Wonderfully wonky Wankels
By James Grahame
"I am mildly shocked to find nothing on the site on my favorite piece of oddball engineering to come out of the fabulous fifties; the Wankel Rotary engine! It took a few models and some creative hand-gesturing from my brother to explain the mechanics of it (and I am no slouch w/ spatial skills), but ever since those Wankels have a special place in my heart.
The last itineration of the RX-7 (and their wonderful Wankels) had a staggered sequential twin-turbo system that Mazda cooked up w/ Hitachi's automotive division. One turbo kicked on in the low RPM band, while the other gave boost in the higher ranges (somewhere around 3800 - 4000 rpm, if I remember right). Between the usual smoothness of the Wankel, Hitachi's changeover magic and the staggered turbos there was virtually none of the usual "turbo lag" that you usually get. It just delivered a smooth band of power."
Right then, Chris. We certainly didn't mean to disappoint you! I'll admit that my only exposure to Wankel powerplants was a well used early 1980s RX-7 owned by a friend. She was a cantankerous old beast (the car, that is).
Felix Wankel first introduced the KKM 57 Wankel Rotary Engine in
1957. It featured a single rotor instead of the reciprocating pistons
found in traditional automobile engines. The rotor is slightly offset
in the engine housing, causing each side of the rotor to move closer
and farther from the wall as it rotates. This serves to compress and
expand the fuel-air mixture in the combustion chamber, similar to the
combustion and exhaust strokes in a piston engine (to see it in action,
check out Matt Keveney's Animated Wankel page).
The design has several advantages. First, a Wankel engine is significantly more powerful than an piston engine of a similar size. Secondly, the rotary design is free to run faster, since there is less friction. A third important benefit is that it greatly reduces the number of moving parts: there are no valves, connecting rods or balanced crankshafts to be found in a Wankel powerplant.
By now you must be wondering why Wankels aren't everywhere. Sadly, they have a couple of significant drawbacks: they're not
as efficient as their piston-powered counterparts and they suffer
from partial combustion that increases their hydrocarbon emissions. It goes without saying that a (relatively) gas-guzzling, pollution spewing beast isn't going to make friends these days.
Mazda was the most famous manufacturer to include Wankel rotaries in their vehicles, starting with the 1967 Cosmo. The company produced a small number of rotary models, but eventually phased out the powerplant in everything but their long-lived RX-7 sports car (1978-2002) and its modern successor, the RX-8. Incidentally, the RX-8 develops an impressive 247 hp from its tiny 1.3 Liter RENESIS rotary.