I think Wankel engines are really fascinating, first off. The whole concept of how they work is perhaps a bit baffling, because they are completely different than a normal piston-based engine. Instead of reciprocating pistons, Wankels use a triangular rotor (or Spinning Dorito as I like to think of it) that moves eccentrically around an elliptical wall — refer to the wiki page for further detail. See below.
As you can see, the engine’s operation is vastly different from a “normal” one, and as we can expect should sound quite different too. The first thing to notice is that for every one revolution of the rotor, there are 3 complete intake/combustion/exhaust cycles happening. However, if you follow the eccentric shaft (“B” in the above photo) you can see that its eccentric motion is 3 revolutions per 1 revolution of the rotor. Therefore when thinking of cylinder fires per output RPM, we are back at a 1 fire per rpm. In a normal one-cylinder 4-stroke engine, by comparison, there is only 1/2 of the cycle per revolution of the crankshaft. So sonically, right away this means we have doubled the root tone of the engine.
The other sonic consideration here is that every revolution is a cylinder fire, meaning the sound output should be even and constant compared to that from a piston engine. It is because of this that rotaries are often described as sounding like 2-stroke engines (like what a weed whacker or some snowmobiles use).
A second thing to consider is that there are far less other moving parts in a Wankel engine. For example, the Wankel has no valves, no valve train, no camshafts and no connecting rods nor pistons. As a result there should be far less “engine noise” by comparison. Here’s a really great example of valvetrain sounds — listen to the whitenoise-y clicking:
A third thing to consider is that rotaries typically allow higher rev ranges than that of all but the highest-end small-displacement piston engines. It is not uncommon for redline to be at 7500 or 8000rpm, with some going all the way to 10,000 and beyond. They’re able to achieve this because the engines have fewer moving parts and fewer inefficiencies as a system.
Typically speaking, rotors can be added together (much like a multiple-cylinder piston engine) to create more power. You can refer to the number of rotors as a 1- 2- 3- or 4-rotor Wankel to be precise. Let’s listen to a few.
Single rotor (these are typically small, used for lawn mowers and etc)
Two-rotor (here we get into car-sized power plants, mostly from Mazda’s RX series)
Three-rotor (Mazda has been racing this in Grand Am and a handful of other GT series for a while now)
Mazda’s engine code for their 3-rotor design is “20b”
(this is from the Belgian Touring Car Series — some beautiful cars in the field there, Seat, Skoda, VW, Volvo, BMW, Audi, Mazda… simply awesome.)
Four-rotor (Mazda R26 or R26b)
And I even found this sample for you of a 6-rotor, the first of its kind I believe.
One thing that all of these videos show is that, especially engines that have been modified, the car sounds like it lopes or surges at idle fairly heavily. I’m researching into why this happens more, but from what I understand it has to do with running lean at low rpm, creating a non-fire on a semiregular interval. Hopefully I can add to this in the near future.