Biodiesel as a fuel has similar combustion properties to normal petroleum diesel, but its physical and chemical properties are significantly different. This raises some legitimate concerns over the performance of biodiesel in automotive engines.

While much attention has been given to fuel issues, such as cold flow properties and the sensitivity of the fuel injection system to the presence of biodiesel, there are also potential consequences for the crankcase lubricant. Biodiesel is less volatile than petroleum diesel; any non-combusted biodiesel that gets past the engine piston rings has a tendency to accumulate in the crankcase oil sump and dilute the engine oil. The risks associated with excessive fuel dilution of the oil by biodiesel are known. Yet, work is ongoing in the industry to fully establish the actual impact of biodiesel-containing fuels in modern diesel engines that are running on modern lubricating oils and may be equipped with exhaust aftertreatment devices such as diesel particulate filters.

Biodiesels Chemistry The distinctive properties of biodiesel result from both its origin and production process. Rather than the wide variety of hydrocarbon compounds found in petroleum diesel, biodiesel specifically comprises a mixture of mono alkyl esters of long chain fatty acids.

Biodiesel is made by reacting natural vegetable oils or animal fats with a lower alcohol (such as methanol) in the presence of a catalyst, producing the biodiesel fatty acid methyl esters (FAME) and releasing glycerol as a byproduct. The purpose of this trans-esterification reaction is to reduce the viscosity of the natural oil triglycerides to a value closer to that of petroleum diesel. Untreated vegetable oils are not an appropriate fuel for modern diesel engines and do not constitute biodiesel.

There are many types of vegetable oils or animal fats available for conversion to biodiesel. The two most common biodiesels are soybean methyl ester (SME) and rapeseed methyl ester (RME), which are derived from soy-bean oil and rapeseed oil, respectively. Other potentially useful raw materials include palm oil and used kitchen oil.

One complication is that the source raw material has a significant impact on the handling, use and performance properties of a given biodiesel fuel. In particular, the oxidative stability of the biodiesel is affected by the degree of unsaturation; that is to say, the proportion of olefinic carbon-carbon double bonds present in the fatty acids groups. However, it is not as simple as choosing a more saturated raw material such as palm oil for better oxidative stability. The more saturated biodiesels have the worst cold flow properties and are the least responsive to flow improver additives.

While fuel specifications for biodiesel such as ASTM D6751 in the United States and EN 14214 in Europe have been defined, fuel quality issues and the inherent variability of biodiesel from differing or even similar raw material sources remain as significant concerns to the industry.

Providing a number of favorable fuel properties, biodiesel is low in sulfur and aromatics, has low toxicity and biodegrades quickly. Biodiesel fuels possess high cetane numbers, and therefore deliver good combustion properties. As an oxygenate, biodiesel helps to reduce emissions of carbon monoxide, hydrocarbons and particulate matter, but tends to exhibit a small debit for emissions of nitrogen oxides. The presence of about 11 percent by weight of oxygen in biodiesel does, though, lower the energy content of fuel, reducing the fuel economy achievable compared with petroleum diesel on an equal volume basis.

Out in the Field

Biodiesel is mainly utilized as a diesel fuel extender in blends with petroleum diesel – with which it is fully miscible – rather than as a neat fuel. Such biodiesel blends are labelled BNN, where NN indicates the percentage of biodiesel that is present. In the United States most experience has been with closed fleets running on B20, although B2 was introduced in Minnesota. In Europe, standard diesel fuel may already contain up to 5 percent biodiesel, although this target is not yet widely achieved, and some fleets have been run on B30 or even higher blends.

Diesel engines may be run on biodiesel blends without any modification of the engine. In general, the problems that could arise would be expected to increase with the proportion of biodiesel present in the fuel.

Fuel injection equipment manufacturers are extremely wary of biodiesel blends above B5. The attitudes of diesel engine manufacturers vary more, but many are still cautious pending full specifications for B20 or higher blends. For engines where the use of biodiesel blends above B5 (such as B20 or B30) is sanctioned, most OEMs recommend close monitoring of the oil condition or suggest a substantial reduction – perhaps halving – of the oil change interval.

The main fuel performance concerns arising from biodiesel use relate to the impact on the fuel injection system and the cold flow characteristics. Biodiesel has poor oxidative stability, restricting the shelf life of biodiesel fuel to six months, and there are material compatibility issues with certain metals and elastomers. Biodiesel oxidation products and contaminants from the biodiesel production process, such as methanol, glycerol, glycerides, free fatty acids, metallic soaps and water, are known to contribute to fuel system problems. These include fuel filter plugging, fuel pump failure, injector coking and corrosion, while inadequate flow properties at low temperatures remain a critical issue.

