By Joe T. Joseph, Ph. D.

Transportation fuels, specifically diesel fuel, play a vital role in the expanding global economy. Governments and social agencies are tightening fuel specifications and the demands from consumers for higher fuel economy and better engine performance are unrelenting. Engine manufacturers are constantly improving engine designs, particularly fuel injectors, and requiring better fuel quality. The demands on fuel get worse when the rulemaking entities cannot agree on reasonable specifications and require inadequate or inappropriate tests. The only help the fuel is getting is from the additive supplier.

The latest challenge for diesel fuel is the numerous complaints about deposit formation on fuel injectors and fuel filters. Of the two, the fuel injector fouling issue is more serious and expensive to rectify. Severely fouled injectors reduce power and lower fuel economy. Cleaning and or replacing fuel injectors is not trivial in terms of cost and downtime. The problem is exacerbated by advanced fuel injector designs, which have ever narrower orifices. Original Equipment Manufacturers (OEMs) are demanding cleaner fuels to prevent Fuel Injector Equipment fouling. The fouling problem seems to be more frequent in biofuels and biofuel blends.

The injectors used in some diesel engines, especially the Peugeot DW10B engines, have a higher frequency of fouling tendencies. This problem is primarily in the EU where most of the Peugeot vehicles are sold. Accordingly, the Commission of the European Communities (CEC) adopted a test based upon this engine. Most European fuels contain a generic detergent package and easily pass this test. However, a variation of this test is made more severe by adding 1-2 ppm zinc in the form of zinc neodecanoate (a soluble form of zinc compound) in the fuel. Consequently, the additive suppliers had to create a unique detergent package, usually at a higher cost, for zinc-doped fuel.

There are several dimensions to the injector fouling issue. It has been proven that severe deposit formation on the FIE robs the engine of performance and hence fuel economy. The problem is serious enough that it begs a thorough evaluation.

What is the nature of deposits? – Laboratories from around the world have analyzed FIE deposits. Deposits differ depending on the fuel source, engine and FIE designs and geographic regions. The deposits can be classified into two general types; organic or carbonaceous deposits and inorganic deposits. The carbonaceous deposit structure may be amorphous or crystalline. Carbonaceous deposits are likely formed by chemical reactions in the fuel injector itself. The inorganic deposits may contain compounds of metals as chlorides, sulfates or oxides. Iron seems to be a universal contaminant and most likely comes from engine parts or refinery and distribution equipment. Calcium, sodium and potassium may come from refinery equipment, road salt contamination or lube oil and there is no doubt that these metal compounds contribute to the deposit build-up. Conversely, controlled experiments have shown that zinc can assist deposit formation. It is believed that zinc is leached out from certain FIE or fuel system components in the engine. Acidic additives such as corrosion inhibitors and mono-acid lubricity improvers can help in dissolving zinc from the hardware. However, none of these metals seem to take part in the chemical reactions that lead to the formation of carbonaceous deposits.

 

How are deposits formed on the fuel injector?

The major theories boil down to the possible thermal reactions of certain types of chemical components of fuel producing insoluble substances. These substances continually build up inside and at the opening of the nozzle. How long it takes catastrophic amounts of deposits to form depends on the nature of the fuel, characteristics of the FIE and the temperature of the nozzle area the FIE experiences during engine operation.

Diesel fuels contain a plethora of hydrocarbons in various molecular configurations. At the relatively high temperatures of the nozzle, some of these hydrocarbon molecules can decompose and create highly reactive fragments that lose hydrogen and combine into larger fragments, which in turn combine to form even larger fragments. At some point these reactions will result in very large molecular species that contain predominantly carbon and smaller proportions of oxygen and hydrogen. The final material resembles amorphous carbon, which does not dissolve in fuel and lays down in the nozzle, effectively restricting fuel flow into the combustion chamber. Ultimately, the deposits grow to a state where they cause operational issues.

The molecular composition of the fuel plays a major role in deposit formation. Fuels that contain higher amounts of aromatics are believed to form deposits rather easily. Although generally true, it has been shown that even fuels containing non-aromatic hydrocarbons (alkanes) can form deposits. The mechanism of deposit formation from alkanes is believed to involve reactions with dissolved oxygen. Note that the fuel is in contact with air under high pressure. Naturally, entrainment of oxygen in the fuel is quite possible. The oxygen reacts with certain types of alkanes to form peroxy intermediates that can start the chain reaction that eventually forms carbonaceous deposits. The oxy and peroxy intermediates can also be produced by partial combustion of alkanes.

Blending of biofuels into petroleum-derived diesels has been shown to accelerate deposit formation. The most common biofuels blended into diesel fuels are soybean oil methyl ester (SME) and rapeseed oil methyl ester (RME). They are oxygenated molecules and can potentially produce the reactive oxy or peroxy molecular fragments. These intermediates can facilitate deposit forming reactions. Additionally, the oxygen in the ester molecules increases their ability to retain moisture and increases the propensity for carrying water soluble salts.

Another major cause of deposit formation is the FIE design itself. The effective temperatures in the nozzle and the dimensions of the nozzle orifices are key to deposit formation. In order to meet the ever increasing demand for fuel economy, noise reduction and increased performance, OEMs are changing the FIE technology on a continuous basis. As a result of such technological advances, the FIE is becoming more sensitive and less tolerant to fuel contamination. It is unlikely that deposits can be prevented altogether from forming in the FIE. However, it is quite feasible to control deposit formation and extend FIE life. The measures taken have to be from the fuel side as well as from the hardware side.