Developing a gasoline-powered rotary engine was a dream for Felix Wankel, and we mean that literally. Some steam engines worked on the same basic principle as far back as the 18th century, but the concept for using gasoline came to Wankel while he was sleeping. And he built it despite not having any formal training, albeit with a little help from his fellow Nazis — official World War II Nazis, not some CEO sued for pushing neo-Nazi conspiracy theories.
That said, there were plenty of real-world difficulties with Wankel's dream motor, including the way the rotor's apex seals vibrated and quickly wore down. Mixing oil and gasoline in a rotary engine helped address the problem, because the oil reduced the friction between the rotor apex and rotor housing. So, automakers turned to oil-metering systems to inject oil into the gasoline. However, these had issues as well. As a result — and depending on a few important factors — you do have to premix gasoline and oil in some rotary engines.
Before we discuss specifics, though, let's take a quick refresher on how a rotary engine works. It's basically a triangle (the rotor) inside an oval housing. As the rotor spins, only its corners (apexes) touch the housing, creating a seal. The sides of the triangle perform the jobs of pistons by pulling in gas and air, allowing combustion to take place.
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After World War II — and after a short stint in an Allied Powers' prison – Wankel teamed up with the German automaker NSU in 1951. This partnership led to the first car with a rotary engine, the 1964 NSU Wankel-Spider. The auto industry took note. Mazda became one of the first Wankel licensees and the only automaker to ever mass produce rotary-engine cars — although Wankel, Citroën, and a few others did sell some, and the Soviets made rotary-powered Ladas for the KGB.
Yet even Mazda couldn't solve the apex-seal issue completely. This was especially true for Mazdas with the electronically-controlled oil-metering system first found on the 1989 RX-7. At the time, Mazda was struggling to make sure the engine had enough oil to remain lubricated but not so much that it exceeded emissions guidelines. Engineers erred on the side of emissions reduction by cutting the amount of oil injected by 75% compared to 1988. The same concerns affected the 2004 RX-8.
For models like those, premixing gas and oil can be a great way to extend engine life. More generally speaking, you should premix if you're preparing for competition. Even the 20 minutes or so you spend autocrossing can create enough heat and friction to damage apex seals without the extra lubrication. The bottom line is that rotary cars with electric oil-metering systems, those with oil-metering system deletes, or vehicles that are going to be put under any kind of extended stress can benefit from premixing gas and oil.
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First off, here's what you shouldn't use: traditional engine oil. It doesn't actually mix that well with gasoline and tends to separate from it in the fuel tank –- not a great feature if you want a consistent supply of oil protecting your apex seals. You can use two-stroke motor oil, but you have to make sure it's the high-quality synthetic stuff with JASO ratings — from the Japanese Automotive Standards Organization – of FC or FD, which indicate it's specifically formulated for high-temperature, high-performance applications.
Experts warn against using two-stroke motor oils intended for lawn equipment. These oils can contain a dye for identification purposes, and that dye can stain the plastic parts of your car. It's better to stick with products engineered specifically for rotary engines, from brands like Idemitsu, Klotz, and Motul.
As for how much oil to premix with the gasoline, the team at Rotary Performance has done the math for us. They determined the amount of oil a typical RX-7 or RX-8 consumes over time, allowing them to calculate how much oil by volume needs to be added. In the case of your average street car, that means about 4 ounces of oil per 10 gallons of gas. That said, Mazda will certainly figure out the oil-consumption issue for the rotary-powered Mazda RX-7 successor – well, hopefully anyway.
