Two stroke engines are favorites of everyone who wants a really compact engine with lots of power, there are fewer parts and power delivery on every stroke. Of course, the blue haze from the exhaust didn't sit well with the enviro-police so they've pretty much disappeared from use in transportation, but, they are slipping back with new technology applied to the new designs.
EcoMotors has a design called OPOC, Opposed Piston Opposed Cylinder. The design is just what it says, as you can see above, with 2 cylinders and 4 pistons. Peter Hofbauer came up with the idea when he was at Volkswagen, the outboard pistons take the place of the cylinder head and each piston only travels half the distance necessary for the full stroke, allowing higher engine speeds. An electrically controlled turbocharger can be used when necessary with adjustable boost, it can even spin up before the engine starts to give instant boost.
The engine can be built in a modular fashion, allowing more cylinders to be added in pairs with a clutch between the modules so one pair can be used at start or light load with the clutch adding more cylinders when required.
Emissions are said to be very low, meeting EPA standards for both gasoline and diesel configurations.
They're looking for 100 mpg in an automotive application, so just think what you could do with something like this in a motorcycle. Very cool!
(I actually thought I wrote about this engine before but found nothing, probably one of those engines I meant to post that got lost in my
stack of post it notes carefully organized filing system)
Additional Update: Several comments are saying this is not a new design. If you followed the link to EcoMotors I originally included, you would see the designer acknowledges that and refers to a WWII German aero engine from Junkers as one example.
Kim Scholer, comments below he sent me an article about an early NSU motorcycle powered by an engine like this. The article is completely in Danish, so I can't tell you anything else about it. The image is below:
The motorcycle is shown here but I can't quite figure how the engine is oriented. Perhaps Kim will be able to fill us in.
The point here is not that this design is entirely new, but, as I said in the first paragraph of the post, that technology is allowing us to utilize old concepts in new ways with low emissions and high power output. Computerized injection, electric turbochargers, clutches to gang modules together, all of these update the old idea into, essentially, a new engine based on what is obviously an older engine design. I think that's pretty neat.