A 'proper' way to have a 24 cylinder liquid-cooled aero engine for the ww2?

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and the Griffon Spitfires they just put streamlined teardrops in the cowl so that the extra width had almost no effect.
 
I really appreciate the detailed erudition in this answer. Thanks!
 
MiTasol said:
Even derated to use regular fuel the Fiat would possibly have topped 2500 hp in 1939
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Going from a racing engine to a production engine involves 3 things (at least).
1. can you get the power that you want from "production" fuel?
2. can you get the power you want (or even 80% ) for long enough periods of time for engine to be a worthwhile service engine?
Or do you have to "beef" up the engine (and hundreds of pounds) and run hundreds of RPMs slower to have decent engine life.
3. Getting engine power at sea level is a whole lot easier than getting power at 13,000-15,000ft.


Napier got over 1000hp from the Lion at the end of the 1920s
The Merlin was estimated to be worth 1500hp at sea level even using 87 octane fuel if the engine had been set up for sea level.
It took a while (several years) for Merlins to be rated at over 100 hours in service use. Merlins were steadily improved and were worth around 400 hours (depending on fighter or bomber transport engine use) by 1945.

 

3,100hp used very special fuel developed by Rod Banks, who had also developed the fuel for the Rolls-Royce R.

The R was capable of >>2,000hp. From memory 2,300hp in 1931 for the Schneider Trophy race and over 2,500hp for the speed record run. A special sprint version managed over 2,700hp

The R was detuned for use as a regular engine, known as the Griffon I. This was rated at ~1,500hp.

Given that, it would seem that the best an AS.6 would probably have done by 1939 would be around 1,800hp.
 

The AS.6 had two separate crankshafts that were independent of each other.

The front crankshaft drove the rear prop, the rear crankshaft drove the front prop.

The rear crankshaft drove the supercharger and accessories. The carburettor was mounted tot the supercharger.

The reduction drives were mounted between the crankshafts and drove the props via extension shafts in the vee.

IIRC the rear half was started first, then the front section.

Mixture distribution was a big problem with the AS.6, and there were problems with back-firing.
 

Good info, wuzak. Here's a rear view out of interest's sake.

Fiat AS.6
 

The mixture problems were solved in 34 by simulating ram air though fuel injection would have been a far better solution. As a concept this was revolutionary and produced a light strong and powerful engine. Fortunately the Italian powers that be decided liquid cooled engine were a thing of the past and never developed this engine. The much later AS.8 had potential but was way too late to prove itself.

On the plus side the AS.6 engine drove contra-rotating propellers which eliminated a number of flight issues like torque rolls, turning ability and the need for the offset fin which created drag which slowed other aircraft down.

We all tend to forget the Italians because most of their aircraft (except the trophy racers) were designed to compete with the French for how heavily the designer could use an ugly stick and it still fly. AFAICR the Italians never won that competition.

We tend to forget that on their engines they thought way outside the box so developed a lot of interesting engines - many that worked well. If my memory is correct the Italians were the first to build a production engine (as apposed to racing engine) that simultaneously conquered the pound per horsepower and horsepower per cubic inch barriers.

Naturally, like everyone else, they also had their spectacular failures - I seem to remember it was an Italian engine that had the supercharger mounted under the engine instead of behind it thus almost doubling frontal area.
 
Two things:

1. In all this, is there any preferred cross section for the engine (presumably to reduce drag)? I mean, would we prefer a 90-degree X, or an X with angles of 60-degrees and 120-degrees? If the 60-120 version, would we have the longer dimension upright or lateral (60-degrees on top, or 120-degrees on top)? Same question for an H configuration...sideways like a Sabre or vertical like a Hispanio-Suiza? Etc.

2. On the Fiat, I'm trying to figure out where the shaft passes through the front section of the engine. How can it be co-axial without interfering with the connecting rods as the crankshaft swings round and round?
 

The reduction gears are in the middle of the engine.

The extension shafts to the propellers run in the vee of the front half. Similar to motor cannon position on the Daimler-Benz DB 601/605/603. The extension shaft in the rear half runs inside the extension shaft from the front half.

Thus the rear crankshaft drives the front propeller, and the rear crankshaft drives the rear propeller.


FIAT AS.6 Aircraft Engine (for the MC.72)

The FIAT AS.6 engine ultimately produced 3,100 hp (2,312 kW) and was used to propel the MC.72 to 440.682 mph (709.209 km/h). But development of the 24-cylinder engine was plagued with issues and co…
oldmachinepress.com
 

Frontal area is the usual key

For a fighter a narrow engine is desirable as that keeps the fuselage narrow - anything more than the width of the cockpit is waste.

For a bomber probably the same as the flat engine makes a bigger undercarraige bulge but on four engines the horizontal would reduce frontal area..
 
