How a 4-Pound Engine Can Replace a 40-Pound Engine | Hacker News Reader

How a 4-Pound Engine Can Replace a 40-Pound Engine

popularmechanics.com

149 pointsytNumbers107 comments

I saw this a couple of years ago, its a pretty nice rotary engine. As others have pointed out, rotary engines have their challenges but their advantage in weight and torque still makes them a good choice for aircraft.

The "secret sauce" that the Liquid Piston folks bring to the party is the upgraded rotor shape. It doesn't need apex seals like the old Wankel version did. The news here is that they had reduced their experimental work into an actual functional engine. Unlike the Wankel, their cylinder design would really support a compact multi-rotor engine but presumably you could gang them together on the same crankshaft if you really wanted to.

Other than the UAV and possibly the paramotor market I'm not sure where these guys are going to find customers though.

I'm going to get laughed at.. but the first thing I thought of was lawn tractor! A lighter tractor would be less of an issue to wrangle about a garden, it would consume less fuel shifting itself about, domestic ones get relatively few engine hour... a smaller engine might allow novel designs that get into tighter corners are simpler and cost less... You need ~10hp for a lawn tractor as is soooooo...

The only problem that I could see there is that you actually kind of need weight on a tractor. You take a 500lb tractor and take out the 150lb engine you're getting a lot closer to the weight of the driver and you're upping the possibility of tipping over significantly.

Couldn't you just adjust the driver/engine position to account for this?

That's a good point!

From my experience with small quadbikes, I'd say that gives the driver more controll. On a light vehicle, you can just lean a bit to counter highly uneven terrain.

Chainsaws and weed-whackers might be good applications.

Looks good for robotics applications. The power density is comparable to the low end of brushless motors, and it can use hydro-carbon fuels which are about 25 times more energy dense than current batteries. At 3 hp, it might be a reasonable drop in for the type of weed-whacker two-stroke that drives big dog.

This is certainly something to consider, and for robots or exoskeletons the power density would make for a pretty compelling solution. I'd be curious to try out its low RPM torque, that was always a challenge on the RX-7, starting in first gear you spun up the engine to 2 - 3K RPM before you let the clutch out.

How many watts can you reasonably generate with 3 hp?

Approximately 2237.

I was going to nitpick about efficiency of the generator not being 100 %, but apparently a generator can be 99 % effective.

Yeah, if you start from the break HP of the motor, pretty much all the work can be converted into electricity. If you start from the energy in the fuel, and go all the way to the generator, its like 50%.

If they can keep the size/weight, go to 5 HP AND make it cheap, this would be perfect for chainsaws, brush cutters/trimmers, blowers, and lots of other portable gas-powered tools... Hell, even small portable generators.

The problem I see is that existing engines are already good enough and cheap, so it could be a hard sell for manufacturers.

The weight of smallish outboard motors (9.9hhp in particular) is really irritating. Making these a lot lighter for the same power would find a customer with me.

I wonder how loud it is compared to traditional engines

"the X2 engine is exceptionally quiet because it burns all of its fuel. In current ICE engines, an alarming amount of fuel goes out the tailpipe. This not only cuts down on fuel efficiency and pollutes the air, it also makes the engine noisy. Since the X2 engine burns its fuel completely, there’s no need for complicated silencing apparatus. (Update: The engine runs quietly due to not needing poppet valves.)"[0]

[0] http://www.gizmag.com/liquidpistol-rotary/24623/

In current ICE engines, an alarming amount of fuel goes out the tailpipe. This not only cuts down on fuel efficiency and pollutes the air, it also makes the engine noisy.

The "an alarming amount of fuel goes out the tailpipe" is reminiscent of the wording associated with various fuel-saver scam gadgets. In practice, this is measured in HC emissions, which are usually in the low tens of PPM range on a properly tuned engine.

Disregarding mechanical sources, engine noise is largely caused by the high-pressure gases flowing out the exhaust with pulsations on each power stroke. The distinctive beating of a low-cylinder-count engine e.g. a motorcycle is entirely this phenomenon. To attribute the noise of a properly functioning (i.e. not backfiring) engine to "unburned fuel" is completely wrong.

A four-stroke engine fires every other crankshaft revolution. Each cylinder creates its own pulses. The exhaust manifold tries, with varying success, to blend those into a single higher-frequency pulse train.

