpm does not look outlandish come mid-ww2.
Better S/Cs will also be needed, especially if the RPM range remains modest, talk 3000. Dry engine weight will easily approach 500 kg here.

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Look closely at the Ranger V-770 powered Bell XP-77. The engine was not supercharged. The aircraft went 330mph at 5000ft. This actually is impressive, and comparable with all the other contemporary aircraft at low altitude. Its performance fell off above 5000ft, rendering it tactically useless for any missions carried out by the USAAF.

Add a supercharger, and you may have a daylight air superiority fighter. The small engine limits the weapons you can carry. The XP-77 was supposed to have a 20mm cannon and a pair of .50 calibre machine guns. Did it have these when it went 330mph? Its bomb load would be pathetic. Could a supercharged XP-77 shoot down heavy bombers?

By WWII standards, the Rolls Royce Merlins were small engines, at a piddling 27_litres.

Howard Gibson said:

Look closely at the Ranger V-770 powered Bell XP-77. The engine was not supercharged. The aircraft went 330mph at 5000ft. This actually is impressive, and comparable with all the other contemporary aircraft at low altitude. Its performance fell off above 5000ft, rendering it tactically useless for any missions carried out by the USAAF.

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Did it get to 330mph?

The XP-77 proved to be difficult to fly, and despite lacking guns and armor, it did not approach its expected performance, mainly because it was woefully underpowered.

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My understanding is that the XP-77 was supposed to use a supercharged Ranger, and with that the expected performance was 330mph.

Still, that's not very good for an aircraft that was being developed in 1944.

GrauGeist said:

Just to add to the mix, what about the Czech engines developed by Praga?

The two I have in mind are the ESV and ESVR series V-12 engines.

Were these dead-ends or did Germany's annexation kill their development?

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I've always wondered about the Czech engines. Here's what I have on the ESVR:

CKD Praga ESVK aircraft engine
Brand: CKD-Praga, the Prague-Liben
Year: 1932

Performance:
Start (k): 770
(RPM): 2100 (BMEP = 151)
Nominal (k): 650

The Ranger V-770 used in the XP-77 was supercharged. It was supposed to make somewhere between 450hp and 520hp at 12,000ft. According to some accounts it was supposed to get a version with a Szydlowsky-Planiol supercharger was supposed to give over 400hp (or more) at around 25,000ft. Strangely this miracle engine never showed up (sarcasm).


The Original sales pitch was that the plane would do 400mph.

Performance figures are somewhat iffy. The initial rate of climb often listed does not match well with the time to 9,000ft sometimes given.

Shortround6 said:

The Ranger V-770 used in the XP-77 was supercharged. It was supposed to make somewhere between 450hp and 520hp at 12,000ft. According to some accounts it was supposed to get a version with a Szydlowsky-Planiol supercharger was supposed to give over 400hp (or more) at around 25,000ft. Strangely this miracle engine never showed up (sarcasm).

Ranger SGV-770

The Original sales pitch was that the plane would do 400mph.

Performance figures are somewhat iffy. The initial rate of climb often listed does not match well with the time to 9,000ft sometimes given.

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Andre Planiol went to US after french collapse in June 1940 and worked for some years for Ranger.

Did you know that a Planiol-Szydlowski supercharger was tested on the Ranger V-770 ?

I did know that tested a Planiol-Szydlowski supercharger on the Ranger engine and I believe they intended if for production on the "D" model. Or one of the "D"s.
I am not sure how far it got as post war aviation boom (everybody was going to fly to work instead of driving) never happened and the market for new 500-600hp engines
pretty much cratered.
There are photos of a possible candidate in several editions of Wilkinson's, at least a very sizable supercharger/housing on the back of the engine compared to some of the other V-770 engines. They may have combined it with a 2 speed supercharger drive?