Regulators worldwide are actively pursuing advances in fuel economy and efficiency, as illustrated by the upcoming API PC-11 and GF-6 specifications for the North American market. Latest reports suggest that these are likely to take effect from 1 March 2017 and 1 December 2017, respectively. Lubricants will play an important role in achieving this goal, providing the issue of dilution can be overcome.

The Dilution Problem

Fuel dilution occurs when unburned fuel gets past the piston rings into the engine oil sump. With increased use of advanced emission control devices, particularly exhaust gas particulate filters, the impact of fuel dilution can also increase as a result of in-cylinder post fuel injection designs used in conjunction with the regeneration cycle of some of these devices.

Problems occur when a portion of the fuel in the post-combustion injection phase is not vaporized in the cylinder and exits via the exhaust valves. As a result, the liquid fuel then comes into contact with the cylinder walls and is washed down into the crankcase engine oil, reducing engine efficiency.

Fuel dilution is also attributed to driving conditions, most notably frequent short trips where the engine operates under cool conditions with excessive amounts of idle time and lack of highway speeds. The impacts of fuel dilution can be severe, often leading to reduced oil viscosity and oil film thickness.

As different fuel qualities are increasingly used to suit specific vehicle performance or emissions requirements, the need to get the lubricant additive and base oil mix right is paramount to reduce the negative effect of dilution.

The Biodiesel Challenge

Recent years have witnessed increased demand for biodiesel to help meet renewable targets, but the use of these fuels can impact the engine oil and vehicle hardware performance. In Europe, EN 590 now allows up to 7 percent (B7) biodiesel to be blended into conventional diesel fuel. By comparison, B5 is generally accepted in the United States, while in Asia biodiesel mandated levels range from B2 in Korea to B10 in Indonesia.

Valuable research is being conducted into biodiesel contamination of the engine oil. Dilution with biodiesel can impact the engine oil in a number of ways.

First, it can increase lubricant oxidation. Oxidation control can be affected by the concentration levels of biodiesel, raw materials, severity of operating conditions and lubricant quality.

Second, high levels of fuel dilution can reduce lubricant viscosity and oil film thickness, thereby increasing the possibility of wear. Third, biodiesel has been known to cause significant corrosion of bearing materials such as copper and lead.

Fourth, biodiesel can increase piston deposits. These deposits can cause ring sticking, leading to cylinder wear in the most extreme cases, and increased fuel dilution. Finally, unburned biodiesel in the oil sump may become oxidized, promoting lubricant degradation and thickening.

As a result, it is important to formulate high-quality lubricants to mitigate the negative effects of biodiesel. However, understanding the theoretical principles behind the impact of fuel dilution only provides part of the solution. To gain a true insight into the potential implications, observations must be made in the field.

Dilution in the Real-World

To understand the wider implications of biodiesel fuel dilution in the real world, Infineum designed a three-and-a-half year, 100,000-mile field trial using B20 fuel in a fleet of 25 medium-duty buses to gain new insights into tackling the issue. The vehicles ran regular stop-go-idle service between the Las Vegas, Nevada, United States, airport and local hotels. Four oils formulated with Infineum technology and one with commercially available reference oil in a range of SAE viscosity grades from 5W-30 to 15W-40 were tested. For each test oil, four engines ran on B20 fuel and one on ultra-low sulfur diesel.

Oil levels were checked regularly during the test, and very high biodiesel dilution of 10 to 50 percent was observed, potentially caused by a number of factors. One such factor was the design of the in-cylinder post fuel injection for diesel particulate filter regeneration, which could increase the risk of unburned fuel entering the oil sump. Another factor was the extreme stop-and-go drive cycle with extensive periods of idling and lack of highway speeds. Finally, the engines were not specifically designed for biofuel use

In response to the excessive biodiesel dilution, two revisions were made to the drain interval during the trial. First, the initial approach of draining once the vehicles had travelled 10,000 miles was replaced with a drain once the sump volume reached 1.9 liters over the full mark. Secondly, oil was drained once levels exceeded 1 inch above the full mark on the dipstick.

Real-World Insights

At the conclusion of the trial, engine valve train regions were inspected, and each engine showed an excellent sludge rating as well as exceptional cylinder liner wear control and no problems with oil screens. Furthermore, as expected, total acid number increased and total base number decreased as the distance traveled increased for all vehicles.

Assessing fuel dilution against viscosity, it was observed that viscosity decreased with increased fuel dilution. In addition, engines running on B20 consistently showed more fuel dilution than those running on ultra-low sulfur diesel.

Meanwhile, metal analysis, which indicates wear, was consistently higher for oils from vehicles running on biodiesel than those running on ultra-low sulfur diesel. Main bearing wear was also consistently higher for engines running on biodiesel, although these engines had fewer piston deposits.

Analysis showed that the OEM specification SAE 5W-30 oil had the lowest total base number throughout the test – almost reaching zero at the end. This means it had less ability to neutralize acids. The highest fuel dilutions were also observed in this oil.

In most cases, engines running on B20 showed higher levels of iron, copper and lead in used oil analysis. API CJ-4, SAE 5W-30 and 15W-40 oils all showed similar trends, which could indicate that viscosity is not the main factor influencing these results.

Tackling Extreme Dilution

From this research, Infineum concluded that the most likely causes of extreme dilution of the lubricant by biodiesel were the design of the in-cylinder post-injection for particulate filter regeneration combined with the severe stop-go-idle driving cycle. The fuel dilution had significant impacts on lubricant performance, specifically in terms of viscosity decrease and wear protection.

However, despite the severity of the stop-go-idle drive cycle in this trial, all test oils demonstrated the ability to provide adequate protection to these medium-duty bus engines running on B20 fuel over 100,000 miles. However it should be noted that careful attention must be paid to drain interval and used oil viscosity.

If the volume of biodiesel blended with conventional diesel continues to rise, we can expect to encounter further crankcase lubricant formulation challenges. It is becoming increasingly important to use high-quality oils tailored for specific biodiesel applications so that the higher levels of biodiesel can be tolerated as a diluent and to mitigate any negative effects from the fuel.

With so many advances and divergences in the fuels and lubricants industries, dilution is likely to remain an issue, but it is being actively addressed through research and close collaboration between OEMs and lubricant manufacturers.

Pui Fun Cheon is lubes deployment technologist for Infineum Inc. She is based in Singapore.