Emulsions Usually Ok, But Equipment Questions Remain

Diesel Fuel News, August 28, 2000 by Jack Peckham

Parts -- Diesel/water emulsions are catching on in the commercial emissions-reduction market, but issues such as impacts on exhaust aftertreatment devices and fuel injection equipment are only beginning to be resolved.

Example: The water in diesel emulsions can lower exhaust temperatures by 10 to 20[degrees]C, possibly below the "balance point" at which a catalyzed soot filter continuously regenerates. Given the soot-plugging sensitivity of PM traps used with certain "colder" engines in low-temperature operations (such as old, two-stroke diesel engines on slow-speed city transit bus routes), this might limit emulsion markets.

However, French refiner Elf (now part of TotalFina) reported at the Society of Automotive Engineers international fuels & lubes conference here (SAE 2000-01-1861) that its Aquazole emulsion (10-13% water in diesel) could perform well. Elf tested a Euro-2 Volvo bus fitted with the Johnson-Matthey (JM) "CRT" soot filter on the standard Millbrook-London Transport Bus (MLTB) test cycle.

"The combination of an emulsified fuel and particle filtration trap gives a particle reduction of over 90%; this result demonstrates that reduction of exhaust gas temperature due to the emulsion is compatible with CRT efficiency," the Elf researchers said.

Still, PM reduction efficiency of a water/ultra-low-sulfur diesel (ULSD of 30 ppm sulfur) emulsion with the PM trap was slightly inferior to ULSD without water emulsion (with the same PM trap). Elf isn't sure why the emulsion wasn't as good on PM reduction, although it's possible that the average 10[degrees]C decrease in exhaust temperature caused by the emulsion may have slightly decreased the PM reduction efficiency of the CRT, Elf researcher Pierre Schmelzle speculated.

During an ADEME-RATP (Paris) urban cycle test, the average catalyst inlet temperature decrease was 7[degrees]C, rather than 10[degrees]C, when Aquazole replaced ordinary diesel fuel. In another test with Aquazole, a Dennis Dart bus fitted with a pre-Euro 1 Cummins B5 engine and an oxidation catalyst produced a 57% reduction in PM and a 73% reduction in smoke compared to the same engine operating on non-emulsified diesel fuel.

Depending upon engine type and test conditions, Aquazole produced a wide variety of impacts on carbon monoxide (CO) and HC emissions -- from -20 to 20%. But these emissions are already very low, and in all cases, combined NOx HC emissions levels were reduced. Even fuel economy showed a slight improvement when consumption of the diesel portion of the emulsion is compared to the non-emulsified fuel, Elf found.

Meantime, biocides are also required for emulsions to avoid fuel filter plugging by microbial growth, as Elf discovered to unpleasant surprise last year. But lubricity hasn't been a problem because the emulsion surfactant also overcomes lubricity problems that might be expected with water emulsions, Schmelzle said.

Meanwhile, Lubrizol's "Purinox P10" 10% water-diesel emulsion "has not led to any decrease in [exhaust] catalyst efficiency," Millbrook Proving Ground investigator Andy Eastlake reported in another emulsion paper (SAE 2000-01-1915) here.

In tests of Purinox emulsion on an Olympian bus with a Euro-2 Volvo B10A engine, Millbrook ran both "urban" and "suburban" duty cycles to find the impact on regulated emissions, non-methane organic gases (NMOG) and formaldehyde.

Compared to ordinary ULSD fuel and oxidation catalyst, the ULSD emulsion reduced NOx and PM further, the tests showed.

In tests with a non-catalyzed PM trap, the emulsions produced lower PM emissions during the "urban" cycle, compared to non-emulsified fuel with PM trap. But the emulsion with PM trap produced slightly higher PM emissions in the "suburban" cycle compared to the non-emulsified fuel with PM trap.

Still, the small differences in total PM mass emissions with the two fuels on the PM trap might be within normal experimental test accuracy, Eastlake reported.

Note: Millbrook never affempted to regenerate this PM trap, so these tests didn't show whether the lower exhaust temperature caused by the Purinox emulsion would have thrown off the "balance point" for continuous trap regeneration. Still, the 10[degrees]C exhaust temperature drop found with Purinox P10 was about the same as the temperature decrease found in Aquazole tests on a bus with CRT PM trap.

While Millbrook/Purinox test measurements didn't focus on the possible impact of emulsions on formation of the smallest ([less than]30 um) "nano-particles," researchers intend to look into that. "The test work showed significant reductions in particle number in all size bands measured, from 0.03 to 10 microns. Further work is anticipated to verify this result in even smaller particle sizes," Eastlake said.

Meantime, worries that emulsions might boost aldehyde or non-methane organic gas (NMOG) emissions were disproved. Instead, the Purinox emulsion itself cut NMOG a further 29% compared to non-emulsified fuels (using comparable exhaust after-treatment), and formaldehyde dropped 22% just from the emulsion, or 58% when combined with an oxi-cat.