Starting in 2010, owners of diesel-powered cars and trucks may have to fill a supplementary tank with urea, an organic compound that fights nitrogen oxide emissions when it's injected into a vehicle's exhaust system, The Washington Post reports. In anticipation of vehicle makers adopting an emissions-reduction system that depends on urea to meet tight diesel pollution-control rules, the Environmental Protection Agency issued guidelines on March 27 telling manufacturers how to earn certification for the new engines. The agency wants to ensure that urea is easily available and that systems will be designed to force owners to keep tanks full. Companies must design a system that would meet Clean Air Act rules by 2010 calling for the virtual elimination of nitrogen oxides and compel owners to maintain emission-control systems. According to The Washington Post, the EPA cautioned that the systems must be designed so they can't be disabled, tampered with or filled with something other than the proper concentration of urea.