The Liquid rotary fires thrice per revolution, or six times faster. The go-kart engine is designed for 10,000 RPM. That puts its fundamental frequency at a less objectionable 500 Hz.

But the go-kart video did not sound like 500 Hz -- it sounded like about 100 Hz.

As for the "alarming amount of fuel out the tailpipe" line, maybe they're comparing a two-stroke engine. I don't really know how unburned fuel affects a two-stroke's sound.

I questioned whether should edit part of that out. Somewhere it does mention over-expansion to near 1 atmosphere in its cycle. So the theory is quiet, the go kart audio not withstanding.

The challenge with that comment from 2012, is that the go cart video from 2016 is pretty noisy. Easily 90 to 100db from the sound on the video. https://vimeo.com/170502635

If true, that would also cut emissions significantly compared to a conventional 2-stroke.

Based on their video (last on this page), that certainly looks like their target:

http://liquidpiston.com/technology/x-mini-gasoline/

If these are indeed quieter, then I could see them commanding a significant premium for lawn tools, generators, etc.

I'm curious. With the air chamber in the cylinder head, why can't they fire 3 charges of fuel per revolution instead of 2? I would think with some appropriate valve configuration they could do that and it would simplify heat and torque problems. Maybe they'd need 2 air chambers for that (but then the counterweight wouldn't be necessary).

Oops! I found a different video that shows three spark plugs. I'm not sure what was going on with the first video I watched.

> UAV

There are a lot of customers there these days.

Rotary engine advantage in torque? This is nonsense.

The Wankel's design limits the eccentricity of the crank shaft which consequently limits torque to relatively low levels compared to a contemporary piston engine of similar fuel consumption. There is no torque advantage to a Wankel, it's at a disadvantage.

Imagine powering an exoskeleton with it.

Given that they are targeting very high efficiency, and they have very low weight, what about something with just a little bit more than the power output to cover highway cruising speeds? With the properties this has, it would make a great range extender.

I always thought torque was an incredible weakness of rotary engines.

For those wondering how it works, here's a visualization: https://www.youtube.com/watch?v=0e785YnDmq0

That's an extremely elegant design in terms of intake/exhaust flow.

It might be fairer to compare to a state of the art piston engine. Brief googling finds a DeNardis 40cc engine weighing 11 lbs and producing 7hp. Still impressive though.

http://www.denardisengines.com/40cc.html

Ouch. Note that the 4 pounder is only outputing 3 HP so at a pound and a third per hp a denardis equivalent would weigh a bit more than 9 lbs. Note that the denardis spec includes the accessories that make the engine work and the picture in the article does not. Likely an installed operating denardis would weigh less.

There's a trick you can play with engines that only engineers really understand. A good engineer can fluidly trade economy, reliability, and mass. So if you want a couple ounce model airplane engine that outputs a couple HP and might run for ten hours before burning out at insane RPMs with awful fuel economy specs, those are widely available and fairly cheap. Or you can buy an inboard diesel that'll push a motorboat for insane cruising distances under hurricane conditions and maintained with a shoebox of cheap tools, but it'll be expensive and heavy, like 5 HP and it'll be 100+ pounds.

I have no question someone can build a 4 pound engine that outputs 3 HP or an 11 pound engine that outputs 7 HP. The only question is how long, and how much gas it'll burn. Something tells me neither of them would run my commuter car for 200+K miles OR get 30+ MPG, which is pretty easy for my extremely high tech, but heavier engine, commuter car.

The denardis 40cc 7hp engine says it burns 610g of fuel per kilometer driving a pocket bike. That is around 3 mpg for those of us with strange units. It's safe to say they traded away efficiency.

It's a two stroke engine with oil mixed into the fuel, I don't think the intended users (there's such a thing as competition racing micro motorcycles??!) care too much about fuel efficiency...

I have no question someone can build a 4 pound engine that outputs 3 HP or an 11 pound engine that outputs 7 HP. The only question is how long, and how much gas it'll burn.

For an even more extreme example, a Top Fuel dragster engine can make over 10000HP and weighs less than 500lb, but has a lifetime measured in seconds and fuel efficiency better measured in GPM instead of MPG.

That's what Moller, the flying car guy, did. His "Rotapower" engine[1] is an overdriven Wankel. Lots of power, but a short life, which is why the engine never went into production.

