LiquidPiston's 'Inside-Out' Rotary X-Engine Wins Army Research Contract (newatlas.com) 97
Connecticut-based company LiquidPiston is developing a portable generator for the US Army that uses its X-Engine, a fresh and extremely powerful take on the rotary engine that'll deliver as much power as the Army's current-gen-set at one-fifth the size. New Atlas reports: We've written a few times before about the fascinating LiquidPiston rotary engine. It's not a Wankel -- indeed, it's closer to an inside-out Wankel -- and with only two moving parts, it's able to deliver extraordinary power density at up to 1.5 horsepower per pound (0.45 kg). 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. Check out the design being used in a go-kart and an unmanned aircraft in the video below.
LiquidPiston demonstrated the technology for the US Army by building a Compact Artillery Power System (CAPS) generator unit designed to power the digital fire control system on an M777 Howitzer artillery piece. It replaced a generator that needed a truck to move it around with something 20 percent the size: a 41-lb (18.6-kg), 1.5-cubic foot (28.3-L) box about the size of a gaming PC that can easily be carried by two men. Smartly designed to work in conjunction with a battery in a hybrid system, the 2-kW CAPS generator impressed the Army enough that LiquidPiston has been awarded a Small Business Innovation Research contract to develop it further as a 2-5 kW Small Tactical Generator for a range of military use cases, running on diesel with compression ignition.
LiquidPiston demonstrated the technology for the US Army by building a Compact Artillery Power System (CAPS) generator unit designed to power the digital fire control system on an M777 Howitzer artillery piece. It replaced a generator that needed a truck to move it around with something 20 percent the size: a 41-lb (18.6-kg), 1.5-cubic foot (28.3-L) box about the size of a gaming PC that can easily be carried by two men. Smartly designed to work in conjunction with a battery in a hybrid system, the 2-kW CAPS generator impressed the Army enough that LiquidPiston has been awarded a Small Business Innovation Research contract to develop it further as a 2-5 kW Small Tactical Generator for a range of military use cases, running on diesel with compression ignition.
Home emergency generator (Score:4, Interesting)
Re:Home emergency generator (Score:4, Interesting)
It's an interesting design all right.
The video ending with the predator-like drone is disingenuous though. The main power consumer in a modern drone are the electronics. The Karabakh war demonstrated armed drones like the Predator being on their way out. Small, low radar cross section drones and long range missiles with terminal guidance warhead (specifically the BM-30 with the the 9M55K1 cassette individually guided submunitions). Drone picks up, missile is fired from 130kms away and arrives in less than 1.5 minutes. No defence. Boom. So going back to the drone and the video - these drones fly very slow (50-100km/h) so very there is very little airflow for an air cooled engines. They are also stuffed to the gills with electronics. In fact, everyone is reversing the idiotic USA decision from the early days of the Predator to remove the radar. It is being put back to track "fellow drones". All of these are power hogs (especially the radar) and I do not quite see how an air-cooled engine with minimum airflow over it will drive all of that. It will end up being water cooled and then the size and weight will become similar to other modern engines.
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Re:Home emergency generator (Score:5, Informative)
That's a very uninformative marketing video, I agree. This one is much better for details: https://www.liquidpiston.com/h... [liquidpiston.com]
I've been following the Liquid Piston for a while now, and if you really dig into the details, this is a fundamentally different system than existing Wankel-style rotary piston engines, and really does solve virtually every shortcoming with traditional designs without adding any extra complexity. Some of the big differences:
- The biggest problem with Wankel efficiency is the long, thin combustion chamber that's rapidly expanding, preventing complete combustion.
In the LP combustion occurs in an arbitrarily small, constant volume combustion chamber with a long dwell time, maximizing combustion efficiency beyond even what's possible in traditional piston engines. It also makes it easy to tune the design for wildly different compression ratios.
- Apex seals are stationary, making them easy to lubricate (they're a nightmare in Wankels)
- All engine chambers experience the full compressions-combustion-expansion cycle, just like in piston engines, greatly reducing thermal stresses from uneven heating
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That's a very uninformative marketing video, I agree. This one is much better for details: https://www.liquidpiston.com/h... [liquidpiston.com]
The animation is also on yootoob: https://www.youtube.com/watch?... [youtube.com]
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It does seem to solve most of Wankel issues. There is still quite a lot of complex shaped surfaces compared to a modern piston engine, but nothing beyond what is possible with modern manufacturing techniques.
