Modern diesel engines are already equipped with sophisticated fuel injection systems, prompting engine manufacturers to utilize the flexibilities resident in these injectors thereby cutting the cost of devising an alternative approach. “As injection systems and injection technologies have advanced since the early 2000s, all the OEMs are using common rail or unit injection systems, which allow multiple injections of fuel during the cycle,” says Alexander Sappok, a researcher working on his Ph.D. at the Sloan Automotive Laboratory at Massachusetts Institute of Technology. “It gives you the flexibility to really modify your injection strategy for a number of different reasons. So some OEMs figured since they’ve already made the investment in these injection systems they would utilize those same systems as a part of their regeneration strategies for aftertreatment systems.”
Not all aftertreatment systems providers utilize post injection in their regeneration strategy. Some incorporate in-stream fuel injection where injectors positioned in the exhaust stream squirt controlled doses of fuel downstream of the combustion process, some coupled with a burner, to create the exothermic reaction needed to incinerate the trapped particulate matter while avoiding the issue of fuel dilution altogether. Caterpillar utilizes this in-stream approach with a diesel burner in the exhaust stream. Currently it’s a matter of economics—OEMs see a cost savings in utilizing existing fuel injection systems rather than tacking on the additional cost of extra equipment. A 2007 Society of Automotive Engineers paper written by engineers from Tenneco Inc. and AirFlow Catalyst Systems stated that, “Although most 2007 systems introduce the fuel for diesel particulate filter (DPF) regeneration through in-cylinder post-injection, it is likely that 2010 systems will also need in-stream fuel injection to help avoid oil dilution and potential premature engine wear.”
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The engineers note this may be especially necessary if the DPF is downstream of the urea-injected selective catalytic reduction (SCR) systems for nitrogen oxide (NOx) abatement.
Even though some predict the need for in-stream injections eventually, as of now many OEMs rely on post-injection to meet their needs.
A New Understanding of Biodiesel’s Dilution Effect
Volkswagen is using post-injection for regeneration and according to Stuart Johnson with the Engineering and Environmental Office of Volkswagen Group of America, the issue of oil dilution from biodiesel is a real concern for the automakers. “We can tolerate up to 50 percent fuel mix in the oil but no more,” he said at the 2008 National Biodiesel Conference.
Volkswagen tests using B5 and post-injection showed 45 percent oil dilution after 10,000 miles, but surprisingly no engine damage was evident upon inspection. “Using B10 at 10,000 miles surpasses that 50 percent threshold—and that is unacceptable,” Johnson said. “We want longer oil change intervals as a car company, so it’s hard for us to talk about this.” The implications are that increased fuel dilution due to biodiesel blends could lead to premature engine wear if oil changes are not done more often.
Senior technical advisor for Cummins Inc., Howard Fang, explains to Biodiesel Magazine exactly how this inordinate accumulation of biodiesel in the crankcase oil takes place.
Cummins has done extensive work to characterize biodiesel’s effects on engines, performance and exhaust. “Biodiesel definitely promotes fuel dilution,” Fang says.
Post-injection of fuel into the cylinders is intended to vaporize in the cylinder but not combust, exiting then through the exhaust valves and traveling downstream where the introduction of the unburned fuel to the catalyst creates an exothermic reaction incinerating the collected soot. Inevitably the heavier fractions of fuel will not vaporize during post-injection and in liquid form can adhere to the cylinder walls. Through the slapping motion of the pistons and oil rings, the unburned fuel from post-injection can make its way through the tight, hot quarters between the piston, rings and cylinder walls. The fuel accumulates in the crankcase and dilutes the oil, which is a major concern regarding engine wear and longevity.
“Using post-injection you will generally see elevated levels of fuel dilution regardless of what fuel you’re using,” Sappok says. Because biodiesel has a higher distillation temperature and boiling point, when it’s present in the post-injected fuel it tends to dilute the oil on a level disproportionate to its blend ratio in the fuel. Fang says this is just now becoming understood.
Through his work at Cummins, Fang has developed a new algorithm to predict the amount of fuel dilution when running on various blend ratios of biodiesel. “The conventional way to derive the calibration to estimate the amount of fuel oil dilution is completely wrong,” Fang tells Biodiesel Magazine. This conventional approach to determining biodiesel’s dilution effects on oil when implemented in a lab suggests mixing 5 percent of B20 in fresh oil, then 10 percent B20, and so on to create a calibration curve through analytical means such as infrared (IR) or gas chromatography (GC), the results from which can then be used to predict unknowns using the slope created by those data points. “We cannot use this way to quantify the fuel dilution from biodiesel in the oil,” Fang asserts. “The problem is in all IR, GC—any analytical technique really—you assume the concentration is B20 but because ultra-low sulfur diesel (ULSD) is easier to vaporize, the concentration of biodiesel on the cylinder walls is higher than 20 percent, maybe way higher.” In fact, Fang has developed a method to more precisely measure this and found that post-injected B20 can lead to as much as 40 percent methyl ester accumulation on the cylinder walls.
Fang’s new method to determine this uses what he calls an oil tracer—a component that is stable, not subject to decomposition or easy oxidation and is compatible with additives. “We monitor the tracer concentration and quantify the decrease and correlate it to a certain fuel percentage and estimate the fuel dilution,” Fang explains. There are many different compounds suitable as oil tracers, but Cummins has been using pentaerythritol ester (PE). “It’s a very common oil component used for aviation oil, so it’s very compatible with oil,” he says. The GC or IR signal of PE is far different from the oil and additive signatures so the amount of the tracer can be easily quantified.
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