> I guess I was specifically referring to car engines which, aside from the Wankel, are the same basic idea

Well, there have been a few commercially available cars driven by rotary engines (which are pistons being turned by a crankshaft, rather than turning a crankshaft) and cars driven by steam engines, but you're right that at the moment, only Otto-cylinder, Diesel, Atkinson, and Otto Wankel engines are on the market for cars, and all of these except for the Wankel are built around pistons turning a crankshaft.

But if you're talking about innovations on the level of more-efficient two-stroke engines, rather than totally new engine designs, I think you can find a lot of those that have made it to mass production in the last few decades. Atkinson-electric hybrids might be the biggest one, but there's electronic ignition control, gasoline direct injection, electronic fuel injection, coaxial springs to prevent valve dwell, continuous-jerk cams, hydraulic tappets, variable valve timing, etc.

That list certainly supports your contention that manufacturers are too conservative, though.

(I don't really know much about this stuff, so let me know if I'm making some kind of goofy mistake.)

It seems to me that in order to make manufacturers less conservative, we need to have more of them. Widespread CAD and simulation facilities and automated fabrication that lowers the cost of producing prototypes could make a big difference.

BTW, the SyTech is another experimental non-crankshaft design: http://www.autospeed.com/cms/A_0948/article.html

It's not easy to change from one engine to another radically different type. I would love for them to go down a path of better fuel economy, torque, weight, and horsepower. But you are looking at manufacturers trying to sell cars that need to be serviced down the line. service centers are not trained to handle X technology and the only place you could get it serviced would be the dealership. As a consumer I probably would not want to take the risk of an unknown longevity engine with unknown costs to fix it so I can save 20% on fuel; the engine design must be > 50%-75% less fuel(or some other reason that would make it a no brainer) used to make it worth taking a gamble on unknown quantities.