Looks cute. It will not remain so small and simple once you add cooling, emission control and all the other modern engine paraphernalia, but it will still be more compact than a normal piston engine.
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Not only that, but I'd imagine you could make the apex seals externally accessible since they're mounted in the engine block instead of the rotor.
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Possibly, though the access port would probably unavoidably mean greater leakage around the end of the seal, and I believe seal leakage (blow-past) is already one of the weaknesses they're still working on. Though I imagine it's not nearly as severe as the Wankel, since the body of the piston protects them from the initial shock of combustion. (And they don't suffer from centrifugal distortion)
Besides, without them being subjected to immense centrifugal forces like they are in a Wankel, I don't know that y
Re:Home emergency generator (Score:4, Insightful)
It will end up being water cooled and then the size and weight will become similar to other modern engines.
Water-cooled Wankel engines are typically smaller and lighter than 4-stroke piston engines of the same power. The 2 bhp/kg mark of a Mazda 13B is only beginning to be matched by modern turbocharged engines (20 years later). This engine should have a similar weight advantage.
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For a stationary engine, just add a cooling fan as per this video [youtube.com].
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It's 2kW, you do not go far with 2kW, a fridge (that need PSW) can spike more than 2000W when compressor starts, a well-known problem RVers are facing when using too small inverters for instance.
With 2000W you can easily power lights especially all LEDs, and some applicances, but that's it. A simple electric heater is 1000 to 2000W, you would need one generator per baseboard heater! One for your coffee maker, one for your toaster, one for your micro-wave, etc Forget water heater and electric stove with 2kW
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Transient spikes are an issue with most any generator, but depending on what you're powering it may not be essential that it can be provided. It can also be easily solved with a UPS or other power buffer.
Meanwhile, average U.S. household power consumption is between about 20-40kWh per day (0.833 - 1.667kW average) depending on state, so even such a small generator could easily provide it, provided you had enough of a buffer to spread the load across the day.
And of course they're also developing a full-size
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Interesting that a portable generator was picked as a demo...during WW2 there was a man-portable generator that ran on a Stirling.
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Given its small size (Score:2)
... and therefor low rotating mass, I'd say its torque is pretty poor.
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Seems like it'd be much better than a traditional piston engine, where the only rotating mass is the small-diameter (= low rotational inertia) crank-shaft - all the pistons are reciprocating, which just adds vibration while actually reducing rotational inertia (since the inertia has to be spent slowing and reversing the piston direction), and most of the mass is in the block, which just sits there.
The LP has a single large spinning piston inside a small block instead, which should act as a flywheel - in fac
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Okay, yeah, for direct-drive applications that could be an issue. But that's why you almost always have a gearbox - to convert high speed, low torque to low speed, high torque. All that really matters is the peak power (=speed*torque) output and, if you're using it in a variable speed application, the width and shape of that power band as a percentage of the peak power speed.
That said, I've yet to see a power curve for an LP engine. The closest I've seen is testimonials of people who've ridden in their g
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You're not going to be using the larger engine for a chainsaw or other handheld tools (which is where this thread started). The X-mini is currently 3hp, with an estimate of 5hp once mature. You don't want to try to manage much more than that with only arm strength to control it.
The larger engine is a 40hp(30kW) diesel designed as the drive component of a generator. And while it does sound like it's going to take a more sophisticated cooling system, it shouldn't be any worse than in a standard ICE of comp
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An example (McCulloch 840) an be seen in operation towards the end of this video https://www.youtube.com/watch?... [youtube.com]
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I stand by my assertion for most people - you're going to want a considerable amount of strength and expertise to handle such a beast.
Also, as the overall size scales up, the weight advantage of a rotary becomes more pronounced, and the added weight of a reduction gear (assuming it's needed) becomes proportionally less significant.
