What is a rotary pulse jet engine?

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Offline McQueen

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What is a rotary pulse jet engine?
« on: 01/09/2007 16:37:57 »
Right! Well you all know that there has been a lot of talk of late on global warming, carbon emission and so on! Well just have a look at this site and tell me what you think!
http://www.geocities.com/rotarypulsejet  Mcqueen
« Last Edit: 24/05/2008 00:42:31 by chris »
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another_someone

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Re: What is a rotary pulse jet engine?
« Reply #1 on: 01/09/2007 17:12:17 »
The problem with all rotary engines is that in principle they seem simple (this is certainly true regarding the Wankel engine), but heavily dependent on the quality of the seals - and it is often the breakdown of all the seals that gives the biggest headache.

I would have thought the other problem with this engine is that so little of the energy generated by the drive is actually used.  The exhaust gasses from the engine will be travelling at high speed, and be very hot - that is a lot of energy in that heat and exhaust gas speed that has not been put to use by the engine.  Conventional IC engines still have some significant residual heat (the catalytic converters, it is now very difficult to reuse that heat because the catalytic converters need that heat to function), but they no not have very much exhaust velocity since they rely on pressure rather than impulse derived from the exhaust velocity to derive their energy.

From what I have read, I cannot see actual efficiency figures for the engine they are describing (or how they would compare with other rotary engines, such as the wankel engine, of gas turbine engines).

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Offline McQueen

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Re: What is a rotary pulse jet engine?
« Reply #2 on: 01/09/2007 17:18:58 »
anothersomeone
Quote
From what I have read, I cannot see actual efficiency figures for the engine they are describing (or how they would compare with other rotary engines, such as the wankel engine, of gas turbine engines).
Well, all I can say is that you have not gone through the web-site. A Wankel Engine for your information is just a glorified version of the Rotary vacuum pump, that has been around for more than a hundred years, while the rotary vacuum pump is extremely efficient, since it runs in an oil bath for effective sealing, the Wankel engine, dealing with much higher pressures, tries to do the smae thing in reverse in air! It is a huge scam, on which a lot of money has been spent. Let's take another look at this site, right! McQueen
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Offline McQueen

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Re: What is a rotary pulse jet engine?
« Reply #3 on: 01/09/2007 17:25:44 »
And another thing, in an effective CDN de laval nozzle, the very thing is to extract heat and energy from that initially high temperature, high velocity gas flow. You do agree I hope that rockets are the most powerful form of engine known to man? Imagine trying to go to the moon in a souped up IC piston engine, or even a jet engine. Still have doubts about the energy of high velocity gases? Let me know McQueen
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another_someone

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Re: What is a rotary pulse jet engine?
« Reply #4 on: 01/09/2007 17:57:53 »
Rockets are not necessarily the most powerful engines known to man, but they are engines designed for a particular purpose.

Nuclear power plants generate far more power than rockets (usually thought the medium of steam turbine engines), but they would not work (at least in the way they are designed for electric power generation) to take a vehicle to the moon.

The advantage of a rocket engine is that the high exhaust gas speed allows the vehicle to be propelled at high speed.  Rocket engines can propel vehicles at high speed, but not necessarily with the highest power (a little like the top gear in a motor car - you use that to travel very fast, but it is useless for carrying a very heavy load up a steep hill - it all depends on what you need to do with that power).

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Offline syhprum

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Re: What is a rotary pulse jet engine?
« Reply #5 on: 01/09/2007 20:51:42 »
The normal IC engine achieves its reasonable thermal efficiency due to the high temperature of combustion compared to exhaust, Wankel engines have an additional problem apart from seals which is the shape of the combustion chamber which makes it hard to achieve a good expansion ratio (commonly  called compression ratio) without pre-ignition.
There are many things to consider in an automobile engine besides thermal efficiency primarily the spurious gases in the exhaust.
Aircraft gas turbines achieve good thermal efficiency by the use of expensive alloys to allow a high combustion temperature and a high expansion rayio but are very expensive and inflexable.
The engine described in the URL given is just a bad joke!   
« Last Edit: 01/09/2007 20:58:59 by syhprum »
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Offline daveshorts

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Re: What is a rotary pulse jet engine?
« Reply #6 on: 01/09/2007 22:30:51 »
The efficiency of a rocket is connected to the mass that it is throwing out of the back. the force you get from throwing something out is proportional to the mass times the velocity where as the energy you use is the mass x velosity2 so the more mass you throw out the back the slower it needs to go so the less energy you need to have the same effect.

A conventional IC engine is essentially pushing the whole weight of the car which is very efficient, where as the engine you draw is only throwing out the gasses from the explosion which will come out very fast but very inefficiently.

