Compatibility of Biodiesel with Petroleum Diesel Engines

Hannu Jääskeläinen, W. Addy Majewski

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• Introduction
• Material Compatibility
• Engine Oil Dilution
• Fuel System Compatibility
• Diesel Aftertreatment Effects
• Manufacturers’ Position

Introduction

A number of factors related to fuel compatibility need to be considered when using biodiesel in any particular engine. Logically, the effect of biodiesel on engines and aftertreatment systems depends on the blend level used. While in many cases the most significant effect would be expected with neat B100 or high level blends, intermediate level blends can be most prone to the precipitation of fuel insolubles and filter plugging. Some cumulative effects can be also caused by prolonged operation with low biodiesel blends.

Biodiesel advocates, including biodiesel manufacturing groups and those organizations that support the use of biodiesel, often claim that biodiesel can be used in existing diesel engines without modifications. While it may be true that most diesel engines can be started and operated with biodiesel fuel (at least under mild weather conditions), engine manufacturers may limit the use of biodiesel in many engine models to ensure no adverse effects over the entire life of the engine. The restrictions on the use of biodiesel fuels are typically imposed through new engine warranties that become void if the engine is operated with a fuel that does not meet the manufacturer’s specifications.

Almost all modern diesel engines can readily use biodiesel blends up to B5 or B7 and diesel fuel standards such as ASTM D975 and EN590 typically specified for these engines cover diesel fuels from 0% to 5% or 7% biodiesel to reflect this. For blend levels up to B20/B30, many manufacturers have engine models that have been certified to these higher blend levels. Some manufacturers may even certify all their common engine models to this blend level. Fuel standards specified for these engines are separate from diesel fuel standards and include ASTM D7467, EN 16734 and EN 16709. For even higher blend levels up to B100, manufacturers may offer engine options that can safely use this fuel, but options are far more limited than for B20 blends. Fuel standards available for engines capable of using B100 are also more limited—ASTM does not have a fuel standard for B100 as a standalone fuel while in Europe, EN 14214 covers biodiesel as a blend component and as a standalone fuel. In the initial period of biodiesel commercialization, from the 1990s to the early 2000s, B100 was readily available at retail pumps in some places such as Germany where tax exemptions that made it significantly cheaper than conventional diesel. However, the market has shifted toward lower blends as fuel tax laws evolved and engine manufacturers tightened warranty requirements.

The potential issues with biodiesel fuels may be grouped as follows:

• Material compatibility—Depending on the engine make model and model year, the engine components that come in contact with the fuel can be made from incompatible materials.
• Oil dilution—A possibility exists with all engines that some fuel will make its way into the engine’s crankcase and dilute the lubricating oil. Over time, the accumulated fuel can amount to a significant proportion of the engine’s oil capacity. What happens to the lubricating oil and engine components that come into contact with the oil/fuel mixture can have an impact on engine durability and longevity.
• Fuel injection equipment—Impacts on fuel injectors, filters and other fuel system components can cause a significant deterioration in engine performance.
• Emission control system—Emission aftertreatment systems, including catalysts and particulate filters, can be negatively affected by biodiesel fuels. Increased engine emissions or shortened durability of emission components can result.

Material Compatibility

Whenever the fuel composition is changed in the fuel system, material compatibility is a major concern. Fuel system designers incorporate materials based on laboratory testing and the available historical data. Thus, changes in fuel composition and the introduction of alternative fuels often create unforeseen problems in seals, gaskets, O-rings, and metallic components in the fuel system. The consequences of changing the fuel composition without prior compatibility studies are best illustrated by the reduction of aromatics in low sulfur diesel fuel in California. This “minor” change resulted in numerous problems due to shrinkage of O-rings made from nitrile rubber, including leakage in pumps and other fuel system components [152]. Aromatics cause nitrile rubber to swell and after the switch to lower aromatic fuel, the nitrile rubber O-rings “dried out” and shrank.

Material compatibility of biodiesel and its blends with elastomers and metals was investigated in an experimental laboratory study [149]. Samples of tested elastomers were stored in biodiesel blends at 51.7°C for up to 694 hours. The tensile strength, elongation, hardness, and swell were determined for each sample after the storage period. It was concluded that the physical properties of nitrile rubber, nylon 6/6, and high density polypropylene were affected by biodiesel and its blends while Teflon, Viton 401-C and Viton GFLT appeared to have good resistance to biodiesel, Table 1.

Metals evaluated in the study included copper, steel, brass, aluminum, and bronze. Samples of metals were stored in the biodiesel and its blends for 6 months at 51.6°C. The samples were rated visually and the total acid number (TAN) of the fuels was determined. Copper containing metals (this included brass and bronze) exhibited severe corrosion when exposed to biodiesel and its blends. They also formed large amounts of gum when stored at elevated temperatures. The steel and aluminum samples did not exhibit gum formation, but did exhibit exceptionally high total acid numbers which could corrode various parts of the fuel system. Zinc is also incompatible with biodiesel.

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