WITH OR WIHOUT VALVES, WHAT IS MORE EFFICIENT

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celsius235
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WITH OR WIHOUT VALVES, WHAT IS MORE EFFICIENT

Post by celsius235 » Sun May 06, 2007 6:12 pm

I would like to know what is the most efficient engine as thrust / weight ratio and SFC.
Running in a plane at 300 km/h

Pulsejet with valves
Pulsejets valve less
Pressure jets
any more engines or ideas ? Turbines not possible.

Regards

tufty
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Re: WITH OR WIHOUT VALVES, WHAT IS MORE EFFICIENT

Post by tufty » Mon May 07, 2007 7:17 am

celsius235 wrote:I would like to know what is the most efficient engine as thrust / weight ratio and SFC.
Running in a plane at 300 km/h

Pulsejet with valves
Pulsejets valve less
Pressure jets
any more engines or ideas ? Turbines not possible.

Regards
2-stroke with tuned pipe.

celsius235
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Post by celsius235 » Mon May 07, 2007 10:25 am

Tufty: 2-stroke with tuned pipe.

but Thrust / weight ratio is worse compared with Argus V1

larry cottrill
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Efficiency?

Post by larry cottrill » Mon May 07, 2007 12:42 pm

celsius -

There is a "conventional wisdom" (based on the laws of physics) that the maximum propulsion efficiency (which I think is what you mean) of any air-breathing jet device is achieved when the forward speed equals one half the exit speed of the jet stream. This has to do with the increase in stream momentum created by the device. At 300 km/hr (somewhat less than 250 MPH, I think), the propulsion efficiency of any air-breathing combustion-driven jet engine will be fairly poor.

This can change somewhat for a pulsejet if augmentors are used. Without an augmentor, the energy represented by the induction of air by the exhaust stream is wasted. With an augmentor, part of that energy becomes useful. The resulting exit stream is slower, cooler and denser and far more massive than the exhaust stream of the unaugmented jet. Because of the lower exit speed, the best speed for propulsion efficiency is much lower. It is often claimed that maximum benefit from an augmentor is realized at zero forward speed (which is also true of an ordinary airplane propeller).

Theoretically, we should be able to make a valveless engine that has better propulsion efficiency at speed than a valved design because of the inevitable losses of air going around the open valves - there simply is no "ideal" pulsejet valve. Unfortunately, there are design limitations to the intake section of valveless engines, too. I used to say that valveless engines have to be much bulkier than valved engines to achieve the same thrust, but we have proven in the last year or so that this is no longer true - the valveless engine only needs a little more volume (especially at the tail end) to achieve comparable thrust. Of course, thrust is not efficiency, and a lot of factors (especially internal and external engine drag) have to be taken into account when we're talking about an engine at speed.

I have postulated that a "passive ramjet shell around pulsejet" approach is the best way to get propulsion efficiency, but this has yet to be proven. By "passive ramjet" we just mean that the heat for expansion is picked up from the pulsejet's radiated "waste" heat, not from burning additional fuel. There has been some experimentation (mostly by Eric) on ramjet sections that "follow" the pulsejet exhaust like an augmentor (a sort of "pulsejet afterburner") which also seems promising.

In terms of overall propulsion efficiency at moderate speeds, I think a properly designed and fully augmented valveless pulsejet (with passive ramjet action) will do very well. At the moment, though, I think the most efficient jet devices are probably fanjets. The little Williams fanjets have a T/W ratio of 11 to 1, but of course they are high priced and need lubrication, regular maintenance, etc., which a valveless pulsejet would not require at all. Note that fanjets successfully combine hot high speed exhaust and massive cold air motion; they are a kind of augmented turbojet achieved via a steep increase in mechanical complexity.

