Hot off the heels of the Supra revitalization, the web is filled with the next possible RX Mazda sports car. Fanboys won’t be happy though, because what everyone wants is a ‘new’ ’93 RX-7. They want a gorgeous low slung lightweight body with two seats, a turbocharged rotary engine, 6-speed manual gearbox, locking differential and traditional Mazda chassis and suspension calibration. Sounds perfect, doesn’t it? It was. Unfortunately, the most important part of that formula to the fanboys is that turbocharged rotary engine.
The rotary engine hasn’t been used in a production car since the RX-8 went out of production nearly 10 years ago and that one was garbage. A turbocharged rotary hasn’t been produced in almost 20 years as the FD was discontinued in Japan in 2002. A rotary engine in the form that we know it won’t be able to meet modern emissions standards, not by a long shot. There are a few deadly flaws in the Wankel design that hinder its ability to burn fuel cleanly. First is the basic shape of its combustion chamber and the rotor itself. Because the rotor comes to a point, it can only have one surface that touches the housing, the dreaded apex seal.
In a piston engine, the pistons use 3 seals against the cylinder walls called piston rings. The top ring is for sealing combustion pressure into the combustion chamber. The second ring is shaped to scrape the oil makes it up the piston skirt back down again. And the bottom ring is there to provide oil control. The rotary as mentioned earlier only gets one ‘ring’, and it better be holding combustion pressures in the chamber or this engine won’t be powering anything. Yes, Wankels use oil seals and such also, but nothing to specifically keep oil from the combustion chamber. In fact, rotaries use an oil injector to inject oil into the rotor housing for lubrication. Burning of this oil creates lots of nasty byproducts that must be filtered before they can leave the tailpipe. Catalytic converters can only deal with this burned oil for a short time before they become clogged up and stop working properly. A much more precisely controlled lubrication system would have to be devised and implemented before the rotary could be considered to power the RX-9.
The next hurdle is timing control, or lack thereof. One way modern piston engines broaden powerbands and meet emissions requirements is electronic control over when the intake valves open and close. Some OEMs even have control over valve lift, or how far and long valves stay open. The rotary uses no valves. Again, like a 2 stroke piston engine, there are ports milled into the side of the rotor housing. When the rotor passes over these ports it creates a vacuum to pull intake air in, or creates positive pressure to push exhaust gas out. A beautiful simplicity, but once the ports are milled into the housings, that’s where they stay. There’s currently no way of keeping ports open longer or shorter depending on engine speed or torque demand from the driver. This would be the equivalent of variable valve timing for a rotary.