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The national average price of diesel fuel was unchanged Monday from a week ago, holding at $4.692 a gallon, the Department of Energy said.

WASHINGTON — Several prototype Class 8 hybrid trucks are currently under development, but the technical and economic obstacles in the way of placing reliable and fuel-efficient heavy hybrid over-the-road trucks in service remain daunting, experts told a House subcommittee.

The U.S. diesel average inched downward for the second consecutive week, dipping 1.5 cents to $4.692 a gallon, while gasoline topped the $4 mark for the first time ever, the Department of Energy reported.

Caterpillar Inc., which said Thursday it has signed an agreement to develop a heavy-duty truck with Navistar International Corp., said it would not develop and sell engines that meet 2010 emissions standards to other truck original equipment manufacturers.

U.S. economic activity remained generally weak in late April and May, and some trucking companies reported having difficulty passing on high fuel costs to their customers, the Federal Reserve said Wednesday.

I joined the company where I work about three months ago, and one of my top-priority jobs has been fuel usage. We had to look at our idling time because all our trucks are sleepers and the drivers run in 48 states.

Police cleared striking truckers who were protesting high fuel prices from the Spanish-French border, and two striking drivers were killed in separate incidents in Spain and Portugal, the Associated Press reported Wednesday.

The International Energy Agency lowered its forecast for global oil demand this year, while thousands of European truckers protested high fuel prices, the Associated Press reported Tuesday.