Naked Science Forum
Non Life Sciences => Technology => Topic started by: McQueen on 09/05/2008 10:57:40
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Here is an experiment that illustrates once and for all that expansion forces and reaction forces are equal. It might seem rather basic but these are the same type of experiments carried out by Robert Goddard during his experiments with rockets. Fig.1 shows the components I have used to test the reaction force.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.geocities.com%2Frotarypulsejet%2Fimages%2Fexp1.jpg&hash=8800200f40b34794b4d6d0592d73fbd8)
1. is an empty 120 ml bottle of Renu, Bausch & Laumb contact lens solution this represents the combustion chamber, it is more or less the right shape but lacks a venturi. 2 is an empty toilet paper roll with a hole cut in it this is used as a stand for the combustion chamber (rocket pod) 3. is a small fire-work squib which represents the explosive fuel air mixture. 4. is some cotton waste which is used to seal the mouth of the combustion chamber and which represents the valve. Fig2.(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.geocities.com%2Frotarypulsejet%2Fimages%2Fexp2.jpg&hash=f98e1a242b109bfc895fbd537c3e5686) Shows how the cotton waste is wrapped around the squib to form an air-tight seal.
The squib is then fitted into the mouth of the bottle as in Fig. 3
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.geocities.com%2Frotarypulsejet%2Fimages%2Fexp3.jpg&hash=3229ecd84ddfac8cfc88bf867999ff06)
The combustion chamber is then fitted into the holder as in Fig.4
and the wick lit as in Fig.8
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.geocities.com%2Frotarypulsejet%2Fimages%2Fexp8.jpg&hash=424695ee87cf0e29f31281eed943a21f)
with the result that the bottle shoots up into the air. (http://www.archive.org/details/Reactionforce)
Fig 5. shows the components I have used to simulate the expansion force as found in a piston engine.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.geocities.com%2Frotarypulsejet%2Fimages%2Fexp5.jpg&hash=ba3a49da6d2621d57ca7da518ca49fb6)
1 is the squib representing the explosive fuel air mixture 2. is double sided tape to fulfil the function of a piston ring. Three is a 40mm PVC union with a hole in it to allow the wick to be lit. 4. represents the piston. The whole together simulate a square piston where the stroke and the bore have the same dimensions.
The double sided tape is wrapped around the bottle about half an inch from the bottom and forms a tight seal within the piston this is the piston ring Fig.6
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.geocities.com%2Frotarypulsejet%2Fimages%2Fexp6.jpg&hash=9082d4af4f2dbc285d8fd9433c9180a2)
Fig. 7 shows how the piston fits into the cylinder.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.geocities.com%2Frotarypulsejet%2Fimages%2Fexp7.jpg&hash=9fc98fbe6fa1ed858d365f1f94b8840f)
the fuse can be seen at the bottom. Fig.9. shows the fuse after it has been lit.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.geocities.com%2Frotarypulsejet%2Fimages%2Fexp9.jpg&hash=c182514e504bb55f04fb7dad16f19e98)
the bottle is propelled into the air. It must be noted that the weight of the object being lifted is the same in both cases. (i.e. The bottle) . Both forces propel the bottle almost equal distances. However, while the piston is working under optimum conditions the combustion chamber has neither a convergent divergent nozzle nor a small enough aperture for the exit hole necessary for maximum performance. Robert Goddard noted in his experiments that the presence of a convergent divergent nozzle resulted in an almost 75% improvement in performance. In fact if a convergent divergent nozzle is added I can state with confidence that the reaction force would work better simply because the force acts over a longer distance than in the expansion force. What does this prove, it proves that the reaction force does provide a force equivalent to, or greater than that of the expansion for (IC piston engine) and this after all is what Wankel had tried to achieve, pure rotary movement, with a force equivalent to or greater than that of a piston engine.
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Why this obsession with force when it is energy / power which count?
Neither configuration is anything like optimal so you can't compare them validly.
The pressure which you would get inside a piston / cylinder in a properly designed engine could be much greater and the resulting pressure X volume change (the energy transferred) would be much more unless the 'projectile' from the reaction system travels very fast.
If you need a rocket (in space), you use a rocket engine, if you want a reaction engine in an atmosphere you use a rocket or jet engine. If you want a high speed engine (e.g. turbo fan) you can use a turbine. If you want a slow speed engine, you use expansion; any high speed reaction engine needs a hideous amount of gearing ratio and this is inefficient. It's horses for courses.
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Force x distance = power. A rocket is a conflagaration device. Pressure and therefore force is created as the fuel burns, increasing pressure inside the combustion chamber, when these gases are vented under pressure through a convergent divergent nozzle the forces of reaction create thrust in the opposite direction creating thrust. The combustion chamber in the Rotary Pulse Jet is sealed until the fuel/air is combusted, therefore pressures rise upto 30Kg/cm before they are released. This gives it the equivalent power of a piston using expansion forces. See this link. (http://www.archive.org/details/Reactionforce) You must forgive me for the poor quality of the movie but I can tell you what happens, the bottle under the explosive reaction force shoots twenty to thirty feet into the air, and this is without a Convergent divergent nozzle. So what I am trying to get across is that reaction forces can be used to develop an impulse force just as a piston does in an IC engine using expansion force.