(The Freedom Motors site looks convincing, until you compare it with their site from 11 years ago. That's Moller - Real Soon Now forever. Over 40 years of Real Soon Now on the flying car.)

[1] http://freedom-motors.com/ [2] https://web.archive.org/web/20050411075152/http://www.freedo...

> but it'll be expensive and heavy, like 5 HP and it'll be 100+ pounds.

tl;dr: affordable, efficient, light weight: Pick any two ?

Here's a 3hp model plane motor. It weighs 750g all up. http://www.hobbyking.com/hobbyking/store/__9969__Turnigy_26H...

And here's 3hp electric motor (550g), although that's probably an unfair comparison. http://www.hobbyking.com/hobbyking/store/__18177__Turnigy_Ae...

The horsepower ratings of engines in the hobby world are optimistic at best. I have personal experience with some RC airplane engines and a dyno, and frankly I don't understand how the manufacturers can keep advertising the numbers that they do.

Source on that? Never heard anyone claim the numbers on these motors were fake before.

I have personal experience with RC electric motors and dynos and they are correct.

I believe those have thermal problems, use more fuel and have a shorter lifespan?

Note that the 40 pound engine includes the tank, gearbox, muffler et al, while the 4 pound engine is just the core. Still an amazing demo, though.

Pretty informative video about rotary engine dis-advantages. https://www.youtube.com/watch?v=v3uGJGzUYCI

Except this isn't a Wankel.

I looked into this a while back, and they're able to get 50%+ thermal efficiencies by using their own modified thermodynamic cycle that promotes more complete fuel combustion, as well as arranging the engine components in such a way that the engine has low heat rejection qualities. Nearly doubling the efficiency of the typical ICE setup and doing so with large weight reductions far outweighs any issues with rotary seals. These things are so compact and simple that it would be feasible to simply drop an entirely new engine into your car and melt down the old one when the seals wear out. Not that it would come to that, but those are the kind of economics we're dealing with here. This is a legitimately nifty invention.

t. mechanical engineer

Trying to understand what is different and found this:

"The basic idea is similar to a Wankel rotary, but turned on its head. Where the rotor holds the seals in a normal Wankel, the housing does that job in the X1 engine. This allows significant reduction in oil consumption over a regular rotary motor. Other enhancements include direct injection, a high compression ratio at 18:1, and a dramatic change to the geometry of the combustion chamber, which maintains a constant volume during ignition. This change means the air-fuel mixture auto-ignites like a diesel, and can be burned much longer than normal. The result is a more complete combustion ending in low emissions and very high chamber pressures. This high pressure is allowed to act on the rotor until it reaches nearly atmospheric pressures, so almost all the available energy is extracted before the exhaust is physically pushed out. Again, this is different than a normal internal combustion engine, which releases very energetic, high-pressure exhaust gas."

Just doing a brief visual comparison from https://www.youtube.com/watch?v=0e785YnDmq0, the most immediate difference is the inversion of the housing and rotor shapes. Instead of a dorito shaped rotor like the wankel, it has a jelly bean shaped rotor. And instead of a jelly bean shaped housing, it has a dorito shaped housing.

Also, my memory of the workings of the wankel is a bit rusty, but I believe it is inherently balanced, so no counterbalance is necessary, but this one requires a counterbalance. On the other hand, unlike the wankel's one combustion chamber, this one has 3, so it doesn't need to deal with asymmetric heating issues. The combustion chambers themselves are very interesting. Due to the dorito shaped nature of the wankel rotor, there seems to be an inherent limit to the compression ratio because of the planar sides of the rotor can't get any closer to the wall of the housing. But on this one, the rotor can get completely flush with the housing, allowing more direct control over the compression ratio. Definitely an intriguing design.

I believe the counterbalance is due to the asymmetric jelly bean shape to create a larger expansion vs intake volume.

This article/company are misrepresenting the truth. Once you add the carburetor, a starter (watch the video, they use a drill to start the engine, obviously real small engines have pull starters, which add weight.), the exhaust, intake manifold, etc. This engine is much heavier than 4 pounds.

This is almost the equivalent of me saying I've invented a 3 pound airplane, which is nothing but a carbon fiber tub. You just have to add wings, engines, cockpit, etc. etc.

