October 14, 2011 Rubbing Out Friction in the Push for Mileage By PAUL STENQUIST Detroit WITH more than three decades of intensive engineering efforts devoted to the cause, automakers have depleted the store of cheap and easy solutions to improving fuel economy. Yet with federal mileage standards rising to 50 miles per gallon and above in coming years, the pressure to do better is only increasing. That is why reducing friction inside engines is becoming crucial. “We have to hit every system on the engine that slides or rotates,” said Mike Anderson, General Motors’ global chief engineer and program manager for 4-cylinder engines. “We have to go after all of these interfaces.” Friction reduction is proving to be an effective pursuit in the struggle to improve engine efficiency, complementing innovations like direct fuel injection, variable valve timing, turbocharging and cylinder-shutdown systems. In recent years, all have contributed to the development of engines that produce more power from less fuel. Internal-combustion engines are by nature high-friction devices. A multitude of mechanical parts turn or rub in metal-to-metal contact, separated only by a thin oil film. Friction nibbles away at engine power, producing heat that is lost to the atmosphere. Limiting those losses is a goal of every automaker, with huge efforts directed toward tiny returns. For instance, the 3.5-liter V-6 engine of the 2011 Honda Odyssey benefits from a suite of advances that improves the minivan’s E.P.A. highway rating to 28 m.p.g. from 25 for the previous version. Among the improvements were measures that cut engine friction by 4 percent, good for 0.15 m.p.g. — a small but worthwhile gain, said Paul DeHart, senior powertrain engineer for the Odyssey. Of all the power-robbing parts in an engine, Mr. DeHart said, the piston assembly was the source of the most significant losses. As the piston moves in the cylinder bore, the piston rings press against the cylinder wall, providing a seal for combustion gases and keeping lubricating oil from being burned. All the while, the piston’s sides, or skirts, slide against the cylinder wall. Considerable progress is being made in reducing the friction losses resulting from piston movement. Mercedes-Benz, for example, has developed a slippery cylinder-coating technology it calls Nanoslide, first used in 2006 on the 6.3-liter AMG V-8. In the Nanoslide coating process, the cylinder walls are sprayed with an ultrathin layer — 4 to 6 one-thousandths of an inch — of a molten iron-carbon alloy. A special finishing process puts a smooth surface on this extremely hard coating, at the same time opening tiny pockets in the metal that retain oil for lubrication. In the diesel V-6 engine of the ML350 BlueTec model, Mercedes says, Nanoslide has reduced fuel consumption by 3 percent. Other automakers have taken different approaches to producing cylinder surfaces that are as smooth and wear-resistant as possible. For example, Honda has developed a technique it calls plateau honing for the cylinder walls of its engines. Rather than a standard single-step machining process with a honing tool — an abrasive that smoothes the cylinder to the required finish — plateau honing uses two stages of grinding to produce a surface that is ultrasmooth yet leaves a pattern of very fine grooves to hold oil. Even that may soon be outdated. The director of advanced powertrains at Chrysler, Chris Cowland, said in a telephone interview that his group was developing a technology that used a laser to burn a honing pattern into cylinders. But the best cylinder-finishing practices are wasted if the bores are subsequently pulled out of round when bolts are tightened during engine assembly or by the expansion of the metal at running temperatures. Powertrain engineers emphasize the importance of designing an engine block that will not exhibit cylinder distortion when in use. Mr. Anderson, the G.M. engineer, said that when designing the engine block for the company’s new 2.5-liter 4-cylinder, engineers did extensive computer modeling of the block’s structure. The modeling included simulations of the conditions that could cause cylinder distortion. “Because thermal and assembly loads handshake with one another, we do coupled analysis,” he said, referring to the practice of doing the two simulations concurrently. “It requires some of the world’s most powerful computers.” With cylinder bores held to tight tolerances and engineered to remain as round as possible in operation, the tension exerted by the piston rings can be reduced, lowering the drag as the piston travels through its stroke. The rings can also be made narrower, reducing their contact area. Further efforts to cut friction losses include shrinking the size of the piston skirts and applying friction-reducing coatings. Pistons in the 1.8-liter engine of the 2012 Honda Civic carry a molybdenum treatment applied in a polka-dot pattern. Such slippery coatings are not just for gas-sippers; they have been used in powerful engines like the 7-liter V-8 of the Chevrolet Corvette Z06. Automakers have also shifted the positioning of the engine’s crankshaft in respect to the cylinder bores to gain an advantage. For a new 1-liter 3-cylinder engine, Ford engineers specified a crankshaft position that was offset, rather than centered, under the cylinders. Ford’s technical leader for gas engine systems, Stephen Russ, said this reduced the force of piston skirts against the cylinder wall, letting the piston slide with less resistance. Oil splashing against the engine’s rotating parts — even in the form of an oil mist — can also create drag that cuts efficiency, so engineers work to minimize lubrication. The flow of oil draining back to the sump from the top end of the engine is carefully routed to keep it away from the spinning crankshaft. Because thick oil requires more power to move, automakers are specifying lighter viscosities. But other steps to keep oil flowing freely are being taken as well. Mr. Anderson noted that G.M. had designed a computer-controlled two-stage cooling-system thermostat for its new 2.5-liter engine. By letting the engine run hotter when conditions are not extreme, the oil remains thinner. Friction savings have been realized on the engine’s top end as well. Many automakers have abandoned once-common bucket-type valve lifters, an arrangement where the lifter is pressed directly by the cam lobe rather than a rocker arm. The sliding motion of the cam across the lifter is a source of considerable friction; today, roller-tipped cam followers are becoming the norm, as they generate less friction. Automakers that have retained the bucket-type valve lifter fight friction with hard and slippery coatings. Nissan, for instance, uses a coating called Diamond-Like Carbon on piston rings, piston pins and cam followers. The hard film binds well with engine oil and, according to Nissan, reduces overall engine friction by 25 percent. Other techniques, including smaller crankshaft bearings and straighter camshaft chain drives, are being applied in the effort to keep gasoline engines alive in a 50 m.p.g. world. How successful has that effort been? It’s difficult to quantify the gains achieved by each technology because the components and systems interrelate. For example, more uniform cylinder bores enable the use of low-tension piston rings and tighter tolerances. Reduced clearance allows the use of lighter-grade oil. Putting a percentage number on any single change is difficult. However, General Motors has quantified the overall friction reduction for three of its 4-cylinder engines. The 2007 2.4-liter 4-cylinder generated 46 percent less friction in low-speed driving than G.M.’s 2-liter 4-cylinder of the early 1980s, despite having more valves, a more complex camshaft drive and a pair of counterbalancer shafts. Friction reduction alone resulted in about 7 percent better fuel economy over that 24-year period. In the new 2.5-liter 4-cylinder that will power the 2013 Chevrolet Malibu, friction has been cut another 16 percent, resulting in a 2 percent engine efficiency gain compared with the 2007 2.4-liter. Each engine developed more power per liter than its predecessor, doing more with less.