Rotary Spark Plugs

I was looking at a photo of a Mazda Cosmo engine the other day, and was surprised to see two distributors and six plug wires. Having never owned a rotary, I assumed they had two (two rotors, two plugs) or none (like a diesel) or they used some other sparking method like friction or something odd.

So I did some Googling, and it seems that each rotor needs two spark plugs. Why? A two-rotor rotary should have one wire into the dist. from the coil, and 4 going out then, correct? But the Cosmo had two distributors, each with 3 outgoing wires. A two rotor 13B has one dist. with 3 outgoing plug wires. Huh? And a 4-rotor has 6:

2-Rotor Cosmo:

2-Rotor 13B:

4 Rotor:

So I am lost now. How does a rotary engine's ignition system work?

You need two plugs because of the shape of the chamber. It's long, and with just one plug the flame wouldn't propagate quickly enough.

I'm not sure why Mazda elected to run separate distributors (and usually coils) for the leading and trailing plugs, though. My guess is that they either fire simultaneously or quick enough in succession that one coil wouldn't be able to recharge quickly enough to fire both properly.

I seem to remember that the trailing plugs only fire under certain conditions, like either only over a certain rpm or under a certain rpm.

Trailing plugs fire a few degrees after the lead plugs for a not-insignificant increase in fuel efficiency.

I think you're miscounting the number of plug wires.

Early 10a and 12a engines had twin distributors until they worked out how to utilize one distributor reliably.

But you still had twin coils.

2 plugs per rotor, no matter the number of rotors (leading and trailing).

Cosmo's had 2-rotor? Thought they had the 20B 3-rotor.

But actually 3 wires in a distributor does make sense. 1 input and 2 output. 1 distibutor for upper plugs and 1 distibutor for lower plugs.

wlkelley3 wrote: Cosmo's had 2-rotor? Thought they had the 20B 3-rotor.

Old Cosmo had 2 rotor, newer Cosmo 3 rotor.

In reply to pilotbraden:

Ahh!! Thanks!!

The Eunos Cosmo could be had with either a 13B or a 20B. They were all automatic though.

IIRC the twin distributor / twin coil was to allow the coils to "recharge" at higher RPM's.

and on that subject... I think the coils were the main limiting factor in the FB's 7k redline... but then again that could just be rumors.

You CAN buy housings with 3-plugs though (like the 787b!)