The background: Transportation is second only to electricity generation as a contributor to global warming. While manufacturers have dramatically cut exhaust emissions from standard cars, they're only starting to do so for 18-wheelers and other commercial vehicles powered by diesel engines.

The solution: Westport Innovations has developed a conversion technology, called high-pressure direct injection (HPDI), that allows diesel engines to run on cleaner-burning liquid natural gas. Last year the EPA certified HPDI for commercial use.

The payoff: Starting this year, operators of 5,000 diesel trucks must convert to natural gas, and Westport, based in British Columbia, has the first-mover advantage.

The opportunity: Global diesel engine sales will hit $160 billion this year. Washington-based Energy Conversions is selling natural-gas retrofits for locomotive engines and power generators used in offshore drilling. And major diesel manufacturers are inventing new filters to meet tough new standards. Other companies, like California-based Clean Energy, are ramping up the infrastructure for LNG filling stations.