Excessive cetane additization could harm USLD stability

Diesel Fuel News, Nov 24, 2003 by Jack Peckham

Pittsburgh -- Putting very large doses of cetane improver (such as 20,000 parts per million of 2EHN) into ultra-low sulfur diesel (ULSD) could cause fuel instability and filter plugging, a research project at Cummins found.

However, "it's not an issue with normal dosing" of octane improver, as now-retired Cummins researcher Dave Stehouwer explained to us in an interview following his presentation of a paper (SAE 2003-01-3140) at Society of Automotive Engineers powertrain conference here.

The main point of the SAE paper is to alert refiners, pipelines and marketers to the possibility of harmful interactions caused by certain fuel additives. Example: Dimer-acids used as pipeline corrosion inhibitors or as lubricity improver can react with certain lube additives to plug fuel filters (see Diesel Fuel News 11/10/03, p9). Fortunately, no-harm alternative lubricity improvers are available.

Likewise, going way overboard on octane improver dosing--either from accident or ignorance--could lead to fuel system problems, the research paper shows.

Tests involved ordinary 400-ppm sulfur fuel, a commercial ULSD fuel (BP's "ECD,") a research ULSD fuel (from Phillips) and a 20% blend of biodiesel in a 300-ppm sulfur non-road commercial diesel fuel. All were additized with 20,000 ppm of 2EHN to accelerate instability for the test.

~Biodiesel The "Least Stable" Among Fuels

The "least stable sample is the B20 biodiesel even [though] it contains a substantial amount of sulfur," the researchers found. "We conclude that the instability of B20 biodiesel blend with off-highway diesel fuel comes from an acid-catalyzed hydrolysis of the ester components from the biodiesel."

Nor are problems likely to arise solely from the deliberate mixing of used oil with fuel, as with the Cummins "Centinel" recycling system. While that system results in about a 3,000-ppm dose of lube oil in fuel, the newer high-pressure fuel injection systems also result in about 1,000-ppm of lube in fuel, Stehouwer explains.

A separate test of cetane improver in 20% biodiesel blend (B20) found even worse instability problems, the researchers found. This is of special concern because modern high-pressure injection systems circulate a lot of fuel through hot injectors (as a heat sink) for return to storage tank.

That's why engine makers are pressing for a tough thermal stability limit in American Society for Testing & Materials (ASTM) D975 diesel fuel standard, as well as similarly tough thermal stability limits for biodiesel blends.

With diesel fuel, careful fuel blending and proper additization could avoid stability problems, Stehouwer points out. Otherwise, thermally-unstable fuels could form gums that could plug filters and cause injectors to stick, resulting in costly repairs, he said.