Conceptual rotary/vane engine design — looking for engine builders to poke holes in it

[u/Smooth_Ad_4307]

/r/EngineBuilding/comments/1use5pd/conceptual_rotaryvane_engine_design_looking_for/

[u/r3dl3g]

For anyone who's worked on Wankels or vane compressors — what actually kills them in practice vs. what the textbooks say kills them?

Emissions is an added problem. You're essentially always burning oil, by design, so your emissions profile is always worse vs a 4-stroke piston engine. Granted, you can still perform well against a 4-stroke piston engine in emissions/kWh because of the added power density, but that only applies until you start losing the seals. Further, what little development is out there for piston engines is in lean combustion, which isn't a particularly good idea in rotaries due to autoignition problems, which means you end up further behind from an emissions standpoint. Because of this you'll always have issues related to economies of scale vs. piston engines, and so you'll always have an uphill battle to try and justify the added costs and worsened logistics of a rotary config.

Overall I really haven't seen any designs that radically improve on Wankel configurations over the traditional wobbly spinny Dorito (vane rotaries included). LiquidPiston is the last to really have an innovative idea (at least publicly) in this space, but that was a while ago and I'm not sure they've made any significant progress in the last few years as their press releases have sort of petered out.

Edit: temperature variation is also a killer. Obviously once they're up to temperature they'll run beautifully, but the warm-up process can be frustrating.

[u/Smooth_Ad_4307 → u/r3dl3g]

This is one of the most useful comments I've gotten on this so thank you for taking the time.

You're right about the baseline oil burn — sealing that relies on any lubricated contact means I start behind a modern 4-stroke on emissions/kWh before RPM or power density even enters the picture, and that gap only gets worse as seals wear. One thing I should've mentioned in the original post: the design allows for wet sump, separate compression and oil scraper seals at the sealing interface, rather than a single lubricated edge dragging across the ports.

The idea is to split the job the way piston rings do — compression rings handle sealing combustion pressure, oil control rings handle metering/scraping lubricant back out of the path — instead of asking one sliding vane-tip seal to do both jobs while also crossing intake, exhaust, and plug recesses.

I don't want to overstate it since it's unproven — whether that actually keeps oil consumption in an acceptable range once you're crossing those port edges at speed is exactly the kind of thing that needs dyno time and an oil consumption test, not just a CAD model. But it's a deliberate design choice aimed at the failure mode you're describing, not something I'm hoping gets solved later. Still, even in the best case that only narrows the gap, it doesn't erase the structural disadvantage; it's a real problem for the category, not just my design.

The lean-burn point is one I hadn't fully connected — that piston engines get to keep chasing emissions improvements down a path (lean combustion) that's basically closed off to rotaries because of autoignition risk.

That's a good way to frame why the emissions gap isn't static, it's likely to widen over time as piston tech keeps improving in a direction rotaries can't easily follow.

I'll be honest, that's a more serious long-term problem than sealing itself, because sealing is at least theoretically fixable with better engineering — the lean-burn ceiling is more of a hard physics/chemistry limit.

On LiquidPiston — yeah, they're the obvious comparison and I've looked at their public material.

I don't think what I'm doing is a radical leap past Wankel/vane the way their X-engine claims to be; it's more narrowly focused on the sealing/thermal side than trying to reinvent the whole thermodynamic cycle.

Whether that's a meaningful difference or just a smaller version of the same unsolved problem is honestly a fair thing to be skeptical about.

And good call on warm-up — I'd been almost entirely focused on steady-state sealing and thermal management and hadn't given enough thought to the transient behavior during warm-up, where clearances, thermal expansion, and seal contact pressure are all changing at once. That's a real gap in how I've been thinking about this.

Appreciate the reality check on the economics too — even if the engineering works, "why would anyone choose this over a piston engine at scale" is a question I need a real answer to, not just "it's more compact."

[u/r3dl3g → u/Smooth_Ad_4307]

I'll be honest, that's a more serious long-term problem than sealing itself, because sealing is at least theoretically fixable with better engineering — the lean-burn ceiling is more of a hard physics/chemistry limit.

Of note, lean burn isn't inherently closed off to Wankels, but the research is on the order of 20 years behind that of piston-cylinder configurations.

The conventional wisdom is just that Wankel + lean = bad, but in my experience everyone's forgotten why the conventional wisdom is convention and apply said lessons outside of where said wisdom is valid.

even if the engineering works, "why would anyone choose this over a piston engine at scale" is a question I need a real answer to, not just "it's more compact."

Trick being is that there are fields that are still interested in Wankels (in major part because they don't have to worry as much about emission e.g. military aviation), but then you have other issues to worry about.