I can’t believe I am explaining this. I’ll let others more qualified explain it better but I insist you are mistaken. Only friction turns energy into heat. In the absence of friction the pendulum would go on forever and the motor would turn forever. friction is what slows it down by turning energy into heat.
When the pendulum is stopped at one end it has potential energy. As it speeds up it converts that potential energy to kinetic energy. At the lowest point it has no potential energy and all the potential energy has been converted to kinetic energy (minus friction losses). On the upswing the cycle is reversed. Minimise friction and your pendulum will swing for a long time.
A reciprocating engine is the same. Only friction causes losses not the reciprocating motion itself. Of course there is a lot of unavoidable friction in a motor, much more than in a pendulum.
Guardian, you are wrong. It is friction that slows the pendulum and in the absence of friction it would swing forever. Just like the planets circle the sun falling from aphelium to perihelium and swinging up again. In the absence of friction they do not slow down. They are very similar to a pendulum in that they exchange height (distance) for speed while conserving energy.
This is very basic physics so you did not take much advantage of that course. I would have failed you. Perhaps you should take it again.
>> So you are saying that perpetual motion machines do indeed exist? COOL!! And I always believed my physics teacher when he said they don’t. Boy, was he an idiot! (any cites?)
Guardian, your are showing your ignorance by not understanding the context of the quote. When it is said that a perpetual motion machine does not exist and cannot be built what we mean is that friction is unavoidable in any machine we humans can build.
As I have pointed out, get rid of friction and you have perpetual motion. Even planets have some friction (tidal forces) but so low that they will keep moving for millions of years. That’s pretty close to perpetual motion but it is not what is meant by the “perpetual motion machine” which refers to machines humans can build and use.
This is a clear case where “a little knowledge is worse than no knowledge at all”.
I stand corrected on the “frictionless” pendulum. However, I would disagree that it applies to an internal combustion engine as simplified as you have stated. If we remove friction from the engine, than we basically only have one energy, the kinetic energy of the rotating crank shaft. This energy is unaided until the ignition of the fuel, and it must overcome the force of the mixture being compressed, as well as puting the piston in motion three times between each ignition. You can not use the argument that the piston gathers potential energy, since many engines do not have pistons that are oriented vertically.
Again, since we are dealing with a real world, friction is also an issue.
Instead of making snide remarks, how about just sticking to the facts. If I’m mistaken, I am perfectly willing to admit it; but there is no reason to insult me simply because you consider yourself superior.
Now if I attach the frictionless pendulum to a frictionless mechanism to make if go in and out…and in and out…and in and out…and guarantee enough lubrication - hmmm…this can go a long time…
L. Guardian, sailor is correct. Potential energy is not merely vertical height. When the pistons are pumping at a certain rate, if it were an ideal, frictionless system, they would remain pumping forever. When the piston heads are fully extended or retracted, their kinetic energy has been converted into potential energy.
Think of a mass attached to a spring, which is attached to a wall, all in a frictionless environment. When you put the mass into an oscillating state, at either end of its journey across the frictionless floor, it is not moving for a split second; all its energy is potential energy, stored in the spring.
Likewise, the pistons, which are attached to a rotating body, have no kinetic energy at the two ends of their journey. All of their kinetic energy has been converted into potential energy, stored in the rotating body.
Guardian, my comment was not in any way intended to be snide, derogatory or offensive. Sorry if it sounded that way. It was just intended to mean literally what it says and I have been there too so it would apply to me just as much.
The issue at hand is whether a reciprocal engine wastes energy because of the alternating motion of the pistons and cranks and the answer is no, this does not affect it. The energy used to slow the piston and crank is stored in the crankshaft and it is returned during the next quarter cycle.
We are not talking about other issues. Thermal motors, whether they be reciprocal, wankel, turbines etc. all have their pluses and minuses and they are best suited to different applications.
But the fact that the motion is reciprocal does not per se mean it is wasteful.
(Note the correct use of “per se”. One of my pet peeves is that 95% of the time it is used totally wrong by people who want to sound like they know what they mean. Even worse when they misspell it as “per say”. Search this site and you’ll know what I mean.)
Punoqllads, the kinetic energy of the reciprocating parts (piston, crank) is stored in the crankshaft also as Kinetic energy by speeding it up. No potential energy here, this is valid in the absence of gravity where a pendulum would not work. For potential energy you need gravity or some other force like a spring. In a motor the energy stays kinetic but is transferred from reciprocating parts to rotating parts.
To further expand on this: the crankshaft has a natural resonance frequency and it is transferring energy back and forth to the pistons at a frequency which is directly related to the RPMs. If this frequency of transfers is in the resonance vicinity you can destroy a crankshaft in short order. That is why some motors will have certain red zones and can operate safely above or below them but not in them. Also, making the oscillating masses smaller (lighter pistons and cranks) means less energy transferred back and forth which means less stress on all moving parts, less forces near resonance, less friction, and a lighter car which will save gas, accelerate better etc.
Uh…yeah, I knew that, I was just…um…just testing you, yeah, testing you. And you did real well, yeah! Great job! Not serious, definitely a test; definitely.
Seeing that your question was answered and you did not post further questions, I am not sure what your complaint is but if you feel we are squatters in this your thread you could ask the mods to close it. I would express my disagreement though. The thread goes where the posters take it and you are welcome to post like the rest of us.
But I will tell you what does bother me: people who post questions with complex answers and then get bored because the answer is something that requires some mental effort. (This is a general observation, not specific to this thread) I have seen several such threads.
But start a thread about sex, beer, sex, Gore, sex, Bush, sex etc and it will go on forever.
I guess we forgot to mention sex, beer, sex, Gore etc in the context of Wankel motors.
Beer would probably be a bad fuel to use in a wankel rotary engine, and is certainly a bad thing to have in the blood of the person operating it.
Sex in an RX7 is probably very uncomfortable
Al Gore once claimed to have invented the wankel rotary engine
If asked for comment, George W. Bush would probably have absolutely no idea what a wankel rotary engine is, even after drawing little diagrams and explaining it in one syllable words.
