Ferrocene: A Smokeless Additive

Ferrocene is an iron-based additive discovered in the early 1950's. It is an unusually stable orange crystalline solid with a formula of FeC₁₀H₁₀ (heat of decomposition 465 ºC). The compound is soluble in common organic solvents and stable towards acids and bases ^[1].

One of the most important applications for ferrocene is its use as an additive for promoting the smokeless combustion of fuels. Comparatively low concentrations of ferrocene have a marked effect upon carbon formation, and appear to catalyze the oxidation of soot. Further, ferrocene has demonstrated antiknock properties for use in fuels for spark-ignition engines ^[1].

The action of an antiknock additive such as ferrocene is the opposite needed for efficient compression-ignition where rapid flame formation and propagation is vital. An antiknock compound will slow flame development by absorbing heat from the flame front, an effect that is undesirable and deleterious to fuel efficiency in a heavy-duty diesel engine. This effect could cause the engine to consume more fuel in order to produce the needed power. This action might also lead to increases in oxides of nitrogen (NO_x) as the increase in fuel input causes increased combustion gas temperature.

Ferrocene has been evaluated for its effect upon fuel efficiency and economy by Southwest Research Institute, the US Bureau of Mines, and the University of Minnesota. These studies confirm that the antiknock properties of ferrocene lead to increased fuel consumption and NO_x emissions, while having a positive effect upon soot removal and oxidation.

Southwest Research Institute tested ferrocene in the early 1980s. The study was conducted in a gas turbine engine. The additive was tested to determine it's affect upon flame radiation (fuel efficiency) and soot formation. The study determined ferrocene had a dramatic effect upon the oxidation and removal of soot, but had no measurable effect upon combustion efficiency. The study did not determine the effect of ferrocene upon particle size or mass. It was also discovered that ferrocene oxidized to form iron oxide. The iron oxide coated the exposed combustion chamber components forming a red rouge. The rouge could act as a polishing agent, and in high concentrations, could create excessive wear to valve stems. The rouge buildup can probably be prevented with the use of a detergent additive ^[2].

The University of Minnesota study determined the addition of ferrocene had a negative impact upon fuel consumption. The study was intended to show ferrocene created greater improvements in specific fuel consumption (faster heat release and higher mean effective pressure) and soot particle reduction over time, as the engine conditioned or was coated with reactive iron. The ferrocene additized diesel fuel showed an immediate increase in specific fuel consumption of 4%. The specific fuel consumption remained greater than the baseline diesel fuel for one of the test engines, regardless of ferrocene concentration. Fuel consumption for the second engine appeared to return to a point near that of baseline diesel fuel after ferrocene concentration was reduced. The University of Minnesota study also showed ferrocene produces soot particles that are smaller, but greater in number (6 to 9 fold increase in ultrafines). No change in particulate mass was observed, and NO_x emissions were increased ^[3].

A third study confirms the findings of those mentioned above. This was done by the US Bureau of Mines. Test results show the addition of ferrocene to diesel fuel caused increases in carbon dioxide (CO₂) of 2% to 8%, and increases in NO_x of 12%, with an associate decline in oxygen (O₂) ^[4]. These data indicate increases in fuel consumption. As more fuel is introduced into the engine, more O₂ is needed to combine with the fuel hydrocarbons, producing greater concentrations of CO₂ and lower concentrations of O₂ in the exhaust gases. This would of necessity generate higher combustion temperatures leading to increases in NO_x. The increase in the fuel-air ratio of 3% observed during the ferrocene treated test confirms this ^[4].

The exhaust gas concentration changes were also confirmed by the trend toward increasing brake specific fuel consumption the longer the engine was operated on ferrocene treated fuel ^[4]. The test engine was examined after 250 hours of running on ferrocene treated diesel and a layer of ferric oxide that was resistant to scratching had collected on the combustion chamber components.

These studies confirm ferrocene to have both antiknock and smokeless additive properties. One action appears to be detrimental to fuel efficiency in compression-ignition engines, while the second action has no effect on efficiency as it takes place post combustion.

Ferrocene is the active ingredient for a number of additive brands. In the past, these have included additives sold by Econalytic Systems, Parish Chemical, Octel/Starreon and Exxon/Nalco. Some brand names include Satacen, Octimax, Catane and Ferox

The active ingredient for FPC® Fuel Performance Catalyst is also iron-based, but is not a ferrocene. Unlike ferrocene, FPC® readily decomposes to form free radicals or ions that initiate faster flame development. No heat is absorbed from the flame front and rather than slow flame propagation, like antiknock additives, flame development is enhanced. Studies by Southwest Research Institute, Brigham Young University, Automotive Testing Laboratories, and a number of international testing institutions confirm the use of FPC® will reduce fuel consumption as much as 9%. These same independent tests show FPC® reduces the emissions of CO₂, carbon monoxide, unburned hydrocarbons, and particulates with no increase in NO_x.

FPC® also functions to reduce engine smoking by inhibiting the formation of particle precursors, and/or preventing the nucleation of these particle precursors. And the oxidized iron exiting the engine remains in close association with the lower soot concentrations that form, catalyzing the oxidation of the soot at lower exhaust temperatures. The net result is a more fuel efficient and cleaner system throughout.

References

1. Rausch, Marvin, Vogel, Martin, and Rosenberg, Harold, Ferrocene: A Novel Organometallic Compound, Journal of Chemical Education.
2. Conversation between the author and Mr. David Naegli and Mr. Vernon Markworth, SwRI, 1993.
3. Du, C.J., Kracklauer, J. and Kittelson, D.B., Influence of an Iron Fuel Additive on Diesel Combustion, SAE Paper 980536, 1998.
4. Zeller, William H., and Westphal, T.E., Effectiveness of Iron-Based Additives for Diesel Soot Control, US Bureau of Mines, RI 9438, 1992.