Kind of weird article that doesn't talk about either the main benefits or design hurdles. The advantage of the LiquidPiston design is that at the time of ignition, the combustion chamber is a roughly round shape, as opposed to a traditional rotary where it's a long crescent. This should theoretically improve the emissions and fuel economy to be more like piston engines.
I don't think Liquid Piston was talking about a 2-stroke version before. That's news to me, and it seems like it would throw out the efficiency/emissions benefits in exchange for more power. Maybe the author of the article was confused and this isn't actually a 2-stroke design?
The main design hurdle has been longevity. Their prototypes in the past only work for a short time and wear out. That's probably just a matter of doing the necessary R&D to figure out the best alloys to make the engine out, and the best ways to lubricate everything. It's not clear from the article if this new version has this solved so it can last a normal amount of time or not.
This is likely all part of a pump and dump. That's why it's a weird article.
They've been advertising this heavily of all places on instagram and taboola chumboxes. Obviously none of those people need this engine, they're looking for marks.
2 stroke has poor unburned emissions because of mix flow-through when ports are open. If air charge / purge is completed before fuel is injected then that doesn't occur; whether that's diesel or something else is irrelevant.
I thought 2-strokes had poor emissions was the injection of lubrication oil into the air fuel mixture. traditional 2-strokes I'm familiar with pull in air into the crank case (which requires lubrication), then uses a portion of the exhaust stroke to compress the fuel/air creating forced induction. By keeping air out of the crank case and providing lubrication delivery outside of the air/fuel mixture, emissions can be significantly improved. The article claims the design allows external lubrication, but it doesn't say how. I'd like to see how the seals are better than apex seals.
The basic crankcase-scavenged two-stroke suffers from both problems. Typical diesel two-strokes use a separate blower for scavenging, so the combustion air inflow never passes through the crankcase and does not come in contact with the lubricating oil, while the fuel is not injected until after the exhaust is closed.
The engine described here can have its shaft and eccentric bearings externally lubricated, but its apex seals, like those of Wankel engines, seem to be in a situation analogous to that of a piston ring. Unlike either of those, however, they are mounted in the stator, not the moving part, and so may be easier to lubricate precisely.
My 1960s cafe racer (years ago!) had oil injection. Some engines do have oil injection and just inject it into the inlet port, but in this engine it was delivered directly to the crank bearings. From there it was flung all over and ultimately burned. Anyway, as the saying goes on the lost continent of Perl: "There's More Than One Way To Do It".
All the numbers in the article were really small and seemed to aim at use cases that are all about small and reliable. Is anything stopping this from being scaled-up to be a light but very powerful engine for a sports or supercar? This could be a huge innovation in racing if it scaled.
The four strokes of a four stroke cylinder are intake, compression, combustion, and exhaust. That's down, up, down, up. Since there's no down or up in any of this engine's combustion chambers, "strokes" is probably inappropriate.
The strokes have nothing to do with up and down (e.g. flat motors, like a boxer), but to do with compression and expansion. The direction of compression doesn't really matter aside from packaging concerns.
As you said, 4 stroke has intake, compression, combustion, and exhaust. That's expansion, compression, expansion, compression. It fires between compression and combustion.
A 2 stroke motor has compression and expansion. It fires between compression and expansion. Intake, exhaust, and combustion happen during the expansion phase.
A rotary motor is traditionally 4 stroke, and I'm not sure a 2 stroke version is feasible. The LiquidPiston rotary appears to be a 4 stroke to me, despite the article text.
Typical Otto cycle 4-stroke piston engines do two rotations for a single cycle. Wankels are more complicated; each rotor face acts as a separate combustion chamber and all four strokes happen in a single rotation. It looks like the situation is similar with LiquidPiston's implementation.
I don't think Liquid Piston was talking about a 2-stroke version before. That's news to me, and it seems like it would throw out the efficiency/emissions benefits in exchange for more power. Maybe the author of the article was confused and this isn't actually a 2-stroke design?
The main design hurdle has been longevity. Their prototypes in the past only work for a short time and wear out. That's probably just a matter of doing the necessary R&D to figure out the best alloys to make the engine out, and the best ways to lubricate everything. It's not clear from the article if this new version has this solved so it can last a normal amount of time or not.