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Piston Assisted Pulsejet Hyrbid

More efficient pulsejet design
  [vote for,

Most engines that produce thrust from fuel (gas turbines, rocket engines, etc) opperate continuously; that is to say compressed air is continuously supplied while fuel is continuously combusted. Drawbacks to this include the inherent inefficiency of the continuous backpressure from combustion towards the compressed air (gas turbines produce thrust because of the difference in gearing between the compressor and combustion blades) and also that a high compression ratio, meaning higher efficiency, is more difficult to achieve (high compression on a gas turbine leads to excess heating of the turbine blades, which is difficult to get around even with today's technology and metalurgy).

Pulsejets are a much more simplistic means of producing thrust, as pulsejets have no turbines or mechanical parts at all save for reed valves, and the majority of pulsejets are valveless altogether. Rather than compressing air/combusting fuel continuously, they opperate in pulses in which air is forced into the chamber from the resonant frequency of the preceding combustion, and fuel is then injected and ignited. The problem is, as they rely on resonant frequencies from the combustion pulses to force air in, they cannot achieve above a 1.2:1 compresion ratio (typical ICE compression ratios range from 10-12, with diesels being as high as 22).

What I propose is a hybrid pulsejet design that couples a reciprocating piston air compressor with a pulsejet. Though not the most efficient way of compressing airThe reciprocating compressor, the reciprocating air compressor would act as a perfect intermitant compressor for the pulsejet. It could be made from lightweight materials such as carbon fiber as it would only have to handle the temperature of ambient air, and would have a flat head with a sliding valve (a thin rectangular slice of titanium with a hole in it to be used as a port) sandwiched into the head and sliding horizontally. A cam lobe would be able to block and unblock the opening into a separate chamber as well as block and unblock the intake valve. The separate chamber, used as the combustion chamber for the pulsejet and containing a high-pressure direct injector, would be a great deal smaller than the compressor to achieve a high compression ratio (if the chamber was made to be 50ccs and the compressor 1000, the pulsejet would have an effective compression ratio of 20:1). Another sliding-port camshaft controlled valve would be utilized at the exit end of this chamber to prevent the compressed air from escaping until the fuel is injected and is begining to detonate. This would all allow for an infinitely more efficient pulsejet engine with an easily controlled frequency and thrust because of the reciprocating compressor. Also, the efficiency should be greater than that of a gas turbine engine because while the gas turbine is always losing efficiency from backpressure into the compressor, this design isolates the air compression from combustion.

Because the air compressor could not be directly driven by the pulsejet since the pulsejet only produces thrust, I propose that a CVT-controlled electric motor be used to power the compressor, and a CVT-controlled variable vane steam turbine to be used to harness a good deal of the waste heat from the pulsejet. Though the summation of these systems is complex overall, it would provide a very efficient overall package that would be very viable in small aircraft, boats, etc.

acurafan07, Apr 07 2011

Valved pulse jet http://en.wikipedia...i/V-1_(flying_bomb)
Somewhat more serious than a hobbyist's toy [Twizz, Apr 07 2011]

Brayton Cycle http://en.wikipedia.../wiki/Brayton_cycle
Compress gas, heat and expand to produce power [Twizz, Apr 07 2011]


       Where to start?   

       Most pulse jets use a valve. The valveless design (Gluhareff) has limited compression as you describe. Valved designs can produce much more than 1.2:1 compression.   

       A seperate compressor would need to handle high temperature. As a gas is compressed, it's temperature is increased. In a diesel, this is enough to cause ignition (around 250 to 300°C)   

       Why would this compressor be any more effecient than the directly driven compressor of the gas turbine?   

       You are proposing a series of less effecient devices to extract energy from the jet engine. There is no 'waste heat' to be recovered - all the thrust is produced by heating and expansion of gas.   

Twizz, Apr 07 2011

       Yes, valved pulsejets can create slightly higher compression ratios, but certainly nowhere near even the 10:1 of a common ICE. Also, because I have yet to see a valved pulsejet that did not burn through the reed valves within a very short amount of time, I consider them extremely impractical for anything other than hobyist toys.   

       Yes, the compressor aspect of this design would be no more efficient and probably less efficient than that of a gas-turbine, but that is not where the benefits of this idea are. The benefit is that you are releasing extremely compressed "shots" as pulses, thus extracting much more thrust per unit fuel compared to any gas-turbine. And yes, of course it produces waste heat- any engine does. If you look up pulsejets on youtube you'll note that they glow red after about a minute of use. Some gas-turbines even use steam to capture some of the waste heat. Though the compressor itself is not overly efficient, as it is utilizing the waste heat to drive it in a way that gas-turbines cannot, it is more efficient.   

       And yes, though part of the reason diesels autoignite is also from the temperature in the cylinder from combustion which would not be present in this design, so I maintain that it could indeed be made from lighter materials that don't have to withstand the heat or pressure of combustion.
acurafan07, Apr 07 2011

       See link re. valved pulse jet.   

       I managed to build one when I was younger and more stupid. Run on acetylene gas, it burnt through it's tailpipe but not the reed valve. The valve is constantly cooled by incoming air.   

       Your compressor would be massively less effecient than the direct driven turbine type. You idea sounds very similar to Brayton's first engines, which achieved only 17% effeciency, compared to over 30% for gas turbines.   

       'Waste' heat is that which does no work in the engine. In a jet engine, all thrust is developed by heating and expanding gas. Removing any heat from this process will reduce expansion and reduce thrust.   

       Gas turbine engines may use mechanisms to regulate temperature so that components and systems operatwe within their design limits. This is quite different from waste heat recovery. If materials allowed, the engine would be heavily insulated and run as hot as possible. Greater temperature = greater effeciency.   

       Look up Boyle's law. this explains how much gasses heat up when compressed. It's a lot. Diesel engines start without residual heat from previous combustion. Glow plugs are necessary because cold cylinders absorb much of the heat of compression.
Twizz, Apr 07 2011

       I see this a bit, as a tube with a piston in it that compresses air into a (imho magically) robust valve at the other end where the fuel injector is. You wouldn't need an engine to operate the piston though: the oomph from the detonation could keep it running, could use a scavenger. But I don't see how you're going to get the thrust (exhaust) valve to operate in a useful fashion.
FlyingToaster, Apr 07 2011

       [FlyingToaster], the piston compresses air through a cutout port in a sliding valve sandwiched into the head and into a separate chamber. When that valve slides shut (the valve slides to a position where the port doesn't align with the chamber), fuel is injected and the same type of valve at the other end of the chamber opens immediately to release the pulse. The valves would only have to be robust enough to handle the pressurized air, as the exhaust valve opens to allow the combusting mixture to escape rather than containing it.   

       [Twizz], this idea is like the original Brayton design only in its use of a piston air compressor. The Brayton design uses an additional piston to harness the combustion. It is essentially a conventional ICE, only with two pistons instead of one. My idea calls for an less than ideal means of compression, but the combusted pulses are used much more effectively than gas-turbines. Instead of producing backpressure into the source of compression, the compressed air is isolated before the fuel is introduced, and provided with a direct path the the outlet in the same way as a conventional pulsejet.
acurafan07, Apr 07 2011

       Back pressure into the compressor of a gas turbine engine is not a loss. Most of the thrust from a modern gas turbine comes from the compressor. In many case, much of the throughput from the compressor bypasses the combustion chamber and is used directly. Back pressure on the compressor simply balances some of the torque generated by the turbine.   

       The Brayton engine uses a piston to harness gas expansion energy, but the principle is the same and modern gas turbines are referred to as Brayton cycle.
Twizz, Apr 08 2011


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