The Norton post about rotary-engined motorcycles got me thinking about motorcycle engines in general. There haven’t been a lot of rotary motorcycle engines–or car engines for that matter. Frankly, despite the valiant efforts of Mazda, the fact is that the Wankel engine has problems.
The seals at the three apexes of the rotor are always problematic. The seal area is relatively small and prone to breakdown, and hence, leakage. Heat differentials are a problem, too, in that the engine casing receives a big differential in heating. Lots of engineering is required to make it reliable, and give it acceptably long engine life. The heat from the exhaust will bake you as well. And efficiency has always been a problem with Wankels. They just tend to use more fuel. For instance, that old Norton commander only got about 30-35 MPG, which is pretty low for a motorcycle. They also have high emissions due to incomplete fuel burning, which of course, contributes to the engine’s poor fuel efficiency.
But the advantages are really great, especially for motorcycles.
Vibration with a rotary engine is almost completely eliminated, because there are no reciprocating parts like a conventional engine. You don’t have to convert the up and down motion of the cylinders to a revolving crankshaft. That eliminates a huge number of moving parts, and, hence, weight. Manufacturers have tried to reduce this vibration with counterbalancers and the like, but, of course, while they do a decent job, they add even more weight. Conversely, the Wankel engine really has only one moving part, the rotor, and it’s already moving in a circle with the crankshaft. Rotary engines are, therefore, far more mechanically efficient. Conventional engines only have about a 30% efficiency rating. Much of the engine’s power is lost in converting reciprocating movement to revolving movement and whatnot. A Wankel engine’s mechanical efficiency approaches 80%. The Wankel engine also has instant power delivery, since the stroke movement of a conventional engine is eliminated. And, of course, when something catastrophic goes wrong in a Wankel engine, it just tends to slow down to a stop. A conventional engine tends to destroy itself and break large numbers of internal parts.
Still, until recently, the higher cost and lower fuel efficiency of the Wankel engine has made them a non-starter as a standard engine. In recent years, Mazda has addressed the fuel efficiency problems with the RENESIS engine for the RX-8. And even independent manufacturers have come up with more efficient designs that can meet modern emissions standards. For instance, Freedom Motors is a US company that has been working on more fuel-efficient and emmissions-compliant wankels. They produce a 1300cc twin rotor engine that weighs 120 pounds, and puts out 120HP.
Still, the Wankel is comparitavely expensive, and less fuel efficient.
So, how do you get a rotary engine that keeps the advantages of the Wankel engine, while eliminating the disadvatages? Is it even possible?
Well, it may be.
A new concept called the DART engine might just do it.
The DART engine has three moving parts. It has two differently-sized rotors, and a sliding connector.
What makes it such a great concept is that, it uses the Atkinson cycle, rather than the Otto cycle for combustion, so there’s a power stroke in every revolution of the engine. Every revolution also scavenges exhast gas, mixes it with intake air, and reburns it, resultion in more complete oxidation, meaning higher fuel efficiency and lower emissions. The sealing problem of the Wankel is also eliminated since a significant portion of each rotor edge seals to the engine casing, as opposed to the small apex seals of the wankel rotor.
This is a very exciting concept and promises to deliver extremely high power output with excellent fuel efficiency, low emissions, instant power delivery, and no vibration from reciprocation. In addition, it can use gas, diesel, ethanol, or any other kind of internal combustion fuel. There’s even a hydrogen burning variant called the HART concept.
And you get the same power output of a conventional engine, with half of the physical size and weight.
You can learn more about the DART engine here.
This is a very exciting engine concept.