The MAN site makes interesting reading on this. The dual fuel (the DF in the engine model) means it will run on both natural gas and fuel oil. The trick seems to be that natural gas won’t burn by itself - I assume the ignition temperature is too high for the diesel to get to - so at least 5% of the fuel burnt is still oil, and at less than 15% power, all of the fuel is oil. They call this a micro-pilot light. The neat trick is that the engine can have the mixture between the two changed dynamically and at load.
Diesels seem to have the best efficiency of pretty much anything - big ones like that get about 50% - which is pretty astonishing. Gas turbines can’t match that. But they can run on pure natural gas. Big steam turbine electricity station with massive cooling towers can get close to 50%, and you can run these on natural gas, but they don’t count as turbines running on methane.
MAN are very bullish about these engines, and are claiming that they expect that most ships will be fitted with LNG fuel systems in the mid term. The ubiquity, low price, lack of sulphur, and long term supply of LNG would make this a good bet it seems. The amount of recoverable methane in the ground is truly game changing. (Sell your oil and nuke shares now).
I follow everything except your comments about the fuels. NG has a much lower ignition temperature than diesel, and vice versa. An air/NG mixture is explosive, while a bucket of diesel can be hard to make catch fire. A lot of it has to do with volatility rather than ignition temp.
I’d suggest that the fuel variations address other issues about keeping the Diesel cycle running at its optimal point. (Diesels are interesting in that they are throttled almost entirely by the amount of fuel injected - air is not throttled or restricted. Inject a wee bit of fuel and the engine idles; inject more and the power curve rises. This is why truckers used to let their rigs idle; they were doing so essentially on fumes.)
Methane has an autoignition temperature of 580 C whilst diesel is 210 C. Fuel oils seem to range over 210 to 262 C.
It is remarkably counterintuitive. Just how a bucket of sludge can have such a low autoignition temperature is difficult to fathom. But there it is. (Then again we are actually talking about vapour, not a gas versus a liquid.)