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Offline McQueen

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Re: What is a rotary pulse jet engine?
« Reply #7 on: 02/09/2007 01:13:42 »
daveshorts
Quote
The efficiency of a rocket is connected to the mass that it is throwing out of the back. the force you get from throwing something out is proportional to the mass times the velocity where as the energy you use is the mass x velosity2 so the more mass you throw out the back the slower it needs to go so the less energy you need to have the same effect.
Exactly the p9oint. Let's look at it another way.
For example, take an open ended cylinder of a certain diameter, equipped with a piston and a cork pushed tightly into one end, and call this cylinder A.  When the piston is pushed up the cylinder it reaches a compression ratio of 3 : 1, or  44 psi approx. before the cork is pushed out of the cylinder. OK< take another cylinder, of exactly the same dimensions as cylinder A and call it cylinder B, it also has a cork stuck into one end. However, instead of a piston, it is equipped with a venturi at one end, the venturi is equipped  with a valve. Cylinder B is then filled with compressed air throughj a compressor until its internal pressure reaches 43 psi (i.e., a pressure just a little less than that needed to blow the cork out of the hole, in cylinder.} The valve to the venturi is opened? What happens ? will the cork fly out of the end of cylinder B. If so how far will it go? My guess is that they will travel the same distance or that B will travel further because it has more compressed air behind it. In order to be exactly equal, when cylinder B is filled with compressed air, its size should also change correspondingly, so that it is the size that remains of cylinder A after the piston has compressed the air to 44 psi.
OK, take another scenario,  now cylinder A is fully sealed at both ends, this time it is fitted with piston rings that enable it to be placed in an air-tight manner down a long barrel. The barrel is equipped with an air-tight piston that allows the air behind the cylinder to be compressed in a ratio of 3 : 1 or to 44psi. When the piston is depressed cylinder A flies out of the barrel. Cylinder B has exactly the same dimensions as Cylinder A, however instead of piston rings it has a venturi equipped with a valve. Cylinder B is filled with compressed air to a pressure of 44 psi and the venturi is opened. Which cylinder would go further, my guess is cylinder B, because it retains the pressure for longer.McQueen

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Offline McQueen

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Re: What is a rotary pulse jet engine?
« Reply #8 on: 02/09/2007 01:18:00 »
Quote
The normal IC engine achieves its reasonable thermal efficiency due to the high temperature of combustion compared to exhaust, Wankel engines have an additional problem apart from seals which is the shape of the combustion chamber which makes it hard to achieve a good expansion ratio (commonly  called compression ratio) without pre-ignition.

shyprum, I have only one question for you. Do you consider the Wankel to be a good engine. Anything at all about it? If yu think the rotary pulse jet is a bad joke, I don't know what you would think of as a good joke! McQueen.
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another_someone

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Re: What is a rotary pulse jet engine?
« Reply #9 on: 02/09/2007 02:52:58 »
anothersomeone
Well, all I can say is that you have not gone through the web-site.

I did look at the entire site, but I don't say I could not have overlooked something - I was merely skimming over what I was reading.

A Wankel Engine for your information is just a glorified version of the Rotary vacuum pump, that has been around for more than a hundred years, while the rotary vacuum pump is extremely efficient, since it runs in an oil bath for effective sealing, the Wankel engine, dealing with much higher pressures, tries to do the smae thing in reverse in air! It is a huge scam, on which a lot of money has been spent.

Tell that to all the people who seem to love the performance of the Mazda RX7.

The Wankel engine has many problems with it, but it has a good power to weight ratio, and very smooth power curve, and very quiet in comparison to a piston engine.  Aside from the RX-7, it is very suitable for motorcycles and microlight aircraft - anywhere where you need a compact light weight engine. 

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Offline syhprum

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Re: What is a rotary pulse jet engine?
« Reply #10 on: 02/09/2007 05:44:58 »
For some niche applications where light weight overides fuel efficiency and emission standards (like the two stroke)the Wankel has its uses, even the the pulse jet found application in WWII in a successfully missile system but as an automobile engine no way.
The Wankel achieved success in a Nissen car at Le Mans about twenty years ago but the present regulation favours Diesels 
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Offline McQueen

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Re: What is a rotary pulse jet engine?
« Reply #11 on: 02/09/2007 06:03:04 »
Syhprum, you seem to be interested in engines, so I suggest that you go through the site I had suggested, it is a quite extraordinary design. The compressed air is supplied to the combustion chambers at a pressure of 125 psi ( compression ratio equivalent to 9: 1) from the air tank, through throtary union and via a poppet valve into the combustion chamber in a completely hermetically sealed environment. Fuel is similarly introduced through the rotarty union, so the fuel and compressed air are kept separate until they enter the passage in the rotor leadi\ng to the combustion chambers. The poppet valve is closed and the fuel/air mixture is ignited. Simultaneously with this or a little later, a gate valve in the CDN (Convergent divergent nozzle) opens and the hot gases of combustion escape at velocity thrusting the rotor in the opposite direction. Much, much more efficient than either the Wankel or an IC Piston engine. McQueen
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Offline McQueen

