Advanced piston skirt coating

Dr. Neil Canter, Contributing Editor | TLT Tech Beat February 2011

A new technology reduces friction and wear to improve efficiencies in combustion engines.

KEY CONCEPTS
• A piston skirt coating reduces friction and provides scuff resistance when a piston moves up and down in a cylinder of an internal combustion engine.
• A new piston skirt coating based on a blend of graphite, molybdenum disulfide and carbon fibers offers superior performance.
• The new piston skirt coating exhibits an 18% reduction in friction and a substantial reduction in wear.

In the operation of an internal combustion engine, the piston performs a critical role in compressing the air fuel mixture so that it can then be ignited and then driven down during the combustion phase. Ensuring that the piston moves up and down in a uniform manner is very important in making sure the engine operates at optimal efficiency.

A piston skirt acts as a guide to ensure that the piston moves up and down in the cylinder in an orderly fashion. This automotive component plays an important role in minimizing the friction and wear generated in an internal combustion engine.

A recent TLT article on GF-5 noted how the new passenger car motor oil specification highlights the problem that some additive chemistries designed to reduce friction have in causing problems with engine deposit control (1). Data shows that some high-performing friction modifiers that can produce better fuel economy do not perform as well in the engine deposit test.

Coatings for the piston skirt have been developed to reduce friction and provide scuff resistance. Dr. Frank Doernenburg, director of technology, Pistons and Pins, for Federal-Mogul Corp. in Nurnberg, Germany, says, “Piston skirt coatings were developed at the end of the 1960s. Since the 1980s, the coatings have been applied to the piston skirt through the use of a screen printing process. This method involves rotating a piston in the horizontal position under a screen made of steel wires. A window in the steel wire screen is opened to enable application of the coating.”

Doernenburg indicates that the traditional coating used is a paste based on graphite. He adds, “For 95% of the piston skirt coatings, graphite has been found to be a suitable coating. An iron coating is used in the case of aluminum engine blocks that are primarily required for high-performance automobiles.”

There is the need to develop an improved piston skirt coating in order to improve the efficiency of the internal combustion engine. Such a coating has now been developed.

SOLID LUBRICANT MIXTURE
Federal-Mogul has developed a new piston skirt coating that provides superior friction reduction and wear resistance compared to existing technology. The coating has been commercialized and is known as ecotough.®

Doernenburg says, “We blend graphite, molybdenum disulfide and carbon fibers to produce a superior-performing coating. The graphite and molybdenum disulfide provided friction reduction while the carbon fiber increased the wear resistance of the coating.”

Friction loss for the entire engine is measured by a standard method that involves the use of an electric brake to run the engine.

A fourth key component in the coating is the resin. Doernenburg indicates that this type of resin enables the coating to be more durable and is also less hazardous than the resins currently used from competitors. In addition, the new coating is formulated with 10 to 12 additives, including a defoamer, to further enhance its performance.

The new piston skirt coating was evaluated in a series of performance tests vs. a standard, graphite-based coating. The Cameron Plint Rig evaluated the friction reduction and antiwear characteristics of the new coating.

Data showed that the new piston skirt coating exhibits an 18% reduction in friction and showed a substantial reduction in wear. Doernenburg says, “The positive friction reduction benefit achieved by the new piston skirt coating also leads directly to a reduction in carbon dioxide emissions.”

Friction loss for the entire engine is measured by a standard method that involves using an electric brake to run the engine. Says Doernenburg: “We use this procedure to measure the electric energy needed to move the piston up and down. The piston skirt coating demonstrated 1% to 3% lower friction losses at engine speeds ranging from 1,000-5,000 rpm.”

Urban driving simulation testing is used to evaluate the fuel consumption of engines with the new piston skirt coating and a conventional coating. One technique used is an ECE cycle, an urban driving cycle characterized by low automobile speed, engine load and exhaust gas temperature. A second procedure known as the EUDC is an extra urban driving cycle that covers more aggressive, high-speed driving modes. In both cases, the new piston skirt coating exhibits reduced fuel consumption compared to a conventional coating.

The scuff resistance of a coating was evaluated through the use of a cold-start cycle that simulates how an engine performs after sitting for an extended period of time at low temperatures. Doernenburg explains, “This test simulates an engine left at a temperature between -5 C and -15 C over a very cold weekend and then driven on a highway for 200 kilometers. The concern is the piston starts moving very fast, which increases its dilation and the risk of scuffing.”

Much lower wear is seen with the new piston skirt coating in testing at full load after 50 cold-start cycles. Greater wear for a piston skirt coated with a conventional coating is seen in the left image in Figure 1. In contrast, the same type of piston skirt prepared with the new coating displays much lower wear, as seen in the right image of Figure 1.


Figure 1. In testing at full load after 50 cold-start cycles, greater wear is seen with a piston skirt coated with a conventional coating (left). The new piston skirt coating displays much lower wear (right). (Courtesy of Federal-Mogul Corp.)

The new piston skirt coating is designed to last for the lifetime of the engine. It has a thickness of 20 microns that is 30% greater than a conventional coating. Dorenenburg says, “This added thickness means more of the coating is present to reduce friction and wear over a longer operating time period.”

The same screen printing process is used to apply the coating, which means there will be no interference or change in the current piston production process. The new piston skirt coating also is prepared with no hazardous components.

Further information on the new piston skirt coating can be obtained by contacting Jim Burke of Federal-Mogul Corp. at jim.burke@federalmogul.com.

REFERENCE
1. Canter, N. (2010), “Proper Additive Balance Needed to Meet GF-5,” TLT, 66 (9), pp. 10–18.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.