Understanding the Post-Injection Problem

Many diesel engine manufacturers have implemented what's called post-injection, the introduction of fuel late in the combustion cycle, as part of an advanced control strategy to reduce emissions. Biodiesel Magazine looks at how post-injection of biodiesel blends facilitates dilution of engine oil while interacting with oil additives to potentially accelerate engine wear.
By Ron Kotrba | April 15, 2008
Fuel dilution in diesel engines is not a new issue, but it's one that's gained greater attention recently as lower emissions thresholds went into effect last year. Meeting the 2007 regulations requires the incorporation of diesel particulate filters to trap soot. To avoid filter plugging, however, the accumulated soot needs to be burned off-a plugged filter will cause high backpressure and operational issues. Many original engine manufacturers (OEMs) use a regeneration scheme in which fuel is introduced late in combustion to create an exothermic reaction downstream in the filter, which burns off the soot. The resulting ash from regeneration of the filter can collect for hundreds of thousands of miles.

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."

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.

Sappok ran across a different analytical problem: Inordinately high oxidation readings gained from fresh oil and fresh biodiesel mixed. It is his conclusion that the ester peak interfered with the oil oxidation spectra.

Biodiesel Interaction with Additives and Trace Concerns
Oil additive packages, which may constitute up to 25 percent of the total lubricant, contain dispersants to suspend soot in the oil, viscosity improvers, over-base detergent used to neutralize any acids built up and zinc dialkyldithiophosphate (ZDDP) as an anti-wear agent. "Look at all of these additives," Fang says. "They are all bipolar molecules-one side of these molecules is very polar while the other is nonpolar. The nonpolar tail is used to sustain the polar head and make it suspended in the oil, which is largely nonpolar." So the additives are relatively polar compounds compared with the base oil, and methyl esters are polar as well. Polar molecules are attracted to polar molecules, so biodiesel in the oil is attracted to the likewise polar additives. "We used spectroscopy to quantify this," Fang says. "The more polar the additives, the more they interact with the biodiesel. And if the biodiesel gets degraded, or experiences thermal or oxidative aging or whatever, the biodiesel will absorb more oxygen and become even more polar, which will generate even more interaction with the additives."

It didn't take long for Fang to discover the adverse effect aged biodiesel has on ZDDP, the anti-wear additive. ZDDP is chemically designed to be attracted to the metal surface of the cylinder walls and to form a protective layer. "But if you've got biodiesel in your oil, and the biodiesel likes to go to the metal surface too because the metal surface is polar, the biodiesel competes with the additive over the metal surface," Fang says. Although there are complex interactions between the biodiesel and additives affecting the functionality of the additive, Fang suggests the mere fact that biodiesel and ZDDP compete for surface area on the cylinder walls is troublesome enough. The effectiveness of detergents is also mitigated by the presence of biodiesel in the oil.

New diesel engines require the use of low-ash oil as found in the CJ-4 oil standard, which limits ash content of the lube to 1 percent. "The ash in the oil is mainly coming from the additives-anti-wear, detergents, dispersants-and currently the oil companies are limited as to how much additives they can use in the oil because, as the additives are consumed through normal operation, they end up in the diesel particulate filter," Sappok tells Biodiesel Magazine.

At MIT, Sappok is interested in how trace contaminants resident in biodiesel-even that which is within the ASTM spec-may cause trouble downstream. Calcium and magnesium are limited to 5 parts per million (ppm) in D 6751, the B100 spec. Sappok says 1 ppm of trace metals in the fuel can equate to as much as 1,000 ppm of the same material in the engine oil. "Oil consumption is about a thousand times less than engine fuel consumption," Sappok says, which is where his calculation comes from. According to him, even biodiesel within the trace metals specification can lead to inordinate concentrations of potentially damaging metallic ash loading in the DPF. Sappok's concern is that ash from trace metals in biodiesel may either contribute to ash buildup in DPFs, facilitate a gradual deactivation of the catalyst used in regeneration, or both. Ongoing research conducted by Sappok will lead to a better understanding of how sodium and potassium ash specifically affect aftertreatment systems. "There's not been a lot of work on this yet," he says.

Fang doesn't buy this argument about trace metals in biodiesel, and the 1:1,000 ratio of contaminants present in the fuel versus that which ends up in the oil. "We've never seen that ash can cause trouble for the DPF," Fang says. "If the regeneration scheme is good, the ash is not an issue. … The MIT estimates are too high from my point of view."

Even though for years biodiesel has been heralded as a lubricity additive helping keep fuel system components like the moving parts inside fuel injection systems operating smoothly, the bitter irony here is that, when post-injected, it tends to dilute engine oil and interacts with additives and increases the possibility of engine wear. Much work remains developing viscosity improving, anti-wear, dispersant and detergent additive packages in which adverse reactions with biodiesel are significantly reduced.

Ron Kotrba is a Biodiesel Magazine senior writer. Reach him at [email protected] or (701) 738-4962.
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