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Re: What is a rotary pulse jet engine?
« Reply #12 on: 02/09/2007 06:10:50 »
Quote
Tell that to all the people who seem to love the performance of the Mazda RX7.
That's just what gets me, granted it is a highly desirable emngine design if it can be sealed. Have you claculated the area that has to be sealed ? An eccentric rotary vacuum pump, runs on exactly the same principle, but runs in an oil bath to achieve proper sealing. No doubt, engineers diud manage to do the almost impossible with the Wankel, but only at hell of a cost! And even then it never sealed perfectly which is why it has never caught on, leave alone the high precision engineering required! McQueen

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Offline syhprum

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Re: What is a rotary pulse jet engine?
« Reply #13 on: 02/09/2007 07:14:43 »
I find it difficult to make any serious criticism of the so called pulse jet engine, compressed air provided by a separate compresser! (I guess a turbo charger could be used), poppet valves operating at 2000°C the exhaust valves in an IC engine have a hard enough time coping with the exhaust gases after they have cooled by expansion.
How are inlet and exhaust valves to be operated (if they could be built) on these moving conbustion chambers?, I think the request for funds to develop it says it all!
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Offline McQueen

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Re: What is a rotary pulse jet engine?
« Reply #14 on: 02/09/2007 07:56:00 »
Quote
I find it difficult to make any serious criticism of the so called pulse jet engine, compressed air provided by a separate compresser! (I guess a turbo charger could be used), poppet valves operating at 2000°C the exhaust valves in an IC engine have a hard enough time coping with the exhaust gases after they have cooled by expansion.
How are inlet and exhaust valves to be operated (if they could be built) on these moving conbustion chambers?, I think the request for funds to develop it says it all!
I can understand the cynicism, but then again, if you don't mind me saying so, you haven't really gone through the site at all. These questions have all been answered in some detail! The poppet valves are used only for the inlet port, because they provide such excellent sealing, the valve to the CDN (exhaust port) is a sliding gate valve. Now if we take the final pressure in the combustion chamber (after ignition) to be 500psi, then the pressure on the gatevalve, since it sealing a hole about a quarted inch in diameter would be about 1/20th that area so the pressure it has to cope with is just 25 psi, right. Commercially available belt run compressorsd are available that deliver 125 pf psi at a flow rate of 8 cu ft/min! The size of this compressor is *' in length and a diameter of just 5". This compressor would be used to replenish the main air tank, with the small amount of air (16 cu in) used for each combustioon cycle. What is so impossible about that! Anyway one man's meat............ as the saying goes. McQueen
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Offline McQueen

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Re: What is a rotary pulse jet engine?
« Reply #15 on: 02/09/2007 07:56:47 »
Soory that should be the compressor length is 8" and its diameter 5"!
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Offline syhprum

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Re: What is a rotary pulse jet engine?
« Reply #16 on: 02/09/2007 08:09:58 »
Belt driven compressers on IC engines have been obsolete for thirty years superseded by ceramic turbos.
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Offline McQueen

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Re: What is a rotary pulse jet engine?
« Reply #17 on: 02/09/2007 09:41:26 »
Quote
Belt driven compressers on IC engines have been obsolete for thirty years superseded by ceramic turbos.
We are obviously talking about two different things, I am referring to an air-compressor to supply compressed air to a tank, while you are referring to a turbo that is used to supply extra air to the engine!!!!!McQueen
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Offline Soul Surfer

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Re: What is a rotary pulse jet engine?
« Reply #18 on: 02/09/2007 10:03:11 »
Rocket engines may be very powerful and the only way to achieve thrust in space but they are not thermodynamically efficient and that is the most important figure when it comes to fuel consumption in terrestrial vehicles.

To properly calculate the true thermodynamic efficiency of an internal combustion engine  one needs to calculate the temperature and pressure of the high pressure gases and temperature and pressure of the exaust gases and the efficiency with which the expansion of these gases have been converted into useful work.

Think of a normal car  the velocity of the exhaust gases do provide in theory a useful thrust out of the back of the car but this is miniscule in proprtion to the much more effective thrust via the expanding gases pushing the cylinder down inside the engine.
« Last Edit: 02/09/2007 10:04:49 by Soul Surfer »
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Re: What is a rotary pulse jet engine?
« Reply #19 on: 02/09/2007 10:08:36 »
Think also of modern aircraft engines.  they are all high bypass ratio turbofans in which most of the energy in the gas turbine is used to suck air from the front of the aircraft and push it out the back and the jet thrust is only about ten pecent of the total thrust of the engine.
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Offline McQueen

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Re: What is a rotary pulse jet engine?
« Reply #20 on: 02/09/2007 11:31:49 »
Rocket engines may be very powerful and the only way to achieve thrust in space but they are not thermodynamically efficient and that is the most important figure when it comes to fuel consumption in terrestrial vehicles.