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Force x distance = power
No!
Force X distance is energy (or work)
Force times speed is power.
Pressure times volume change is energy
and, possibly, Force times time is IMPULSE which equals change in momentum
All these may be relevant in this discussion - they need to be quoted accurately.
We are not dealing with highfalootin' modern Science here. We are dealing with facts not opinions. If your exhaust gases are zapping out of your machine at higher speed than the exhaust gases from an ordinary IC engine then they are carrying with them a load of WASTED energy. This is also true if they are HOTTER.
Do you not see how important the energy considerations are in this discussion? Force is almost irrelevant - at least it is secondary. A simple lever can give you any force you want but it can't suddenly give you more energy.
What relevance does your exploding piston have to a constrained piston in a 4stroke engine? The velocities are different and so are the pressures etc etc.
By the time your projectile has emerged from the cylinder there is still loads of available energy which just dissipates into the surrounding air. You would need a long 'barrel' for all the energy to be transferred to the projectile via your 'compressive force'. Whatever the force at the start of the experiment, it is turned off much too soon for a fair comparison. Like I said, it's energy that counts.
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Depressingly you are right as usual! (Or OK once in a while) Force x distance = work and the time within which that distance is covered equals power . What this experiment was supposed ( and hopefully does prove) to illustrate is that in both instances, expansion and reaction, the results are almost equal, with the forces of reaction edging out expansion in the amount of power developed. In other words, under exactly equal conditions, both forces would be equal but in the conditions as applied to the IC piston engine and the rotary pulse jet engine, the reaction force would be stronger. I can’t understand where you get the idea that reaction forces result in a slow start and then slowly accelerate, maybe if you are talking about rockets it might be true, but what about recoil forces? In a rocket the acceleration is slow because it is designed to be slow, in the case of the RPJ the reaction forces are designed to yield full force in the smallest amount of time and the result is more than equal to an expansion force, as the experiment illustrates. Further, you must be aware that in an IC piston engine the exhaust valve opens about 20 degrees before BDC, it has to do this if it doesn’t crack the crankshaft. What a waste of energy! And one which does not occur in the Rotary Pulse Jet Engine.
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I'm sorry to be so pedantic McQ.
Here you go again, though, talking in terms of forces - it is not really relevant that you compress the gas before TDC; that energy is stored and you get it back as the piston moves down again - it's only like pushing against a spring. It is quick enough to be adiabatic - so no energy loss.
As we agree, pressure times speed is power. When a rocket starts off slowly the engine cannot transfer much power to it because it is going so slowly. At high speeds it is working much better; this is why turbines (and your engine) work best at high revs because the pressure is high and so is the speed.
Your test, as you say, gives similar conditions for both systems but the two engines operate under totally different conditions so it isn't really relevant.
There is a further point. If your force only acts for a short time then the change of momentum (impulse) will be shorter than optimum. You are, effectively wasting 'pushing time'. You would be better to operate it as a Catherine wheel when the force would be applied all the time - like a turbine.
Try looking at the things in terms of energy transfer - which is what all engines are for. Forces are at work, of course, but they can lead you up the garden path if you ignore the other considerations.
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If you have an identical weight, propelled to an identical distance in the identical amount of time, and motivated by two different types of force, what conclusion can be drawn from the circumstance ? Either there are some esoteric forces at work OR the amount of power, the work done and the force generated by the two different motivating forces are equal and equivalent !
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In an experiment in which a three year old child throws a ball the same distance as an Olympic athlete, it is just possible that the athlete is not using all his capabilities. The test proves nothing other than that the athlete was not working optimally - possibly with his hands tied behind his back. Yes, of course they both transferred the same amount of energy. Not, of course, necessarily involving the same force; the distance and time for which the force was applied may be different in each case.
In a fair test, however, in which they are both working optimally, you might find there would be a significant difference in their performances. Would not the fair test result be more relevant?
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sophiecentaur,
Just think of a scuba tank filled to 300 bars, would you like to be anywhere in the vicinity when it was punctured??? I am talking about the same type of force, but channeled and used usefully!!! Of course I am talking in t5he region of 35 bars, but maybe at 3000 rpm!!!
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I can't comment until you can bring yourself to talk in terms of power, energy and efficiency. They are the three relevant quantities in transport engineering.
Like I keep telling you, you can produce as much force as you like and transfer no energy at all. Your rotor can rotate as fast as it likes yet it may transfer no energy.
Do you really not understand the significance of the preceding paragraph?