Properties of diesel fuel can affect performance

Jeffery W. Janka
Posted on October 1, 2000

The diesel engines powering the vast majority of school buses are highly efficient, reliable sources of mechanical power. But their longevity and performance are affected by many factors — including fuel quality. This article will look at the characteristics of diesel fuel and provide information on how to choose the best fuel for your fleet. Let’s start with the basics. There are two grades of on-road diesel fuel as defined by the American Society for Testing and Materials (ASTM) D 975.

1-D: For on and off-road use in motor vehicles. It is a lighter grade fuel blended for low-temperature operating conditions. (Lower cloud point, see below)

2-D: For on and off-road use in motor vehicles, industrial engines and off-road machines operating in normal temperature conditions. This fuel has higher heat content and will provide better fuel economy than 1-D fuel. The main point to remember is that all fuels are not “created equal” and may not be appropriate for use in your engine or application. It’s important to understand the following diesel fuel properties and how they affect performance.

API gravity — The ratio of the density of fuel to the density of water. The lower the API gravity, the heavier the fuel and the harder it is to burn. Conversely, the higher the API gravity, the lighter the fuel and the poorer the fuel mileage. The API gravity can be measured with a special fuel hydrometer. A good API gravity would be 34 to 36 (measured at 60 degrees F).

Cloud point — The temperature at which hydrocarbons in the fuel cannot be dissolved and form wax crystals. This crystallization causes the fuel to cloud or haze. The crystals could clog filters and fuel lines. A good rule of thumb is to purchase fuel that has a cloud point at least 10 degrees below the lowest temperature you anticipate during the run of the fuel.

Pour point — The temperature at which diesel fuel begins to solidify or gel. The pour point is normally lower than the cloud point. To be on the safe side, fuel should have a pour point 20 to 25 degrees F below the lowest expected temperature.

Cetane index or rating — The quality of diesel fuel ignition; how easily it ignites. In 1993 the EPA mandated a minimum 40 cetane index rating based on distillation temperatures and density. A minimum rating of 45 is best for performance. A higher number means shorter time between spray and burn. Refiners use additives to meet the minimum index, and supplementary additives can be added to attain the suggested rating of 45.

Other considerations
Fuel quality depends on proper storage. Stored fuel forms corrosives and varnishes as it breaks down. Each load of fuel has specific conditions that can affect its compatibility with fuel already in storage. Condensation forms in storage containers as temperatures change; this can be corrected with additives containing methyl carbitol or butyl cellusolve. Alcohol should not be used as it reduces fuel lubricity. Fuel/water separators should be installed on vehicles and serviced regularly where water contamination is possible. Finally, remember that nothing prevents fuel-related problems better than a good preventive maintenance program. Quality filters go along with quality fuel. However, do not rely on filters to clean up a below-grade fuel system. Fuel tanks, pumps and hoses should all be constantly monitored and inspected. Fuel pumped into vehicles should be filtered at the island.

Jeffrey W. Janka is senior mechanic for the transportation department at Cleveland Municipal School District.

Related Topics: preventive maintenance

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