To properly calculate the true thermodynamic efficiency of an internal combustion engine  one needs to calculate the temperature and pressure of the high pressure gases and temperature and pressure of the exaust gases and the efficiency with which the expansion of these gases have been converted into useful work.

Think of a normal car  the velocity of the exhaust gases do provide in theory a useful thrust out of the back of the car but this is miniscule in proprtion to the much more effective thrust via the expanding gases pushing the cylinder down inside the engine.
Have you read my post at http://www.thenakedscientists.com/forum/index.php?topic=9816.msg120599#msg120599
?
Think also of modern aircraft engines.  they are all high bypass ratio turbofans in which most of the energy in the gas turbine is used to suck air from the front of the aircraft and push it out the back and the jet thrust is only about ten pecent of the total thrust of the engine.
This is not a jet engine, but functions more on the principles of a rocket engine. McQueen
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Offline Soul Surfer

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Re: What is a rotary pulse jet engine?
« Reply #21 on: 02/09/2007 11:52:09 »
What you are describing in that thought experiment has nothing to do with calculating the true thermodaynamic efficency of converting heat energy into useful work and contains several unfair and invalid assumptions.

The jet engine example was just to show that the high bypss turbojet engine can create much more thrust by using the heat enrgy to move more air than just using the heat energy to create thrust in its own right like a rocket.
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Offline McQueen

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Re: What is a rotary pulse jet engine?
« Reply #22 on: 02/09/2007 16:06:36 »
Quote
The jet engine example was just to show that the high bypss turbojet engine can create much more thrust by using the heat enrgy to move more air than just using the heat energy to create thrust in its own right like a rocket.
I entirely agree with you soulsurfer, you are perfectly right. A bypass turbjet does infact create most of its thrust from the extra air it pulls in with its turbine, the jet component is comparably small. What I am trying to demonstrate is the equivalency between an IC Piston engine and the thrust that is produced by the Rotary Pulse Jet Engine. In fact there is a practical demonstration of this in the form of recoilless guns. There is a good article on the subject in Wikipedia. McQueen
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Offline Soul Surfer

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Re: What is a rotary pulse jet engine?
« Reply #23 on: 02/09/2007 17:40:19 »
So you agree that your rotary pulse jet engine cannot be a thermodynamically efficient way of turning the heat energy of burning fuel into useful work for driving a car along a road.  Presumably you just find the rotary pulse jet emgine to be an interesting engine like the first steam engine designed by the ancient greek Hero which by using opposed steam jets similar to your propsed design created a fast rotary motion but could produce very little useful work.
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lyner

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Re: What is a rotary pulse jet engine?
« Reply #24 on: 02/09/2007 22:43:24 »
McQueen wrote
Quote
while the rotary vacuum pump is extremely efficient, since it runs in an oil bath for effective sealing,
The 'efficiency' of a vacuum pump is not a relevant thermodynamic concept. It is not a 'heat engine' in the conventional meaning of the term - you don't get mechanical work from internal energy.
However good your seals etc, are, you maximum efficiency is defined by the range of temperatures in the gases involved.
Hero (the greek not the member) used steam at a bit over 400K in his rotary jet so the engine wuld not have been very efficient. The jet engine - operating at  2000K (?) would be more thermodynamically  efficient than Hero's engine but there's much more involved if you actually want to realise anything near that efficiency.
I cannot (carnot?) find any reference to  the overall efficiency of the pulse jet engine in the article. This would be absolutely critical  before it would be worth while  considering it as an alternative to the  internal combustion engine, with its well known range of efficiencies.

As others have pointed out, jet engines are only used in certain applications and NOT because of their efficiency. As soon as you can 'push' against something solid - e.g. with tyres against a road , you do much better than a jet engine (a reaction engine)  can hope to. This is basically because, the MOMENTUM  given to the Earth is equal  (and opposite) to the MOMENTUM given to the vehicle. The Kinetic Energy  - which is proportional to velocity squared - is hogged, almost exclusively by the vehicle; the Earth's velocity change is virtually zero , so no KE wasted. The energy, necessarily,  given to the very fast exhaust gases of a jet engine is a huge proportion of the available energy and means LOW efficiency. You only use them when there is no alternative. Think of the wasted energy in getting a Space Shuttle the first 10m off its launch pad!
As this engine would use conventional fuels, I wonder why, if it's so good, Ford haven't tried it in their new models. They are always after good ideas and it wouldn't upset any vested interests.
« Last Edit: 03/09/2007 13:36:16 by sophiecentaur »

