Free Piston Compressor for Turbofan? ("pulse turbine")

[schwee]

Has anyone on these forums heard of using a free-piston compressor to boost the compression ratio of a turboprop or turbofan?

The concept would be to refit the combustion chamber of an existing jet engine with a cylinder, valves or ports, and a free piston, using the "air cushion" on the non-combustion side of the piston to provide the compressing force. Oscillations could be introduced by allowing a portion of the escaping exhaust gases (thrust) to press against the air cushions, which would then spring back and compress a fresh charge once the "stroke" is completed.

The result, it seems, might be a "pulse turbine," where a multi-stage turbine would act as a compressor for low compression, high volume, ambient density air, and a free piston oscillating at high rpm (10,000+ ?), would further compress the now lower volume, higher density, higher compression air, and combust it intermittently. There would still be no connecting rods and no crankshaft -- the addition of a free piston to the combustion chamber would simply be an attempt to increase the compression ratio, and hence the fuel efficiency, of the turbofan/ turboprop while preserving its characteristics of light weight and high power.

Variations on this scheme might include one or more dual-ended free pistons, in which there would be no air cushions but rather combustion on each end; or, alternately, a single cylinder with two opposing free pistons, each with its own air cushion. This latter variation might be useful in balancing the moving weight of the pistons and thus minimizing vibration.

I would welcome any speculation on the effects that a "pulsed" type of thrust vs. continuous combustion might have on the functioning and longevity of the (rear) turbine blades.

 

[Michael996]

Typically, a turbine engine has an over sized compressor. Most modern turbine aircraft extract bleed air off the compressor to be used for power. Examples:

F-16 Bleed air off at 1100 Deg F. Uses this are for ECS and the EPU.

F-22, X-35 Bleeds are off at 1100 for main engine start and ECS.

APUs provide bleed air also. They include a secondary bleed system that is controlled with a valve. This is the surge control system. As with the case of the 450 hp G250. The mass air flow for the APU is around (working from memory here) 7 lbs/sec. The surge control valve can allow up to 1.2 lbs/sec in bleed flow. This bleed is required because the excess air would cause the engine to surge. And under mass flow caused exhaust gas temperatures to be too high. The power extraction of the bleed is in the neighborhood of 4 lbs/sec. producing around 40 Isentropic hp.

Most APUs that I have worked with, consume 99.% of the available power in the form of shaft power (G250 ~ 23,000 rpm)via the turbine. This power is converted into hydraulic, electric, or pneumatic power.

In the diagram you referenced, they are replacing the combustion chamber with a reciprocating engine. An AUP equivelent would be a 450 hp Free-Piston Diesel Engine. But, look at the internal aerodynamics. Get your thermo book, and your fluids books out. Do some simple canculations. And, rememeber all your flow must stay below Mach .5 to prevent shock wave choking.

An easy test of your application would be to size the inlet and exit valves to the diesel compresser that would allow 7 lbs/sec flow.

Several year ago, I was working with Greyhound to develop a new bus. The research you are looking at, I too visited. From a slightly different angle, but, very similar. Effectively extracting the excess power from oversized turbos, and applying the power to the fluids in the transmission.

 

[schwee]

Michael --

Thanks for your response and your info. I'll break out the books!

--Paul