LiquidPiston's “inside-out” rotary X-Engine wins Army research contract
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Durability and reliability and maintenance are the issues with these type engines.
For some military applications that's not an issue. If you want to build large number of cheap lightweight drones for high intensity warfare, durability is secondary. They will be shot down eventually anyway. Operating smaller number for training purposes during peacetime is not that expensive.
I presume that having those seals on the non-moving part is a big win.
I've had to replace numerous things under the hood due to the heat output though (wiring harness, everything rubber) or otherwise poorly designed (the weak alternators...).
If you discount design flaws in the early RX-8s wrt lubrication, turbo timing issues in the FDs, the oil metering pumps in FCs & FDs, the engines are pretty solid. Almost all of the blown engines I've seen start wth people trying to increase boost.
The problem is that you can modify the car little by little and every way you do this puts stress on the engine. You have to modify the car a lot (fuel pump, injectors, engine porting, turbo wastegate modification, new exhaust, bigger alternator, ecu chipping/megasquirt, cooling upgrades) before the car can really handle the new power that it has without tearing itself apart.
(Of course I think the motor was geared and I think 6000 rpm on the tach was really 2000 rpm, but hey)
After about 60k miles it started getting little puffs of blue smoke when it started up (they went away in a moment).
I got a good deal on another car and traded it in, but I still think about it.
Newer models came along, and finally the RX-8. Now the RX-8 was nice but I believe the last generation RX-7 is one of the most beautiful cars ever.
The engine referred to by my parent poster isn't available anywhere that they could comment on.
I still don't understand how it seals better than a Wankel. I understand how it's turned inside out, and it's very clever, but it seems like it would have the same problem with seals.
Yes. However, the apex seals of the Wankel are the most troublesome, from what I've read about the RX-7/8.
This design has a smooth surface against the seals at all times, and they can be lubricated directly.
What you're describing is a challenge on peripheral port (race) engines where very large intake and exhaust ports must be bridged by the apex seals, and in the pursuit of performance their radius is often less than ideal making the transition onto and out of the port area more abrupt.
The spark plug hole however is relatively small and circular.
High-mileage Mazda Wankels typically require overhauling because the side seals become seized in a compressed state (their springs are meager segments of ~sinusoidal wire, and the motor burns oil by design causing excess carbon/coking clogging them up), resulting in excess blow-by and unintended oil burning. Once the side seals go, the combustion gases start reaching the oil control rings, baking their soft interior seals into brittle plastic and it's all downhill from there.
source: former rx-7 enthusiast with multiple diy engine rebuilds in his past.
Edit:
BTW something that's often misunderstood about Wankels is that the rotor turns lazily at 1/3rd the engine RPM. So despite it being a "high revving" engine, there isn't actually that much centrifugal force acting on the apex seals at conventional engine speeds. The pain point in this department is the stationary gears responsible for converting that 3X eccentric shaft speed into a mix of orbital and rotational motion at the rotors.
1000+Hp Turbo 4rotor AWD [All-Wheel-Drive]
https://m.youtube.com/results?search_query=4rotor+turbo+1200... There's definitely a market for rotary engines, time will tell how this new form takes shape.
http://what-when-how.com/automobile/wankel-engines-automobil...
See also Pengaru's comment about problems with side seals, specifically:
I'm curious what the emissions look like. And I'm not sure if the apex seals actually change how much distance they rub over (though stationary may be enough to make them easier to deal with, and probably cheaper to replace the rotor rather than the housing if it gets scratched).
I don't expect emissions to be any good since it's fundamentally a 2-stroke, which is perfect for non-road based applications. I don't think we'll see this on the road... ever.
To make a 2 stroke meet emissions while maintaining its primary gain - thermal efficiency, and matching the power density a lot of complexity is needed.
This quora post[0] largely sums that up, but quora is only so trustworthy.
[0] https://www.quora.com/Why-arent-2-stroke-engines-used-in-car...
It's very small and lightweight, so it could serve as a range extender running a generator in an electric vehicle.
2 strokes have to go oil-sump + direct injection with supercharger to be able to match a 4 stroke when it comes to maintaining thermal efficiency, match power density while meeting emission standards.
All that complexity and the 4 stroke atkinson's cycle looks really attractive again. A huge benefit in of the Atkinson's setup is it can switch back to the Otto seamlesssly and directly drive the car for additional power (i.e Hybrid vehciles)
If you can parse through the noise of Quora posts, this highlights above[0].
Tesla won't use it gasoline for range extension - it's against their religion - which I subscribe to.
[0] https://www.quora.com/Why-arent-2-stroke-engines-used-in-car...