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Offline syhprum

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Re: What is a rotary pulse jet engine?
« Reply #25 on: 03/09/2007 09:59:31 »
Otto got it right first time, he did have the experience of 50 years of steam locomotives to draw on.
Wankel engines have been around for about 100 years but have only caught on for niche applications, about every 10 years an eccentric rotary design is published but none ever get built.
Just after WWII a few specialist cars were built with true pulse jet engines of the type used on the V1 missile (not very practical!)
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another_someone

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Re: What is a rotary pulse jet engine?
« Reply #26 on: 03/09/2007 13:22:59 »
Otto got it right first time, he did have the experience of 50 years of steam locomotives to draw on.
Wankel engines have been around for about 100 years but have only caught on for niche applications

The point is that every engine design serves a particular functionality.

I do not doubt that Wankel engines would not be suitable to drive a heavy goods vehicle; but equally, most electricity generation is now done using steam or gas turbines, and not using the Otto cycle either.  It is very much about horses for courses.

True, our own personal experience of power generation is for medium to heavy road transport, and in this context, the Otto cycle engines tend to predominate; but that is not a reflection of all oil based power generators (from aeroplanes, to gas powered electricity generating plants).

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lyner

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Re: What is a rotary pulse jet engine?
« Reply #27 on: 03/09/2007 13:42:37 »
I wonder if and when nanotechnology will  ever give us an internal combustion engine.
You could imaging all sorts of advantages (and problems, of course) with a distributed engine with, in effect, thousands of tiny reciprocating parts, all working independently and thinking for themselves (well you know what I mean). No vibration, easy cooling and easy replacement of parts. Sounds a bit like muscles.

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another_someone

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Re: What is a rotary pulse jet engine?
« Reply #28 on: 03/09/2007 13:52:19 »
I wonder if and when nanotechnology will  ever give us an internal combustion engine.
You could imaging all sorts of advantages (and problems, of course) with a distributed engine with, in effect, thousands of tiny reciprocating parts, all working independently and thinking for themselves (well you know what I mean). No vibration, easy cooling and easy replacement of parts. Sounds a bit like muscles.

Micro sized internal combustion - maybe; but nano sized, I would rather wonder if we could even fit a single molecule of heavy fuel oil into such an engine (not a problem for methane, but certainly not a diesel engine).  Would also wonder how you could guarantee proper air flow for each engine, and proper separation of intake and outflow.

I would have actually thought it probably easier to have nano sized fuel cells, generating electricity, rather than generating mechanical power (it would be easier to channel electrical power out of the system than to channel mechanical power out of such a system).

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lyner

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Re: What is a rotary pulse jet engine?
« Reply #29 on: 03/09/2007 15:32:01 »
Good points another someone.
It would have to operate in some sort of Wave mode; adjacent cells would have to be inletting or exhausting at similar times - some sort of circulation system (capillaries? spiracles?) would have to deliver the fuel and air. With the big surface area / volume ratio, catalysts would be effective. Sounds more and more like muscles. The engines  need not be reciprocating, of course.  The Wankel idea could be taken further and  developed into a peristaltic system., rather than a rotary system.
I must get that patent written before someone else takes it on.
Think how quiet it would be.
The snag is that most IC engines are better as they get bigger.

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Offline McQueen

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Re: What is a rotary pulse jet engine?
« Reply #30 on: 03/09/2007 17:24:02 »
syphrum
Quote
Otto got it right first time, he did have the experience of 50 years of steam locomotives to draw on.
Wankel engines have been around for about 100 years but have only caught on for niche applications, about every 10 years an eccentric rotary design is published but none ever get built.
You know I've been thinking about this! 150 years of the IC piston engine , based on the one Otto built, and hundreds of 'improvements, including overhead cams, double overhead, cams, multiple valves, MPFI and still the efficiency remains about 20% under actual road conditions ( although diesels are reputedly more efficient). My point is whatever can be done using piston technology has already been done, one cannot even imagine the money spent on trying to improve its performance or the number of people involved. So, a breakthrough, if one is going to come, would have to involve some new type of technology, such as that suggested in the Rotary Pulse Jet Engine. McQueen.
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Offline McQueen

