Potential of motorjets?

[FlyingPete]

So recently I learned about an early type of jet engine, developed mainly by the Italians in the 1940s- the motorjet. It works similarly to the turbojet, except rather than the compressor being driven by an exhaust turbine, it's driven by a separate piston engine. The hot gas from the combustion chamber goes directly to the nozzle- with increased energy due to not driving a turbine on the way. It's basically something between a boosted ramjet and an afterburning ducted fan- depending on how the compressor is set up.

The thing that interests me about this design is that you have two operating modes- the 'cold' jet thrust provided by the compressed air, and afterburning mode where the compressed air is heated for increased thrust. You'd have two distinct combustion processes, and so two degrees of freedom to control the thrust produced.

Now, these engines pretty much died off when a functioning turbojet became practical, but I wonder if they'd still be useful today. Particularly, part-load efficiency of a piston engine doesn't drop off as much as it does with a gas turbine, plus there's the option to reduce fuelling or switch off the combustion chamber and run on the compressed air thrust only. There's also the maintenance reduction associated with not having to deal with a gas turbine.

I reckon there's potential here for fairly small, fast aircraft doing short hops between remote areas. Most likely you'd use a turbodiesel engine so that both combustion processes can use the same fuel.

[Bill Phil]

There's not much of a maintenance reduction. Piston engines are very complicated, when compared with turbojets. Both are complicated mechanisms, however, using a piston engine in conjunction with a turbine will only add to the complication.

By the way, it would have a turbine. But it's not run by the compressor as in other types of jets. The motor runs it.

[linkxsc]

I kinda fail to see the advantage... youd still need a fairly complicated compressor, and unlike a jet (centrifugal compressor, or axial flow) there would be extremely high chances of flameout due to the piston engine not feeding the proper amount of air.

Like I get it and it could be done, hell could probably build something along these lines out of an old 8c radial (2 rows of 4) RC engine ive got (take the cam out of 1, have a new 1 made that opens the valves every stroke)

Run the exhausts from those into a CO2 canister (to stabilize the pressure, less pulsing) and then feed that compressed air into a pulsejet.

Convoluted as hell. But would be efficient at flying low speeds due to the prop still working. Then compressed air is there ready to help start the pulsejet.

Ofcourse, by that point though... I could have just spent $1500 on an actual RC jet engine which would be several times as efficient, and less likely to break.

[pincushionman]

You're trading one expensive system for another. Jets are expensive because of the tight tolerances required. The airflow considerations are quite complicated, but they essentially only have one moving part in the whole system.*

Piston engines have lots of moving parts, and they're heavy. And if you're trying to take any kind of advantage of that pseudo-afterburning duct, it would really need to be a high-performance engine, which also has those pesky tight tolerances to deal with.

*in concept, at least

[sgt_flyer]

Could be useful for one thing - getting a ramjet up to speed - the motor driven compressor would have a separate closable intake (to limit drag once the ramjet has enough speed to not need the compressor anymore) and feed air directly into the combustion chamber. (Though, you'll need to keep it running afterwards to drive the fuel pumps too)

However, it would require extremely complex set of injection nozzles inside the ramjet's combustion chamber - as you'll need to have both kind of injection nozzles (fuel + compressed air) properly placed in the combustion chamber (to have a proper stable combustion) - so, having the two sets of injection nozzles mixed up will - reduce the number of fuel nozzles compared to a classic jet engine, and tremendeously increase the plumbing complexity.

[DaveofDefeat]

The problem with using pistons for anything is you run into speed and efficiency limitations. motorjets died for a reason.

[pincushionman]

...The hot gas from the combustion chamber goes directly to the nozzle- with increased energy due to not driving a turbine on the way. It's basically something between a boosted ramjet and an afterburning ducted fan- depending on how the compressor is set up...

