Most trucks use chemical additives to break down tailpipe pollutants and meet the clean-air requirements of the Environmental Protection Agency -- but Navistar is taking an alternative route to compliance by developing engines that don't produce smog-causing pollutants. Hauling companies have yet to embrace Navistar's vehicles, but some industry watchers say that there are reasons to prefer the firm's simple, clean engines to conventional engines that require post-combustion pollution-busting systems.

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