Effects of Biodiesel Fuel on In-Service Engine Oil

By David Doyle, CLS

Vice President & Operations Manager, North America

As the use of biodiesel fuel increases, there are more questions regarding the effects these fuels may have on the engine crankcase oil. Fuel dilution and its effects when mixed with the engine lubricating oil are the primary concerns with biodiesel fuel blends. Issues that relate to fuel dilution in crankcase lubricants become more predominant as the concentration of biodiesel is blended into mineral oil-refined diesel fuel. Generally, most negative effects on in-service crankcase lubricants are minimal in lower biodiesel fuel blend concentrations, such as B5 through B11 blends.

Biodiesel has a higher density, surface tension, and viscosity than petroleum-based diesel fuel; therefore, injector spray patterns are affected due to larger droplet size. Droplet size, combined with lower volatility, promotes a greater occurrence of fuel dilution when the concentration of biodiesel increases in the fuel blend. There is a tendency for fuel droplets that remain and adhere to the cylinder walls to be scraped by the rings into the oil sump. There are also fewer light ends in biodiesel fuel than in mineral diesel fuel. Consequently, once fuel dilution occurs in crankcase oil the tendency diminishes for the product to burn off as quickly, so it has a greater effect because it does not become volatilized. 

It is thought that since viscosity is higher in biodiesel the oil thinning effect would not be as noticeable when fuel dilution occurs. This is only true up to a point. Even though B100 biodiesel viscosity may be higher than mineral-based diesel fuel (approximately double), biodiesel blends still have to be within #2 diesel fuel viscosity specifications when meeting standard diesel fuel requirements (whether it is ASTM D975 for up to B5 or ASTM D7467 for B6 through B20), and the difference in the engine oil viscosity as compared to the fuel type is too great not to be noticed.

Even though B100 has greater instability than mineral-based diesel fuel, it possesses better solvency characteristics. Mineral diesel fuel has less solvency than biodiesel fuel but greater stability. Biodiesel B20 through B30 blends may be at the threshold where the contribution of insoluble and degradation materials created by the biodiesel component overcomes the poorer solvency characteristics of the mineral oil-based diesel component. This can lead to the promotion of insoluble and degradation materials in the contaminated crankcase oil.

Engine wear can result from excessive biodiesel fuel dilution in crankcase oil. In this case, we are not just talking about viscosity thinning of the lubricant. High enough concentrations of biodiesel fuel dilution in crankcase oil can lead to the formation of deposits on pistons and cylinder walls. These deposits can be hard enough to cause engine failure.

Since biodiesel fuel is inherently less stable, it may create degradation products and insoluble material quicker when diluted in engine oil. This can affect the life of the crankcase oil additive protection. Fuel dilution from biodiesel in crankcase oil can also create organic acids, which adds to the sulfuric and nitric acids engine oils have had to deal with in the past. Studies have shown that lead content increases dramatically from reaction with engine components when there is a significant amount of biodiesel fuel dilution in the crankcase oil. Apparently, copper content may also show some increase, but it does not have nearly the impact in comparison to lead components.

There are many fleets successfully operating on biodiesel, and its effects should be considered on a case-by-case basis. As previously mentioned, the adverse effects on crankcase oil are proportional to the blend concentrations. Higher blends of biodiesel fuel may require shorter drain intervals for in-service crankcase oil. This is why lubricant suppliers and engine manufactures strongly recommend used oil analysis