Rotary Engine Evolution - Tech

The four stages of combustion, starting with the intake charge entering the rotary housing via the intake port (light blue), the intake charge being compressed (dark blue), the compressed air/fuel mixture igniting with the aid of a spark plug (red) and the byproducts of combustion exiting the exhaust port (yellow).

The four stages of combustion, starting with the intake charge entering the rotary housing

Recognizing the limitations (and in particular the lack of low-end torque) of a single-rotor engine design, Mazda began investigating two-rotor, three-rotor and four-rotor designs while working on solving the apex seal vibration issue causing the chatter marks and the oil consumption problem caused by a leaky oil seal. Just three years after signing the licensing agreement with Wankel/NSU, their second two-rotor test engine, called the Type 3820 (2 x 491cc), was built. This engine evolved into the 10A mass-production two-rotor engine featured in the now famous and highly collectible '67 Mazda Cosmo Sport. Pumping out 110 hp, the 10A two-rotor engine was equipped with newly developed, high-strength, carbon-based apex seals that showed only slight wear and none of the dreaded chatter marks after 100,000 km of testing. It would seem that Mazda had solved the dreaded "nail marks of the Devil" and had also cured the oil consumption issue by developing a unique oil seal in conjunction with the Nippon Piston Ring Co. and Nippon Oil Seal Co.

How the Rotary Engine Works So that's how the Wankel rotary engine was born and eventually became synonymous with Mazda. But how exactly does a rotary engine work? There are some excellent videos on the Internet that illustrate how this unique piece of engineering comes together, but here's a quick overview to get you started.

At the heart of a rotary engine is a trochoid-shaped rotor (looks a bit like a bloated triangle) that spins on an eccentric shaft within an oblong or cocoon-shaped rotor housing. This design results in three spaces between the rotor and housing wall, creating the required chambers in which the four parts of the combustion process (intake, compression, ignition and exhaust) take place.

Part of the genius of the Wankel rotary design is the way the eccentric shaft interfaces with the rotor. There's an inner-toothed gear ring fixed on the inside of the rotor and an outer-toothed gear fixed on the eccentric shaft, with the turning speed between the rotor and shaft being 1:3. In other words, the rotor rotates once for every three revolutions of the eccentric output shaft.

What this means is that with the engine running at 9000 rpm, the rotor itself is only spinning at 3000 rpm. This allows the relatively small displacement of, say, the 13B two-rotor engine featured in FC and FD RX-7 (654cc per rotor for a total displacement of 1.3 liters) to produce very impressive peak horsepower figures. It's this rpm overdrive or multiplier effect from the way the rotor interfaces with the eccentric shaft that creates such tremendous volumetric efficiency from these very compact engines.

The output shaft multiplier effect is also part of what makes rotary engines feel so smooth and quiet compared to a traditional piston engine. During a single combustion cycle the eccentric output shaft of a rotary engine spins three times and the rotor itself completes just a single revolution of the rotor housing. Meanwhile, in a traditional piston engine the crankshaft (output shaft) makes two full revolutions to complete a single combustion cycle and each piston travels up and down its cylinder three times. The very high piston speeds that result from this, along with all the additional moving parts in the cylinder head(s), means there's a lot more noise and vibration produced by a piston engine as compared to a rotary engine.

The larger-diameter gear inside the rotor provides a three-times multiplier effect to the speed at which the eccentric output shaft spins.

The larger-diameter gear inside the rotor provides a three-times multiplier effect to the

The simplicity and efficiency of the rotary engine design is impossible to deny. It's an elegant and intriguing piece of engineering, and although Mazda has succeeded admirably in producing reliable and powerful rotary engines for the last 43 years, as modifiers and tuners the question becomes what can be done to improve upon the reliability and power of these amazing little engines? For that you'll have to tune in to Part 2 of this story in next month's issue, where Jim Mederer of Racing Beat chimes in with some very useful insights based on his over 30 years of rotary tuning experience.