Various Questions about X-24 engines/other exotic layouts, from a guy who doesn't know much about engines

I've been curious lately about the design of the various "weird" engine layouts being tested in WWII, mostly the X-24

First, some things I want to make sure I'm correct on, so that I'm not working from faulty assumptions:

1: Larger Cylinder Displacement means lower max RPM because the combustion travels at a finite speed, and for max power, it needs to not only ignite the entire fuel-air mixture, but have enough time for the entire mixture to do work on the piston

2: This practical limit on Displacement per cylinder is probably around 4 Liters/250 cubic inches, give or take, _maybe_ up to 5L or 300 cubic inches or so.

3: in the mid-late 1940s, the amount of power that could be _practically_ obtained per liter of displacement (ie, suitably reliable for long term military use) was approaching a limit.


As for the actual questions:


1: why do most of the X-24 engine attempts appear to be Oversquare? I would assume an Undersquare design would have fewer issues fitting sufficiently strong connecting rod assemblies, and that the lower max rpm would be more than compensated for by the greater torque per power stroke, and the sheer number of power strokes per revolution

2: were the "Inline radials" ever going to be practical?

3: if Turbine engines never came into service, would we see X-24 designs in use alongside H-24, or was the H-block just better, even if more development time were poured into the X layout?

I'm not an expert either but I think larger displacement means bigger pistons so you have more mass to move and as the speed goes up so does the momentum of that mass and all the associated forces.

Spindash64 said:

I've been curious lately about the design of the various "weird" engine layouts being tested in WWII, mostly the X-24

First, some things I want to make sure I'm correct on, so that I'm not working from faulty assumptions:

1: Larger Cylinder Displacement means lower max RPM because the combustion travels at a finite speed, and for max power, it needs to not only ignite the entire fuel-air mixture, but have enough time for the entire mixture to do work on the piston

2: This practical limit on Displacement per cylinder is probably around 4 Liters/250 cubic inches, give or take, _maybe_ up to 5L or 300 cubic inches or so.

3: in the mid-late 1940s, the amount of power that could be _practically_ obtained per liter of displacement (ie, suitably reliable for long term military use) was approaching a limit.


As for the actual questions:


1: why do most of the X-24 engine attempts appear to be Oversquare? I would assume an Undersquare design would have fewer issues fitting sufficiently strong connecting rod assemblies, and that the lower max rpm would be more than compensated for by the greater torque per power stroke, and the sheer number of power strokes per revolution

2: were the "Inline radials" ever going to be practical?

3: if Turbine engines never came into service, would we see X-24 designs in use alongside H-24, or was the H-block just better, even if more development time were poured into the X layout?

Click to expand...

Some thoughts also...

- Indeed, the combustion travel is important and limits cylinder dimensions, but above all what matters is the moving parts mass.

- The different parameters that affect the performance of an engine - displacement, mass of the moving parts, section of the parts subject to constraints such as connecting rods, bearings and cylinder walls, gas speed in the pipes, etc., do not evolve in parallel, but are proportional sometimes to the linear dimensions, sometimes to their square, sometimes to their cube. This is why increasing the cylinder dimensions is never the right solution.

- H-engines have a superiority over X-engines : they distribute the forces on two crankshafts, while the others have a single crankshaft which, therefore, is subject to greater stress.

- A catastrophic example of an X-engine failure is the Vulture, whose connecting rod problems Rolls-Royce was never able to overcome. The "star" connecting rods were apparently subjected to unprecedented stresses that were neither met in V engines, nor radials - nor in flat engines, which are the basic principle of H engines.

- The Vulture was based on the Peregrine dimensions (bore & stroke 5 x 5.5 in) and it had been planned that later they could be upscaled up to those of the Merlin (5.4 x 6 in). In both cases, these were not oversquare dimensions at all.

My understanding is that they were looking to increase the HP levels by using proven design items but increasing the RPM of the engine. This meant shorter stroke engines, and the easiest way to reach those goals would be increasing the number of cylinders.
The thought process being you could take a smaller displacement V-12 and just double it but use a single crank shaft, so the weight does not double it weighs about 3/4 of what two V-12's would weigh.
There is no reason to believe that a multi row radial engine with inline cylinders could not be made to work, probably liquid cooled. But as you have already stated Large aircooled Radials filled the immediate need, and Jets filled it long term. So after 1943 I think the correct decisions were made to drop most of the odd engine configurations and spend your time and money on other projects.

