Partnering to Improve Biodiesel's Outlook

While previous advances in diesel engine technology and exhaust aftertreatment systems were on separate and parallel development paths by the engine developers, fuels, lubricants and additive industries, it's increasingly apparent that further significant improvements will require an integrated, or systems, approach. Engine makers face ever-increasing requirements that are often in conflict with one another. As engine-out and tailpipe emissions regulations tighten, alternative fuels are being introduced, which have different physical, chemical and combustion properties than conventional diesel fuels for which engines and aftertreatment systems were originally designed. To find solutions, regulators, engine builders, equipment providers, biodiesel producers/suppliers, oil companies and lubricant additive developers must all work together towards common objectives. (Note: Be sure to check with your attorney regarding antitrust/anticompetition advice in advance.) We believe performance additives can play the important role of biodiesel "enabler" in existing and future engines.

Diesel emissions standards currently focus on two primary areas: particulate matter (PM) and nitrogen oxide (NOx) emissions. Aftertreatment systems are needed to meet the most stringent standards. Diesel particulate filters (DPF) are highly effective at removing PM from the exhaust stream and are typically used to meet the toughest regulations. For NOx emissions, there are two primary methods currently in use: lean NOx traps (LNT) and selective catalyst reduction (SCR).

Aftertreatment systems require a certain amount of heat to operate effectively. During idle and low-load conditions, diesels operate at high air-to-fuel ratios and the exhaust temperature is relatively low. As a result, plugging can occur. Therefore, most diesel aftertreatment systems require periodic regeneration typically achieved by introducing raw or partially combusted fuel into the exhaust stream. As the fuel burns, it heats the exhaust and facilitates regeneration.

Two primary methods are used to heat diesel exhaust for regeneration: late post-injection, in which fuel is injected into the cylinders well after the primary combustion event; and direct injection into the exhaust stream through a dosing injector. Internal surfaces of DPFs are typically coated with a catalyst allowing for continuous regeneration under normal, cruise conditions. Sensors monitor pressure drop as an indication of plugging across the DPF. When the pressure differential reaches a predetermined level, regeneration is started to burn off PM and reduce DPF plugging. The frequency of DPF regeneration is highly dependent on the driving cycle and ambient conditions. LNTs also require a form of regeneration called desulfurization. The frequency of LNT regeneration is based largely on the amount of fuel sulfur, but can be as frequent as every 90 seconds. Due to packaging and other concerns, light-duty diesels typically incorporate late post-injection while heavy-duty diesels use dosing injectors.

Late post-injection leads to increased cylinder-wall wetting with unburned fuel. Some of the fuel then finds itself past the rings during the compression stroke and enters the crankcase as fuel dilution. This is a physical phenomenon and is mostly independent of fuel type. However, once the fuel enters the crankcase, there are large differences in behavior depending upon which type of fuel is used. For petroleum diesel, the light compounds volatilize as the oil-to-fuel mixture reaches the hotter regions of the engine. The volatilized fuel escapes through the crankcase breather system. The heavier fractions remain mixed with the crankcase oil as fuel dilution. For biodiesel, which has a higher, narrower boiling range, none of the internal engine surface temperatures are high enough to volatilize the fuel, therefore virtually all biodiesel entering the crankcase remains as fuel dilution.

Fuel dilution of crankcase oil can reduce the ability of engine oil to protect from wear and keep the engine clean. Initially, the fuel dilution reduces the viscosity of the oil, which results in a thinner oil film. As the fuel in the oil ages, it starts to decompose and can lead to increased viscosity, crankcase deposits and acids, which attack certain metal parts. Biodiesel decomposes at a different rate and in a different manner than mineral diesel.

Although additives cannot solve the physical part of fuel dilution, they can be used to help reduce the likelihood of fuel dilution from harming the engine. Antioxidants in the oil additive package help reduce fuel oxidation and formation of acids and deposit precursors. Detergents help neutralize crankcase acids and keep metal surfaces deposit-free. The additive chemistry best suited to prevent oxidation, neutralize acids and keep metal surfaces clean is complicated by the fact that the reaction mechanisms for mineral diesel and biodiesel are different. Oil additive formulations can be tailored to be more effective when biodiesel fuel dilution is present.

By working together, growing pains from new and emerging fuel, engine, and exhaust aftertreatment technologies can be reduced or eliminated. One example of the positive impact of collaboration in the biodiesel industry is the development of ASTM specifications for biodiesel and biodiesel blends. These specifications have resulted in greater biodiesel consistency and original equipment manufacturer confidence regarding biodiesel use in their equipment. The BQ9000 accreditation program is another example of industry collaboration to develop a recognized quality program, benefiting individuals and the industry. To be part of the solution, partner with others to develop reasonable solutions before unreasonable solutions are forced upon us.

Gary Parsons is the global OEM and industry liaison manager for Chevron Oronite Co. LLC. Reach him at GMPA@chevron.com.