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The do mention combining this with electric engines as a hybrid, so you can use the electric engine for the torque when needed, which is only at infrequent times. For a generator the torque isn't needed as much as in a vehicle. But many hybrid automative designs work best at reducing fuel use by keeping the engine at a constant speed more often, limiting the acceleration which requires more fuel. So it won't be the design desired by the racing industry, but it seems to have great uses in generators alrea
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I may be misremembering, but my understanding of their hybrid discussions was more that they envisioned their larger model (the one they're developing under another military grant for use in a small portable 30kW generator) being used in series-hybrid BEVs, so the rotary engine never drives the wheels directly. A Tesla at highway speeds consumes about 15kW, so at 30kW they could charge it as fast as it would otherwise be discharging. Which could be very handy for occasional range extension, allowing cons
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"all the pistons are reciprocating, which just adds vibration while actually reducing rotational inertia (since the inertia has to be spent slowing and reversing the piston direction"
Your reasoning is flawed. The whole rotating mass is connected together and physics doesn't know which part is a piston and which isn't as far as inertia goes. You might as well say the crank doesn't provide any torque because its inertia has to be spent turning the wheels!
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>The whole rotating mass is connected together and physics doesn't know which part is a piston and which isn't as far as inertia goes.
Thinking further, you're right - the pistons will contribute inertia, but not via a straightforward method. Physics absolutely knows the difference between linear and rotary motion. The imaginary centrifugal (outward) force is the most dramatic example - there's no such force, but there appears to be because centripetal (inward) force is constantly accelerating everything
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In the video there's a shot of the go-kart moving from a near stop, and it's obvious that the engine doesn't make nearly enough torque for that particular use. No surprise, low displacement, low rotating mass. Still a cool idea, and they're in my area. If I was younger and not so established, it looks like something I'd want to be involved in.
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Replacement for ICE in hybrid electric cars (Score:4, Interesting)
From TFA:
It would certainly look attractive as an ultra-lightweight range extender for electric cars
This could be useful for hybrid electric cars, buses and trucks where they have the worst of both worlds with not enough battery capacity for mid distance and ongoing maintenance costs of the engine. With less moving parts it reduces the maintenance and with a smaller size allows for more battery.
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The Wankel too had many fewer moving parts than piston engines, but it never was cheap to operate and maintain. Why should this one be any better within the next 5 years? After that the market for non-BEV cars will be shrinking increasingly quickly.
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Actually, this very much is NOT a Wankel design - Wankels move the piston chambers around the engine, and have a long, thin, rapidly expanding combustion chamber during ignition, which is terrible for complete combustion.
In contrast, the LP has three independent stationary piston chambers with tiny, constant-volume (long-dwell) ignition chambers whose size can be tuned independently from the size of the rest of the piston chamber, ensuring complete combustion.
I'm not sure what "sealing ring" you're talking
Re:Replacement for ICE in hybrid electric cars (Score:5, Informative)
The Wankel too had many fewer moving parts than piston engines, but it never was cheap to operate and maintain. Why should this one be any better within the next 5 years?
Flat out better design.
- 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.
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Yeah theoretically it looks like they addresssed most of the issues with the normal dorito Wankel. But as alawys I think the trick for making an actual production engine will be in the details. How does it scale, what's the efficiency over a wide range of loads and RPM, cold or hot conditions, long idling, what are the emissions, etc, etc. There are all sorts of tricks to increase efficiency in ICEs but most come with significant compromises or caveats.
Still this is very cool development and I'm really inte
A quantum leap? (Score:3)
Ah, a quantum leap. Sounds really impressive until you realise that it means the smallest change possible...
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Apparently dozens of people laboured, I mean really laboured for over a year on it. They used state-of-the-art CAD tools, so you know you it's good.
Take a look at the design: It's dog simple.
Sometimes people come up with a breakthrough where the KISS principle works. All the design elements fall out of the fundamental concept, there are a small number of parts to design and get to work together, and a small number of aspects to be researched. A small team assisted by good tools can get the job done well
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Agreed the marketing video isn't great.
However, if you watch the more useful video linked below (nicked from another poster), at around 1:55 you'll see that they claim combustion happens entirely while the combustion-end of the rotor is stationary, in the time period when the intake/exhaust end is moving from one chamber to the next. Shrinking arc is the compression cycle and growing arc is the expansion cycle.
https://www.liquidpiston.com/h... [liquidpiston.com]
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As this is a diesel style compression ignition engine, the whole fuel air mix burns at the same time, making combustion much faster than spark ignition, therefore doing more work. That’s the main reason diesels are more efficient too.
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As this is a diesel style compression ignition engine, ...
