5 ms·

Seems like needing the piston to change directions is a waste of momentum. What about a similar design where a ball is propelled around the inside of a circular

by mentos 12y ago

Seems like needing the piston to change directions is a waste of momentum. What about a similar design where a ball is propelled around the inside of a circular doughnut to generate electricity?

codfrantic 12y ago
Or a Wankel engine[1] [1] http://en.wikipedia.org/wiki/Wankel_engine http://en.wikipedia.org/wiki/Wankel_engine
Zigurd 12y ago
A rotary engine isn't free-piston, but it also doesn't have reciprocating parts. But it also shows there are other challenges to efficiency than reciprocating pistons.
sharemywin 12y ago
I think the pressure would go around the donut on both sides and equalize.
yxhuvud 12y ago
Wouldn't a more obvious solution be to change the direction by adding a second combustion chamber on the other side?
- kabdib 12y ago
  That works great until you miss ignition on the opposite side, then wham it bottoms out and destroys the other side of the engine. A gas spring is a pretty simple mechanism and can't misbehave (too bad you can't put it to work, though -- probably a lot of heat being wasted compressing it).
  - Already__Taken 12y ago
    This would be why you wouldn't run 2 pistons facing each other sharing common valve and ignition set to balance?
luch 12y ago
Is your question rhetorical ? Because that's how ITER (nuclear fusion reactor) generate electricity in a nutshell, if you replace the ball by plasma. source : https://www.iter-india.org/images/fusion_experiments2.jpg https://www.iter-india.org/images/fusion_experiments2.jpg
- fanf2 12y ago
  That diagram is about the energy inputs to heat the plasma. Energy output to generate electricity is quite indirect: hot neutrons escape the magnetic containment and are caught in a lithium blanket. This produces tritium from the nuclear reaction between the neutrons and lithium nuclei, and heat. The blanket is cooled with water which is used to drive a conventional steam turbine generator set. See the last paragraph of the reactor overview at http://en.m.wikipedia.org/wiki/ITER http://en.m.wikipedia.org/wiki/ITER and note that they do not plan to generate electricity from ITER: it is purely experimental.
chiph 12y ago
Too hard to seal. Ask a RX8 owner about rotor apex seals sometime. The bane of their existence. In this new motor there's nitrogen on the other end of the piston that provides "rebound". It's probably inside the coolant jacket as well to prevent expansion and the loss of efficiency that would bring.
- kabdib 12y ago
  Ex RX-7 owner here. Not much room in the back of one of those things, but I always had room for cans of oil :-) Wonderful cars. I'd buy an RX-9 if they ever built one, but they're never going to. So I have to motorcycle instead, could be worse :-)
  - nicholassmith 12y ago
    I know a few people who've owned RX-7s and 8s, one said it was the only car he worried about running out of oil before fuel. Beautiful cars, but they're a bit high maintenance.
    - busterarm 12y ago
      10th Anniversary RX-7 owner here. If you actually maintain your car and don't treat it like a toaster, it's not too bad. They combust oil to lubricate the engine, so you just have to make sure it has oil. If your oil metering pump is electronic ('89 and after), run it with premix when you fill up. And drive your car hard & rev past 6k once a drive or so to clear out the carbon. Yes, I've gotten a speeding ticket for doing this. I love my car. It's about to hit 100k miles and it's a Turbo and despite what the naysayers say, it hasn't blown up yet and still has good compression. The electrical issues are far worse than the engine ones. The engine is bulletproof if you do your basic maintenance and listen to your car. Every sports car is high maintenance. The Alfa, Porsche and Ferrari owners have it as bad or worse - people just treat exotic powerplants like voodoo - I sure paid a hell of a lot less than they did though. I would rather pull and rebuild my entire engine than change the power steering fluid on a recent Porsche. The latter has a nightmarish set of steps likely to break all sorts of tiny plastic bits that cost more to replace than my engine rebuild kit would...
pbhjpbhj 12y ago
You could have a reciprocating engine instead. Wankel engines have problems due to sealing being difficult (as would your idea, explosive gases escape at the edge of the sphere), as I understand (I've not studied it in depth), but a reciprocating engine would seemingly have no more [in fact less] issues with being sealed than 2 parallel pistons. Have the first combustion chamber push the piston one direction past the coils and have a second chamber at the opposite end push the piston back. Any excess movement might be able to be used for pre-compression? Of course many people have thought of this before (don't you hate it when that happens! ;0)) - http://www.youtube.com/watch?v=2SlwoBLspTQ http://www.youtube.com/watch?v=2SlwoBLspTQ shows 2 such engines. http://www.youtube.com/watch?feature=player_detailpage&v=yV09wMXTSIc#t=7 http://www.youtube.com/watch?feature=player_detailpage&v=yV0... shows another idea, 2 pistons either side of a single combustion chamber.
nabla9 12y ago
If you follow that line of thought to it's conclusion, you end up with gas turbine (Brayton cycle).
- Sami_Lehtinen 12y ago
  Yes, that's what I thought. Many people seem to ignore gas turbine option, even if those are very reliable and efficient power sources. If there's interest towards different engine designs, it might be a good idea to check this out too: https://en.wikipedia.org/wiki/Wave_disk_engine https://en.wikipedia.org/wiki/Wave_disk_engine and another interesting design: https://en.wikipedia.org/wiki/Swing-piston_engine https://en.wikipedia.org/wiki/Swing-piston_engine There surely isn't shortage of engine designs, most of these designs are from 1800's. So nothing new.
susi22 12y ago
Don't forget that for an electric motor you need a sine shaped AC and not DC so it's pretty convenient that this design produces exactly that. Whereas with a "continuous" design you'd need to make it AC.
- randallsquared 12y ago
  If your motor needs AC, you already have an inverter, since the battery produces DC.
derekp7 12y ago
I hear this quite often, but I don't quite see how this is true. Each piston is hooked to a crankshaft, which is hooked to a flywheel. Even with a single piston, the energy of it slowing down and reversing is absorbed in the flywheel, which is in turn used to propel it in the opposite direction. Add multiple pistons, and the energy of slowing down is again used to accelerate another piston. Just think of any spot on a wheel representing an arbitrary amount of mass. Spin the wheel, and the only thing slowing it down is friction -- not that spot constantly changing direction.
- maxerickson 12y ago
  This article is specifically about a piston engine that does not have a crankshaft.
bnegreve 12y ago
> Seems like needing the piston to change directions is a waste of momentum. It's not, in a closed system momentum is conserved [1] so without friction, a free engine would rotate forever. Also there is not conceptual difference between an engine and a ball sniping in a donut. In both case, the piston or the ball have to change direction (the piston has to change direction more abruptly though) [1] http://en.wikipedia.org/wiki/Momentum#Conservation http://en.wikipedia.org/wiki/Momentum#Conservation

