Winter is here, which means the “gelling” of diesel fuel is once again a concern for Stuart Porter, quality control coordinator for Suncor Energy Products, a Candadian refinery that makes a variety of products from oil, including gasoline and diesel fuel. Porter is concerned about the cold weather performance of diesel fuel blends, and in particular what the trucking industry knows about cold-flow fuel analysis.
“When fuel gets cold, the heavier components solidify the same way water freezes into ice,” he explains. “This solid component, commonly called paraffin wax, precipitates out of the liquid fuel and adheres onto cold surfaces in the engine or fuel line. This is particularly harmful for fuel filters as the waxes can block the small openings.”
Porter contends that while most fleets understand the effects of cold weather on diesel fuel, they may be confused about the best way to prevent gelling.
To help fleets prevent cold-flow problems before they start, Porter is trying to help clarify the information available to the industry. He is chairman of the task group for cloud-, pour- and freeze-point testing for the American Society for Testing and Materials (ASTM).
Understanding the cold flow has taken on even greater importance because diesel fuel is undergoing a fundamental change in 2006, when federal regulations mandate the production of a low-sulfur product. In addition, the sulfur content of fuel will be lowered to 15 ppm, quite a change from the 350- to 500- ppm fuels in use today.
“Fleets have to realize that the composition of the diesel fuel they use depends on the oil feedstock and the blending process used to produce it,” he explains. Also, diesel fuel is blended to meet particular cloud point specifications that are tied to the temperatures in different regions of North America. That's why diesel fuel purchased in the southern U.S. may run into gelling problems when used in regions to the far north.
While there are many fuel additives on the market today designed to reduce the gelling of diesel at cold temperatures, Porter says it's important for fleets to be aware of the cold flow threshold temperature for the brand of diesel they use. This will enable them to know exactly when they need to take steps to prevent their fuel from gelling.
There are several tests available for measuring the cold flow threshold of diesel. According to Porter, the one used by refineries is the cloud point (CP) of a fuel, defined by ASTM as the temperature at which waxes appear. In short, above its CP, a fuel will not form waxes that lead to gelling. Below the CP, components within a fuel will begin to form wax crystals, which could lead to engine problems. Porter feels that the fuel's CP is a fail-safe limit if you want to avoid gelling.
The pour point (PP) is the lowest temperature at which a fuel will move or pour, he explains. However, it's not a very practical measure because it can be affected by meshes and filters in the fuel system.
Other measures are called filter tests, including the cold filter plugging point (CFPP) and low temperature filterability test (LTFT), which use wire meshes in the testing. These meshes represent the fuel filters used in today's truck engines. The CFPP and LTFT are the lowest temperature at which a fixed amount of fuel successfully passes through the mesh within a pre-determined amount of time.
But back to the CP test. According to Porter, this is considered the primary cold flow indicator for diesel fuel. Industry research confirmed that the CP test correctly predicted the minimum operating temperature of test vehicles 98% of the time. By contrast, LTFT measurements correctly predicted cold flow 89% of the time, and CFPP 64% of the time.
“On a purely technical basis, we produce and blend fuel to the CP spec,” he says. “That is the diesel cold-flow fail-safe point.”
Each month this column looks at emerging truck technology issues through the eyes of a leading engineer.
Name: Stuart Porter, Quality Control Coordinator, Suncor Energy
Background: Degree in Chemistry from the University of Western Ontario, Canada, 1982. Joined the Sunoco refining division of Sun Oil in 1982 as a laboratory technician. Served as production scheduler for Suncor Energy, the renamed Canadian refinery subsidiary of Sunoco, from 1990 to 1992. Worked as a refinery chemist from 1993 to 1997, when he was appointed quality control coordinator.
