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Comment by mentos | Hacker News Reader

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?

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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.

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 :-)

Same. Mine didn't burn much oil, but would refuse to start when the engine was warm because of the weak compression. Would fire right up when it was cool though.

Loved how it handled and the 9,000 rpm redline.

It sure is a pain in the ass when that happens. I have a chipped ECU so I don't have to get out and pull the fuse to get it started, but I haven't installed it yet.

One of my cats is gutted and my exhaust is rusted to hell - right now my fuel economy is shit (~10mpg). This should actually improve when I put the Racing Beat exhaust in but then I have to upgrade the injectors and air intake and port the turbo.

My 10th Anniversary is still bone-stock and I'm sad about having to change it.

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.

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...

> If you actually maintain your car and don't treat it like a toaster, it's not too bad.

This. Never forget basic maintenance. I had a 1981 RX-7 and had over 194,000 miles on it when I got rid of it in 1998. (I still have the concrete blocks I'd put in the back when it snowed.)

The problem with the RX-8s isn't the rotor apex seals (any more so than any other rotary), it's actually injection. A design oversight with how they inject oil in the '04-'06 models causes the engine to not be lubricated properly and this will cause a lot of problems. Even though they fixed it after '06, it's still not perfect. Like the '89 and after RX-7s, the RX-8 needs to run with premix.

But generally you want to avoid buying an '04-'06 RX-8 used.

Elaborate?

With the initial design they used 2 oil injection nozzles per rotor and basically there was insufficient coverage due to that and the way they were angled.

After the '06 model year they went to 3 per rotor and angled third nozzle to compensate.

That's how I understand it from the reading I've done and conversations with mechanics & owners I know. I've yet to work on one of these with my own two hands. I'm strictly interested in 7s and earlier. I inspected a few 8s for potential purchase but they all had problems that I wasn't interested in dealing with.

Ah, ok.

What problems does the 8 have?

Nono, each car I looked at had problems individually.

What I talked about earlier is the main problem for RX-8s. Beyond that they're about as reliable as other cars of the same era and classification. They do come with about as much performance as you can get out of them from the factory though. They're not that interesting as anything other than a street cruiser.

> The bane of their existence.

Just like spun rod bearings and thrown rods are for Subaru owners, amirite?

At least when a seal goes, I can replace the rotor, walls and side housing; if it even needs that.

A good comparison for Subie owners might be head gaskets. :)

Those too! :D

Or a Wankel engine[1]

[1] http://en.wikipedia.org/wiki/Wankel_engine

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

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).

This would be why you wouldn't run 2 pistons facing each other sharing common valve and ignition set to balance?

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 shows 2 such engines.

http://www.youtube.com/watch?feature=player_detailpage&v=yV0... shows another idea, 2 pistons either side of a single combustion chamber.

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

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 and another interesting design: 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.

> 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

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.

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

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.

This article is specifically about a piston engine that does not have a crankshaft.

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

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 and note that they do not plan to generate electricity from ITER: it is purely experimental.

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

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.

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