In the Sump

For the engine lubricant, the main concerns have been the effect of biodiesel on engine cleanliness and the consequences of fuel dilution. Any non-combusted fuel striking the cylinder wall is scraped down past the piston rings and enters the engine oil sump. The rate of fuel dilution is expected to be somewhat higher with biodiesel fuel as it has a higher viscosity, density and surface tension than petroleum diesel, factors that increase fuel droplet size. Biodiesel tends to accumulate in the oil sump due to its lower volatility and narrower boiling range than petroleum diesel, and this concentrates the biodiesel contribution to fuel dilution. In addition, any severe fuel injector deposits caused by biodiesel that disrupt the fuel spray pattern will further exacerbate the rate of fuel dilution.

The risk areas associated with biodiesel fuel dilution in the lubricant are the impact on wear, corrosion, engine deposits and oil degradation. The initial effect of fuel dilution will be reduction in the viscosity of the lubricant. Loss of viscosity can reduce the oil film thickness, which could increase abrasive wear. This does not appear to be a major issue currently as the biodiesel itself has some inherent lubricity properties. Biodiesel may actually be used to improve the lubricity of ultra low sulfur diesel fuels, though the treat rate required is substantially higher than for conventional lubricity additives.

Increased corrosion of engine bearings is a significant concern. Biodiesel oxidation products and any free fatty acids present in biodiesel are known to be aggressive towards the soft metals such as lead and copper used in bearings. Although evidence for increased bearing corrosion with biodiesel is mixed, corrosion is a known issue in the fuel system.

There does seem to be a tendency towards increased engine fouling from piston deposits or sludge precipitation from biodiesel use. These issues again relate to the poor oxidative stability of biodiesel. The unsaturated and polyunsaturated esters present in biodiesel can undergo oxidative polymerization, resulting in oil thickening and deposit formation. Fuel quality may also be a factor in contributing to sludge formation. Excessive upper piston deposits can have further consequences when they cause the piston rings to stick. Stuck rings will increase the quantity of soot and blow-by gases entering the lubricant, thereby promoting further viscosity increase and degradation of the lubricant. The crankcase lubricant has to provide sufficient antioxidancy and dispersancy to help combat these effects.

DPF Woes

A pressing issue that dramatically increases fuel dilution rates is the use of fuel post-injection to regenerate diesel particulate filters, DPFs. While DPFs reduce particulate matter emissions, the carbonaceous soot that accumulates in the filter has to be periodically burnt off. One favored method of doing this is to post-inject a small amount of fuel into the exhaust stroke, where it burns and raises the temperature of the exhaust gas such that DPF regenerationcan occur. Such post-injection tends to cause significant fuel dilution. The resultant volume of fuel that builds up can be such that the sump overflows, suggesting that engineering changes as well as lubricant solutions may be required to solve this problem.

Biodiesel itself may assist DPF regeneration due to its producing less soot or a more combustible type of soot, but metallic contaminants in biodiesel can also contribute significantly to the build-up of metallic ash in the DPF.

Why Go There?

The rather unfavorable nature of some biodiesel properties raises the question, Why go down the biodiesel route at all? given the likely reduction in fuel economy and increased stress on engine and lubricant systems.

Concerns over the security of energy supply and the effect of fossil fuels on climate change have coincided to create an unstoppable momentum towards the use of more renewable energy resources, including biofuels. The use of biodiesel is thought to contribute to an overall reduction in greenhouse gas emissions, compared with petroleum diesel, over the entire life cycle of the fuels.

In the short- and medium-term, biodiesel will be the second main biofuel after ethanol. Europe has taken the lead on the use of biodiesel, and the United States and Asia are now starting to follow this trend. Some cost benefits may be available to the consumer where fiscal incentives exist to encourage a switch to biofuels. Longer term, there are next-generation biofuels on the horizon, such as hydrogenated vegetable oils, that may overcome some of the shortcomings currently associated with biodiesel.

The use of B5 does not require a shortened oil drain interval, and several fleets have been successfully run on higher biodiesel blends without significant lubricant issues arising. Along with these positive signs, the additives industry is also working to help ensure consumer protection, supporting development of standard industry engine tests that will use biodiesel-containing fuel. This includes the OM501LA engine test which examines piston cleanliness and wear.

Infineum has been actively conducting research to understand the impact of biodiesel on current and future automotive hardware, and is committed to ensuring that this exciting innovation is facilitated through availability of appropriate lubricant and fuel additives.