by mentos 12y ago

Seems like needing the piston to change directions is a waste of momentum. What about a similar design where a ball is propelled around the inside of a circular doughnut to generate electricity?

codfrantic 12y ago

Or a Wankel engine[1] [1] http://en.wikipedia.org/wiki/Wankel_engine http://en.wikipedia.org/wiki/Wankel_engine

Zigurd 12y ago

A rotary engine isn't free-piston, but it also doesn't have reciprocating parts. But it also shows there are other challenges to efficiency than reciprocating pistons.

sharemywin 12y ago

I think the pressure would go around the donut on both sides and equalize.

yxhuvud 12y ago

Wouldn't a more obvious solution be to change the direction by adding a second combustion chamber on the other side?

kabdib 12y ago
That works great until you miss ignition on the opposite side, then wham it bottoms out and destroys the other side of the engine. A gas spring is a pretty simple mechanism and can't misbehave (too bad you can't put it to work, though -- probably a lot of heat being wasted compressing it).
- Already__Taken 12y ago
  This would be why you wouldn't run 2 pistons facing each other sharing common valve and ignition set to balance?

luch 12y ago

Is your question rhetorical ? Because that's how ITER (nuclear fusion reactor) generate electricity in a nutshell, if you replace the ball by plasma. source : https://www.iter-india.org/images/fusion_experiments2.jpg https://www.iter-india.org/images/fusion_experiments2.jpg

fanf2 12y ago
That diagram is about the energy inputs to heat the plasma. Energy output to generate electricity is quite indirect: hot neutrons escape the magnetic containment and are caught in a lithium blanket. This produces tritium from the nuclear reaction between the neutrons and lithium nuclei, and heat. The blanket is cooled with water which is used to drive a conventional steam turbine generator set. See the last paragraph of the reactor overview at http://en.m.wikipedia.org/wiki/ITER http://en.m.wikipedia.org/wiki/ITER and note that they do not plan to generate electricity from ITER: it is purely experimental.

chiph 12y ago

Too hard to seal. Ask a RX8 owner about rotor apex seals sometime. The bane of their existence. In this new motor there's nitrogen on the other end of the piston that provides "rebound". It's probably inside the coolant jacket as well to prevent expansion and the loss of efficiency that would bring.