It's lighter, so less power is required to move the whole car, so it's fine if it provides less power than a regular diesel.
It is also easier and cheaper to make.
Is durability and maintenance the real issue?
So using it for constant RPM applications such as a generator can make sense. Using it as the prime mover where a wide power band is needed is a bad choice.
Does this design require oil in the fuel? Certainly in the case of burning diesel I would expect that alone is a good enough lubricant.
> https://www.liquidpiston.com/how-it-works
At 53 seconds, it shows the intake cycle beginning. In the same lobe, the exhaust & the intake ports are both open or "connected" at the same time, which is the 2-stroke cycle.
They need to be 100% isolated and it's not super obvious that they are.
I'm pretty in awe of the cleverness. Hope it works out.
Both intake and exhaust is done via two separate ports in the rotor. Due to their positioning, there are separate intake and exhaust strokes.
> a one rotor X engine behaves like a 3-cylinder 4-stroke
That's actually quite clever -- increase the capacity, without increasing the size.
The new piston design looks to have a much larger sealing area against the walls of the block, and the lack of distinct edges -- the ever-so-fragile apex seals -- should increase longevity, although IANAME, so have your salt-shaker ready for action.
These look fairly simple to build as well.
My understanding of thermodynamics was routinely humbled by my coworkers. I remember one time rolling into the office in the morning to find my coworker twirling with excitement and saying something like "I had a realization at 3AM and came to the office and figured out my enthalpy problem!"
Working with your buddies to create a working engine from scratch that is unlike any engine that has ever been created before it is a real rush. I recommend it for anyone.
Here's a slo-mo video of combustion inside an LP engine. We bolted a thick piece of quartz onto the engine instead of one of the side pieces. https://vimeo.com/111796558
The compact dimension of the demonstrator engines make that kind of agile development possible--similar to the idea behind Boom's scale demonstrator.
Is there a video somewhere showing the air flow of all the cycles? Thought I saw something in one video that showed the inside of the rotor being used for something.
https://uploads-ssl.webflow.com/5f6086e9bbbabd41a20f2984/5f9...
They seem to think the apex seal wear issue will be better with the apex seals stationary. In addition to that, I wonder if it might also be possible to design the engine such that you can just slide the apex seals out sideways through a slot in the engine housing (sealed off in normal operation) without disassembling the whole engine? That by itself could be a pretty big improvement even if the seals wear down the same as they do in other rotaries.
I also wonder if they have a good solution to having more than two rotors? Mazda made a 3-rotor but due to the way the engine fits together the eccentric shaft had to be in two pieces that bolt together.
I hope as soon as this company makes a car-sized engine someone swaps one into a Mazda RX-7 or RX-8 just for fun. Maybe see if they can beat these guy's record [2] for longest RX-8 trip on a single tank of gas. Or swap one into one of the old Mazda rotary pickups.
Unlike the face of an engine block, you can't get away with a gasket to mask imperfections. You need a hard, smooth bearing surface. That means expensive materials, which makes them even harder to flatten. Then you need the actual sealing elements, and they'll be at the worst spots possible- far away from the shaft. The sealed edge is in fact the fastest-moving part of the engine.
Bearing surfaces are rated on their PV- pressure times velocity. Both pressure and velocity create heat, and the life of a bearing surface is very roughly inversely proportional to the 4th power of the PV it's run at. Naturally, you want to put bearings as close to the shaft as possible.
> They seem to think the apex seal wear issue will be better with the apex seals stationary.
They're almost certainly right, although the actual details of implementing the seal are very complicated. They have to seal the corners as well as the sliding edge, so you need multi-part seals that are cut so that spring tension pushes them all outwards, and they still have to seal against each other[1]. They also need to be pressed sideways into their groove and kept from being dragged diagonally.
However, normal Wankels burn oil to keep the seals lubricated. Since these are stationary, they can be fed from oil in the crankcase. They can also be fenced in with scraper rings like a normal piston has, which keeps oil on the bearing surface and prevents it from being burnt off. Normal Wankel rotors lift off too much to keep the oil contained.
[1]: https://i.ebayimg.com/images/g/gRYAAOSw8-tWVSxv/s-l1600.jpg
> I also wonder if they have a good solution to having more than two rotors? Mazda made a 3-rotor but due to the way the engine fits together the eccentric shaft had to be in two pieces that bolt together.
IIRC they'll have pretty bad second-order vibration, but either way the exhaust/intake is in the way to a significant degree.
On a related note, does anybody know of DARPA-funded projects that eventually become consumer-grade? It seems that cool stuff like this never makes it out of the military due to the classified nature, which is sad.
