A rotary Wankel engine consists of a triangular rotor spinning inside an epitrochoid housing. A liquid piston engine consists of an epitrochoid rotor spinning inside a triangular housing and this makes it better in every way.

So today we will take an in-depth look at this engine, we will analyze its benefits, its drawbacks, we will compare it with traditional piston engines and Wankel rotary engines to measure it’s potential to change everything.

If we observe the animation of the engine in operation we can observe that the Non-Wankel X engine which has a fundamentally different thermodynamic cycle, architecture and operation completes three combustion events for a single rotation of the rotor and does intake, compression, combustion and exhaust. Just like a Wankel engine.

Is this nonsense? It’s not and that’s because the inverted geometry of the x-engine enables it to overcome a major limiting factor of the Wankel engine. Kenichi Yamamoto is the father of Mazda’s Wankel engine, he is the man behind Mazda’s inspirational endeavor to make Wankel engines viable for mass production and in 1981 Mr. Yamamoto wrote a book called the Rotary engine. In this book he discusses and calculates the compression ratio for Wankel engine. And it turns out that the practical compression ratio limit for a Wankel engine is around 12:1. This limit is a consequence of the geometry of the Rotary Wankel and the resulting shape of the combustion chambers. This compression ratio limit also limits the maximum efficiency of the Wankel engine and it also makes a diesel Wankel rotary unfeasible. Yamamoto’s calculations stand as correct more than 40 years later because Mazda’s latest and only currently produced rotary engine which is used as a range extender in the MX30-REV has a compression ratio of 11.9:1

But the different geometry of the liquid piston engine means that it does not have a compression ratio limit which means that a diesel version is possible and that’s exactly what the liquid piston has done with their XTS-210 engine, which is a compression ignition version of their design.

But the unique geometry of liquid piston engines enables another benefit, which is the main source of the engine’s potential for improved efficiency. And that is a piston-less implementation of the Atkinson cycle, Liquid piston calls this a High Efficiency Hybrid Cycle, because obviously this sounds far more sexy for marketing and investor attracting purposes. But in reality it is a pistonless Atkinson cycle.

The entire premise of the Atkinson cycle is to have a greater expansion or combustion stroke and a smaller compression stroke. A compression stroke saps power whereas a combustion stroke generates power. So if we create a greater combustion or expansion stroke than we give the engine the possibility to extract as much energy from the combustion as possible which means reduced energy losses and improved efficiency.

Liquid piston engines have resolved the compression ratio limitation of rotary engines but they have not resolved the apex seals. The x-engine still has apex seals they have simply changed location. Instead of being in the rotor they are now in the housing. Liquid piston claims that this is a significant benefit because the seals no longer have to withstand centrifugal forces. According to a technical paper they wrote their models show a blowby reduction of 35% over a traditional Wankel however liquid piston believes that ultimately they can achieve around 65% blowby reduction compared to a Wankel. This simply is not enough for a truly widespread application in many different markets.

But the real problem with lubrication is with the crankshaft. Because air comes into the engine through the crankshaft it means that we cannot expose the crankshaft to a constant oil bath or even pressurized oil. Instead, as we can see from their how it’s made video, the engine uses sealed bearings instead of lubricated journal or ball bearings. In terms of longevity this is an inferior solution and this together with the apex seals is the reason why even the mature design of the engine is only expected to last 1000 hours between rebuilds.

So overall, this is no doubt a very clever design and I genuinely like the reverse Wankel idea of the Liquid Piston Rotary Engine. I’m also sympathetic of the fact that new engine designs need to claim very widespread potential applications to attract investors but outside of a few niche applications, where this engine will likely excel and offer genuine benefits, I personally don’t see a lot of potential for widespread use.

A special thank you to my patrons:
Daniel
Pepe
Brian Alvarez
Peter Della Flora
Dave Westwood
Joe C
Zwoa Meda Beda
Toma Marini
Cole Philips

#d4a
00:00 Working principle
03:06 Rotary Diesel
05:27 Pistonless Atkinson
08:07 Power density
09:53 Apex seals
11:11 Lubrication issues
14:08 Efficiency
15:52 Torque and VVT
17:18 You're in the Army Now

#d4a