One engine that I've come to greatly admire is the Wankel, a rotary engine that is relatively small in size, light in weight, has few moving parts (a two rotor Wankel has only six moving parts) and has a lot of power for its displacement (between 100 and 225 hp from1.3-liters).
Mazda was one of the first automakers to produce vehicles employing Wankel engines, with the first one I remember being the RX-2, followed by the RX-3, and then later by the RX-7 and RX-8 sports cars. Yamaha fielded a Wankel-powered motorcycle, the RE-5, from 1975 to 1976.
I've read about boats re-powered with a marine version of the venerable Mazda 13-B two-rotor Wankel. (The repowered boat had incredible power – about 225 hp, normally aspirated – in a small package.) I had hoped at one point to replace the bulky 4.3-liter V6 in my boat with one of those Wankels, but it was not to be – too expensive and the marine Wankels were one-offs.
As far as I know, no one is producing automotive Wankel engines any more, with Mazda ending the production run of its RX-8 a few years ago. But that hasn't stopped anyone from trying to improve upon the design.
The latest spin on this venerable engine design has been developed by an MIT start-up, Liquid Piston. Unlike its previous configurations, Liquid Piston has created what looks nothing more like an inverse Wankel. While the standard Wankel has a roughly triangular rotor contained in what looks like a pinch-waisted oval chamber, the inverse Wankel has an oval-shaped rotor in a triangular chamber.
LiquidPiston's 70-cubic-cm engine, the X Mini, produces about 3.5 hp at 10,000 RPM; at 4 lb, it's also about 30 percent smaller than the four-stroke, 50-cubic-cm piston ICEs it aims to replace. When fully complete and optimized, Shkolnik says, the X Mini could churn out about 5 hp at 15,000 revolutions per minute, and weigh 3 lb.I figure it can be scaled up even more to the point where it can power the same kind of vehicles standard Wankels have powered.
Initial applications will be handheld lawn and garden equipment, Shkolnik says. But the engine can be scaled and modified for other applications, including mopeds, drones, marine power equipment, robotics, range extenders, and auxiliary power units for boats, planes, and other vehicles. The company has also demonstrated proof-of-concept for high-efficiency diesel versions of the engine, including the 70-hp X1 and the 40-hp X2, for generator and other applications. The company hopes to eventually develop small diesel versions of the X Mini engine for military applications.
"If you look at a 3-kw military generator, it's a 270-lb gorilla that takes five people to move around," Shkolnik says. "You can imagine if we can make that into a 15-lb device, it's pretty revolutionary for them."