It's a small engine, yes, that's nice. But don't blow it's carbon emissions up my ass and expect me to believe the hype.

The starter may be a non-issue. E.g. starting a drone that way (with a drill) would be fine. Maybe even leave off the exhaust. Depends on the application, which they admit.

This is true but the engines they're comparing to have all of these things. You could take the starter etc. off of those engines and it would weigh less, too- it's a misleading comparison.

You take off all those things, those engines still weigh 20 pounds. You put the mini-versions of all those things on theirs like they show in the video, and I doubt it breaks 10. It's probably overstated bragging, but they've got much to brag about.

Then don't cheat by comparing the weight of the motor with starter to the one without a starter. They are comparing two very different things to produce a spectacular head line.

0.75hp/lb is better than the 0.1625hp/lb engine they replaced with it, but doesn't seem so revolutionary compared to the power-to-weight of existing engines:

https://en.wikipedia.org/wiki/Power-to-weight_ratio#Engines

Even the well-known Mazda rotary is slightly better at 0.92hp/lb, and various aviation engines have higher power-to-weight ratios still.

I think the big difference is this is really, really light weight and tiny compared to a small aviation engine. More in the category of something you would power a 100 kilogram drone with than an ultralight or LSA category aircraft.

This is another take on the rotary/Wankel engine. Mazda and many others have tried to develop durable, efficient versions of this type for decades, with only limited success.

The reason Mazda struggled with the rotary engine is due to what their application was. Rotaries work most efficiently and reliably when run for long periods of time (hours) at a consistent RPM. Vehicles with traditional manual or automatic transmissions (what most Mazda rotary cars had afaik) do anything but keep consistent RPM. Mazda subsequently struggled with keeping emissions down, and fuel economy up, while prolonging the life of the engine.

If you tie a rotary to a generator, or true CVT transmission, they're much better suited to the task.

That sounds like a good choice for a hybrid that uses the rotary ICE to periodically charge the battery.

Or a range extender generator like in the BMW i3

Mazda actually had a Mazda2 EV prototype with a rotary range extender that they unveiled in 2013. Not sure what ever happened with it, though.

I first saw this on the Audi A1 e-tron concept; it was a 250cc wankel for the range extender. I thought that was the future when I saw it, and here I am now with a Vauxhall Ampera (Chevy Volt)!

> or true CVT transmission

What I never understood: why don't do it like on a ship, attach a generator to the fossil-fuel engine, add a battery and supercap pack, and four small-ish wheel electric motors?

That should save the weight of a gear-shift box, complex mechanics on FWD cars (because the axle transmitting the movement energy must be flexible), and it would allow the motor to run at its most efficient RPM count, as well as providing full torque from 0 km/h.

In essence, a Tesla with a fossil-fuel engine in the frunk...

I think a more apt comparison would be with a Diesel-Electric locomotive. They're already the kings of transportation efficiency (or possibly a close second to container ships).

I don't know why tractor trailers haven't followed this same diesel-electric pattern. Max torque from 0 RPM plus the elimination of 18 speed manual gear boxes?

> I don't know why tractor trailers haven't followed this same diesel-electric pattern.

A high-load tractor trailer can deliver around 500 kW of power. I believe the main problem is that there is no wheel-mountable motor available to transfer that kind of power. The Tesla, for example, is rated 345 kW peak in the P85D, but a tractor trailer needs this amount of power continuously. When we do the math and assume 500V (which is a ridiculous high voltage anyway!), you'll need 1000A-rated cables, switches and motors/generators.

That's not your home 2.5mm2 cable, and discarding the waste heat (assuming 95% end-to-end efficiency) at this scale is a hard task as you have, essentially, a rolling 25 kW heater.

With ships there is MASSIVELY more space available, and the heat can be (and often is) used for heat distribution, and a diesel loco also has lots of space available to put components in.

A loco can take up to eight times the same space as a single tractor-trailer...

In ships a portion of the waste heat from the engine is also used to pre-heat the fuel tank and lines to make it easier to pump, as heavy bunker oil is extremely thick at 'room' temperature.

No one uses wheel motors on cars; they sound good at first, but the problem is that motors are heavy, and the last thing you want on car wheels is more unsprung weight: it's very bad for ride and handling. So you still need flexible axles to transmit power from the motor to the wheels. Go look at a Tesla at the showroom and you'll see this in the bare-chassis floor models they exhibit.