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Re: What is a rotary pulse jet engine?
« Reply #31 on: 03/09/2007 17:50:28 »
The thing that I find impressive about the Rotary Pulse Jet Engine is that it provides equivalent power to that of a piston engine. In effect what the Rotary Pulse Jet Engine does is to take only the combustion chamber volume of the IC piston engine and discards the rest, cylinder, pistons, piston rings etc., It then supplies compressed air to this combustion chamber (rocket pods) at 125 psi from an external tank, the compressed air supplied to the combustion chambers by the external tank( a very small volume 18 cu ins) is constantly replenished by a commercially available belt driven air compressor that can deliver 8.5 cu ft min at 125 psi ! The thing that makes the rotary Jet Engine possible is the use of rotary union, this makes possible the introduction of fuel and compressed air to the rotor even though the rotor is rotating at high speed! Otherwise imagine the situation, here is the rotor spinning at several thousand rpm and you are trying to introduce fuel and air into it ! Sounds ridiculous and it is the rotary union that makes it possible. The rest is simple, the compressed air fuel mixture is fed into the combustion chambers through a poppet valve, (125 psi works out to a compression ratio of 9:). The poppet valve is closed, sealing the combustion chamber and the fuel/air mixture is ignited using a piezo electric spark plug. A Valve to the CDN (Convergent divergent nozzle ) is opened and the hot gases of expansion escape at velocity driving the rotor forward through reactive forces. McQueen
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Offline syhprum

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« Last Edit: 03/09/2007 19:12:11 by syhprum »
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lyner

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Re: What is a rotary pulse jet engine?
« Reply #33 on: 04/09/2007 12:10:24 »
The problem, McQueen, is still that you are ignoring the efficiency issue. All those hot gases you are sending out somewhere have a lot of KE. That had to be transferred from the fuel energy and can't be used for useful work. Bang goes your efficiency. The exhaust gases from an IC engine have nothing like the same amount of KE -so the efficiency is not significantly affected.
All your practical details about how it will operate can't help with that basic problem.

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Offline syhprum

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Re: What is a rotary pulse jet engine?
« Reply #34 on: 04/09/2007 14:18:08 »
What is the difference between your belt driven compressor and a supercharger?, it would absorb a large proportion of the output power and would be much better replaced by an exhaust driven turbine.
If your engine was to produce the same power as a conventional 3 litre engine running at 5000 rpm it would require 140 cubic feet per minute of air.

wracking my brains I seem to recall that an engine of this type using a hypoglopic fuel mixture was used to drive the fuel pumps on the the Saturn 5 rockets but I cannot quote chapter and verse, of course thermal efficiency does not figure very largely in this application!
« Last Edit: 04/09/2007 14:50:08 by syhprum »
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Re: What is a rotary pulse jet engine?
« Reply #35 on: 05/09/2007 09:23:28 »
Syhprum

It might surprise you to know that I, at one time thought as you did, that there could not possibly be much energy in reactive forces. Looking at some of the posts by other members in this thread, this seems to be a widely held view. What changed my mind, and set me thinking was an article on the Recoilless Gun

The Recoilles rifle (gun) was originally invented by an American naval Commander by the name of John Cleland Davis, his purpose was to design a gun that would eliminate the forces of recoil, which necessitated a bulky gun carriage and heavy weight to overcome the forces of recoil. Originally he deigned a twin barreled gun, with the two barrels pointing in opposite directions. In one of the barrels he placed the shell and in the other an equivalent weight of grease, in the center was the charge (propellant). When the charge was ignited it exerted equal force in both barrels sending the shell out of one barrel and the grease out of the other with equal force.  Eventually Cleland came to the conclusion that since mass1 x velocity1 = mass2 x velocity2 it would be possible to eliminate the grease in the second barrel and to replace it with gases escaping at high velocity! Thus was born the recoilless gun in 1866 as it happens this was also the year in which Nicholaus Otto invented the IC piston engine!  Now as you can see from the article the recoilless gun propels the same caliber shell as an ordinary cannon, to aprrox. The same distance. However one very important point is that in the recoilless gun, the gases are left to exit freely through the Venturii (CDN) without interruption. If one thinks about it this makes sense. If one did try to impede the exit of the gases, as say by the placing of a valve in the venturi, the purpose of the gun would be defeated and the forces of recoil would be present. In the Rotary Pulse Jet Engine, since we are not concerned with the forces of recoil a valve is present in the venturi (CDN). Therefore the power of the gases escaping at velocity from the combustion chamber should yield an equivalent power to that of a similar sized IC piston engine. In this way the recoilless gun was the fore runner of the modern grenade launcher and all other rocket based type of armaments, including missiles.  Forgive the long winded post, but I hope you see where I am coming from. McQueen.
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another_someone

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Re: What is a rotary pulse jet engine?
« Reply #36 on: 05/09/2007 11:36:02 »
You cannot compare a gun to an engine of this type.

The purpose of a gun is to fire a projectile at a given velocity, and the power output is incidental; whereas for what you are trying to create, power is all, and it is velocity that is incidental.

In order to drive a projectile at a given velocity, it is a prerequisite that the exhaust gasses behind the projectile are travelling at the same velocity as the desired projectile at the point where the projectile leaves the muzzle of the gun.  If you wish to fire a projectile at Mach 2, then the gasses behind it must be travelling at Mach 2, and that is a prerequisite, from which all else will follow.

