LiquidPiston X Mini 70cc Engine

[IamIan]

Originally Posted by cRiPpLe_rOoStEr

Had them been reliable at all, don't you believe they would have taken over the stationary/industrial market already?

no

I doubt it's that simple.

There are numerous times a 'reliable' option .. sometimes even a more reliable option .. did not 'take over the market'.

[cRiPpLe_rOoStEr]

Sure there might be "better" engine layouts that we don't have easy access owing either to an inherently higher overall cost or to political pressures, but the Wankel has proven itself not so dependable. Anyway, I'd still want to see how a similar engine employing the LiquidPiston configuration would fare in real-world driving conditions.

[IamIan]

Originally Posted by cRiPpLe_rOoStEr

Anyway, I'd still want to see how a similar engine employing the LiquidPiston configuration would fare in real-world driving conditions.

+1

I look forward to their continued development/improvement .. so it might someday be good enough to try in my own ride

[cRiPpLe_rOoStEr]

As long as they overcome those apex seal issues that plagued most Wankel engines, this engine design would be quite competitive. BTW even though the water cooling seems to fare better due to a more accurate thermal management, the simple air-cooling layout might sound attractive for the general aviation market that still relies mostly on outdated designs.

[serialk11r]

I just saw this somewhere on Youtube or something and went through all their papers.

The concept looks decent. Apex seals in the housing instead of the rotor should make them more reliable I'm guessing.

Compression ratio can be pretty much as high as you want, that's good. Could set it at 18:1 expansion, 12:1 effective compression for similar VE to a Mazda Skyactiv-G.

Increased dwell actually isn't good for low speed given the surface area to volume ratio is still poor, but you can just run the engine at high speed only as a generator or something.

Since you have most of the combustion chamber in a quench/squish zone and a tiny pocket to ignite most of the fuel, combustion speed is not a problem. Then again, combustion speed is less important if you have a really big expansion ratio.

Now the bad:
XMv3 indicates 18% peak efficiency, 5 bar IMEP, 4 bar BMEP without Atkinson cycle intake "timing". That is BAD. A similar sized Wankel from AIE gets almost 7 bar BMEP: https://www.aieuk.com/40s-5bhp-wankel-rotary-engine/ The sealing must not be working, or something. A dyno chart would make it easier to figure out what's wrong.

The rotor has a similar problem to the Wankel: one side has the exhaust constantly going through it, the other side goes through intake/compression/expansion. There's definitely going to be a big temperature difference, which might contribute to poor sealing. Since you can't liquid cool the rotor, this is probably a big problem, since even if you're using inconel, the exhaust side would expand by several thousandths, that's going to create some bad vibration for the apex seals. A carbon ceramic rotor with dry lubricant ceramic seals might work, at very high expense. Another idea is to use intake air to cool the rotor, but that reduces power density and WOT efficiency.

It looks like the rotor air cooling blows out the exhaust? That can't work for any emissions controlled engine, can't be mixing fresh air into the exhaust.

I'd like to see what happens with a bigger sized demonstrator if they ever come out with one. The bigger size alone would make sealing, cooling loss, and pumping loss less of a problem.

[cRiPpLe_rOoStEr]

Originally Posted by serialk11r

Increased dwell actually isn't good for low speed given the surface area to volume ratio is still poor, but you can just run the engine at high speed only as a generator or something.

Most likely it could serve the purpose of a hybrid, or being matched to a CVT.

[samwichse]

They've been awarded a military research contract to make a very compact generator for howitzers:

https://newatlas.com/military/liquid...rmy-generator/

This posting has a nice summary of the advantages over a standard wankel
https://tech.slashdot.org/comments.p...0&cid=60840426

- Seals: Wankel seals are on the rotor, shaken, hard to lubricate. Were a big maintenance problem. LiquidPiston seals are on the stator, unmoving, constantly rubbing in one direction. Can be lubed easily via lube passages in the stator. Better than piston rings for reliability.
- Two moving parts, one just rotating smoothly, the other rotating smoothly around an axis on a smoothly rotating crank offset. (Note the gear on the rotor riding a circular gear track, like a planetary transmission with one planet gear and no sun gear.) Add counterweights to the crank and no vibration from the moving parts. Harmonics only come from the pressure curve of the combustion gasses. With one rolling race, three bearings, a few sliding seals, all lubricated, and smooth motion you have minimal opportunities for wear or early breakdown mechanisms.

Besides the factors pushing longer life there are other advantages:

- Well shaped combustion chambers and constant-volume combustion for substantially more efficient pressure/volume cycle. Also: The high compression ratio and long-time constant volume combustion mean you can burn slow burning stuff like diesel fuel (and design it to run as a diesel), or darn near anything else you can atomize.
- Lightweight small rotor driving THREE combustion chambers for three power strokes on two turns of the shaft. Equivalent to a three piston engine but with only two moving parts. Makes for a smother power cycle, too. (The Wankel needed two rotors to avoid major fluctuations in torque.)
- Tiny weight for a phenomenal 1.5 horsepower per pound! Call it 1,115 watts per pound, or close to a kilowatt after generator inefficiencies. Two pounds of engine drives a house. 14 pounds drives a hybrid car.
- RPM suitable for direct drive of a generator on a common shaft.

[cRiPpLe_rOoStEr]

Originally Posted by samwichse

They've been awarded a military research contract to make a very compact generator for howitzers

Like it or not, as long as the military gets involved, a ground-breaking tech becomes more likely to succeed and eventually appealing to the average Joe at a later stage. Just look at how the Jeep CJ and the Wrangler evolved.

[cRiPpLe_rOoStEr]

It's been a while, and LiquidPiston even tried a 2-stroke Diesel prototype, still based on its inverted Wankel design. Sure trying to sell such engines for military applications, not only gensets but also UAVs and eventually outboard motors too, might generate a spontaneous marketing just like the Jeep after WWII, but LiquidPiston is wasting time not making some 2-rotor and 3-rotor engines which could cater to the Mazda fanbase as a crate engine, and for general aviation to compete with those flat-4 and flat-6 gassers too.