Summary of findings regarding use of biodiesel
in vehicles employing late post injection
Introduction
Biodiesel is a promising alternative fuel with many environmental and social benefits. It has been in use in diesel vehicles for many years, by individual consumers as well as in public and private fleet applications. Biodiesel can be used alone as a diesel fuel replacement, or blended with regular diesel in any proportion. Biodiesel blends are called "BXX", where XX stands for the percentage of biodiesel relative to the percentage of ultra-low sulfur diesel (ULSD, the kind of diesel sold in North America and Europe for on-road vehicles). For example, a twenty-percent biodiesel blend is B20, and pure biodiesel is called B100.
Biodiesel blends from B2 to B100 are increasingly available throughout the United States. Cities like Portland and states like Minnesota have biodiesel blend mandates, with other cities and states in talks to adopt similar mandates, which require biodiesel blends to be sold at public filling stations.
New compatibility issues are arising with the use of biodiesel in engines equipped with actively regenerated diesel particulate filters, a new type of emissions system used in most diesel vehicles since the year 2007. Biodiesel incompatibility has been seen with blends as low as B5. This problem needs to be addressed and remedied in order to maintain the bright future of biodiesel in our economy.
Description of challenges
In 2007 new emissions regulations went into effect for on highway diesel vehicles. In complying with the lower emission thresholds, engine manufacturers have incorporated a diesel particulate filter (DPF) to trap soot generated during combustion. The DPFs periodically require regeneration to oxidize the trapped particulate matter and avoid filter plugging. Regeneration typically requires filter temperatures of 550°C .
Two primary methods of achieving regeneration temperatures have been employed by OEMs. The first is commonly called "post injection" or "late post injection". This method is being used by nearly all passenger and light duty diesel vehicles year 2007.5 and newer. During post injection, fuel is injected late in the engine cycle near the exhaust valve. Ideally this fuel is meant to vaporize producing unburned hydrocarbons which exit through the exhaust valve. These unburned hydrocarbons travel downstream to an oxidation catalyst where an exothermic reaction takes place elevating the temperature sufficiently for DPF regeneration. The second method of achieving regeneration temperatures is commonly called "exhaust stream" injection. This method involves injecting fuel in the exhaust stream between the cylinder and the DPF.
The post injection process of inducing regeneration has the problem of leading to engine oil dilution. Engine oil dilution occurs if any portion of the fuel is not vaporized and evacuated via the exhaust valves during the post-combustion injection. Liquid fuel will then adhere to the cylinder walls, and make its way past the pistons and rings into the crankcase oil.
Engine oil dilution takes place with both diesel and biodiesel in any blend. There are two processes which contribute to the overall level of dilution in engine oil. The first is the rate at which fuel is added to the engine oil and the second is the rate at which fuel in the oil is evaporated. While there are significant experimental challenges involved in measuring fuel dilution, evidence has been presented showing a significant level of fuel dilution for B20 relative to ULSD. In addition, ULSD evaporates from the engine oil at a higher rate than B20. In both the case of B20 and ULSD, the overall rate of oil dilution is dependant on RPM and load of the engine. Lower RPM’s and lighter loads result in higher levels of oil dilution due to the lower operating temperatures. For both B20 and ULSD, negligible oil dilution is seen during base operating mode. Oil dilution occurs during regeneration mode only, due to the post-combustion injection.
There are also other factors that influence the performance of the DPF. The amount and character of the emissions from biodiesel are different than those of ULSD. The use of biodiesel significantly reduces the balance point temperature (BPT) of the DPF. The BPT is defined as the temperature at the particle oxidation rate equals the rate of particle collection. The BPT has been shown to drop by 112°C when using B100 and 45°C when using B20, relative to ULSD . In addition the regeneration rate is increased with biodiesel. This has been attributed to the particulate matter from biodiesel oxidizing more quickly and at a lower temperature than the particulate matter from ULSD. The lowering of the BPT and the increase in regeneration rate may actually allow the development of passive DPFs for use with biodiesel, potentially eliminating or reducing the need for actively regenerated DPFs.
There is another concern related to oil dilution from biodiesel. Today’s engine oil contains a large percentage of additives, about 25% by volume. The additives are designed to increase performance and reduce wear. The additives generally tend to be polar molecules. Methyl ester (the chemical name for biodiesel) is also a polar molecule and has strong attractions to the oil additives. This can adversely affect the functionality of the additives, leading to increased engine wear.
Potential solutions
While there are significant challenges to making advanced engines fully compatible with high blend biodiesel, there are also significant advantages to doing so. The advantages range from environmental (lower emissions, lower carbon footprint) to economic (stimulating a new economy and industry, domestic production and sourcing, reducing dependence on foreign oil).
One potential solution involves reconfiguring the injection process to avoid engine oil dilution. This may mean incorporating an exhaust stream injection valve, perhaps with an additional source of heat such as electric heating at the DPF, to facilitate the regeneration.
Another option would be to further research the idea of passive regeneration of the DPF, particularly for use with high blend biodiesel. This has been mentioned in several referenced papers as a possibility, due largely to the lower balance point temperature and the higher regeneration rate seen with high blend biodiesel.
Finally, reformulating engine oil additives to be compatible with biodiesel dilution may help address some problems associated with biodiesel use in new engines.
References
- C. Harvey, et al., Comparison of Nitro-polycyclic Aromatic Hydrocarbon Levels in Conventional Diesel and Alternative Diesel Fuels, NREL/CP-540-38494
- Andreae, M., et al., Biodiesel and Fuel Dilution of Engine Oil, SAE 2007-01-4036
- A. Williams, et al., Biodiesel effects on Diesel Particle Filter Performance, Milestone Report, NREL/TP-540-39606
- A. Williams, et al., Effect of Biodiesel Blends on Diesel Particulate Filter Performance, SAE 2006-01-3280
- M. Tatur, et al., Effects of Biodiesel Operation on Light-Duty Tier 2 Engine and Emission Control Systems, NREL/CP-540-42928
- Biodiesel Magazine, www.biodieselmagazine.com/article.jsp?article_id=2290
- Sloan Automotive Laboratory,
www1.eere.energy.gov/vehiclesandfuels/pdfs/deer_2007/session5/deer07_sappok.pdf - BiodieselSMARTER Magazine,
www.biodieselsmarter.com/archives/2008/12/the_saga_of_the_09_tdi_and_its.php - Gary M. Parsons, Chevron Oronite Corp., Clean Diesel and After-Treatment Systems