The hard and slippery diamond-like coating, or DLC, holds promise for cylinder liners, piston rings, and wrist pins, where it can eliminate the need for bearings between the pin and connecting rod. Lower ring friction can unlock as much as 0.15 horsepower per cylinder.Īutomakers are also hungry for new friction-reducing finishes between parts that rub or rotate against each other. Piston supplier Federal-Mogul is experimenting with a tapered face on the oil ring that allows a reduction in the ring tension without increasing oil consumption. Friction-reducing, graphite-impregnated resin patches screen-printed onto the skirt are now nearly universal. Pistons account for at least 60 percent of the engine’s friction, and improvements here have a direct impact on fuel consumption. Creating a hollow passage means casting the piston as two pieces and joining them via friction or laser welding. The seemingly simple technology isn’t easy to manufacture, though. The squirters shoot oil into a small opening on the bottom of the piston that feeds the gallery. The hottest gasoline engines will soon require a cooling gallery, or an enclosed channel on the underside of the crown that’s more efficient at removing heat than today’s method of simply spraying the piston’s underside with oil. To compensate for the 50-percent increase in peak cylinder pressures, some aluminum pistons now have an iron or steel insert to support the top ring. In many ways, gasoline-engine development is following the path laid out by diesels 15 years ago. It is these three concerns-durability, friction, and mass-that consume the piston suppliers’ workdays. In the quest for improved fuel economy and lower emissions, automakers are asking for lighter, lower-friction pistons with the stamina to endure tougher operating conditions. To coin a maxim: As the automobile goes, so goes the engine and as the engine goes, so goes the piston.
Yet a piston’s design and technology can also say a lot about the wider trends and challenges facing the auto industry. The crown can reveal the bore, the number of valves, and whether or not the fuel is directly injected into the cylinder. In the past decade, piston operating temperatures have climbed 120 degrees, while peak cylinder pressures have swollen from 1500 psi to 2200.Ī piston tells a story about the engine in which it resides. With turbocharging and increased outputs come even tougher conditions. As automakers chase higher efficiency, piston manufacturers are preparing for a future in which the most-potent naturally aspirated gasoline engines produce 175 horsepower per liter, up from 130 today.
These days, that cylindrical Hades is hotter and more intense than ever, and it’s only likely to get worse for pistons.
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At full throttle and 6000 rpm, a piston in a gasoline engine is subjected to nearly 10 tons of force every 0.02 second as repeated explosions heat the metal to more than 600 degrees Fahrenheit. The slugs of aluminum inside your engine live in a fiery hell.
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From the October 2014 Issue of Car and Driver
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