1. The max cylinder bore is also limited by cooling, particularly air-cooled engines. Wright were able to get to 6.125" by the genius of Sam Heron. P&W tried with the Hornet, however it ran ok but it was never quite there. Bristol used 5.75", and P&W followed.

3. It is worth reading "The Avro Manchester, The legend Behind the Lancaster" by Robert Kirby, as it tells of the history of the Vulture. It started out being very unreliable, but by May to August 1941 most of the bugs had been fixed and it was about as reliable as the other engines in service at the time.

Spindash64 said:

I've been curious lately about the design of the various "weird" engine layouts being tested in WWII, mostly the X-24

First, some things I want to make sure I'm correct on, so that I'm not working from faulty assumptions:

1: Larger Cylinder Displacement means lower max RPM because the combustion travels at a finite speed, and for max power, it needs to not only ignite the entire fuel-air mixture, but have enough time for the entire mixture to do work on the piston

2: This practical limit on Displacement per cylinder is probably around 4 Liters/250 cubic inches, give or take, _maybe_ up to 5L or 300 cubic inches or so.

3: in the mid-late 1940s, the amount of power that could be _practically_ obtained per liter of displacement (ie, suitably reliable for long term military use) was approaching a limit.


As for the actual questions:


1: why do most of the X-24 engine attempts appear to be Oversquare? I would assume an Undersquare design would have fewer issues fitting sufficiently strong connecting rod assemblies, and that the lower max rpm would be more than compensated for by the greater torque per power stroke, and the sheer number of power strokes per revolution

2: were the "Inline radials" ever going to be practical?

3: if Turbine engines never came into service, would we see X-24 designs in use alongside H-24, or was the H-block just better, even if more development time were poured into the X layout?

Click to expand...

1. Combustion traveling at a finite speed puts an upper limit on cylinder bore for a spark-ignition engine (Otto cycle). Does not really affect RPM. RPM is mostly constrained by piston speed. Piston speed is constrained by expected life of the engine. RPM is also constrained by the ability to get air in and out of the engine, but that is also a complicated result of bore/stroke ratio, valve layout, etc.

2. Not sure of the practical limit but Sterling TCG-8 motor ran about 450 in3 per cylinder, powering an 83-foot Coast Guard boat in WWII. Each cylinder produced less than 100 hp, so much about it was antiquated even for WWII.

3. Yes and no. This may have been because most of the engines flying had several years of development, and none of the new wonder engines were available -- and most of them would be discontinued at the end of the war as the jet age dawned. There simply is no market for WWII aviation-sized gasoline engines, as their niche has been taken over by turboprops. Because of that, it's hard to say what they may have evolved to. Had the amount of money spent making auto engines more powerful and efficient been spent on making piston aviation engines more powerful and efficient, we might have seen considerable growth.

1. X-24s were "cutting edge" for the time, and engine development had just passed the threshold from being undersquare to oversquare. Auto engines followed in the 1950s.

1.a. Do not get fixated on engine RPM. All these engines were geared, anyway, and any feasible engine RPM could be matched to propeller RPM in this way. HP = Torque x RPM (x a constant). Trading RPM for torque or vice versa buys you nothing. Besides, the ultimate optimization is frontal area and weight, not displacement.

2. We don't know. Water cooled, possibly. Air cooled, I don't think you were going much past an 11-cylinder (44-cylinder) version of a Wasp Major. Very iffy if you could perfect a 5- or 6-row air cooled radial engine.

3. We don't know. The H-24 and X-24 designs that were produced were so much of an apples and oranges thing (sleeve valves vs. poppet valves, for example) that I'm not sure a meaningful comparison is possible. My opinion was that there wasn't much to be gained over an H-24 by a 90-90-90-90 X-24 layout, but there may have been some merit to a 60-120-60-120 layout. But we don't really know.

3.a. Crankshaft details seem to have really devilled X-24 designs. Should we have had master cylinder X-24s (nearly every design I know of)? Offset blade-and-fork cranks with 12 throws? Offset cranks with 24 throws? etc. Theoretically, X-24s should have had a frontal area advantage if the cylinder angle was kept small enough, and 1 crankshaft should have been lighter than 2. But neither of those advantages ever materialized in practice.