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Miller Motor Jet

Ironically could be the most efficient
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If the jet engine's inception was taken another direction, surely we'd have its form half-way closer to its perfection; if you see the same connection let it see a resurrection- or else hit me with rejection, add your fishbone to my collection- nonetheless herein lies a half-assed high efficiency projection.

In the early 1900s, what were formerly known as "thermo-jets"- now referred to as "motor-jets" to avoid confusion with unrelated pulse-jets- were the precursor to the turbojets of today. As the technology of the day was not yet advanced enough for the concept of a bladed turbine that could withstand the high temperatures and pressures of deflagration, motor-jets utilized a separate reciprocating internal combustion engine to drive an axial compressor feeding into a combustion chamber and ejected straight through a nozzle and out the back; in functionality somewhat similar to an air-breathing turbo-rocket.

Obviously, the gutless behemoths of iron proved to be a rather heavy means of powering an air compressor. Nonetheless, the unparalleled ability of a reciprocating engine to operate efficiently under a ride range of load and speeds allowed low-speed flexibility and performance unmatched even today.

Let's first focus on the reciprocating engine driving the compressor itself; we'll call it a 3.8L direct-injected Flat-6 out of a newer Porsche 911. Only let's up the geometric compression ratio to 50:1.

Obviously, this compression ratio would blast the bejesus out of an aluminum block flat-6 with the first gentle opening of the throttle. The trick here is that the engine is not actually powering a compressor; the engine is both power and a compressor. Upon intake stroke, it inhales its full 3800ccs of air. However, it subsequently belches anywhere from 3/4 to 9/10 of it back out during the compression stroke through the exhaust valve or valves. As it is directly injected, fuel can be introduced within the final portion of the compression stroke that occurs with the exhaust valves closed- which would be the final 1/10 of travel at very light load to the final 1/4 permitted at full load- corresponding to a maximum dynamic compression ratio of 12.5:1.

At full load with a dynamic compression of 12.5:1, the 3800cc motor would be left with ~950ccs at 100% volumetric efficiency for combustion compressed into a total of 76ccs; 12.6ccs per cylinder. Though the engine only retained 1/4 total volume compressed to 12.5 original, it harnesses its expansion at a ratio of 50:1 and a final volume that is 4x greater.

Now what happens with the ejected 2850ccs of air? Well, each bank of three cylinders has its own set of short tube headers feeding into a de-Laval nozzle immediately before a 3-1 collector. Fuel is injected through a carburetor-style venture- into the throat and immediately before the exhaust valve allows the air ejection; by the time the deflagration/detonation wave reaches the exhaust valve, the engine is undergoing its own combustion stroke- preventing backpressure from forcing the valve back open.

When each cylinder exhausts its own combustion event, it does so through the second, smaller exhaust valve. Each secondary exhaust valve leads to a pipe that is jacketed around the length of the primary; allowing it to maintain sufficient cooling of the inner combustion event.

At the outlets of both 3-1 collectors, the continuous flow of pulsed combustion events may be ducted to generate thrust, or could be combined into a turbine variably coupled to the engine for a combined power output (turbo-compounded).

acurafan07, Jul 12 2013

Motorjet http://en.wikipedia.org/wiki/Motorjet
[acurafan07, Jul 13 2013]


       Run that past me again.
MaxwellBuchanan, Jul 12 2013

       It sounds like it should work, but I'm not sure of what principles would make this more efficient than the turbine engines we currently use. For example, I thougth that the efficientcy of a turbine engine was typically higher than the efficiency of a piston engine. I assumed that was because the compression or expansion of the gas was more efficeint using turbines rather than pistons. Or is it some other aspect of the design tha allows turbines to be more eficient that would not be diminished by replacing the compressor turbine with pistons.
scad mientist, Jul 12 2013

       You're basically using a piston engine of any given size (with a 50:1 geometric compression ratio; 50:1 pistons) as 75% a reciprocating compressor, 25% self serving; feeding the compressed portion to a high-efficiency pulsed afterburner (one per cylinder).   

       [scad] turbines provide the greatest power to weight ratio; while they may be more efficient within a narrow range, a reciprocating engine used even conventionally in a motorjet setup is likely to be more efficient overall. This should blow both out of the water so to speak.
acurafan07, Jul 12 2013

       Diagram please.
not_morrison_rm, Jul 13 2013

       I thought the theoretical efficiency advantage of a turbine engine was because it is a continuous process. The thermal mass and batch nature of a piston engine means that the effective hot and cold temperatures are less different.
spidermother, Jul 13 2013

       OK, I've read the idea now.   

       What's the reason for that configuration of de-Laval nozzles and collectors? You are ducting and merging supersonic flows. Wouldn't it be better to duct the hot compressed air to a single de-Laval nozzle:   


       or to 6 nozzles in a plane:   

spidermother, Jul 13 2013

       In practice, AFAIK, turbine engines are limited by the heat tolerance of the exhaust turbine. This idea (and other motor-jets) don't have that problem; the combustion chamber could theoretically be cooled, as in rocket motors.
spidermother, Jul 13 2013

       Hang on. It's possible that I've finally understood this idea.   