Also, core exhaust (the hot combustion gas going directly out the nozzle) is about worthless in a jet engine, compared with what bypass air is capable of doing. Nobody except nobody* uses even ordinary turbojets anymore, because it's so much more efficient to use that gas to drive even more turbine, which in turn drives a ducted fan. Basically everybody is driving turbofans.

*except for some niche applications like cruise missiles, where having a simple engine is great because the whole thing is supposed to be disposeable

[magnemoe]

The problem with using pistons for anything is you run into speed and efficiency limitations. motorjets died for a reason.

Yes, jet engines has higher performance for their weight. their downside is higher price. This is why every plane except small private planes like Cessna uses jets.

Also as pincushionman says, you use turbofans, exceptions are also fighter jets and other supersonic planes. On the other hand turboprop planes like C-130 and all helicopters except the cheapest ones uses jet engines too.

[prophet_01]

Well, it might be handy for very specialised planes with ram jets. The SR-71 used compressors on the ground in order to get it's engines running, but if I remember corectly they didn't took those motors up in the air, but left them behind instead. Can somebody confirm this? I don't trust wikipedia with those details.

[sgt_flyer]

Well, it might be handy for very specialised planes with ram jets. The SR-71 used compressors on the ground in order to get it's engines running, but if I remember corectly they didn't took those motors up in the air, but left them behind instead. Can somebody confirm this? I don't trust wikipedia with those details.

regarding the SR-71, they used a power cart, but it was to directly drive the aircraft's own jet engine (JP-7 was extremely hard to burn, it required both a special additive, triethylborane (ignites with air...) and that the turbine was spinning at 3200 rpm before it was able to sustain the reaction - the power cart engines were simply used to make the turbine spin fast enough

[K^2]

By the way, it would have a turbine. But it's not run by the compressor as in other types of jets. The motor runs it.

I think you got these backwards. A motorjet has a compressor, but not a turbine.

The only modern application of motorjets that I can think of is for RC models, using an EDF in place of a motor/compressor stage. It'd be horribly inefficient, but it removes most of the costs associated with model turbojets, so I'm actually quite puzzled that we don't see a bunch of these on the market.

[mrfox]

Motorjets was a workaround due to 2 technological limitations of its day;

1- the lack of high temperature/high strength materials to construct the turbine, and

2- material and aerodynamics for efficient, large diameter fan blades.

[K^2]

It's more the former than the later, because early jet designs relied on a centrifugal compressor stage to compensate for inability of axial compressors to generate sufficient compression ratio alone.

[mrfox]

But imagine a world/situation where high temperature alloys are in short supply. Given the aerodynamics and composites avlb today - you can make a reasonably fast aircraft driven by a ducted fan that is powered by a recriprocating engine. Or electricity for that matter. No need for additional fuel injection after the compressor/fan.

Edited April 20, 2015 by mrfox

[Naten]

Well, considering that the aircraft was one of the slowest, if not the slowest, jet-powered aircraft... I doubt that the idea will get anywhere.

(I'm 13 and learning over time, so I might not get this right. )

Piston engines are not very fuel efficient, and using one to power a compressor rather than using a gas turbine seems much less efficient. Why not just use a small starter motor, like conventional jet engines do, to start the fan. When the fan is started and fuel is injected into the air, and the compressor ignites the fuel-air mixture, the hot exhaust gases would power the turbine and keep the fan cycle going. That way, the fuel that is burned to keep the thrust coming and the aircraft moving also keeps the turbine running. It just doesn't seem like the motorjet would work any better, nor would it be practical because of the maintenance issues associated with "moar parts" (We all know that feel, KSP players; "moar parts" is not always better. (Even though it usually is. )) like previous posters said.

So, from what I read, hear, and know, this thing is just an "inverse turboprop" of sorts? Like, the other way around; a [propeller driven by a*] piston engine driving the jet engine('s compressor) is a motorjet, and the jet mainly provides thrust. And the turboprop is a propeller driven by a jet engine's turbine.

*Sometimes.

Why not use turboprops instead?