In order to maximise the efficiency of the engine you desire, the requirement is to maximise impulse power, and the only requisite minimum velocity that constrains you is that the gasses should leave the combustion chamber should be only slightly faster than the movement of the combustion chamber itself (i.e. that the exhaust velocity should be as close to zero when compared to the stationary parts of the engine as it is possible to be, with just enough energy left to force the gasses through the exhaust pipes).  Since you have no intention of blasting projectiles for several kilometres out of the nozzle of the combustion chambers, any excess velocity is simply waste.

In any event, guns are anything but the epitome of energy efficiency - they have other standards by which they are judged (when was the last time you ever saw energy efficiency of a gun of any kind being quoted as a pertinent statistic?).
« Last Edit: 05/09/2007 11:37:57 by another_someone »

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Offline McQueen

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Re: What is a rotary pulse jet engine?
« Reply #37 on: 05/09/2007 13:31:20 »
Quote
You cannot compare a gun to an engine of this type.
Let's have a poll ! Does the IC Piston engine resermble a gun, or are you blind? McQueen.
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Offline daveshorts

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Re: What is a rotary pulse jet engine?
« Reply #38 on: 05/09/2007 18:22:15 »
But a recoiless rifle needs several times more propellant, and therefore several times more energy, than a conventional gun of the same design. This is why recoiless weapons are restricted mostly to infantry weapons that are not fired very often so the lighter weight of barrel is more of an advantage than the much heavier ammunition that you need to fire in it. If you are going to fire a lot of rounds it rapidly becomes easier to transport a heavier gun and lighter ammunition.

I don't see how your design of internal combustion engine would be much lighter than a normal one and it would definitely need far more fuel in order to accelerate the exhaust gasses to the immense speeds you would need to get any useful power out of it.

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lyner

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Re: What is a rotary pulse jet engine?
« Reply #39 on: 05/09/2007 19:35:35 »
And have you an answer about the efficiency, yet, McQueen?
It is, after all, quite a relevant question.

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Offline syhprum

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Re: What is a rotary pulse jet engine?
« Reply #40 on: 05/09/2007 21:21:43 »
And McQueen why your insistence on a power hungry belt driven compressor when a high velocity exhaust stream is available that could drive a readilly available turbine.
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Re: What is a rotary pulse jet engine?
« Reply #41 on: 06/09/2007 01:14:59 »
Daveshorts
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But a recoiless rifle needs several times more propellant, and therefore several times more energy, than a conventional gun of the same design. This is why recoiless weapons are restricted mostly to infantry weapons that are not fired very often so the lighter weight of barrel is more of an advantage than the much heavier ammunition that you need to fire in it. If you are going to fire a lot of rounds it rapidly becomes easier to transport a heavier gun and lighter ammunition.
When you say "several times the propelant" exactly how many "times" more are you talking about ? I have researched the subject and can tell you that at most a recpoilless gun uses about 2/3 more propellant than a normal cannon to achieve the same performance as that of an ordinary gun.  Also consider, what I have been saying, the gases of combustion in a recoilles gun cannont be restricted in any way, as for instance by a valve because the moment the gases of combustion are restricted,the forces of recoil come into play. If it were possible to restrict the gases of combustion before release then the pressure would build up and the amount of propellant needed would be less, while the performanace would be the same as that of a normal gun.. McQueen
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Re: What is a rotary pulse jet engine?
« Reply #42 on: 06/09/2007 01:21:00 »
sophiecentaur
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....and have you an answer about the efficiency yet..?
I don't have the exact figures but the efficiency of a rotary pulse jet should be far greater than that of an IC Piston engine, it stands to reason. In an IC piston engine the fuel/air is ignited and the piston moves down with great force for a distance of a bout 3 ins. before being brought to a halt and being hauled up again by the fly=wheel or crankshaft. It can't be all that efficient! McQueen
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Offline syhprum

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Re: What is a rotary pulse jet engine?
« Reply #43 on: 06/09/2007 07:38:03 »


May I suggest for your reading

The high-speed internal-combustion engine
by Harry Ralph Ricardo

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Offline McQueen

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Re: What is a rotary pulse jet engine?
« Reply #44 on: 06/09/2007 10:38:46 »
Syhprum
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....May I suggest for your reading....
The efficiency of the gasoline IC Piston engine is only about 20%, you can look it uip wherever you want on the net, by comparison a rocket or jet engine has an efficiency of over 70%. McQueen
P.S I get the idea of using the exhaust to run a turbine, at the moment what is being checked out is the feasability of the RPJ.
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another_someone

