Splitting Late Post-Injection

New study may get to the bottom of biodiesel dilution issue
By Ron Kotrba | January 12, 2011

Work is underway that could eventually help approval of higher biodiesel blends in new diesel vehicles equipped with particulate filters that employ late post-injection for regeneration.  A joint study between the German oilseed council UFOP and Volkswagen is investigating ways to partition late-post injection to help reduce dilution of engine oil.

Diesel particulate filter regeneration is required to burn off soot, or particulate matter (PM), collected in the filter to help meet U.S. EPA’s PM reduction requirements. Extra fuel is injected late in the combustion cycle not to be burned but rather to be sent through the exhaust valves and straight into the filter, located downstream in the exhaust system, in order to create a rich condition for soot burn-off. In late post-injection, some of the fuel injected late in the combustion cycle, especially fuel sprayed onto the cylinder walls, can make its way past the oil rings and accumulate in the engine oil crankcase. Petroleum diesel fuel volatizes off, but due to biodiesel’s different properties and higher distillation temperature, the bio portion of the fuel that enters into the crankcase tends to accumulate, causing dilution issues and premature engine wear from degradation of engine oil. Some OEMs avoid this issue altogether by injecting fuel directly into the exhaust system, just upstream of the filter, rather than using the already existing fuel injection infrastructure on the engine. The downstream injection approach, however, costs more due to the added hardware.

Regarding dilution with varying biodiesel blends, the UFOP/Volkswagen study found that “the splitting of the late post-injection in two partial injections lowered the fuel entry into the engine oil in comparison with a late, undivided post-injection during operation of the engine with B7 by approximately 20 percent, and with B30 by approximately 27 percent.”

While this is positive, and shows that automakers are serious about understanding and incorporating biodiesel, UFOP further states that the tests “clearly demonstrated that the early post-injections subsequent to the main injection can have a considerable share in the engine oil dilution if partial amounts of the fuel jets encounter the cylinder wall. The early post-injections should be incorporated in future potential investigations. However, the emissions, consumption and vehicle handling must not [be] negatively [impacted].”

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