Ralph Sarich invented the orbital engine [0] almost 40 years ago, formed a company and spent millions on the idea but it went nowhere. Interestingly, they knew when to change direction and ended up making a number of breakthroughs in the field of two stroke engines. The company is still in business today [1]
That's not actually true. There have been an enormous variety of "impacts" from alternative engine designs. It's just that the engines that made the biggest "impacts" aren't "alternative" any more.
External-combustion engine: prior to 70 AD (aeolipile).
Internal-combustion rocket engine: 1200s.
Steam turbine (impact-turbine steam-jack): Taqi al-Din, 1600s.
Piston-vacuum steam engine: Newcomen, 1712.
Separate-condenser vacuum steam engine: Watt, 1765.
High-pressure steam engine: early 1800s, although maybe we should assign the credit to Hornblower, 1785.
Stirling engine: Stirling, 1816.
Otto engine: Otto, 1861 (or maybe some Italians in the 1850s.)
Two-stroke engine: Clerk, 1881.
Atkinson-cycle engine: Atkinson, 1882.
Rotary engine: Millet, 1889.
Diesel engine: Diesel, 1892.
Progressive-expansion turbine: Parsons, 1897.
Four-stroke diesel engine: Diesel, 1897.
Combustion-gas turbine: Elling, 1903.
Pulsejet (used in buzz-bombs): Karavodin, 1907.
Monosoupape, used in the Sopwith Camel: 1913.
Liquid-fueled rocket: Goddard, 1926.
Turbojet: Whittle and von Ohain, 1930s.
Ramjet: Leduc, 1936.
Turbofan: 1943.
Wankel engine: Wankel, 1954.
I've omitted minor changes like turbochargers, nozzle shapes, fuel injection, continuous-jerk camshafts, and so on; even though they're very practically important, I don't think they represent "alternative combustion engines". I've also omitted designs that haven't seen widespread use.
Now, from this, it looks like nothing has happened for 50 years. That is an illusion. The Atkinson engine only became significant in the last decade or two; Wankel's device only became significant in the 1970s; the ramjet didn't become significant until the 1950s; steam turbines went from a curiosity in the 1600s to the prime mover for much of the world's economy today, but it wasn't until Parsons' redesign that they really mattered.
Over the last 50 years, many new heat engine designs have been created: nutating-disc engines, thermoacoustic engines, the quasiturbine, scramjets, Stelzer's engine, the fluidyne engine, Sarich's orbital engine, the IRIS engine, and now this "wave disc generator". It's too early to tell which, if any, of these will turn out to be useful in the long run — at least as useful as the Wankel engine was in Mazda’s cars.
I guess I was specifically referring to car engines which, aside from the Wankel, are the same basic idea (pistons turning a crankshaft) that's been around forever. Yes, the technology has improved out of sight, and there's been a number of significant innovations like you've mentioned, but I've never seen anything outside the piston/crankshaft paradigm take hold.
About 15 years ago I was able to tour the Orbital Engine workshops and have a look at their stuff and chat to their engineers. At the time (mid 90's) they had developed a two stroke car engine they had running in a bmw. The engine made more power and torque, used about 20% less fuel and was about 30% lighter than the one it replaced, yet nothing ever came of it. [0]
I'd love to see this stuff happen, but I think the problem lies not with the technologies but rather that manufacturers are too conservative
[0] bloody hell, I've found one of the cars and it's on the market at the moment http://www.carsales.com.au/all-cars/private/details.aspx?seo...
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 obviously can still fail but I don't think it's possible to draw historical parallels.