Trying to figure out the plausibility of a fictional aircraft I created; specifically the engine.

[MH4UAstragon]

In this design, the engine is a combination of diesel and jet engine technology.

 

A 36-cylinder diesel engine is the core part of the concept, operating like a pair of linked up 18-cylinder motors. The front engine is a bit wider than the rear one, but has a wider, hollow crankshaft rotating one way while the rear engine crankshaft rotates the other way and extends through the first one.

 

Questions regarding the piston portion of the engine:

- In this idea, the engine uses lightweight but strong high temperature alloys. I'm imagining it to be a mix of beryllium, titanium, and magnesium, maybe the cylinders could be made from high-temperature strength steel if need be. I do know that beryllium greatly reduces fire hazards from other metals it is alloyed with, so is this combination viable?

- The engine is a "star" configuration, or liquid-cooled radial. The aircraft it is mounted on would be able to go well past mach 2. Air passes around a shock cone akin to what the SR71 used in its engines, one or more Stirling Coolers powered by the engine itself cool down the intake air, some of the air gets combusted while the rest flows around the engine inside its cowling, passing over a ring of radiators. The two halves of the engine drive a pair of conta-rotating ducted fans. One half of the engine also drives a centrifugal supercharger, while the other drives the Stirling coolers. The whole engine is turbo-compound, driving an Axial Compressor in front of the supercharger. The main question here is about the Stirling Coolers: can they remove the heat from the air before it reaches the engine and release it behind it fast enough to keep up with this?

- As an alternative idea to cool the motor, is regenerative cooling (like how rocket engines keep their nozzles cool) actually feasible in a piston-type engine, where fuel would flow around the cylinders, remove a good deal of heat, and then the hot fuel would combust in the cylinders (for a possible efficiency gain in combustion?)?

 

Once air has flown through this part of the engine, it then can be compressed by another, larger Axial Compressor, mixed with fuel, and combusted for jet thrust.

 

If the reader is having trouble wrapping their mind around this, picture it as a turbofan engine fused with the motorjet concept. The motor powers the fan part of the engine and the jet combustion can be turned on or off as needed. Due to alloy choice, the motor part of the engine would produce at least 6000 or more HP while weighing roughly half that. The total weight of the engine would not be all that much higher than modern jet engines.

 

The aircraft this engine would go in is part of an idea for a video game I have, but I want the idea to be scientifically possible and not leap headfirst into scifi stuff unless there is no other alternative.

Edited February 26, 2016 by MH4UAstragon

[Enthalpy]

Ahum for the crankshafts. That won't be so easy.

If it looks like in a car (or boat, or plane) then a crankshaft can't have any matter at its center, or the rods will bang against it. So the other shaft going though looks impossible.

But if it looks like in a locomotive, yes. Try to find a picture: the eccentric runway is very broad and covers the centerline everywhere. It could conceivably contain the shaft from the other engine.

Question is whether one wants such a crankshaft that lets the bearings operate at a higher speed and is heavier.

For contrarotative fans, you could consider electric motors. They are light, small, reliable, silent. Create the electricity from a combustion engine if you wish (...or as you can!).
http://www.scienceforums.net/topic/73798-quick-electric-machines/

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"Strong high temperature alloy" doesn't go together presently, so it depends on how fictional your work is.

Mg has nothing to do at high temperature.

Ti more or less, but not enough for the hot parts of an engine up to now.

Be is toxic and its oxide more so (piston engines produce dust). It's also brittle, expensive, and so on.

Presently, where steel and copper alloys don't suffice, we use nickel alloys. That's the case in aeroplane turbines because some parts feel the heat permanently, as opposed to a piston engine where heat lasts for short and the material encounters cooler air the rest of the time. Active cooling does the rest, both in piston and turbine engines.

Cobalt hasn't improved on nickel. The next big thing might possibly be molybdenum. I advocate its use in rocket turbopumps.
http://www.scienceforums.net/topic/73571-rocket-engine-with-electric-pumps/page-2#entry781023
How realistic and mature its technology is remains to see.

The next available high-temp metals are niobium, tantalum, tungsten.

None of them is light.

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If the work is fiction, the next almost-realistic materials would be ceramics. Some engine hot parts have been prototyped. Zirconia isn't bad. SiC works orange hot for a few seconds in car brakes. Maybe a fictional surface treatment is half-realistic and makes them usable.

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Regenerative cooling for piston engines: no, as a gut feeling. Fuels produce much heat. Even if only 10% is lost to the walls (I don't know the present figures) that would suffice to destroy the fuel. But you might use the heat extracted by an other coolant to run a separate engine. It's just that this doesn't look like aeroplane technology.

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Mach 2 with piston engines is definitely fiction. Some WWII piston planes achieved Mach 1 during dives. Their engines weren't bad to present standards. Twice the speed needs 8 times the power.

But over that, why the hell should one prefer a piston engine? Turbines are better for nearly every aspect.

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Stirling coolers are big, heavy, clumsy. I'd reject them intuitively. Anyway, every cooler must dump the heat it absorbed, and even more. Is there still a benefit in a cooler then?

In the amount of compressed air is limited, like in a truck engine, you can cool it with a passive heat exchanger. It's called an intercooler. But it needs more air flow through the cooler than through the compression thing, which isn't easy at an aircraft.

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Yes, I do have trouble imaging said engine. 6000HP was a realistic figure for WWII combat planes, but not at the mass of a turbine presently, and it won't fly at Mach 2.

Why not a ramjet? Add a turbine for takeoff, but at 2<Mach<3 a ramjet is perfect.