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Re: What is a rotary pulse jet engine?
« Reply #45 on: 06/09/2007 22:03:29 »
http://en.wikipedia.org/wiki/Jet_engine
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The problem was that rockets are simply too inefficient at low speeds to be useful for general aviation.

http://en.wikipedia.org/wiki/Turbojet
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Turbojets are quite inefficient (if flown below about Mach 2) and very noisy. Most modern aircraft use turbofans instead.

http://en.wikipedia.org/wiki/Turbofans
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Depending on specific thrust (i.e. net thrust/intake airflow), ducted fans operate best from about 400 to 2000 km/h (250 to 1300 mph), which is why turbofans are the most common type of engine for aviation use today in airliners, as well as subsonic/supersonic military fighter and trainer aircraft. It should be noted, however, that turbofans use extensive ducting to force incoming air to subsonic velocities (thus reducing shockwaves throughout the engine).

Not sure where and in what contexts you regard jet and rocket engines have an efficiency of 70%, but like all efficiency issues, it depends on context.

IC piston engines can gain efficiencies significantly higher 20%, but this may be applicable to large IC engines that run higher internal temperatures and pressures (and pre-cool the air coming into the cylinder), but these are not practical within the constraints of a small motor vehicle.  It should also be remembered that rockets use unusual fuels (energy efficiency when burning cryogenic hydrogen and oxygen, and ignoring the energy costs involved in creating those fuels, can appear to be far higher than burning petrol in natural air that is not cryogenicly cooled).

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Offline McQueen

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Re: What is a rotary pulse jet engine?
« Reply #46 on: 07/09/2007 00:13:33 »
Quote
The problem was that rockets are simply too inefficient at low speeds to be useful for general aviation.
Well the most obvious reason for that inefficiency, as I have been trying to point out, is that most rockets are simply not equipped with a valve, they burn fuel continuosly at tremendous pressures and temperatures and have hitherto been impractical. If the tremendous power of a rocket design can be harnessed in the manner that is described for the Rotary Pulse Jet Engine, it would definitely be a great improvement over present technology such as the IC piston engine. here's what wikipedia has to say:
en.wikipedia.org/wiki/Pulse_detonation_engine
quote:In theory the design can produce an engine with an efficiency far surpassing more complex gas turbine Brayton cycle engines, but with almost no moving parts.
Consider the idea as it stands, namely ignition of fuel resulting in increase in pressure and then release of that pressure to turn the potential energy of the pressure into kinetic energy of the escaping gases, producing thrust through reactive forces. McQueen

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Re: What is a rotary pulse jet engine?
« Reply #47 on: 07/09/2007 00:45:43 »
What is interesting is that you earlier quote jet engines as having 30% efficiency, whereas the Wikipedia article you quote refers to conventional jets having only 30% efficiency, and the pulse jets (when someone actually succeeds in making one) having a theoretical limit of 50% (still short of your 70%).

But it seems very unlikely that what you are referring to in your design is a true pulsed detonation engine.  The reason I say this is three fold:

Firstly, although there are development projects to develop pulsed detonation engines, the peculiar complexities of the required supersonic flame front required for these engines means that none have yet been commercially produced.  I do not recollect anything in your documentation that suggests you have made any particular breakthrough in this problem.

Secondly, since the flame front, and exhaust gasses, are supersonic, it therefore follows that maximum efficiency (the quoted 50% efficiency) will only be attained when driving supersonic aircraft (the Wikipedia article suggests it might be used to drive an aircraft at Mach 5).  There is nothing in that you have said that has lead me to believe that you intended to propel your combustion chambers at supersonic speeds.

Finally, the benefit of the pulsed detonation design is that it obviates the need for a compressor (which inevitably absorbs much of the energy of the jet); yet your design clearly does require a compressor.

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another_someone

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Re: What is a rotary pulse jet engine?
« Reply #48 on: 07/09/2007 01:05:12 »
If you are interested in pulse detonation (as described in the Wikipedia article), one of the links off from the Wikipedia article is to a radio interview on pulse detonation undertaken by a New Zeeland radio station (it does make clear that you would not use pulse detonation direction for subsonic flight, although you could fit a pulse detonation combustion chamber inside a turbofan engine).

http://www.publicaddress.net/assets/upload/13073/224975781/PAS_PulseDetonationEngines.mp3


There are also issues raised about noise levels associated with pulse detonation engines.

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Re: What is a rotary pulse jet engine?
« Reply #49 on: 07/09/2007 09:22:40 »
The only way that I could envisage the rotary pulse jet engine becoming efficient is to use what is effectively a very lean burn having a small volume near the ignitor exploding violently with most of the air simply being used as mass to be ejected from the cylinder.  The big problem is the energy needed to compress the gas to provide the air for the combustion chamber.  The range of revs it will operate efficiently may be small because of the assymmetry between the two combustion chambers in the illustrated design.
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