Diesel Fuel for Marine Engines

I had a couple of enquiries asking me to talk a bit about Diesel Fuel as it pertains to marine usage so here it is.

Diesel fuel is described as any fuel used in diesel engines which can be ignited by the heat of compression pressure (without the use of a spark plug or other ignition source). The most common type is a petroleum distillate but there are other types such as biodiesel that are becoming increasingly popular.

Diesel fuel is classified by its Cetane Number. A higher cetane number indicates that the fuel ignites more readily when sprayed into hot compressed air and provides more heat energy (power) per unit of fuel. A higher Cetane number also means that the fuel a higher viscosity (thicker) and won’t flow as readily at low temperatures. In extreme conditions it can even gel or wax in the fuel lines and pumps. This is why, for cold weather operation a lower viscosity (thinner) diesel fuel is necessary.

In North America diesel fuel is offered in two grades. Number 2 Diesel is the higher viscosity stuff used in the warmer weather and is what we should all be using for marine applications in the summer. Number 1 Diesel is the thinner winter fuel grade that offsets gelling in the cold temperatures. When used in warm weather it won’t produce the same power or provide as good cold engine staring as the Number 2 diesel will.

Each year as the Fall season rolls around and the weather cools off here in the Northern Climates fuel dealers will begin to blend #1 and #2 fuels and as the weather cools even more eventually be offering straight #1 diesel for retail sale.

Fuel Classification           Cetane#

Thick #2                              46-50

Blend                                  41-45

Thin #1                              38-40

Jet A (Extra Thin). You guessed it, used in jet aircraft engines and sometimes sold as #1 diesel.

Farm Grade (Red, quality ?????)

When does this happen? It’s hard to say but it depend largely on your location but to be safe I would consider filling my tanks for winter storage at least by September 30. Leaving #1 diesel in your boat over the winter months will not be a problem because if does gel in the lines it will quickly turn to liquid again as the weather warms up in the spring.

The next aspect that we need to look at regarding diesel fuel is Sulphur Content. Sulphur is a by-product of the refining process used in the manufacture of diesel fuels and it can also act as a lubricant to many internal engine and fuel system parts. Unfortunately it is also a major source of the pollutants emitted by diesel engines. Since 2006 in North America the only diesel fuel sold at the retail level is classified as Ultra Low Sulphur Diesel Fuel (ULSD). Since many of our marine diesels were manufactured prior to 2006 they were not intended for low sulphur fuel and if used it can accelerate wear on fuel system and engine components. This is why a quality fuel additive is recommended. There are many of these on the market today and as such some re-search is required. The best recomendation that I can give is to stay away from any that use alcohol as an emulsifier.

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