The Wankel rotary engine has always been one of the most fascinating alternatives to the traditional piston engine. It is compact, smooth, lightweight, and capable of producing impressive power for its size. That combination made it famous in Mazda sports cars and useful in certain UAV applications, where power-to-weight ratio matters.
But the Wankel also has well-known weaknesses. Sealing, lubrication, emissions, oil consumption, and long-term durability have always been part of the rotary engine conversation. In a recent Motor Oil Geek video, Lake Speed Jr. visits LiquidPiston to look at a very different approach: what happens when the rotary engine concept is effectively turned inside out?
The result is LiquidPiston’s X-Engine, an “inverted rotary” design that keeps some of the rotary engine’s best traits while attempting to solve some of the tribological problems that limited the traditional Wankel.
Why Tribology Is the Real Story
Tribology is the study of interacting surfaces in relative motion, including friction, wear, and lubrication. STLE defines the field around those three core subjects: friction, wear, and lubrication, with lubrication used to minimize friction and wear.
That makes engines one of the best real-world examples of tribology in action. Every internal combustion engine depends on controlling friction, maintaining oil films, reducing wear, removing heat, and keeping contamination under control. Whether the engine is a conventional piston engine, a Wankel rotary, or LiquidPiston’s inverted rotary, the same basic questions apply: where are the sliding surfaces, how are they loaded, how are they lubricated, and how well are they sealed?
Lake opens the video by reminding viewers that in a reciprocating piston engine, one of the biggest sources of friction is the piston ring rubbing against the cylinder wall. Near top dead center, the ring sees high combustion load while its sliding speed is low. That combination of high load and low speed is a difficult lubrication condition and a major source of friction and wear.
The Wankel rotary was one attempt to avoid some of the drawbacks of reciprocating motion. Instead of pistons moving up and down, the rotor turns smoothly in one direction. That gives the rotary engine excellent smoothness and power density. But changing the geometry does not eliminate tribology problems. It simply changes them.
The Wankel Rotary’s Strengths and Weaknesses
The traditional Wankel rotary uses a roughly triangular rotor moving inside a peanut-shaped, trochoid housing. The design is compact, smooth, responsive, and capable of making a lot of power from a small package. That is why rotary engines earned such a loyal following in performance cars and why they have been attractive for UAVs and other applications where size and weight matter.
The problem is that the Wankel’s sealing and lubrication requirements are challenging. The apex seals move around the housing at high speed, and the combustion chamber is long, thin, and constantly moving. That geometry makes it harder to achieve high compression, direct fuel injection, consistent combustion, clean emissions, and ideal seal lubrication.
In the video, LiquidPiston co-founder Alec Shkolnik explains that Wankel engines have struggled with fuel burning, sealing, cooling, lubrication, emissions, efficiency, and durability. Lake points out that those problems are tribological in nature because they involve surfaces, seals, oil films, deposits, friction, and wear.
A traditional Wankel often lubricates the apex seals by injecting or metering oil into the combustion chamber. That oil is then burned, at least partially. This creates emissions challenges and can also contribute to deposits. Lake adds an important oil-related point: some synthetic oils, especially uniform PAO-based synthetics, can form harder varnish-like deposits when burned, which may be damaging to rotary seals. This is one reason many traditional rotary owners have historically preferred conventional oils.
That does not mean synthetic oil is “bad” in all engines. It means the lubrication environment matters. An oil that works well in one engine design may not be ideal in another if that engine intentionally burns oil as part of its seal lubrication strategy. Same goes with assumption that as long as you use a modern oil that you will be protected.
LiquidPiston’s Inverted Rotary Concept
LiquidPiston’s X-Engine changes the rotary layout. Instead of a triangular rotor inside a peanut-shaped housing, it uses a peanut-shaped rotor inside a three-lobed housing. In the video, Shkolnik describes it as the “un-Wankel” or “inverted rotary.”
That geometric inversion matters because it changes the engine’s combustion and sealing behavior. LiquidPiston says its X-Engine platform is intended to deliver high power density, lower vibration, lower noise, and reduced size and weight compared with conventional engines, with a focus on military, aerospace, mobility, and drone applications.
The company’s XTS-210 variant is a 25-horsepower, 210cc, two-stroke, supercharged, liquid-cooled X-Engine under development. LiquidPiston describes it as heavy-fuel compatible with diesel, Jet A, JP-8, and kerosene, as well as multi-fuel capable with fuels such as gasoline, propane, and hydrogen. The company also states that the XTS-210 is intended to provide much higher power density than piston diesel engines.
For military and UAV applications, that is a big deal. If a small engine can run on heavy fuels like JP-8 or Jet A while remaining compact and lightweight, it can simplify fuel logistics and improve power density in applications where every pound matters.
Stationary Combustion Chamber, Better Combustion Potential
One of the most important differences between the Wankel and LiquidPiston design is the combustion chamber. In a Wankel, the combustion chamber is long, narrow, and moving. That creates challenges for compression ratio, direct injection, flame travel, and complete combustion.
LiquidPiston’s inverted design allows for a more stationary combustion chamber. In the video, Shkolnik explains that this makes it more suitable for high compression and direct injection into a stationary target. Lake notes that a more controllable combustion chamber can improve air-fuel mixing and reduce the kind of incomplete combustion that creates contamination.
That point matters for lubrication. Poor combustion does not just hurt power and emissions. It also contaminates the oil. Fuel dilution, soot, partially burned hydrocarbons, water, and combustion byproducts can all shorten oil life and increase wear risk.
