>We're running engines for tens of hours, dozens of hours, we're not yet in the hundreds of hours where we want to be.
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 have a turbo'd rotary engine in a mostly-unmodified car that's at 140k miles without an engine rebuild.
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.
Swashplate engines are essentially the current go-to: they're extremely compact and have many other good properties, but wear out quickly. I believe they are significantly more compact than rotaries (and this), though.
Still an interesting comment, though, because while most people know that regular apples exist, not as many people know that liquid piston engines exist.
A picture is worth a thousand words. (Well, video in this case)
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.
A Wankel has seals attached to the rotor that rub against the wall of the chamber. This engine only has three points on the chamber wall that touch the rotor, so the seals can be installed on the chamber wall and it is significant easier to cool and lubricate a fixed seal rather than a rotating seal.
As I understand it from friends who're in to rotary engines, the biggest problem is the spark plug hole. The seal - which is being forced outwards by rotation - will fall in to the hole slightly and then wear against the edges of the hole as it rotates past. This gradually increases blow-by and reduces efficiency until it no longer seals at all.
This design has a smooth surface against the seals at all times, and they can be lubricated directly.
That is not the primary source of wear problems on Mazda Wankels.
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.
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).
The thing that makes this so interesting is that it's a 2-stroke, 3-lobe engine. The traditional wankel being a 4-stroke, 1-lobe.
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.
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.
Look forward to about 1:48 where they run through the whole sequence, flashing intake/compression/expansion/exhaust on the screen for each stroke. Each chamber has four full strokes. It seems clear that there shouldn't be any overlap between the ports.
I'm pretty in awe of the cleverness. Hope it works out.
Yeah, that later sequence is clearer now that I look at it again. The first one animating both intake gasses (presumably into the intake chamber, not the compression area) and exhaust at the same time was just confusing me for some reason.
Two strokes don't meet emissions because they use the fuel as a lubricant, which means they have to add oil to the fuel and obviously that doesn't burn cleanly.
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.
The primary limiting factor is emissions for road cars and 2-strokes.
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.
It has the properties of a two stroke engine. I hesitate to call it a two stroke because that isn't strictly true, apparently. However, it has a narrow power band, like a conventional two stroke. This means that it's a poor engine outside its power band; excessive emissions, inefficient, etc. Inside its power band it can be tuned to be clean and efficient.
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.
Really small 2 strokes (like leaf blowers) use mixed oil-fuel which is absolutely terrible for emissions.
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.
It looks like the rotor design is quite different -- much more rounded and oblong, as opposed to triangular, with the combustion happening inside the rotor.
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.
It was just like any startup really, a great team of really smart and dedicated folks trying to disrupt an entrenched industry using new ideas. Lots of simulation, lots of practical prototyping, lots of test rig time. We spent so, so many hours in test chambers observing how a particular engine type behaved with incremental changes to its design, changes which were backed up by simulation. The resulting data was used to strengthen the simulation and we repeated this loop many times. The challenge with this is that there are seemingly infinite different parameters to observe and control in each loop in both the physical and simulated engines, which must be done with high fidelity to learn things.
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
Yes, mostly in house except for specialty materials, high labor components, etc, similar to the workflow at many small hardware firms with machine shops. We just had more combustion involved than your average hardware company.
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.
Very interesting. No mention in the article if this has better fuel efficiency than existing rotaries, though their FAQ blames the poor efficiency and emissions issues of the Wankel on the weird shape of the combustion chamber[1], so if this design resolves those issues maybe we'll see these in cars eventually. (They'd better act quick if they want to get in on that market; I expect it'll be essentially all electric sooner than most people think.)
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.
Sliding friction is an issue with efficiency, I suspect. The flat sides of the rotor are holding back hot, compressed, abrasive gas. That means the faces need to be very flat and very rigid.
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.
> 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.
Interesting idea - is there a technological reason why this design is possible now, or did the designers of the orignal rotary engine just not consider this layout?
Found this awhile ago when researching if there were any diesel wankel engines. Glad to see it's got some updates.
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.
FWIW lots of DARPA stuff becomes commercial/consumer grade. Exhibit A is probably the entire Internet but close behind would be high bypass turbofan engines, LiDAR systems, laser range finders, "space" blankets, etc. There is a long line of these things.
It's intentional too. Many programs start with basic research, but also have specific milestones related to commercialization.
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.
Haha, this has to be the funniest thing I've read on the INTERNET today.
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!
The filter in the MSR Guardian water filter was the result of a DARPA research contract iirc. It’s unique on account of being able to filter out viruses (traditional filters can’t go that small) and also has some benefits like being freeze-proof.
Looks like it would be useful as a compact engine for small planes or drones. The "through" flow of air and exhaust would be easy to ingrate into an airframe.
I know there is a niche of homebuilders in the US fitting Wankel engines. I'm not sure that could be described as popular though. Is there something I'm missing?
Anyone know where there is more complete description of the power cycle?
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
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.
I wonder if it's literally an inverted Wankel engine, as in taking the geometry of the Wankel engine, applying inversion with respect to a circle[1] then fixing the frame of reference to the outer geometry, which was originally the rotor in the Wankel engine.