       Tell me if the following is the correct interpretation:   

       It's an internal combustion engine, but during the compression cycle most of the intaken air is squirted out and supplemented with extra fuel, and ignited to work like a rocket; a smaller part of the air stays in the cylinder and is supplemented with fuel to drive the power stroke as in a conventional engine.   

       Is that it?   

       If so, two questions: with a manageable engine size and speed, can you move enough air to give a good rocket effect? And is the compression of the air feeding the rocket part high enough?
MaxwellBuchanan, Jul 13 2013

       the piston engine still represents a pretty poor show in terms of efficiency, particularly power-to- weight. Why not use a small, conventional turbo shaft, to power a much larger turbine-free engine.   

       You could bury the turboshaft in the core of the engine, making it, in concept at least, similar to a turbofan. However, instead of gaining a whole lot of air with a low exhaust velocity, you'd be gaining an exhaust stream with a relative ly high exhaust velocity. So, more suitable for high speed flight. I'll keep this in mind for when I start my funding drive for "Concorde C".
bs0u0155, Jul 13 2013

       I might be missing something here. I'm aware that if an engine uses a fuel injector, it can in theory be done just about anywhere during a 4-stroke engine cycle. (Of course you can't expect much useful result if you inject fuel during the exhaust stroke.)   

       So, I can imagine two different ways of taking advantage of a 50:1 compression stroke.   

       First, you could inject a very small amount of fuel at a "normal" place relative to the piston being at top-dead-center. You wouldn't need a spark because of the high heat of the compressed air. You might avoid blowing the cylinder apart with the explosion. And, of course, you would not be using a lot of the oxygen in the compressed air, which you want mostly to provide to the jet engine instead of the piston engine. It would be OK to send the entire exhaust of the piston engine into the jet engine, since most of the oxygen would still be there (not counting any that ended up as part of a nitrogen oxide).   

       Second, at some point during the expansion stroke you could inject a larger amount of fuel, and apply a spark. The compression ratio at this injection point might have dropped from 50:1 to the 12.5 :1 that [acurafan07] had mentioned. You wouldn't want to send this exhaust into the jet engine, but since we are talking about just one cylinder of the piston engine, some other cylinder could be feeding the jet engine. Use as many cylinders as needed.
Vernon, Jul 13 2013

       [Spider] You're right; a single nozzle would definitely be better in thinking about it. Six would create large unnecessary restriction.   

       [Vernon] I think I can envision what you describe; however I feel that one of the huge advantages of the astronomical compression is the fact that, relative to the piston energy, only 1/5 the air/energy is used to compress is as is harnessed through expansion; what you describe would actually be 50:1 compression with a 12.5:1 expansion.   

       [Maxwell] you are spot on! I certainly believe so; for the arbitrary 3.8L engine mentioned, 2.8L or more will be going to the rocket.   

       If this "rocket" feeds into a Tesla style disk turbine, detonation would not deteriorate the disks unlike bladed turbines; and the remaining exhaust stream after the tesla could likely power an additional turbocharger feeding into the engine, which could up to double that 2.8L feeding the rocket (the valve timing would have to force out even more air during compression since boost on 12.5 would be pushing it).
acurafan07, Jul 13 2013

       [bigsleep] Find me any type of bladed compressor that achieves constant compression off idle to maximum operating speed...no? how about a positive displacement type resistant to the backpressure of combustion? These are both tremendous advantages.   

       I might also add that no turbofan could withstand pulsed detonations while this, in conjunction with a disk turbine, would be immune. Jet engines must take in far more air than they consume for dilution and cooling purposes if the jet is to attain even a moderate pressure ratio (which is not to be confused with compression ratio as pressure ratios convert to much lower compression values). Those really thin, relatively fragile blades on those turbine thingies really are quite particular about what conditions will not destroy them.
acurafan07, Jul 13 2013

       Why not instead build a single-cylinder engine, with the cylinder equal to the length of your desired journey, and the car fixed securely to the piston?
MaxwellBuchanan, Jul 13 2013

       Like having a bunch of small intermittent, yet high pressure compressors ducted into the same combustion chamber to provide a constant flow?   

       [Maxwell] In answer to your second question which I must have missed, yes; pressure ratio achieved in the rocket section is determined by factors like pipe diameter from each cylinder, geometry and volume of rocket combustion chamber, nozzle taper, etc. If there are two rockets with combustion chamber volume of 100cc each totaling 200; the pipe diameter coming from the engine reasonably small- the effective compression ratio of the rocket will be 14:1 with the air pressurized @ 206 psi in varying SCFM (dependent on engine speed) coming from the reciprocating engine.   

       And I tried that a couple of times! Couldn't ever get the piston to stop in time, though. Always ended up right where I started.
acurafan07, Jul 13 2013


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