Despite recently declining gasoline prices, efforts to produce more fuel-efficient cars and trucks are continuing — as they should.

Last month, an encouraging announcement came from Honda Motor Co. whose engineers have developed a super-clean diesel engine that will be available in some Honda cars in the United States by 2009. Other carmakers are no doubt working on similar projects.

Honda's announcement is important because diesel engines offer an average of about 30 percent better fuel economy over gasoline-powered engines of a similar size. That fuel-efficiency advantage is a big part of the reason that nearly half of all new cars sold in Europe, where fuel costs an equivalent of $4 a gallon or more, are powered by diesel engines.

Honda's new diesel engine is reported to be as clean burning as a comparable gasoline-powered engine, and Honda says that the new diesel engine will meet California's emission standards, the toughest in the world.

Nine states, including Pennsylvania, New York and New Jersey, have adopted California's tough emission standards, which are more stringent than federal standards. With those nine states representing about a third of all U.S. drivers, automakers are highly motivated to comply with California's tough standards.

To accomplish its clean-diesel advance, Honda's scientists looked more to chemistry than engineering. The key to the new engine is a catalytic converter technology that produces ammonia, which turns nitrogen oxide from the car's exhaust into harmless nitrogen.

In addition to getting better mileage, diesel engines also produce about 30 percent less carbon dioxide than a comparable gasoline engine. So, advanced diesel engines such as the one to be produced by Honda will help reduce greenhouse gases linked to global warming as cars powered by the clean-diesel engines begin to replace gasoline-powered cars on highways in the United States and around the world.

Today's clean diesel engines still exhibit the traditional high-torque performance that will satisfy all but the most demanding drivers.

And as the world's largest manufacturer of engines, Honda's clean-engine advances could have a truly global impact. Volkswagen, which has been selling diesel-powered cars in the United States in recent years, is retooling its system to achieve super-clean emission results similar to Honda's. No doubt, DaimlerChrysler, Ford and General Motors are also looking to clean diesel engines as part of a program to raise the fuel efficiency of their cars while also reducing harmful emissions. It is also possible that Honda might license its latest diesel technology to other carmakers.

With clean-diesel engines finally coming to the United States, there is added incentive for a rapid expansion of the domestic biodiesel industry to help reduce the amount of oil being imported into the country to make fuel. Such a move will also help American farmers by creating a new market for soybeans.

There is a similar agricultural push involving corn and ethanol, but biodiesel has a considerable fuel efficiency advantage over ethanol.

Last month's announcement by Honda will no doubt be followed by others. Competition among the world's automakers to produce cleaner-burning and more fuel-efficient cars and trucks is important in helping the United States become more energy self-sufficient while reducing the impact American drivers have on the environment.