Oil Life Comes Down to Heat and Contamination
One of the best takeaways from Lake’s video is his explanation of oil degradation. Motor oil typically degrades through two primary pathways: temperature and contamination. He explains that for roughly every 10°C, or 18°F, increase in operating temperature, the oxidation rate doubles and oil life is effectively cut in half. Contamination is the other major factor, including blowby, soot, water, dirt, and other materials that enter the lubricant. This is a great takeaway - when choosing the right engine oil, you need to take into consideration the application and operating environment.
This is where LiquidPiston’s design becomes especially interesting. Lake says that in a 50-hour test, the engine oil showed essentially no change from the baseline in oxidation, viscosity, or wear metals. He attributes that to effective sealing, controlled temperature, and limited contamination reaching the crankcase and bearing oil.
That is a powerful lesson for any engine owner, not just someone interested in rotary engines. Oil life is not only about mileage. It is about operating temperature, fuel dilution, contamination, oxidation, and wear. Two engines can run the same oil for the same number of hours and produce very different oil analysis results depending on how cleanly they burn fuel, how well they seal, and how hot the oil runs.
Why Seal Location Changes the Lubrication Problem
A traditional Wankel has apex seals moving at high speed around the housing. Those seals are hard to lubricate, and because oil is typically introduced into the combustion chamber, some of it burns. That contributes to hydrocarbon emissions and deposit formation.
LiquidPiston still has sealing challenges, but the geometry changes where those challenges occur. In the X-Engine, the apex seals are stationary in the housing, while other sealing elements, including side seals, still require lubrication. Shkolnik explains that because the seals are stationary, it should be easier to deliver oil exactly where it is needed.
That is the tribology lesson: lubrication is not just about choosing an oil. It is about getting the right oil, in the right amount, to the right surface, at the right time, under the right load and temperature. Geometry can make that easier or harder.
In the Wankel, the moving seal geometry makes lubrication difficult and contributes to oil consumption and emissions. In LiquidPiston’s design, the goal is to keep the rotary engine’s compactness and smoothness while making combustion, sealing, and lubrication more manageable.
Why This Matters Even If You Do Not Own a Rotary
This video is not about a Porsche engine, but it is still relevant to anyone interested in Porsche engine durability, oil selection, used oil analysis, and proper lubrication. The same principles apply to flat-sixes, air-cooled engines, water-cooled M96/M97 engines, GT engines, race engines, and modern direct-injected engines.
The lesson is that engine design and oil choice cannot be separated. Oil is not just a generic fluid poured into the engine. It is part of the mechanical system. It must handle heat, contamination, fuel dilution, blowby, sliding friction, boundary lubrication, oxidation, deposit control, and wear protection.
In a Porsche engine, those concerns may show up as bore scoring risk, fuel dilution from short trips, timing chain wear, high oil temperatures, lifter noise, camshaft wear, ring seal issues, or deposit formation. In a rotary engine, they may show up as apex seal wear, oil consumption, hydrocarbon emissions, and varnish deposits. The hardware is different, but the tribology questions are familiar.
Used Oil Analysis Is How You Verify What Is Happening
One of the most important points in the video is that Lake does not just talk about oil condition in theory. He discusses measured oil data from testing. The claim that the LiquidPiston oil remained stable over 50 hours is based on oil analysis indicators such as oxidation, viscosity, and wear metals.
That is exactly why used oil analysis is valuable. It helps separate assumptions from evidence. Instead of guessing whether an oil is holding up, testing can show whether the oil is oxidizing, thinning, thickening, accumulating fuel, showing elevated wear metals, or becoming contaminated.
For Porsche owners, oil analysis can be especially useful because many problems develop gradually. A single report can provide a snapshot, but trending reports over time are even more useful. Changes in iron, aluminum, copper, fuel dilution, viscosity, insolubles, or oxidation may reveal changes in operating conditions or mechanical health before a major symptom appears.
Oil Choice Still Has to Match the Application
The video also reinforces an important point about oil selection. The right oil depends on the engine’s design, operating environment, and lubrication strategy.
Lake’s discussion of conventional versus synthetic oil in Wankel engines is a good example. In many piston engines, a high-quality synthetic oil may offer better oxidation resistance, cold-start performance, and high-temperature stability. But in a traditional rotary where some oil is intentionally burned to lubricate seals, deposit characteristics become extremely important. An oil that burns into hard deposits can create problems in that specific application.
That is why blanket statements about oil are risky. “Synthetic is always better” or “conventional is always safer” oversimplifies the issue. A better question is: what does this engine need the oil to do?
For Porsche applications, that means considering engine family, bearing clearances, oil temperature, catalyst compatibility, fuel dilution, track use, bore material, ring package, age, and known failure modes. For a rotary, it means considering seal lubrication, deposit formation, combustion contamination, and whether oil is being burned as part of normal operation.
The Bigger Takeaway
LiquidPiston’s X-Engine is interesting because it is not just another rotary engine. It is an attempt to rethink the geometry that created many of the Wankel’s tribological problems in the first place. By turning the layout inside out, the design seeks to preserve rotary advantages such as compact size, low vibration, high power density, and rapid response while improving combustion, sealing, lubrication, emissions, and oil life.
Whether the X-Engine becomes widely adopted remains to be seen. But as a teaching tool, it is excellent. It shows that lubrication problems are often design problems. It shows that oil life depends heavily on contamination control. It shows that seal geometry affects emissions and deposits. It shows why used oil analysis matters. And it shows why tribology is central to every internal combustion engine, from a tiny UAV powerplant to a Porsche flat-six.
For enthusiasts, the most useful lesson is simple: oil is not just maintenance. Oil is part of the engineering. Choose it carefully, test it when possible, and remember that every engine’s lubrication needs are shaped by its design.