Otherwise, you're basically describing the Chevy Volt. There's a reason these cars aren't more popular: they're expensive, and have a lot of drawbacks. Instead of a single drive system like the Tesla (electric motor, axles, single-speed gearbox, and motor controller, plus a big battery pack for energy storage), now you need two drive systems (the former, plus a traditional fossil-fuel engine, and a generator, and a fuel tank). All that hardware takes up space, and costs money, so you end up with a car that's a lot more expensive than a comparably-sized and appointed gasoline car. You could downsize the battery pack greatly to save cost and space and eliminate the 50+ mile electric-only range, but now you're stuck with unimpressive fuel economy: take a look at the Volt's fuel economy when in gas-only mode, it's not anything special. The Prius gets much better fuel economy than the Volt when comparing gas-only operation, and for good reason: serial hybrids are inherently inefficient because of all the power conversions. The Prius does better because it's a parallel hybrid.

Now moving away from the Volt's gas-powered piston engine to a far more compact diesel-powered rotary optimized for single-speed operation might change this equation, I don't know. Of course, as we've seen with VW, diesel has problems with emissions.

I still want a serial hybrid with 4 motors.

The motors wouldn't have to be unsprung weight. They could be inboard, as on the Tesla cars but in pairs. They could be inboard, tipping as needed to eliminate 1 joint per wheel. They could be out at the wheel, affixed to the body, missing the top and bottom to allow the wheel to move around them -- the car effectively being a mag-lev inside centerless wheels.

I want to not have sloppy differentials. Power to each side of the vehicle should be adjusted according to the steering input.

I don't need a battery. Give me a supercapacitor that is just big enough to handle regenerative breaking from 80 MPH to a stop, followed by running the engine while waiting for a drawbridge or freight train.

It's not about fuel economy. It's about acceleration, including 4-wheel fully independent anti-lock acceleration. It's about never feeling the jolt of changing gears, and doing so without the wimpyness of a CVT. It's about getting all this when there is no place to charge an electric car.

If your 4-motor idea really made sense, I think Tesla would have done that. They didn't: they use a single large motor (or two, for the P85D, front and rear), driving a single-speed gearbox, through a differential, to two standard CV axles.

The Chevy Volt, too, uses a single motor. I'm not sure about the Bolt or the LEAF but I suspect they're the same.

>I don't need a battery. Give me a supercapacitor that is just big enough to handle regenerative breaking from 80 MPH to a stop, followed by running the engine while waiting for a drawbridge or freight train.

Then you're going to have lousy fuel economy. If this idea made sense, the Chevy Volt would have used it. They didn't. No one wants to buy an serial hybrid car that has crappy fuel economy. They can just buy a gasoline car for a lot less money.

>It's not about fuel economy. It's about acceleration

The Tesla P85D has this already, and without all the complexity you're proposing.

>It's about getting all this when there is no place to charge an electric car.

If you have a house with electricity, you have a place to charge your car. If you have an apartment with a parking space and no way to get power there for a charging station, then you're too poor for anyone to bother making a car like this for you.

>They could be out at the wheel, affixed to the body, missing the top and bottom to allow the wheel to move around them -- the car effectively being a mag-lev inside centerless wheels.

As an engineer, I have no idea what you're talking about here.

Prior to the P85D, one could have said "if a 2-motor idea really made sense, somebody would have done that". One would have been wrong to say it doesn't make sense.

Prior to the Prius, one could have said the same about hybrids. Prior to Tesla, once could have said the same about electric cars -- and at the time it was true, with nobody wanting what little was offered.

There are numerous city dwellers with plenty of money and no place to charge. Often they live in historical neighborhoods that are all the rage these days. There are also homeowners without garages, and with garages full of stuff like boats and model railroads and other cars.

Also, range anxiety is real. No, I don't want to drive along a route defined by Tesla's superchargers.

That third option for motors probably requires an animation to explain. Never mind; there are at least two other ways that work fine without the unsprung weight problem.

I'm assuming all that weight still gets better mileage than what would basically be a petrol->kinetic->electric->kinetic convertor.

Modern engines are pretty efficient, adding an electric stage would likely reduce that efficiency.