Turboprop fanboy mode activated.

TURBOPROP IS BEST PROP TURBOPROP BEST JET TURBOPROP RULE WORLD HAIL TURBOPROP

MORE EFFICIENT AND BETTER THAN EVERY JET ENGINE. PROVIDE LESS THRUST BUT WHO CARES, JUST ADD MOAAAAAAR AND MAKE SMALLER AIRPLANE AND YOU BE FINE

TURBOPROP BEST ENGINE 2K15

"10/10 WOULD BUILD AIRCRAFT WITH TURBOPROP" -ATR

Edited April 20, 2015 by Naten

[mrfox]

Piston diesels are actually more efficient compared to gas turbines, because their compression ratio is independent of rpm, and their intermittent combusion allows less expensive materials to be used due to lower steady state temps Their big drawback is their power to weight ratio.

[magnemoe]

Turboprop fanboy mode activated.

TURBOPROP IS BEST PROP TURBOPROP BEST JET TURBOPROP RULE WORLD HAIL TURBOPROP

MORE EFFICIENT AND BETTER THAN EVERY JET ENGINE. PROVIDE LESS THRUST BUT WHO CARES, JUST ADD MOAAAAAAR AND MAKE SMALLER AIRPLANE AND YOU BE FINE

TURBOPROP BEST ENGINE 2K15

"10/10 WOULD BUILD AIRCRAFT WITH TURBOPROP" -ATR

I would like an turboprop I could run on internal oxidizer and rocket fuel, real world think an turbopump who is not running an pump but an propeller.

This should have huge benefits getting out from Eve,

[Sandworm]

Motorjets was a workaround due to 2 technological limitations of its day;

1- the lack of high temperature/high strength materials to construct the turbine, and

2- material and aerodynamics for efficient, large diameter fan blades.

Exactly. It was an intermediary/developmental technology to solve a very narrow problem. It never had time to develop and those working on it probably knew it was doomed.

[bs1110101]

A somewhat related question: What about a motorjet driven by a turboshaft/a turbojet where only part of the air goes to the turbine. The idea being that you'd need more or less no cooling air and could burn all the oxygen in the non-turbine nozzle, giving far more thrust for the size and likely more efficient then an afterburner seeing as all the oxygen is burnt at once instead of the two part combustion of an afterburner.

[FlyingPete]

A somewhat related question: What about a motorjet driven by a turboshaft/a turbojet where only part of the air goes to the turbine. The idea being that you'd need more or less no cooling air and could burn all the oxygen in the non-turbine nozzle, giving far more thrust for the size and likely more efficient then an afterburner seeing as all the oxygen is burnt at once instead of the two part combustion of an afterburner.

This sounds similar to an afterburning turbofan, though for some reason these seem to place the afterburner in the gas turbine core rather than in the bypass.

[mrfox]

Afterburners are not limited by the amount of oxygen - they are temperature limited - just like the motorjets were. But since an afterburner is just a nozzle, they can be built to tolerate much higher temperatures then a loaded, rotating turbine disk... just like the limitation the motorjet was trying to work around.

So in a way, the afterburner is what really became of the motorjet. I have never thought of it quite this way - very interesting!

[Iskierka]

Piston diesels are actually more efficient compared to gas turbines, because their compression ratio is independent of rpm, and their intermittent combusion allows less expensive materials to be used due to lower steady state temps Their big drawback is their power to weight ratio.

Maybe compared to small gas turbines, but all piston engines are hugely inefficient compared to any gas turbine that can get its compression high. Pistons are drastically limited in how much power can be extracted as they have to use the same stroke for expansion and compression, despite hugely different pressures on either side, so they leave much gas unexpanded and wasted. Turbochargers help by using that gas for an additional compression stage, but at that point, why not use a turbine? Turbine engines don't have to match compression either side of combustion, so while a diesel may compress by 20, it can only expand by maybe 10, while the turbine will do 20 and 20. Or possibly higher - modern large turbofans are exceeding compression ratios of 50.

