In the early noughties, the European Union was gung ho about biofuels and blazed a trail with its 2003 Directive on the promotion of the use of biofuels or other renewable fuels for transport, also known as the EU Biofuels Directive. This forced member nations to introduce legislative measures such as the United Kingdoms Renewable Transport Fuels Obligation, stipulating progressive percentage targets (based on equivalent energy content) for the replacement of petroleum gasoline and diesel with renewable alternatives.

Automakers and fuel system manufacturers were lukewarm about the idea, at best. The widespread use of bioethanol in Brazil and the United States meant that the technical issues around gasoline replacement were fairly old news. The use of biodiesel on the same scale, however, represented a different and more fertile can of worms.

Moreover, environmentalists claimed that the cultivation of biofuels did more harm than good. This viewpoint caused the EU to put the brakes on its biofuels program pending further analysis of the environmental and social consequences. Amidst the confusion, even oil companies became unhappy. Indeed, some that had already invested heavily in new biofuels facilities were apoplectic.

Against a backdrop of fluctuating crude oil prices, biodiesel producers pressed ahead, with global production capacity – 70 percent based in the EU – exceeding 55 billion liters per year by the close of 2009. Although this represented a significant amount of over-capacity at the time, some current estimates suggest that the uptake of biodiesel will lead to global demand of around 100 billion L/y as early as 2015. Diesel blends with 5 percent biodiesel (so-called B5) are already prevalent in Europe. B20, B30 and even B100 are also available in some markets.

So while the political, socio-economic and environmental arguments drag on about whether or not biofuels – and biodiesel in particular – are a good idea, biodiesel has become an established and increasingly important fact of industry and motoring life. Notwithstanding, there is one point on which those in the know seem unanimous. For todays meticulously formulated engine oils, biodiesel can create a positively unfriendly environment. To understand why and, in turn, explain how this has become a hot issue for the Coordinating European Council for the Development of Performance Tests for Fuels, Lubricants and Other Fluids, we need to take a closer look at our can of worms. As it happens, the problem really does originate quite literally with a can. The worm in this particular can is the diesel particulate filter.

Coping with Soot

As the name suggests, the DPF is a device incorporated into the exhaust system of diesel-fueled vehicles to remove combustion-derived soot or particulate matter from the exhaust gas stream. Over time, the soot can build up and start to block the DPF, giving rise to excessive exhaust back pressure and deterioration in engine performance. Periodically, therefore, the soot needs to be burnt off and reduced to ash – a process known as DPF regeneration.

Regeneration may be passive, occurring during periods of sustained high-speed driving, when the exhaust gas temperature is sufficiently elevated. Sometimes, active regeneration is required, whereby unburnt fuel is introduced to forcibly increase exhaust temperature. Often, this is achieved using a process called late in-cylinder fuel injection or simply post injection. An additional dose of fuel is injected late in combustion or during the exhaust stroke.

There are other regeneration technologies involving injection directly into the exhaust pipe, or even heated DPFs. But with highly sophisticated injection systems already in use, the in-cylinder approach represents a costeffective solution for OEMs, requiring little more than software engineering to implement and no additional hardware. There is another price to pay, however, for this seemingly elegant approach which remains in common use, even on new designs.

Most of the post-injected fuel is vaporized and passes out of the combustion chamber, as intended, with the exhaust gases. Inevitably, though, some unvaporized fuel is deposited on the cylinder liner and, from there, is transferred by the action of the piston and rings to the oil sump.

Biofuel Buildup

The result is fuel dilution of the engine oil. Biodiesel amplifies the level of dilution because it is less volatile than conventional diesel and hence more resistant to vaporization, both initially in the cylinder and thereafter when mixed with the lubricant. Thus, use of a B5 fuel can lead to biodiesel concentrations in oil sumps reaching anywhere from 20 percent to 50 percent, or even higher. Absolute levels of fuel dilution can also be significant, with figures approaching 50 percent being reported for a routine oil drain interval of 16,000 kilometers.

Lets rephrase that and think about the implications for a moment. We are saying that up to half the engine oil could be displaced by fuel, a significant portion of which might be a non-petroleum fluid. So the additive treat rate could effectively be halved, even before the effects on viscosity, oxidation (biodiesel oxidizes readily) and other chemical or physical degradation mechanisms have been considered. Its enough to give nightmares to an automotive engineer. Its also worth sparing a thought for the oil formulator, which has spent a kings ransom on testing and no trivial amount of time perfecting the requisite balance of lubricant components.

Since March 2010, the CEC has been developing a new test to add further to the formulators burden, with an assessment of engine oil tolerance to biofuel dilution. Actually, this is not completely uncharted territory for CEC. In 2008, the organization adopted CEC L-099-08, a replacement for wear test CEC L-51-A-98, which ran on the aging Mercedes-Benz OM602A engine. The organization prescribed that L-099-08 use another Mercedes-Benz unit, the OM646LA, which employed post injection, and that it run on a B5 fuel.

The new test, being developed under the stewardship of Test Development Group TDG-L-104 and led by German test laboratory APL, is based on the same OM-646LA engine. This time, starting with B15 fuel and a test oil charge understood to be diluted with approximately 7 percent B100, the objectives are to develop a procedure to measure levels of sludge and piston cleanliness control.

Interestingly, these criteria were also monitored in the aforementioned L-099-08 test, appearing as report requirements in ACEA Oil Sequences from 2008 onwards. Apparently, CEC L-099-08 was judged unable to provide the desired level of quality assurance for biodiesel compatibility in these performance areas.

Tall Task

Initial work by TDG-L-104 focused on sludge control, with CEC reporting in its March 2011 newsletter that the group had established a good understanding of engine behavior in this respect and was moving on to a second phase of the development, concentrating on piston cleanliness. Despite the optimism expressed about the sludge criteria, the March report described the test as the most significant and the most difficult test development under way in CEC. This point was reaffirmed in a brief June update advising that the group was trying to balance a number of variables in an effort to achieve the correct test severity and that members were being canvassed for suggestions on how to proceed.

Unlike ASTM, CEC activities such as meetings of test development groups are covered by confidentiality. The exact nature of the difficulties that TDG-L-104 is trying to manage has not therefore been publically disclosed. But the technical challenges of developing such a test are clearly substantial: from the uncertainties and problems associated with using oxidatively unstable biodiesel to ensuring a consistent and meaningful level of test-to-test contamination; from avoiding oil sump overfilling to making sure its content includes sufficient candidate oil to make a difference in sludge and piston cleanliness control.

This sounds like a balancing act bordering on a high wire routine, and TDG-L-104 will deserve applause indeed if it pulls off a repeatable and reproducible test in time for the 2012 ACEA Oil Sequences. LubesnGreases will be following its progress with interest.