With recent advances in diesel engine technology, Nissan Motor Co in September released Japan's first diesel-powered passenger car that meets the latest domestic emission control standards, to be implemented in 2009.

Positioning it as a "clean diesel car", the industry, environment and transport ministries and industrial organizations in July decided on a plan to promote sales of such environmentally friendly diesel-engine vehicles.

The promotional measures include providing subsidies to buyers and encouraging government offices to introduce such cars for official use. But are diesel-powered cars really clean?