Greening the world’s transportation fleet will take a host of technologies, and they’re not all powered by electricity. There are plenty of VC-backed startups trying to improve the efficiency of the good old internal combustion engine.

One of them, EcoMotors, announced a $32.5 million Series C round on Tuesday to turbo-charge its efforts. Braemar Energy Ventures led the round as a new investor, and was joined by previous investors Khosla Ventures and Bill Gates. The new round brings the Allen Park, Mich.-based startup’s total investment to about $66 million.

EcoMotors has designed the OPOC engine, so named for its opposed-piston, opposed-cylinder architecture, which promises to be "15% to 50% more efficient, smaller, lighter, and less expensive" than today's diesel engines. The new funding will be aimed at building out the “next stage of development” of engines for existing customers Navistar and Chinese auto parts maker Zhongding Holding Group, the company announced Tuesday, though it didn’t give details on just what that next stage entailed.

EcoMotors is also developing a “multi-fuel family” of engines for both automotive and non-automotive markets. Diesel-fueled gensets for on-site power, gasoline-powered versions and liquid natural gas (LNG)-powered commercial vehicles were three examples cited in Tuesday’s release.

EcoMotors has a great deal of competition, and not just from the major automakers’ ongoing quests for better efficiency. Startup Achates Power is designing its own opposed-piston diesel engine that it says can improve fuel economy by up to 15 percent at a cost 10 percent to 15 percent lower than the standard. Pinnacle Engines is using regular gasoline for its own high-powered opposed-piston engine design, known as a Cleaves Cycle engine, and claims a potential 25 percent to 50 percent fuel efficiency improvement over today's models.

The opposed-piston concept actually dates back to World War II, but the mainstream transportation sector hasn't picked it up, largely due to concerns about reliability and ease of maintenance. Startups like EcoMotors, Achates and Pinnacle will have a long road to proving their technology can meet the standards required to bring it to mass market.

That isn't to say that the major automakers are relying on startups to make their ICE more efficient, either. Ford is working on its EcoBoost engine, a diesel-like gas engine that can boost fuel economy by 10 percent to 15 percent, and GM has a new diesel-fueled Chevy Cruze coming in 2013 that's supposed to get 50 miles per gallon. European and Asian automakers have more efficient diesels in development to meet emissions reductions mandates. In the United States, new fuel economy standards could push more diesels onto the road. (Diesels may invade from Canada, as well.)

Diesel, of course, is the fuel of choice for commercial fleets, and also powers about half of Europe’s personal cars. It could be a major fuel for new mass automotive markets in China, India and other developing economies as well. But EcoMotors stressed that it’s accelerating its gasoline-powered engines for cars and light trucks, as well as for hybrids and extended-range EVs -- a good example of how electric cars and more efficient internal combustion engines can cooperate, rather than compete, for new green markets.