It's not only about the weight. An electric intermediate circuit can:

a) absorb braking energy - that's why I mentioned super-caps, because batteries can't ingest the vast amounts of energy that braking could theoretically produce (usually the brake power is 4x engine power, which means for a 250 kW motor a minimum brake power of 1 MW, and for sportscars likely muuuch more)

b) keep the engine, if not in idle (i.e. you're stuck in traffic), at its most fuel-efficient RPM range - this is something any cheap-ass scooter does, and every diesel-electro locomotive and ship, but no car! Not to mention it isn't just about fuel-efficiency, but also many exhaust gas treatments only work at peak performance under very specific driving conditions (esp. temperature).

The efficiency loss by conversion (97% and better in efficiency class IE4, see https://de.wikipedia.org/wiki/Elektromotor#Wirkungsgrad_und_...) is, taking especially the recovered braking energy into account, smaller than the efficiency delta gained by loss of hundreds of kg of weight (batteries are heavier, yes, but they can also replace "dumb weights" needed before to keep the car on the ground in the "elk test").

Also, less parts that are prone to (dirt) failure, e.g. the power-loaded joints on FWD cars, clutches, bearings, the entire gearbox system, differentials (in 4WD/multi-WD systems!). And you save on tires because in curves the individual motors can adjust speed. If done well, it should be pretty cost-efficient (less stops in the shop), too.

That's a different take on hybrids then? I dunno, I guess manufacturers went for straight electric and skipped that idea, since you can have ICE+fuel OR the same energy in batteries?

Straight electric is usually done without supercaps, thus losing the ability to fully recover braking energy, and batteries still don't have the same range as a tank of fuel, and can't be recharged as fast.

There is one thing that has the potential to kickstart the EV industry: sell the cars, but rent the battery packs, with an industrywide standard on how battery packs look like and how they are swappable by machines.

It's no good if a gas station has to carry swap stations and swap batteries for dozens of different models.

That is what the Nikola Motors One is doing, except it is using a turbine engine.

https://nikolamotor.com/one

How is their maintenance/durability? My understanding on rotary engines is that in order to maintain compression, they have to have a solid seal on very fast moving surface contact points, which is incredibly hard to do. Naturally, the material that produces the seal is going to wear down over time, and to replace it the entire engine will have to be taken apart.

I love the concept of a Wankel engine, and it would be awesome if they've overcome that particular aspect.

Mazda eventually got their engines to consistently last over 100,000 miles with the Renesis in the RX-8. The biggest thing is lubricating the seals; as a result all Mazda rotaries burn a little bit of engine oil to lubricate things, via a metering oil pump. That's the biggest reason why they struggled to keep emissions down.

Perhaps they could come up with some system like what we see in traditional piston engines piston seals - a replaceable seal that goes around the rotary triangular err piston for lack of a better word.

If I remember correctly all the other wankel / rotary engines have just had a lump.

Mazda's automotive Wankels have spring-loaded seals on the apex of the rotor and on the sides.

See this image: https://hackadaycom.files.wordpress.com/2016/02/wankel-insid...

Didn't know that - thanks for the information. Was it wear against the side of the casing that led to the issues then?

I don't know, but just looking at that photo, and comparing to a traditional piston engine with its piston rings, I can see a bunch of problems:

1) there's a bunch of irregular shapes here to seal: each "chamber" has shorter seals in two places (the vertices), and two longer seals on the sides. So 4 separate seals per chamber (two of which are re-used for adjacent chambers). In a piston engine, each chamber has usually 2 or 3 seals, with one being the main seal (top of the piston) and the others being either a backup or an oil ring (for scraping off most of the oil). A circle is a much simpler design.

2) 4 seals means 4 places where there's going to be gaps where gases can escape between chambers, causing extra emissions. In a piston engine, there's only 1 gap in each piston ring, and you can rotate the rings so the gaps are not aligned. The sealing on piston engines is extremely good.

3) The piston engine is pretty simple really, as far as sealing and lubrication: oil is either squirted (some engines) or splashed up on the backsides of the pistons and drawn up on the piston walls to lubricate the rings as the piston rises, and then the oil is almost 100% cleaned off by the rings as it travels on the downward stroke. This mechanism works so well that modern piston engines burn a truly negligible amount of oil. Oil burning has always been a problem on rotaries, and looking at that photo it's fairly obvious why.

