Regarding Espee ' dual - fuel' units, a paragraph from Joe Strapac's ' Southern Pacific Historic Diesels Volume 7 - GP - 9 Locomotives ':
" Dual fuel was successful, at least in the sense that it did indeed reduce fuel costs. Bunker - C oil delivered more BTU's per gallon, so in theory the railroad was getting free horsepower by using cheap oil. On the other hand, engine maintenance costs zoomed; Bunker - C burned hotter than diesel, and it tended to clog fuel injectors and other delicate parts. It turned virtually solid if allowed to cool, so engineers were instructed to run the engine on diesel fuel for the last 20 - 30 minutes before shutdown. "
Mention is also made of the hot water piping required in the Bunker - C fuel tank, to keep the stuff hot & liquid enough to actually flow properly. Lotsa money spent to save a little; another clever idea that didn't work out.