Claiming it now has the largest green fleet in the nation, the city of San Francisco this week completed a yearlong project to convert its entire array of diesel vehicles — from ambulances to street sweepers — to biodiesel, a clean-burning and renewable fuel that holds promise for helping to reduce greenhouse gases.

Using virgin soy oil bought from producers in the Midwest, officials said that as of Friday, all of the city’s 1,500 diesel vehicles were powered with the environmentally friendlier fuel, intended to sharply reduce toxic diesel exhaust linked to a higher risk of asthma and premature death.

 “Just like secondhand smoke, diesel is one of the worst things we can breathe,” said the city’s clean vehicle manager, Vandana Bali of the Department of the Environment.

The announcement came without fanfare from Mayor Gavin Newsom’s office late Thursday, even as Congressional lawmakers dickered over the particulars of an energy bill that would give automakers incentives to produce cars that burn biofuels.

Ms. Bali said the city’s diesel vehicles now all used a fuel known as B20, a mix of 20 percent soy-based biofuel and 80 percent petroleum diesel fuel, which reduces toxic emissions of carbon monoxide, hydrocarbons and other pollutants that lead to global warming.

A spokesman for the mayor, Nathan Ballard, said the goal was to cut such emissions to 20 percent below 1990 levels by 2012.

In November, Mr. Newsom announced a new project called SFGreasecycle, a program to collect fats and cooking oils from restaurants, at no charge.