But what does that change? Major flaw of Wankel engines, apex seals are still there, only now they are fixed in place not on the piston. I think "flipping" the design allows for smaller engine but reliability seems to stay the same. Please correct me if I'm wrong.
“ "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'm just wondering if there is a strong mathematical link between the two. Like flipping the sign somewhere in the description of the geometry of the Wankel engine results in this.
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.
Pretty neat. Seems like a “pseudo” four cycle engine in a two cycle form. I can definitely see this being used in a drone/aerospace application. Even without the efficiency benefit of four cycle vs two, not having piston heads is already a huge plus (moving parts are a pain when you have nothing to ground to but the air). Though I would guess the engine will have to run at a much higher rpm since the air displaced seems lower.
About 4 years ago some friends and I were taking an Uber out in the Hartford area. The driver had a sweet BMW 535 wagon. He worked for this company and we got to talking about how performant and efficient it was. Really cool to see the technology making big waves!
If it's compact enough to compete with 2-strokes, it's likely way quieter than that. In theory a rotary would be quieter than a similarly powerful spoark-ignited engine due to reduced vibrations, but I don't suspect it's a huge difference.
You can hear it in the video and it is pretty quiet. They've also developed a system where you can shut off the motor and cruise on battery power for short bursts for stealth type applications.
> 40-lb (18-kg), 6.5-hp engine out of the go-kart in the video above, and replaced it with a 4.5-lb (2-kg) X-Engine making 3 hp.
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.
It does look like that, but it may simply have been drawn with those proportions for illustrative purposes. On the other hand, perhaps the diagram has correct proportions but they're misleading: the combustion area might be roughly spherical rather than cylindrical, and we're just seeing the cross section that shows its maximum area.
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.
Sure, their engine and they can claim whatever they want.But I have the right to disagree. V4,v6,v8, inline twin, inline 4, 2-stroke, 4-stroke they all have different torque,power curves and characteristics, and there is big difference between Petrol/Gasoline and Diesel engines as well. BUT, they all are considered as internal combustion engines that convert translational motion into rotary motion.
No, you don't really have the "right to disagree". This is clearly not a Wankel engine. In a Wankel engine, the rotor is approximately Reuleaux triangle-shaped. In this engine, the rotor is pill-shaped (obround). Yes, it is a rotary engine and it is an internal combustion engine, but it is not Wankel.
Oh really? this is fun, I absolutely have the "right to disagree". Even with your comment. You can claim that this engine is built by the Martians, nobody is stopping you. You have the same "right to claim" whatever you want.
This isn't a subjective issue. A Wankel engine is a certain type of engine with certain key characteristics, by definition. The LiquidPiston engine does not share all of these key characteristics, so it is not a Wankel engine. Of course you can call whatever you want a Wankel engine, nobody can stop you. You can call the SpaceX Raptor engine a Wankel engine. But we will ignore you, because you are objectively wrong.
See? this comment is an improvement over your previous ludicrous comment of "No, you don't really have the right to disagree". Keep working on it.
Meanwhile, read this.
https://news.mit.edu/2014/liquidpiston-small-efficient-rotar...
Especially, this part.
"The X Mini is essentially an upgrade in design and efficiency of the compact Wankel rotary engine, invented in the 1950s and used today in sports cars, boats, and some aircraft."
Yes, I see how the article literally says this is not a Wankel, but rather an "inverse Wankel". Taking many of the main ideas behind another engine and using those ideas to make your own (with different key features) does not make it the same 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.
The rotor in the center of the LiquidMotion device is shaped more like an oval (or race track) while the Wankel rotor is shaped like a bowed-out triangle. The LiquidMotion rotor has volume within that is carries gases, while the Wankel rotor is solid (or at least if it's hollow the volume inside isn't used). I don't recall ever seeing a counterweight on a Wankel but then I'm not even a little acquainted with Wankels in the field so it could be that Mazda forgot to tell me.
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.
There's nothing inside-out about that design. They've just moved the location of the exhaust/intake porting. It's a neat reconfiguration idea, but it's still a Wankel engine.
Yep. Instead of the seals on the rotor, the seals are on the chamber (much better cooling and lubing opportunities). Instead of the intake/exhaust ports in the chamber, the intake/exhaust goes through the rotor (at first...)
The Mazda RX-8 uses a counterweight. (Actually, on the manual-transmission version it's a flywheel that has a sort of lip around the rim that's thicker on one side than the other.)
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.)
Yeah, but it basically is. The most important difference, as it appears, is they've ported the intake and exhaust through the piston rather than through the block. I don't quite understand the claim that it has a different thermodynamic cycle...
Here is their diagram[1] of the cycle. Wankel engine uses the otto cycle (blue), theirs is what they are calling the High Efficiency Hybrid Cycle, or HEHC (in red). In terms of shape they do seem approx. the same, so when LiquidPiston says theirs is different I think they are just saying it is a more efficient cycle, not fundamentally different in terms of stages.
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.