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Piston engine thrust augmentation


Toxn
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Here is an interesting paper on optimising a piston engine exhaust stack for thrust. Here is another and another.

 

Between using the exhaust stack for direct thrust and things like the meredith effect, it seems that a small but significant fraction of the thrust produced by a piston aircraft ends up coming from things other than the prop.

 

All of which makes me wonder: what would happen if you decided to ditch the prop alltogether (or at least reduce it to a first compressor stage/something to keep the radiator running). What would a piston engine designed to produce thrust mainly from the exhaust look like?

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Like a motorjet, I reckon.  Or like a regular jet engine, but worse.

 

Off the top of my head, and without deriving the equations, the efficiency of a jet nozzle is partially a function of how high the pressure of the gas entering it is.  The nozzle basically converts the static pressure of the gas into velocity, and like most things in thermo this works best if the pressure/temperature differences are really high.

 

Piston engines don't produce particularly high pressure gas.  I would imagine that with WWII jet technology the differences were smaller, but these days the differences are pretty stark.  Contemporary airliner engines are in the 40:1 pressure ratio neighborhood when they're operating at design point, and the best diesel engines only manage a 25:1 compression ratio.  Compression ratio and pressure ratio are not the same thing, and when you crunch the numbers the piston engines come off looking even worse.

 

That said, I think the engine in the LeClerc works like this, except instead of producing thrust the gas generated by a piston motor is used to spin a turbine.

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I think regular jet but worse is a good summary - since you want high exhaust pressure you want a super high chamber pressure and a very low compression ratio (think of it as an expansion ratio taking energy from the hot gases), along with masses of forced induction to get the overall compression ratio somewhere workable. Imagine the front half of a jet engine driven from & feeding a very silly-looking piston engine. I wager a piston engine that takes a given amount of airflow per second is waaay heavier than a similar combustion chamber & turbine assembly from a jet engine, so you'll end up with something very heavy and very anaemic - the number of right-angle turns alone needed to link up all your cylinders with the compressor is bound to sap any useful power

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I think regular jet but worse is a good summary - since you want high exhaust pressure you want a super high chamber pressure and a very low compression ratio (think of it as an expansion ratio taking energy from the hot gases), along with masses of forced induction to get the overall compression ratio somewhere workable. Imagine the front half of a jet engine driven from & feeding a very silly-looking piston engine. I wager a piston engine that takes a given amount of airflow per second is waaay heavier than a similar combustion chamber & turbine assembly from a jet engine, so you'll end up with something very heavy and very anaemic - the number of right-angle turns alone needed to link up all your cylinders with the compressor is bound to sap any useful power

 

Wow, what the fuck is this a new poster or something?

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I think regular jet but worse is a good summary - since you want high exhaust pressure you want a super high chamber pressure and a very low compression ratio (think of it as an expansion ratio taking energy from the hot gases), along with masses of forced induction to get the overall compression ratio somewhere workable. Imagine the front half of a jet engine driven from & feeding a very silly-looking piston engine. I wager a piston engine that takes a given amount of airflow per second is waaay heavier than a similar combustion chamber & turbine assembly from a jet engine, so you'll end up with something very heavy and very anaemic - the number of right-angle turns alone needed to link up all your cylinders with the compressor is bound to sap any useful power

Welcome to SH, comrade!

 

 

 

 

 

 

 

 

 

 

:ph34r:

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The Soviets built two motorjet designs, the I-250 and Su-5. Their performance seems pretty unimpressive; wiki says they both had a planned maximum speed of about 800 km/h, which is pretty good for a piston engine but not earth shattering (the -J version of the Thunderbolt was capable about the same performance). I'm not sure how optimized the exhausts were for thrust, but I would assume they were since Russians aren't stupid. Overall, it's good for an incremental performance at best I'd think.

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Motorjets, at least in as much as they were developed, never seemed like a good solution given that you're giving up the advantages of a prop for the dubious benefits offered by a ducted fan. Even so, I wonder if they could have been improved upon by recapturing as much wasted energy as possible. This would involve putting the engine and radiators in the duct, and then using the waste heat/exhaust to heat the outgoing air.