For example, a biomedical program might have specific timelines for filing patents, meeting with the FDA for Investigational New Drug/Device Exemptions and thence clinical trials. This is rather different from the NIH and NSF, which tend to want smaller, self-contained projects from individual labs rather than a consortium that'll take a single idea from end-to-end.
Do you consider Google to be "consumer grade"? That's up for debate I suppose after recent outages :)
On the serious side though. Research projects rarely make it to commercial success because they're risky, not because they're secret. And that's why the state funds most of it. Generally speaking the private-sector prefers the exploitation phase rather than the research phase, for obvious reasons, it's much more profitable!
A 5.5kw generator from tractor supply runs $650 and weighs ~150lbs.
I’m sure this engine has some big advantages, but lbs/steady state watt doesn’t seem to be one. What am I missing?
From the article: >>> According to co-founder and CEO Alec Schkolnik, the X Engine design combines the high compression ratio and direct injection of a diesel engine with the constant volume combustion process of an Otto cycle engine and the over-expansion abilities of an Atkinson cycle engine, while solving the lubrication and sealing issues of the Wankel rotary engine and delivering huge power and efficiency
https://www.liquidpiston.com/how-it-works
1. air flows in as the chamber is expanding
2. fuel is injected when it is at maximum size
3. the chamber contracts to a tiny size to compress the fuel-air mix
4. combustion occurs, and runs to completion, in the tiny chamber
5. the chamber expands to extract work from the high-pressure gas
6. the chamber contracts to exhaust the waste gas,
The video makes the geometry clear. The way that the chamber contracts down to a tiny nub gives you the high compression ratio. The nubs being fixed in the stator lets you have injection. The fact that the combustion happens while the tip of the rotor is sliding over the nub gives you constant-volume combustion. I suppose the rotor being asymmetric lets you have overexpansion.
[1] https://en.wikipedia.org/wiki/Inversive_geometry
edit: on second though this would probably be too simple to be true.
“ "If you recall the Wankel," says Schkolnik, "they have a triangular rotor inside a peanut-shaped housing. We have the opposite, a peanut-shaped rotor in a tri-lobed housing. So take everything you know about the Wankel and turn it literally inside out. They have a long, skinny, moving combustion chamber, we have a stationary combustion chamber that's nice and round. You can drive it to a high compression, just by making the chamber smaller. And because it's stationary, we can directly inject fuel where the Wankel could not. So those are the two key advantages of the diesel: high compression ratio and direct injection.”
I did some crude geometric measurement on the geometry based on the video and the chamber doesn't seem to have the shape of an Wankel rotor inverted with respect to a circle.
Not sure this is the strapline I'd choose for my weapons manufacturing company.
This is a completely flawed comparison power and weight don't interchange linearly as cooling requirements and seam inefficiencies drive size up and power down as power grows.
>..."they have a triangular rotor inside a peanut-shaped housing. We have the opposite, a peanut-shaped rotor in a tri-lobed housing. So take everything you know about the Wankel and turn it literally inside out. They have a long, skinny, moving combustion chamber, we have a stationary combustion chamber that's nice and round...
Left is a Wankel engine, right is the LiquidPiston engine. The red areas on both sides are the combustion chambers during ignition. The chamber is clearly a lot rounder.
edit: actually, this is covered in their FAQ:
> What is the compression ratio?
> For the X Mini 70 cc spark-ignited prototype, the compression ratio is 9:1. The diesel prototypes have operated at between 14:1 and 18:1, and should be able to go higher, to 25:1 or more.
[1] https://uploads-ssl.webflow.com/5f6086e9bbbabd41a20f2984/5f7...
There is of course the possibility that this is a Wankel as far as patents would say. As different from a Wankel as a true Hemi is from a regular V8.
Aftermarket flywheels are generally symmetrical, with a way of bolting on a separate counterweight piece.
(The reason I know this is that I'm working on converting an RX-8 to electric. I got the car cheap with a messed-up engine, and I bought an aftermarket flywheel to replace the lopsided stock one because I don't want the car to shake itself to pieces. I haven't gotten around to ordering the adapter plate yet, but CanEV makes an RX7/8 transmission adapter plate along with two different versions of a coupler that attaches the motor to either a tapered-shaft style flywheel, which is what the car had originally, or a six-bolt variant for some other version of the engine.)
My saying you don't have the "right to disagree" was a response to your ludicrous assertion that an engine can be whatever you want it to be, not what it is. Any ludicrousness coming from that is purely a function of how ridiculous your original response was.