A low-cholesterol diet for diesels?A low-cholesterol diet for diesels?

Detroit--Certain veggie oils that result in less clogging of human arteries may one day also help diesel engines run healthier. So say researchers at Penn State, who presented a paper at the recent SAE conference in Detroit on the use of these unconventional lubricants in diesel engines.

Preliminary results from research, conducted at the Laboratory for Hydrocarbon Process Chemistry, indicate that using high content oleic sunflower oil instead of traditional lubricants reduces particulate emissions of low heat rejection and uncooled (adiabatic) diesel engines.

The idea for performing these veggie oil tests came from Dr. Joseph Perez, a tribologist at Penn State. Noting that such oils have a lower volatility than petroleum-based lubricants, he surmised that less of it would be liberated through the action of the pistons and the combustion gases during the combustion process. And the less oil that burns, of course, the fewer particulate emissions.

Perez suggested that his colleague, Andre Boehman, an assistant professor of Fuel Science, test out the hypothesis in his laboratory. "We were very interested in the idea because of all the work being done to develop cleaner-burning engines," recalls Boehman. "Once you've optimized the combustion cycle of an engine, the lubricant accounts for an increasing percentage of the emissions. What we saw with the engines we've been studying is that we were running into a plateau in terms of further particulate reduction."

Boehman's research group conducted tests on a Yanmar 15-hp, single-cylinder diesel engine with components having thin (300 mm) thermal barrier coatings, as well as uncoated components. Compared with a standard SAE 10W-30 commercial lubricant, the sunflower oil resulted in fewer emissions under a variety of engine operating conditions.

However, researchers did come across one puzzling finding: Although sunflower oil has superior antiwear properties, test data revealed that it actually led to reduced fuel efficiency. Differences in viscosity may be the culprit, says Boehman, but he stresses that this apparent contradiction requires further analysis. If this phenomenon turns out to be real, increased fuel consumption is not likely to be an acceptable tradeoff for most operators of diesel engines.

Down the road, the research group plans to find out whether it can reproduce its findings using multi-cylinder engines and various lubricant formulations.