 

From the other direction, one of the problems with pulse jets is that they simply cannot generate much pressure during their combustion cycle. This results in them wasting a lot of their energy (ie: they burn fuel exceedingly efficiently* but convert nearly all of it to heat and noise) and has resulted in a bunch of attempted fixes over the years. One - trying to make the combustion process more efficient by having a valve at both inlet and outlet - leads naturally into something like a bizarro two-stroke engine: a manually-operated valve assembly and fuel system up front, one big cylinder/combustion chamber behind it, an extremely large-bore and lightweight piston acting as the rear valve, a long tailpipe feeding from the rear valve and a spark ignition system powered by the crank. Pulse jets seem strangely unaffected by turns in the tailpipe so that aspect wouldn't be a problem here.

 

From these two examples, you can come to a sort of optimised design resembling a big-bore radial inside a duct. The engine itself would barely spin a fan up front for forced cooling, and would instead produce most of its power via the tuned exhausts coming off the whole assembly. I have no idea how practical this would be, but it represents something that nobody (except, perhaps, Harley Davidson) seems to have deliberately tried to achieve: a piston engine whose sole purpose is to make as much exhaust as possible.

 

 

* To give you an idea, one of the most efficient thrust (but not thrust/mass) configurations for a pulse jet is to simply mix water into the fuel system to generate steam from the waste heat. You can get a well-designed pulse jet to run in this configuration even when your 'fuel' is 80% water.

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Adding the water is a way to cheat the efficiency of the nozzle higher by increasing the specific heat capacity of the working fluid, but at the expense of fuel energy density since the water obviously does not burn.  Or you can think of it as reducing specific fuel consumption, but also reducing fuel fraction.  The math works out the same.

 

In proper, modern jet engines there's very little problem with unburned fuel.  The fuel/air mix is very lean at about half the stoichiometric ideal, since the jet is gobbling down stonking enormous amounts of air (600 kg/s in a 747's CF6), so there should be more than enough oxygen to get everything burned.  There are some sacrifices and trade-offs in burner design that can lead to a bit of smoke.  So just like pulse jets, the issue isn't getting the fuel burned, it's optimizing the thermodynamic cycle.

 

The Brayton cycle is hard to beat as a gas generator, at least using hydrocarbon fuels and air, because you don't have to worry about knocking at all.  The fuel is getting burned right the second it enters the engine and the air has already been compressed.  The pressure you can achieve prior to ignition is a function of how sharp your engineers and materials scientists are; there's no worry about keeping the fuel air mixture from igniting prematurely.  If you're using a piston engine as a gas generator it's hard to see how the pressure could possibly get cranked up as high as you can get it with a jet engine.  That's the really fundamental problem; nozzles work best with very high pressure gas.

 

But like I said, I think the Leclerc's engine basically works this way.  There's an itty bitty piston engine that acts as a gas generator that spins a turbine.  It's not very efficient (AMX-56 gets about the same or worse fuel economy as an Abrams), but for some reason it gives excellent throttle response and its a very compact power plant.

 

 

Edit:  Actually, maybe that's not how hyperbar engines work.  I'm having a hell of a time finding a good explanation.

 

Edit Edit:

Here's a PV diagram for the Brayton Cycle:

 

braytonpv.gif

 

This is pressure on the Y axis and volume on the X axis.  Pressure is force divided by area, and area is distance squared.  Volume is distance cubed, so the product of pressure and volume is force divided by distance squared times distance cubed, or force times distance, which is work.  So the area inside of that Nike swoosh looking shape is work.  It works about the same whether we're spinning a turbine in a turboshaft or using a propelling nozzle in a turbojet, or somewhere in-between as in a turbofan.

 

Any sort of piston jet hybrid that uses the piston part of the engine as a gas generator is going to have the fundamental problem that it can't make the swoosh as tall as a Brayton Cycle compressor and gas generator can because of fuel knocking.

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  • 2 weeks later...
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The last example in this article illustrates one approach to boosting a piston engine: make the turbocharger into a turbine.

 

https://oldmachinepress.com/2016/03/24/fkfs-gruppen-flugmotor-a-c-and-d/

 

 

This is not Sparta, this is definitely madness:

 

 

 

This engine consisted of five V-12 engine sections mounted around a central turbine. The V-12 engine sections were based on an extremely-high-output diesel engine Kamm had helped design while at the Stevens Institute of Technology in Hoboken, New Jersey in the early 1950s. The V-12s were air-cooled, two-stroke, loop-scavenged engines with side-by-side connecting rods. The turbine had a nine-stage axial compressor section, a combustion section, and a five-stage exhaust turbine section. High-pressure air from the compressor section would provide the incoming charge for the diesel engine. The diesel’s exhaust would be expelled into the exhaust section of the turbine.
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