No, it is a single 12-cylinder engine. Design-wise it is basically two of VW's VR6 engines mated at the crankshaft, but it is a single engine with I believe a single block.
Rotaries are a little strange in that you get basically twice the work out of each rotation as there is a full cycle each rotation, so the 4-rotor 787B was in some ways similar to an 8-cylinder piston engine (with the nominal displacement doubled for rule purposes, just as the 1.3 liter Wankel in the RX-8 is often treated as a 2.6 liter engine). But it, too, is a single engine with all four rotors turning a common crankshaft.
It's stranger than that, even! There are
three ignition events per rotation of the
rotor. The rotor is mounted on an eccentric shaft, kept in sync by
epicyclic gears whose ratio of teeth is 3:2. Epicyclic means that the gears traverse around each other (as opposed to being stationary, e.g. in a gearbox).
The smaller gears in a rotary engine are fixed in the housings. Because one gear is stationary, the effective ratio is now a function of the eccentricity of the gears, which is equivalent to the difference in diameter. It turns out that for a gearset with ratio of 3:2, having the inner (smaller) gear stationary results in the larger gear
orbiting the smaller one at a rate of 3 times its actual rotation. I still haven't got my head around a mathematical proof for this.
The purpose of these gears, remember, is to maintain the synchronisation between the rotor and eccentric shaft, so their eccentricity must be the same. Therefore, the eccentric shaft, which is also the output shaft, has the same 3:1 ratio as the epicyclic gears, since the "orbit" of the rotor becomes the rotation of the eccentric shaft. See
here for a nice animation, which really is better than the thousand words I'd need to properly explain it!
In short, the output shaft rotates at three times the rate of the rotor. There are three ignition events per rotation of the rotor; hence one per rotation of the output shaft. This is twice as many as there are for an ordinary, "four-stroke" piston engine, or the same as a "two-stroke" piston engine.
Since an ignition event represents the start / end point for the cycling of the working volume in an engine, and hence the extraction of power from it, the fact that two strokes and rotaries "use" their volumes twice as frequently as four strokes (with respect to the output shaft), their displacements are often doubled for regulatory purposes.
Heh, off topic... sorry.
, it has 4 Rotors.
)