L Cottrill
Last edited by larry cottrill on Tue May 08, 2007 4:03 pm, edited 1 time in total.

celsius235
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Post by celsius235 » Mon May 07, 2007 3:58 pm

Well, I have been the last years working with solid and liquid rocket engines, for me the efficient is easy = ISP specific impulse, when you work with engines that use air as oxidizer same things changes. In a rocket two factors are very important.
1º Propellant
2º Chamber pressure

With solid fuel you can reduce the nozzle as you want, normally to obtain a pressure in the combustion chamber under 1500psi. Well but with air-fuel engines the problem is that the efficient depend of the maximum pressure chamber that you can obtain, modern turbojet can work at 20bar but when you work with:

Ramjet (subsonic) or pulsejet, the principal problems is that the pressure chamber is very bad under 50psi, this is because I thought that pulse jet with valves will be more efficient that other system.

for example:

Fuel = Propane
Oxidizer = Air

Mixture 1/20 fuel / air

Pressure chamber at:

7 psi = fuel ISP = 1407s 1kg of propane give 1407 kg of thrust per second
15 psi = fuel ISP = 1860s
30 psi = fuel ISP = 2289s
50 psi = fuel ISP = 2604s 1kg of propane give 2604 kg of thrust per second

As you see the chamber pressure is very important to obtain a good efficient engine..

I have read that valves pulse jets have a TSFC = 1.8 kg/h/N or lbs/h/lbs but this is difficult to believe, if turbojet has 1.2-0.7 TSFC.

Resume. I thought that valves pulsejets could obtain more pressure chamber that valveless. And valve less at high speed are less efficient by the difficult to obtain air and not use a ram inlet. but maybe this is not true.

What is a real TSFC for both pulsejets? Not for amateur construction

Argus V1 TSFC

Hiller valve less TSFC

Jim Berquist
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valved vers valveless

Post by Jim Berquist » Mon May 07, 2007 5:53 pm

I fist favored the valved engine for it power and weight aspect. The valves seem to be the akillies heal.

On this forum over the past year or so, I have seen people raise the valveless engine to new levels. No valves to contend with. Power levels that are starting to rival those of the valved engines. A valveless engine with 100 lb of thrust is much simpler and cheaper to build then a valved engine of the same . NO maintenance required.

There are a few people here that are going to fly a valveless. Including my self if I can.

The valveless engine is coming of age.

Jim
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Thrust and TSFC - Argus

Post by larry cottrill » Mon May 07, 2007 6:06 pm

celcius -

I'll have to let someone else answer for the Lockwood-Hiller engine. For the Argus, the answer depends on the original German design flown on the V-1 vs the improved post-war revisions. My old book from 1948 shows a little table relating the following information:

1944 and Prior:
Thrust per unit major cross-sectional area: 250 psf
Specific fuel consumption: 4.0 lb/hr-lb
Effective valve life: 0.75 hrs

July 1946:
Thrust per unit major cross-sectional area: 570 psf
Specific fuel consumption: 2.6 lb/hr-lb
Effective valve life: 7.00 hrs and up

Even if the units are unfamiliar, I'm sure you can see the level of improvement being claimed, within just a few years' development effort.
Of course, the credibility of the latter figures may be challenged by others ...

L Cottrill

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valved

Post by Jim Berquist » Mon May 07, 2007 6:17 pm

Larry:

What do you think about the valved engine with external valves.


http://www.pulse-jets.com/phpbb2/viewtopic.php?t=4250





Jim
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larry cottrill
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valved

Post by larry cottrill » Mon May 07, 2007 7:53 pm

Jim -

That works - BUT, I don't think it's ever been used n a propulsion engine. My guess is that the forward chamber 'softens' the momentum transfer at the front end, so the thrust is slightly reduced for the size of the engine. I'm just speculating, of course.

If there were no "down side", everyone would be doing it.

L Cottrill

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external valved engine

Post by Jim Berquist » Mon May 07, 2007 8:25 pm

Larry:

It looked like the C.C. was a bit long. Sort of elongated! As you know I like most out here are after the ZOOM factor. I do like the idea of the external valve system. If I took the Chines that Eric built and placed some external valves on it , I bet it would run better. His C.C. was like a long convergence cone, some tube and divergence exhaust. It just didn't look like a C.C.

Like Irvine: I want to see something in the air.

Jim
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Post by tufty » Mon May 07, 2007 9:36 pm

celsius235 wrote:What is a real TSFC for both pulsejets? Not for amateur construction

Argus V1 TSFC

Hiller valve less TSFC
Hiller *claimed* 1.0 Kg/Kph, but I've yet to see anyone who has verified that, despite it being one of the most widely built valveless machines.
Messerschmidt stated the Argus as 3.0Kg/kph. They also have documented test results of the Escopette at 1.8.

