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 aluminum alloys,
while future pistons could use composite materials such as fiber-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.
43 J. T. Kovach et al. "Engine Friction Reduction for Improved Fuel.. Economy,” SAE paper 820085, 1982. Friction meaneffective pressure is a
measure of theamount of engine power that is used to overcomefriction rather than to provide usable torque at the engine’s output shaft.
44 A. Tanaka et al., “Development of Toyota JZ Type Engine," SAE paper 930881, 1990.