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Author Topic: What is a rotary pulse jet engine?  (Read 44728 times)

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 »


 

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).
 

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
 

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
 

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).
 

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 »
 

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.
 

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

 

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.
 

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. 
 

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 
 

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
 

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

 

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 2000C 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!
 

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 2000C 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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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"!
 

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.
 

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
 

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 »
 

Offline 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.
 

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
 

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.
 

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
 

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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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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Re: What is a rotary pulse jet engine?
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