Mazda (and others) has been working on rotaries with very smart engineers for literally decades, and they still haven't fixed these problems. I don't think they ever will; these problems seem pretty fundamental to the layout of a rotary engine. They had some real promise and advantages, but like many technologies, the small details made it impossible to beat incumbent technologies. Piston engines were bad too, ages ago, before they figured out a lot of small details like how to seal them well, how to optimize intake and exhaust valve design and timing, how to best design combustion chambers to maximize compression ratio and eliminate hot spots causing detonation, etc. But some problems just can't be overcome.

The other challenge vs reciprocating engines is that a piston engine cylinder has two volumes; the area above the piston where combustion occurs, and the area below, where there isn't any combustion. So during the stroke, you can spray the cylinder walls with lubricant, the bottom of piston, etc. So the seals can run on a layer of lubricant, and the piston can be cooled by the lubricant. In a rotary, you don't have that. Both sides of each seal are filled with fuel-air mixture, so to lubricate, you have to spray oil into the mixture. It's the same issue as with two strokes. They have to burn oil, and that is really tough on emissions, and it's really difficult to get heat away from the seal (the only place it can go is to the rotor).

That's an excellent point; I mentioned squirting oil before, but I overlooked that it serves another purpose in cooling the piston and the seals. Lots of modern engines now have oil squirters underneath the pistons to do this.

However, since, as you say, they have to burn oil to cool and lubricate the seals, that does make me wonder if this kind of engine wouldn't work much better with a lubricating fuel, such as diesel or kerosene (jet fuel), rather than gasoline. Kerosene is used in jet turbine engines largely because it is a lubricating fuel, so it serves the purpose of both fueling and oiling. The engine in TFA appears to be a rotary derivative, but they do say that it's designed to run on JP-8 jet fuel, not gasoline.

The sibling post to this covered much of it. As a previous RX-7 owner and racer, I can say the apex seals were prone to damage, especially if the engine was ever overheated. Much more so that overheating piston designs. The side seals didn't tend to cause as much problem.

And of course, with the Wankel, if any single seal fails, the entire engine needs to be torn down. With a piston engine, you can often pull the head to perform a valve job or re-ring a cylinder, without pull the main engine block. Not such a big deal on newer vehicles, but as cars aged, it tended to limit their value in the used car market.

"Where the rotor holds the seals in a normal Wankel, the housing does that job in the X1 engine."

I'm not sure how that will affect durability though.

While that's certainly true, others have developed similar small Wankel derivatives and had little luck moving beyond a prototype phase.

LiquidPiston themselves have developed other engines with no market success that I've seen.

There may be a market for these in small UAVs, but I don't see anything here to indicate LiquidPiston is any more likely to "win" than anybody else.

I suspect you are right, but this one does have a couple of distinctions working for it.

They've reversed the cylinder and chamber shapes, and added an air reservoir to the cylinder, giving it two combustion chambers. The seal problem is probably almost the same but the thermal gradient should be a little less pronounced, and the fuel charge looks like it has longer to burn fully.

If Mazda produced the RX Vision, I think many would forgive some loss of reliability. It's gorgeous, though rather close to the F Type.

This is potentially very interesting for medium sized drones that need 12+ hour endurance, as lithium ion/lipo batteries just don't have the energy density of gasoline... Even when you lose 50% to heat in a typical internal combustion engine the kilojoules per kilogram is so much greater in a liquid hydrocarbon fuel.

When I read it was 70 cc I was a little taken aback. In Italy there are a ton of people moving on scooters (mopeds) that are limited to 50 cc. It's true that they can't have more than 1.5 hp of power, but that's the law not the physical maximum for a piston engine.

A video with a slightly different design, including hollow cylinder head:

https://vimeo.com/163543761

Clickbait. It's a 4 pound engine that makes 3 horsepower. Impressive, but the article says the 40 pound engine makes 6.5 horsepower. Also, one costs $50 and the other probably $50,000. And no information given about the 4 pounder.

> LiquidPiston says that their X-mini is still in a testing phase, and they hope to get the weight down to 3 pounds and the power up to 5 hp. (The 40-pound piston engine produces about 6.5 hp.)

That's an unkind reading, for many applications a 5hp 3lb engine would replace a 6.5hp 40lb engine (not least because you put two 3lb engines at 5hp in for a total of 10hp at 6lbs..).

