Reducing Mechanical Friction

Mechanical friction losses can be reduced by converting sliding metal contacts to rolling contacts, reducing the weight of moving parts, reducing production tolerances to improve the fit between pistons and bore, and improving the lubrication between sliding or rolling parts. Friction reduction has focused on the valvetrain, pistons, rings, crankshaft, crankpin bearings, and the oil pump. This is an area where OTA found considerable disagreement among manufacturers interviewed.

Rolling contacts and lighter valvetrain

Roller cam followers to reduce valvetrain friction are already widely used in most U.S. engines. In OTA interviews, some manufacturers claimed that once roller cams are adopted, there is very little fiction left in the valvetrain. Other manufacturers are pursuing the use of lightweight valves made of ceramics or titanium. The lightweight valves reduce valvetrain inertia and also permit the use of lighter springs with lower tension. Titanium alloys are also being considered for valve springs. A secondary benefit associated with lighter valves and springs is that the erratic valve motion at high rpm is reduced, allowing increased engine rpm range and power output.

Fewer rings

Pistons and rings contribute to approximately half of total fiction. The primary function of the rings is to minimize leakage of the air-fuel mixture from the combustion chamber to the crankcase, and oil leakage from the crankcase to the combustion chamber. The ring pack for most current engines is composed of two compression rings and an oil ring. The rings have been shown to operate hydrodynamically over the cycle, but metal-to-metal contact occurs often at the top and bottom of the stroke. The outward radial force of the rings is a result of installed ring tension, and contributes to effective sealing as well as fiction. Various low-tension ring designs were introduced during the 1980s, especially since the need to conform to axial diameter variations or bore distortions has been reduced by improved cylinder manufacturing techniques. Elimination of one of the two compression rings has also been tried on some engines, and two-ring pistons may be the low friction concept for the future. Here again, we found considerable disagreement, with some manufacturers stating that two-ring pistons provided no friction benefits, while others suggested fiction reduction of 5 to 10 percent.

Lighter pistons

Reducing piston mass is the key to reducing piston fiction, and engine designers have continuously reduced mass since the 1980s. Analytical results indicate that a 25 percent mass reduction reduces fiction mean effective pressure by 0.7 kilopascals at 1500 rpm.43 Secondary benefits include reduced engine weight and reduced vibration. Use of advanced materials also results in piston weight reduction. Current lightweight pistons use hypereutectic aluminium alloys, while future pistons could use composite materials such as fibre-reinforced plastics. Advanced materials can also reduce the weight of the connecting rod, which also contributes to the side force on a piston. Manufacturers agreed that a 25 to 30 percent reduction in piston and connecting rod weight could occur by 2015.

Coatings

Coating the piston and ring surfaces with materials to reduce wear also contributes to fiction reduction. The top ring, for example, is normally coated with molybdenum, and new proprietary coating materials with lower fiction are being introduced. Piston coatings of advanced high temperature plastics or resin have recently entered the market, and are claimed to reduce fiction by 5 percent and fuel consumption by 1 percent.44 Some manufacturers claimed that coatings wear off quickly, but others suggested that advanced coatings were durable for the life of the engine. These differences may be owing to proprietary advantages in coating technology with some manufacturers.

Improved oil pump

Friction in the oil pump can be reduced by optimizing oil flow rates and reducing tolerances for rotor clearance. Some manufacturers suggested fiction can be reduced by 2 to 3 percent with improved oil pump designs, for a 0.3 to 0.4 percent fuel economy benefit.

Lubricants

Improvements to lubricants used in the engine also contribute to reduced fiction and improved fuel economy. Friction modifiers containing molybdenum compounds have reduced friction without affecting wear or oil consumption. Some manufacturers stated that future synthetic oils combining reduced viscosity and fiction modifiers could offer good wear protection, low oil consumption, and extended drain capability, as well as small improvements to fuel economy in the range of 1 percent over current 5W-30 oils.