Seems like the H engine was the most attainable with the least amount of cutting edge technology. Despite the Packard X engine, the gestation of most X engines was long and ended badly. An H engine (with two crankshafts) is more of a packaging and systems development with no cutting edge technology. One crank is better than two, but only if it works as reliably. Given the difficulties most countries/companies faced in securing sufficient engineering staff, limiting the technological hurdles is a way to keep a project on track. As 5 years seems to have been a common development time, trying to meet Tomo's timeline of 1939 requires starting in 1934. A way to save some time is to use the already developed components from the V12, such as the piston, valve, combustion chamber shape, intake and exhaust sizes, timing and valve duration etc. This can be done with both X or H, but the H has the potential to use the head casting general shape, the same bore spacing etc. If independent operation is considered useful, a counter rotating prop hub would be the major technological challenge. Starting with a V12, the VV24 (such as the DB604, 610 or V3420) is definitely easier to develop than an H24. However, the H is a better planform for single engine aircraft than the VV as it is narrower. It is important to keep in mind that the flat 12, as used in an H24 uses a 6 throw crankshaft and is not a boxer engine. Both connecting rods of the paired cylinders share the same journal, just like an automotive V8.

The Chrysler V16 was laid out in the pattern of the Fiat AS.6. It was started way to late to make it into the war, and it suffered from the issues of excessive length. For a single engine aircraft, placing the pilot far back behind the engine and fuel tanks can be a major disadvantage. I enjoyed this discussion.
 
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Another variant not mentioned, nor seen, might have been an inverted V next to an upright V. With the exhaust of open end of the V passing over the crankcase of the other V. Development of the inverted or upright V (depending on what was available in the engines of that country) would be relatively easy, and the rest of the development would be similar to the VV (either upright or inverted). An advantage might be narrowness. A disadvantage might be engine fires given the Luftwaffe experience of the He 177. The crankshafts could be the upper and lower points of the engine rectangle. Less development cost than an H, maybe some challenging packaging. The counter rotating prop would be a benefit, but not a requirement.
 
MiTasol said:
Interesting concept. Could make contra prop easy and height and width would be only marginally larger than the V and H engines. Should be less than the X an W engines. Engine mounts may be an issue though.
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The width will depend on how much offset there was between the two crankshafts.

The V-3420 had 10-11 inches between its crankshafts. If your upright and inverted vee engines had a similar spacing, side to side, then it would not be wider than an X-engine.

To be narrower this arrangement would need the offset to be vertical, keeping teh width of the V-12, but making the powerplant extremely tall.
 
sotaro said:
The Chrysler V16 was laid out in the pattern of the Fiat AS.6.
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It was, in that the power take-off was in the centre of the engine.

However, the Chrysler had a single crank (made of more than one piece), whereas the AS.6 had two independent crankshafts, driving two independent coaxial shafts.

Central power take-off or camshaft drive can be found in many motorcycle designs.
 
Post 112 specified the engine offset so that the exhaust passed over the top of the lower engine (With the exhaust of open end of the V passing over the crankcase of the other V) so something like this if 15 degrees rotation added. Obviously a common crank case would be narrower and by redesigning the oil sump on the top engine and removing it from the bottom engine some height could be removed also. Don't think I would like to work on it but it would probably still be more maintenance friendly than any H or VV.

I should have butchered a cross-section instead of a photo

 
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MiTasol and Wuzak, thank you for your interest in this engine layout idea. I love the photo of the end of the engine rearranged. Here is a diagram I found. I am surprised at how long the counterweights are. The intention is minimal area without excessive development time. I was thinking each V would have a vertical cylinder bank with the lower, inverted V's crankcase above the upper upright V's crankcase, but with clearance for the exhaust. I think that layout would require that none of the crankcase and block castings could be joined for the two engines. If a taller engine were acceptable, a common crankcase could be cast holding the two crankshafts side by side assuring good alignment and saving some weight.

https://www.thetruthaboutcars.com/wp-content/uploads/2014/03/Allison9.jpg

Wuzak, what do you think the weight of the Allison crank, either 6 or 12 counterweights is?
 
The length of the counterweights is no greater than the crank throw so the length is not a limiting factor. I do not know what the weight is but it is reasonably large as it is to counterbalance not just the crank throw but also to compensate in part for the rods and pistons.
 
The DB610 had the individual crankshaft centerlines about as close together as you're going to get with coupled/paired engines, regardless of the configuration (positioning of the engines).



Another factor to consider, is the size of the coupler and it's interior components needed to transfer power from the engines to the drive shaft.
In the attached photo of the DB610, you can see the size of the coupler housing in relation to the engines and it's size was relevant to the gear sets as well as drive disconnects for each engine.
 
I just measured the drawing of the Allison end on. From the outside end of the counterweight to the center of the crankshaft was almost exactly (measuring with a ruler) as the length of the outside of the connecting rod as it is around the crankshaft, to the center of the crankshaft. I was surprised.
 
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