The Environmental Protection Agency (EPA) has sanctioned diesel engines from four major automakers, determining that they pass its strict 2007 emissions regulations.

Toyota Motor Corp. and Volkswagen AG were among those to successfully meet the 2007 standards.

However, while the emissions systems on all four engines have passed tests using European low-sulphur diesel, they still need to pass durability tests to ensure performance does not begin to wilt, with ten years or 150,000 miles required before final certification is granted for the engines to be sold in the US.

Fuel-efficient diesels are a leading technology to help automakers meet rising fuel-economy standards, especially for light trucks, which must average 22.2 mpg by 2007, up from 20.7 mpg today.

Regulatory pressure has seen the priority for diesel shift away from technological development, placing a greater emphasis on price and emissions.

Car manufacturers at the Society of Automotive Engineers, such as Graham Hoare, Ford's director of powertrain research and advanced engineering, said tackling costs would be the big 'leap' for the industry.

Walter McManus, head of global powertrain forecasting for J.D.Power and Associates, agreed that negotiating such factors could open up potential in the US diesel market, estimating that diesels could attain 12 per cent of US sales by 2012.

However, Lindsay Brooke of CSM Worldwide was more cautious, warning that there was 'no guarantee' customers would switch to diesel cars, citing barriers of cost, manufacturing capacity and image.