Cummins Inc. put its stamp of approval on clean fuels by offering warranty coverage for engines filled up with 20% biodiesel. The Columbus, Ind.-based engine maker says it now covers highway truck engines manufactured from 2002 onward and new off-road engines meeting federal “Tier-3” emissions. The National Biodiesel Board calls the move “a major milestone.” Traditionally, manufacturers were conservative in approving biodiesel due to variances in regional blends, problems in older engines and lack of testing. But national standards for biodiesel have allowed engine makers to expand coverage.