HFO in Diesel Generators Getting a diesel to run well on heavy fuel or bunker oil is no mean feat. Heavy Fuel Oil (HFO) has almost the consistency of treacle and in some parts of the world, has an excessively high sulphur content. Burning HFO is about as good for a diesel, as kerosene is for a petrol engine. A minimum operating temperature must be reached before the fuel will burn cleanly in the combustion chamber; for example, with the direct-reversing two-stroke Sulzer in the first ship I served on, we had to start the engine on distillate (diesel) then switch to HFO at about half normal operating revs. But when you're operating a heavy-duty commercial vessel, where the engine may be running 24 hours a day for weeks at a time during an ocean passage, the savings by using HFO are too good to ignore. This is where Yanmar's four-stroke direct-injection inline HFO diesels come in. Outputs range from 197hp (147kW) to 4437hp (3310kW). Yanmar supplies its own single-speed hydraulic reverse-reduction gearboxes with ratios from 2.08:1 to 4.15:1 depending on the model, but the 6N260, 6N280/8N280 and 6N330/8N330 series are available with reduction gears only if needed for use with controllable pitch propellers. Yanmar's HFO models are measured by their cylinder bore diameter and not piston displacement. For example, the smallest model (the S165) has a bore of 165mm, though its stroke is a massive 210mm. In ship-speak, smaller models in the range are regarded as high-speed diesels, while the larger models are medium-speed diesels.