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Comment by canbus | Hacker News Reader

hmmm.. still uses apex seals, would need seriously small tolerances to make it work effectively. Call me skeptical but I don't see how this is really any better than the traditional wankel/rotary design

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It inverts where the apex seal goes. So instead of having it on the rotor, and hard to lubricate, it goes on the housing, which is much easier to lubricate.

It's effectively an inside-out wankel, so the seals are stationary. This is a much easier problem to solve, and now you don't need to throw oil in the combustion chamber to lubricate the moving seals.

problem one will be balancing forces if combustion is taking place on one side. also i don't see why this should be burning less oil. you are smearing the oil constantly around.

This isn't all that different from a four stroke, at some level. With a piston engine you lubricate from below, the oil never getting above the rings. Practically some does and gets burned away, but that's negligible usually. Anyway, the seals on the liquidpiston engine supposedly do the same, keeping the oil from leaking around the seals (in a significant quantity anyway; I'm sure it's higher than a four stroke though).

Here's what their webpage says:

> The seals do not experience centrifugal forces, and can be lubricated directly by metering small amounts of oil directly to the sealing surface through the housings, which means that oil consumption can be reduced to levels potentially comparable to that of a 4-stroke piston engine (essentially negligible).

Obviously take all this with a grain of salt. I haven't read any practical studies on this.

Thanks both, I'm not very mechanical and came to the comments to find out what makes this not a wankel.

right, but any float in the main eccentric shaft is still going to wear them out

Yeah but "just replacing $50 seal every few years" is smaller problem than "taking apart whole engine to replace the $50 seal". And now they can technically be directly oiled instead of whole oil in petrol thing.

to me it still looks like its principally a wankel motor just with a few changes.

If you look at an image, you can see the three apex seals on the stator, which is shaped the same way as a wankel rotor. The rotor of the liquidpiston engine is shaped similarly to the wankel stator and has no seals. It's almost the same just turned inside-out.

true - now i see it.

Small, multi-fuel, engines do have interesting applications. I tend to agree though. The other big thing, these rotary engines have traditionally had poor combustion. Direct injection and turbulent jet ignition have focused on improving combustion efficiency and made tremendous gains. I’m not sure how liquidpiston addresses that, especially with the fuel claims. Traditionally you’d add fuel additives to help with combustion efficiency.

You are right - it is not better - not even close.

It is 2-4X WORSE on Liquid Piston's main advertised claim to fame "lightweight" "power/weight" than many available actual Wankel engines.

And the fact that this is still in development a decade+ later and 7-8 years after I was first vetting them for a hybrid multirotor drone company indicates that your suggestion is correct — they have many nagging and serious not-easily-solved problems.

Well, they are on outside. Which means possible way to replace it without tearing the engine apart. And we did get better at making them. And they could possibly even be oiled directly from there.

But the fact companies make normal wankel engines that are better at everything is worrying for them

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