Emissions and Performance Implications of Biodiesel Use in an SCR-equipped Caterpillar C6.6

Tier 4 Final legislation commences from 2008 - 2015, depending on engine power. At the same time the use of biodiesel is being incentivized or mandated in many countries. This is driving up the proportion of biodiesel available to the diesel engine fleet and so it is important to understand its impact on possible Tier 4 Final engine and aftertreatment systems. One of the solutions being explored to meet Tier 4 Final emissions regulations is selective catalytic reduction (SCR) using urea and an appropriate catalyst.

Previous researchers have highlighted the potential for biodiesel to have a much greater impact on percentage increase in tailpipe NO_x on engines equipped with Urea SCR aftertreatment than has historically been the case for engine-out NO_x increase. Increases of as much as 80% have been presented, but without knowledge of the engine-out or absolute NO_x emission data, it has not been possible to draw any conclusions from some of these publications.

As a result, a Cat 6.6l, common rail, DI engine fitted with a urea SCR, vanadium-based catalyst system and DOC was operated on B20, B100 and ULSD. The engine was equipped with NO_x sensors at engine-out and SCR-out. The engine was also fitted with piezo cylinder pressure sensors and standard temperature and pressure sensors. The test cell had Horiba MEXA 7000 series benches at both engine- and SCR-out and CDS6 Siemens laser analyzer at SCR-out to monitor ammonia slip.

The engine was run on each fuel over the European non-road transient cycle (NRTC). Each set of tests on a biodiesel blend was book-ended with a ULSD test to track any engine degradation, which was not observed.

It was found that B20 caused engine-out NO_x to increase by less than 1% while the SCR-out NO_x increased by 12.5% for an absolute increase of 3 ppm. For B100 the increase in NO_x at engine-out was 4.8%, which is at the low-end of expectations based on historical trends. The SCR-out NO_x increase was 105.7%, for an increase in absolute terms of 25 ppm.

Also related to the SCR operation, it was found that the conversion efficiency of the vanadium SCR system dropped by less than 1% point when operating on B20 but dropped by 6% points when operating on B100. This can be explained by the difference in NO₂:NO_x ratio which changes due to the different chemical structure of biodiesel resulting in changes to NO_x speciation during combustion and across the DOC as well as changes in exhaust gas temperature.