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Comment by ChuckMcM | Hacker News Reader

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.

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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.

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