Potential of motorjets?

[mrfox]

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.

A good part of the jets efficiency comes from the effect of ram recovery - great if you already need to go fast, but going fast does mean consuming all that efficient power that has been generated to overcome drag.

Another concept that had been developed during the 50s to improve the expansion ratio of the piston is the power recovery turbine in turbo-compound engines - using the piston exhaust energy to drive a turbine to drive the propeller - pretty much the exact opposite of a motorjet!

Most direct current electric start motors today are small and only drive the high pressure turbine, and only up to around 30 percent. To reduce lag, you would need to drive the much more massive low pressure turbine/fan assenbly, and accelerate all that to 100 percent, so they would need to be massively overbuilt.

I think we are in agreement that the modern turbine's niche is pretty secure.

Edited April 24, 2015 by mrfox

[Specialist]

I tried my engineering skills at this Motorjet stuff and threw it online for others to learn about. I wish I had something like this when I was getting into it!

https://www.youtube.com/watch?v=sogEUBnqb6o&sns=em

[wumpus]

On 4/20/2015 at 4:43 AM, K^2 said:

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.

Count me in here as well.  For hobbyist sized motors (and even to small plane sized), a piston engine will be more efficient, cheaper, and probably even easier to maintain than a turbine.  I'd guess that the same purists who insist on a jet engine (when a prop would be more powerful) don't want to go half way, although I think most of the reasons for inefficiency are on the turbine side, not the compressor side.

On 4/20/2015 at 11:12 AM, Naten said:

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

I might be pretty unclear about the point of high-bypass turbofans, but they sound like turboprops with better marketing (and a tiny bit of jet action).  People don't complain the way they do when they see an open propeller when the "propeller" looks like a proper jet compressor and has a jet's shroud.

I also suspect that that "last little bit of jet action" is a net gain over a turboprop, and so is the shroud around the fans (propellers drive air in many directions).

[sevenperforce]

17 minutes ago, wumpus said:

I might be pretty unclear about the point of high-bypass turbofans, but they sound like turboprops with better marketing (and a tiny bit of jet action).  People don't complain the way they do when they see an open propeller when the "propeller" looks like a proper jet compressor and has a jet's shroud.

I also suspect that that "last little bit of jet action" is a net gain over a turboprop, and so is the shroud around the fans (propellers drive air in many directions).

A high-bypass turbofan is simpler than a turboprop because it is (typically) ungeared; the bypass fan runs at the same RPM as the turbine. It is ducted, so it can operate safely at higher speeds and produce less noise. Finally, the bypass airstream mixes with the hot exhaust, allowing the hot exhaust to expand a little bit more and squeeze out a touch more efficiency, while yet again reducing noise.