Anyone building gas turbines?

[Blunt Eversmoke]

Hi all, here is the ages and ages-old idea: Couple a gas turbine (turboshaft engine) to a free-piston gas generator, and enjoy the best of both worlds. Part-load inefficiency can be combated by utilizing a pneumatic recuperating system(you'll need a gas bottle to start the free-piston engine, anyway).

Now, I realize that a gas turbo already needs all sorts of additional equipment: high-pressure oil pump and gauge, reduction gearing, RPM gauge - but a free-piston gas generator of the simplest kind has but one moving part and comes in place of the combustion chamber, so you aren't really adding anything. So, anyone here building gas turbines?

[BertTrack]

The free piston gas generator would work without the gas-turbine part. Why add a gas-turbine?

Free piston generators are already on the market in the form of Stirling engines.

[Blunt Eversmoke]

The free piston gas generator would work without the gas-turbine part. Why add a gas-turbine?

Free piston generators are already on the market in the form of Stirling engines.

No-no, not a free piston electricity generator - just generator of hot, high-pressure gas to feed the gas turbine that performs work (such as powering a bike, hehe).

[BertTrack]

Ah!

Like the Germans tried.

A free piston compressor for a diesel engine and also as compression stage for a gas turbine.

[Blunt Eversmoke]

Ah!

Like the Germans tried.

A free piston compressor for a diesel engine and also as compression stage for a gas turbine.

Right! As the Italians(without the gas turbine, just the FPE to produce jet thrust), Germans, Russians, Frenchmen and Those Of Sam(Ford's small tractor, 50 HP, governed down from 100) tried and used, in different applications and time. Comes with pros and cons, tho.

Pros: High efficiency from upper mid load upward, reaching 40% with little hustle AND low specific weight per HP, increased durability of both the turbine and the piston component: first because of lower gas temperature compared to classic turboshaft engines, second because no crank, no bearings, no sideloads on the piston - this decrease in friction is, together with higher possible compression, the cause for the higher efficiency compared to normal diesels. One can build a gas turbine from a turbocharger - many guys actually do; but it will only last little in a classic turboshaft engine with low-pressure combustion chamber. Not so here: Cheap materials can be used without durability decrease, just like with turbochargers.

Cons: Quite low efficiency at any load lower than upper middle RPM range for the turbo; dunno about the efficiency losses on the piston side; probably less elastic when it comes to RPM changes. Countering: Variable vane geometry? One turbine for low-load work, one for flat out? Hybrid? If yes, pneumatic or electric?

Finally, is here anyone interested in or actually building a gas turbine? Might be interesting to go for an FP gas generator instead of a combustion chamber...