MDO vs. No. 2 DO

[marineprojectgroup]

I was sailing on a VLCC a few weeks ago, the Syrian 2/E stated he filled the emergency generator tank using the MDO tank. It is my understanding that MDO is not compatible with small engines example Em generator and lifeboat engines as it will cause the pistons to seize due lack of lubrication. No. 2 DO is the correct fuel for these small engines. Perhaps I am wrong, can anyone referree this one ? Thank you.

[Big Pete]

Hi "Marineprojectgroup", welcome to the site.

I can see several problems with running small engines on MDO rather than "Derv" or auto grade Diesel oil.

However, in this context, the emergency generator on a VLCC may be larger than the main engine on a coaster!!! (or even the main generators on a medium size ship) so you have to consider the maker's fuel spec for the particular engine.

MDO is usually supplied in the USA as a "straight cut" distillate fuel, elsewhere it is a blend of Heavy Fuel Oil and Gas oil to achieve the required specification.
Either way, it is denser and more viscous than No. 2 D.O.

In general I can see the follwing problems:

The higher viscosity will affect atomisation and probably cause less good combustion and may have a slightly longer ignition delay.

The higher density will make it possible to overload the engine. The stops on the fuel rack are adjusted to give the maximum permitted volume of the specified fuel. If denser fuel is used, the volume of fuel will remain the same but the mass will increase and so will the energy value.

The dirtier and more viscous fuel will tend to block the filters more quickly.

Fuel for lifeboats on worldwide trading trading ships, should be "winter grade" to avoid waxing up in cold climates. I have never heard of "Winter Grade" MDO.
In the case of the emergency D/A it is normally in an enclosed compartment and fitted with electric heaters so this may not be such a problem there.

There are more impurities in the fuel, so the "Blow Past" into the crankcase will be dirtier, shortening L.O. life. It is also likely to have a higher Sulphur content.

I am not sure about lubrication, usually on Diesel Engines the fuel has nothing to do with this unless poor combustion causes the L.O. to be burnt off the cylinder walls.
Maybe some one else can help you on that.

I hope you get plenty of different answers.

Big Pete.

[TxMarEng]

Most regulatory bodies require both the emergency diesel generator and lifeboat tanks to be filled with MGO grade DMA. While this fuel does have a flashpoint lower than 60 DegC it does have a lower pour point maing it suitable for use in lower temperatures. I would defiitely not use MDO in either application particularly grade DMC.