Also, the power to weight is a big one - aircraft are highly sensitive to weight, so a considerably less efficient engine is often more efficient in practice if it's lighter than the alternative.

This sounds similar to an afterburning turbofan, though for some reason these seem to place the afterburner in the gas turbine core rather than in the bypass.

That's because the bypass of a turbofan doesn't provide significant pressure to the air, only velocity. You need pressure in the gas flow to expand it for more velocity - the core flow has less oxygen to burn, but it has lots of pressure available, so it's the far more effective, and actually more efficient one to add fuel to. I believe often when set to afterburning mode they may also reduce fuel into the main combustion chamber so that more oxygen is available for the afterburner - the afterburner is considerably less efficient than the core, but it's more powerful, which is what you want if you turned it on anyway.

[mrfox]

Maybe compared to small gas turbines, but all piston engines are hugely inefficient compared to any gas turbine that can get its compression high. Pistons are drastically limited in how much power can be extracted as they have to use the same stroke for expansion and compression, despite hugely different pressures on either side, so they leave much gas unexpanded and wasted. Turbochargers help by using that gas for an additional compression stage, but at that point, why not use a turbine? Turbine engines don't have to match compression either side of combustion, so while a diesel may compress by 20, it can only expand by maybe 10, while the turbine will do 20 and 20. Or possibly higher - modern large turbofans are exceeding compression ratios of 50.

Also, the power to weight is a big one - aircraft are highly sensitive to weight, so a considerably less efficient engine is often more efficient in practice if it's lighter than the alternative.

Turbines for mobile applications are also very under-expanded - trading efficiency for weight. Stationary applications generally incorprate some sort of exhaust regeneration to increase expansion and heat recovery.

Quite a few modern pistons increase their expansion ratio beyond its compression ratio using miller or atkinson cycles - increasing the stroke while delaying the closing of the intake valve.

[linkxsc]

Well since efficiency is part of the topic. Opposed piston diesel anybody?

[Iskierka]

Turbines for mobile applications are also very under-expanded - trading efficiency for weight. Stationary applications generally incorprate some sort of exhaust regeneration to increase expansion and heat recovery.

Quite a few modern pistons increase their expansion ratio beyond its compression ratio using miller or atkinson cycles - increasing the stroke while delaying the closing of the intake valve.

Miniature turbines are, but anything large enough to power a useful aircraft isn't particularly so. In the 400-1000 hp range that you'll see for small private/business planes, they're already happily competitive with pistons in efficiency, but are lighter and have a much longer maintenance cycle. They do, however, cost much more to begin with, due to turbine's requirements of much better materials and tolerances.

And while that is a trick that is used, yes, it only reduces the problem, doesn't remove it. If you match expansion to compression by limiting the intake valve, then you do improve energy extracted, but the engine is now fighting vacuum pressure against itself, wasting power on the intake cycle. You can balance this to get close to the theoretical maximum of the thermodynamic processes' performance, but not nearly close enough to beat larger turbine engines. Micro turbines, sure, but that's because turbines are very hard to miniaturise. And to be back on the original subject, motorjets won't particularly be able to bypass this flaw, as the limiting factors will still exist, only you now have to fit a piston engine around it too.

One thought might be that you could use something similar to a motorjet to alleviate the biggest disadvantage of turbine engines, which is throttle time. You could reverse the electric generator that most turbines already have fitted somewhere to give a boost up to max rpm, with a sufficient battery to provide that power. The engine would still be a gas turbine in core function, but a relatively easy-to-fit and, importantly, lightweight electric motor would provide boosting. Current F1 turbochargers do the same trick to eliminate turbo lag, so that full power is immediately available out of corners. Then at high speed, rather than have to waste power by opening a waste-gate that limits turbo pressure, the same device extracts electrical power to the exact amount that no pressure is lost, and the energy is stored for the electric motors in their hybrid engine.