In any case it's a fascinating engine and the application for things like drones are pretty obvious.

I wonder how it compares to fuel cells on drones, in terms of weight of the engine and energy density of the fuel

The problem with fuel cells is power output. They are very efficient and reliable but they generate much less power than a mechanical engine of the same weight. Power/weight ratio is very important for aircraft engines.

You are right, power/weight ratio is important. I've seen news pop up in the last 12 months of fuel cell powered drones with several hours flight time (compared to under an hour with li-ion batteries) so I'm assuming they can push out enough power for a multirotor for decent lift without weighing too much.

Fixed wings should definitely see good gains, as it requires less power to fly than multirotors. I haven't followed it closely but 5-10x flight times vs li-ion sounds realistic considering the energy density of hydrogen. http://www.hes.sg/

Having a conditional "they hope to" makes that information worthless. I hope be to be a billionaire for sitting on my ass, it doesn't mean it's likely or that I'm even working towards it.

> but the article says the 40 pound engine makes 6.5 horsepower.

So? Every production car engine today weighs more than 40 pounds and makes more than 6.5 horsepower. That doesn't obviate the need to lighter engines, even if they are less powerful.

> Also, one costs $50 and the other probably $50,000.

Doubtful (and where are you getting $50k from?). You can see most of what goes into the engine in the video. Their advances seem to be mostly process and shape changes, not due to some space age material. It may cost $1000 in its current incarnation, but that's what you get when you manually machine all the parts instead of have them mass produced.

1. My point was that it wasn't a replacement. It's not even capable of getting half the power. Hence my claim that the title was clickbait, it's patently false. That's like saying I invented a replacement motor for the Bugatti Veyron at half the cost, oh by the way it makes 400 horsepower instead of 1000.

2. Again it's my demonstration of clickbait. A 40 pound engine that makes 6.5 horsepower will never be replaced by an engine featured in the article. At least, not in the next several decades. An engine that weighs 40 pounds and makes 6.5 horsepower is called a lawnmower engine. It's cheap to make and maintain and people use it because it's affordable. People aren't going to replace their $50 lawn mower engine with one that's $1000.

I'm not sure why you think replacing an engine has to entail having the exact same performance as before.

> That's like saying I invented a replacement motor for the Bugatti Veyron at half the cost, oh by the way it makes 400 horsepower instead of 1000.

You can. What's wrong with that. Maybe there's something else about the engine that makes it noteworthy, such as it being electric? In this case, it's a way to showcase specific aspects of the engine, it's size and weight.

> A 40 pound engine that makes 6.5 horsepower will never be replaced by an engine featured in the article.

Might it be replaced by two 4 pound engines with a total of 6 horsepower for 8 pounds? Or maybe three for 9 horsepower for 12 pounds? Perhaps the efficiency can scale and they can do a single motor for ~9 horsepower and ~12 pounds?

> It's cheap to make and maintain and people use it because it's affordable.

It's cheap because it's mass produced. You can't compare how useful this engine is to existing engines without considering the cost when built at the expected scale.

> People aren't going to replace their $50 lawn mower engine with one that's $1000.

I'll just repeat myself with inflection: "It may cost $1000 in its current incarnation, but that's what you get when you manually machine all the parts instead of have them mass produced."

Actually the current "development kit" starts at USD$30,000

http://liquidpiston.com/dev-kit/

That seems to include IP licensing as well, since it stipulates any new IP will be jointly owned.

It's sort of like licensing a new engine design from a major automaker (e.g. Ford) along with any proprietary technology they implement, with the agreement that any advances you make are jointly shared by you and the licensor. This might be a good deal for an F1 racing team or something, but doesn't really relate in any meaningful way to how much the auto company will charge for their engine to regular consumers.

I don't think they mean you get the IP. I think they mean they get the IP you generate while conducting research on the engine; though I seriously doubt much will be created with a clause that says you can't reverse-engineer it!

The purchaser agrees that any intellectual property that results of any development on the engine will belong jointly to LiquidPiston.

I read that as shared, as you can't jointly own something as one party. There are probably more explicit terms on something you have to sign.

I read it to say that they will jointly, with you, own new IP that you generate during research on the engine. That is to say, you own the IP from your own research by default, and the license says they will own it too.

Yeah, that's what I said (meant) in the beginning. I think we're just saying the same thing.

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