If you look at several of their YouTube videos you'll see that they make it both as a diesel with fuel injectors (shoot it in so it burnsduring the "constant volume" time at max compression rather than starting as it's compressed) and as a spark-ignited version (not clear whether that also has combustion-chamber fuel injection or if it's fed a fuel-air mix to compress). ... the whole fuel air mix burns at the same time, making combustion much faster
Some claim numbers. (Score:2)
This video [youtube.com] claims the diesel version, compared to a "traditional" diesel engine, is:
- 1/10th the size and weight
-30% more fuel efficient.
Also mentions "overexpansion" which should cool the exhaust substantially.
You can see that the rotor doesn't roll over the combustion chamber, but has a circular section (arc) that slides, leaving the combustion chamber essentially sealed off for maybe 20 degrees of shaft rotation.
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I don't think it solves the problem with the seals. I mean, it's an IC engine, so it releases greenhouse gas which will make sea ice melt. What will the seals do when there is no sea ice left?
Why not everywhere? (Score:4, Insightful)
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Does it suffer from the same "unburnt fuel leaking past the seals" problem (and the resulting very bad emissions) that caused Mazda to discontinue their rotary engine cars? That might be why they are showing this to the military (who don't care so much about emissions) and not shouting from the rooftops about potential civilian applications (where emissions matter)
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Re: Why not everywhere? (Score:2, Insightful)
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>Wankels don't run on mix either
Actually I believe they do - the mixing just happens within the engine, rather than in the tank. There's just no other reliable way to get lubricant to the spinning apex seals.
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No, it's really not.
A Wankel moves the "piston chambers" around the engine, with each phase of the combustion cycle (intake, compression, combustion, expansion) taking place in a different part of the housing. And due to geometry constraints the lines between cycles gets a little blurry. Plus, the combustion phase takes place in a long, thin, rapidly expanding chamber, practically guaranteeing incomplete combustion. And since the apex seals are mounted to the fast-spinning rotor, lubricant needs to be a
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Because they use a non-spinning dorito and a peanut shaped spinny bit, the apex seals are on the housing and are still, and it also means the combustion chamber is still so direct injection is a possibility.
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The part I never understood about Wankel, and I see it here, is how you seal and lubricate the side of the rotor. A cross section of the 3D model doesn't show what happens perpendicular to that. Is it a flat surface moving against a flat surface?
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Ah, thanks. So with the liquidpiston engine, they could have a more continuous oval side seal, instead of three separate side seals.
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It's neither a recent innovation or a new company so I'd say you're probably right https://www.liquidpiston.com/a... [liquidpiston.com]
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Automotive companies easily spend hundreds of millions developing a new engine family, and that's with a massive legacy to build on. I'm not surprised they're going at the 'deep pockets' end of the market first.
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The fact that they are still fishing for government "development" money instead of selling them on every street corner tells me that despite the simplicity, there is still significant issues with the design that they are not mentioning in the flashy videos.
I am also suspicious of anyone looking for government money. Private citizens with money to spend on development got their money by picking where to invest it very carefully. Government money comes from senators looking to buy votes in their state. It's likely far easier to get money from the government than any private investors.
The military though is made of very practical and cautious people. They have to be because war is hell and poorly designed hardware costs lives. If the military approves then
Looking for an animation? (Score:5, Informative)
Check out the link for an animated demonstration of it working. I am not too sure about the "we've solved the seals issue" but it is a very cool concept
https://www.youtube.com/watch?... [youtube.com]
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It is a hot concept! High power density means challenging to get it cooled.
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That's the one thing that really bugs me about this design: the exhaust gas flows out through the rotor. So that rotor is going to get pretty hot, and the only cooling I see is the intake gas...
After 120 years, the pistons on a crank model... (Score:2)
.... still hasn't been surpassed. Other designs have come and gone but they all had one too many compromises in the end that proved their undoing. I wish these guys luck but I won't be laying any money on a bet that they'll triumph just yet.
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Carnot Cycle (Score:1)
I had to laugh in their video where they state that their patented thermodynamic cycle has an ideal efficiency of 75%. That is the carnot cycle efficiency for gasoline (TH of ~1200C), so it's a bit of a meaningless statement. Why do marketing people produce this sort of useless information? It otherwise looks like an interesting design that solves some of the issues with the Wankel engine.
I think the biggest issue it has would be the apex seal lubrication and blow-by. There will be oil getting into the comb
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How is apex seal lubrication a significantly different problem than with ring seals on a piston? Certainly it's a huge problem on Wankels, since the seals are attached to the spinning rotor, and thus oil must be added to the combustion mixture to lubricate them, but an LP's stationary seals can be lubricated directly, so you should only have whtever oil comes off them getting into the mix, just as with piston seals.