As for the amateurs; there are a few on here who have broken the 1.0 TSFC barrier with homebrewed designs.

celsius235
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Post by celsius235 » Mon May 07, 2007 11:06 pm

But do you think that all aviations companies that invest a lot of money to develop new engines technologies as Pulse Detonator Engine, with rotary valves or rotary chambers need in all cases to fill the combustion or detonation chamber with fresh air using normally axial compressor with turbine or supersonic Ramjet or Scramjet and the air inlet always is opposite to the air direction to obtain the maximum air flow.

I do not know exactly how valveless works but I am sure if works with deflagrations the wave shock pressure must be very little and low pressure in the combustion chamber, but I would like that my opinion be wrong because the valve less is very easy to make and high thrust / weight ratio.
In all case I appreciate yours opinion or conclusions

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A valved enigine is the way to go fast

Post by cliff » Tue May 08, 2007 3:19 pm

I have noticed that all the focus in the prior replies was on airstream output and efficiency

When you compare a valved to a valveless engine, it is clear to see that the valved engine is much sleaker than the valveless.

When you add the drag and weight factor of the valveless engine to your calculations, you will find that nothing beats the valved pulsejet when it comes to finding speed.

Valved is my answer.

celsius235
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Post by celsius235 » Tue May 08, 2007 5:55 pm

my target is with valves, but I was not sure. I will begin with a new age of high pressure pulsejets with high air flow valves. will be very funny to work in the edge of defregration to detaonartion transition.

Why I think in this way.

Pipe diameter with 2.4 inch diameter = 2.89 inch ^2

Speed 156 milles/h

You have 0.40 pounds of air per second

20/1 air/fuel ratio at 50 psi give 75.54 pounds of thrust

efficient engine coefficient 0.5 = 37.54 pounds of thrust

2.4 inch diameter if give 37.54 pounds of thrust at 156 milles/h the work can be interesting.

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Post by tufty » Tue May 08, 2007 8:11 pm

I don't want to be a wet blanket or anything, but I think you're in for a disappointment, at least with some of your calculations.

Flow through a pulsejet is not continuous. Simplifying things somewhat, we can assume it's "breathing" for half the combustion cycle, which halves the amount of fuel and air you're going to be able to have it use, with a corresponding drop in theoretical thrust you're going to produce.

Also, your calculation assumes that there's nothing impeding flow, which is most certainly not the case. Not only is a pulsejet a tube, but you'll have a bunch of valves in the way. Ain't no way your jet is going to swallow the amount of mixture you think.

On top of all that, unless you do something *really* funky, you're never going to achieve 50psi in the chamber, at least for any meaningful period of time. The Messerschmidt and SNECMA testing showed peaks of 1.5atm or so (about 22psi) in a "biggish" pulsejet, but average chamber pressure over the entire cycle was (very) slightly below atmospheric, and during the overpressure phase, average was about 2/3 of peak, or just under 19psi. Remember, the main structural worry with pulsejets is not explosion, but collapse...

Even assuming you can somehow instantaneously fill the chamber with air without it running out the back, I can't see how you're going to manage to get more than stagnation pressure in there before valve closure without some serious trickery. Stagnation pressure is given by our old friend Bernoulli thus : p + 0.5*r*v^2 where p is atmospheric pressure, r is atmospheric air density, and v is the freestream velocity.

Ignoring static pressure, at 168mph, or 7.15m/s, assuming sea level atmospheric conditions at 20c, this gives you a pressure rise due to stagnation of: 0.5 * 1.29 * 7.15^2 == about 33Pa or about .005psi

Yes, the ram pressure you're gonna see really is that small.

Really.

You can get more by speeding up, but not that much, and (especially valved) pulsejets have a significant issue with maximum speed; the Argus simply "blew itself out" at around Mach 0.6 or so.

Then there's the fact pulsejets act in an oscillating fashion. By which I mean that you're not producing a steady stream of accelerated gases coming out of the tailpipe, but rather you're accelerating the column of gas in the tailpipe back and forth, even pulling external air back into the tailpipe during certain points of the cycle.

This doesn't mean you can't see 70lb of thrust from a pulsejet of manageable size, but what it does mean is that you're liable to be using an awful lot more fuel than you think. Something like Eric's TP180 with afterburner might do the trick.

Simon

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