kabdib 12y ago
Ex RX-7 owner here. Not much room in the back of one of those things, but I always had room for cans of oil :-) Wonderful cars. I'd buy an RX-9 if they ever built one, but they're never going to. So I have to motorcycle instead, could be worse :-)
- nicholassmith 12y ago
  I know a few people who've owned RX-7s and 8s, one said it was the only car he worried about running out of oil before fuel. Beautiful cars, but they're a bit high maintenance.
  - busterarm 12y ago
    10th Anniversary RX-7 owner here. If you actually maintain your car and don't treat it like a toaster, it's not too bad. They combust oil to lubricate the engine, so you just have to make sure it has oil. If your oil metering pump is electronic ('89 and after), run it with premix when you fill up. And drive your car hard & rev past 6k once a drive or so to clear out the carbon. Yes, I've gotten a speeding ticket for doing this. I love my car. It's about to hit 100k miles and it's a Turbo and despite what the naysayers say, it hasn't blown up yet and still has good compression. The electrical issues are far worse than the engine ones. The engine is bulletproof if you do your basic maintenance and listen to your car. Every sports car is high maintenance. The Alfa, Porsche and Ferrari owners have it as bad or worse - people just treat exotic powerplants like voodoo - I sure paid a hell of a lot less than they did though. I would rather pull and rebuild my entire engine than change the power steering fluid on a recent Porsche. The latter has a nightmarish set of steps likely to break all sorts of tiny plastic bits that cost more to replace than my engine rebuild kit would...

pbhjpbhj 12y ago

You could have a reciprocating engine instead. Wankel engines have problems due to sealing being difficult (as would your idea, explosive gases escape at the edge of the sphere), as I understand (I've not studied it in depth), but a reciprocating engine would seemingly have no more [in fact less] issues with being sealed than 2 parallel pistons. Have the first combustion chamber push the piston one direction past the coils and have a second chamber at the opposite end push the piston back. Any excess movement might be able to be used for pre-compression? Of course many people have thought of this before (don't you hate it when that happens! ;0)) - http://www.youtube.com/watch?v=2SlwoBLspTQ http://www.youtube.com/watch?v=2SlwoBLspTQ shows 2 such engines. http://www.youtube.com/watch?feature=player_detailpage&v=yV09wMXTSIc#t=7 http://www.youtube.com/watch?feature=player_detailpage&v=yV0... shows another idea, 2 pistons either side of a single combustion chamber.

nabla9 12y ago

If you follow that line of thought to it's conclusion, you end up with gas turbine (Brayton cycle).

Sami_Lehtinen 12y ago
Yes, that's what I thought. Many people seem to ignore gas turbine option, even if those are very reliable and efficient power sources. If there's interest towards different engine designs, it might be a good idea to check this out too: https://en.wikipedia.org/wiki/Wave_disk_engine https://en.wikipedia.org/wiki/Wave_disk_engine and another interesting design: https://en.wikipedia.org/wiki/Swing-piston_engine https://en.wikipedia.org/wiki/Swing-piston_engine There surely isn't shortage of engine designs, most of these designs are from 1800's. So nothing new.

susi22 12y ago

Don't forget that for an electric motor you need a sine shaped AC and not DC so it's pretty convenient that this design produces exactly that. Whereas with a "continuous" design you'd need to make it AC.

randallsquared 12y ago
If your motor needs AC, you already have an inverter, since the battery produces DC.

derekp7 12y ago

I hear this quite often, but I don't quite see how this is true. Each piston is hooked to a crankshaft, which is hooked to a flywheel. Even with a single piston, the energy of it slowing down and reversing is absorbed in the flywheel, which is in turn used to propel it in the opposite direction. Add multiple pistons, and the energy of slowing down is again used to accelerate another piston. Just think of any spot on a wheel representing an arbitrary amount of mass. Spin the wheel, and the only thing slowing it down is friction -- not that spot constantly changing direction.

maxerickson 12y ago
This article is specifically about a piston engine that does not have a crankshaft.

bnegreve 12y ago

> Seems like needing the piston to change directions is a waste of momentum. It's not, in a closed system momentum is conserved [1] so without friction, a free engine would rotate forever. Also there is not conceptual difference between an engine and a ball sniping in a donut. In both case, the piston or the ball have to change direction (the piston has to change direction more abruptly though) [1] http://en.wikipedia.org/wiki/Momentum#Conservation http://en.wikipedia.org/wiki/Momentum#Conservation