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Quite. Another big one is that you don't have centrifugal force stretching the seals outward, into the housing surface, like you do with a Wankel. Stationary seals just sit there, subjected only to friction/adhesion forces with the passing surface.
As I understand it, almost all the longevity problem with Wankel apex seals can be traced to a combination of centrifugal stretching, and the difficulty keeping them adequately lubricated.
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One thing that I can't really come up with is a way of getting a continuous supply of oil to conve
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Yeah, Wankel apex seals seem a sticky problem - and given how long they've been being developed, I suspect that if there were a cost-effective method to lubricate them without burning oil it would have been found. I suppose a secondary face seal would let you keep oil from leaking out onto the working faces of the case, where it would be burned, but I suspect having much oil within the gearing chamber would actually present a potentially serious efficiency loss.
That's hardly the only problem with the Wanke
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Hmm, actually, what about three concentric face seals on the Wankel? The usual chamber-sealing one around the rim, and two concentric rings that form an oil channel between case and rotor? That'd keep the oil clear of both the chambers and the gearing. Of course since the rotor doesn't have a well-defined rotational axis you'd need a sufficiently wide oil channel to keep the intake within it for the full rotation, which might be a challenge. You'd also be introducing considerably more friction and adhesi
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I'm no expert, but that would make sense, use the softer rings as bushings to avoid damaging the barrel. I guess I'm including everything that moves with the piston as part of the piston.
Point is -
In a traditional piston engine the piston is guided by rubbing between something on the piston and the barrel - there is no other mechanical constraint keeping things aligned (some pistons are mounted on a rod that passes through a guide at the non-working end of the barrel, but usually not in car engines), and
How it's made (Score:1)
Yup, they made one of those episodes for this engine. Video on the manufacturer's page https://www.liquidpiston.com/ [liquidpiston.com]
https://player.vimeo.com/video... [vimeo.com]
Seems to me that if they have to machine the seals from a big chunk of steel it will be quite expensive. Hope they can punch them in the future.
Also, the way the seals are steel on aluminium rubbs me the wrong way. (pun intended)
Doesn't look stackable (Score:3)
I sort of recall that one of the cool things about the Wankel was that you could stack several of them together on the same crankshaft. With the airflow coming in from and out of the faces of the disc, it doesn't look possible with this configuration. You might be able to cook up some manifold arrangement where there's an inflow and outflow disc on either side of the power disc, but that ends up making the engine three times as long, mitigating any advantage. You COULD put two together, with inflow coming from their exteriors and a common outflow (or vice versa) for a slightly smaller "length penalty."
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There wouldn't be as much of an advantage to stacking this one though. Might as well just use a bigger version when you need more power.
Ford/Mazda (Score:2)
Mil-spec RX-9?
Used in tandem? 2 in parallel with offset firing? (Score:2)
That's something they should be investigating and if reasonable, testing.
This somewhat reminds me of the old steam locomotives. The old classic ones have 2 bi-directional steam cylinders, (1 per side). So they get power at 0, 90, 180 & 270 degrees on the wheels. The 0 & 180 are from one cylinder, and 90 & 270 are from the other cylinder. Other variations of the ste
So many "clever" IC engine designs (Score:2, Insightful)
There have been so many of these clever mechanical alternatives to a standard piston engine. Maybe this one will actually make it, but so far none of this endless series has really gone anywhere.
Modern IC engines are very compact, usually cooling limited. They have very good lifetime.
The liquid piston engine has constant volume combustion, which is not what you want at high compression - the pressures get too high for the power generated.
The military will want a diesel version because they want single-fuel
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The military will want a diesel version because they want single-fuel operation.
I'm not so sure diesel cycle is a requirement. Being able to run on JP-8 is a requirement since in wartime they don't want to be bothered with having to manage multiple kinds of fuel. In practice though even in war (so far at least) cost saving measures means using common diesel where possible and saving the JP-8 for the aircraft.
I had a conversation with a coworker that grew up in India and he said it was quite common for people to use kerosene in their spark ignition motorcycles and such. The result wa
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Agree, it needs to run on standard kerosene like fuel. Diesel is a lot more efficient than a low compression spark engine that runs on that fuel, but might not be critical here.
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