a dual fuel engine system and method of operating the engine reduces emission rates of polutants . the dual fuel engine includes a fuel oil supply connected to the engine and a fuel gas supply connected to the engine . means connected between the fuel oil supply and the engine inject low levels of diesel fuel into the engine . in particular , the injection of diesel fuel is limited to pilot injection quantities only . the exhaust emission of nox is maintained at less than 1 gm / hp - hr , where natural gas is used as the primary fuel , and less than 0 . 5 gm / hp - hr where digester gas is used as the primary fuel .