No part of an engine works harder than its pistons. As a piston rises on compression, the pressure of the fuel-air mixture confined above it rises to 200 psi. Then a spark ignites the mixture. In the next 45 degrees of crank rotation, the mixture burns, reaching a peak pressure of perhaps 1,200 psi. Gas temperature jumps up 4,500 degrees Fahrenheit. The melting point of pure aluminum is just under 1,200 degrees Fahrenheit, and alloying for greater strength reduces that to a bare 1,000 degrees. How can the metal survive? First, pistons are exposed to combustion only one-tenth of the time—60 to 80 degrees out of the 720 of the 4-stroke cycle. Second, the piston’s surface is protected by a thin “boundary layer” of stagnant gas that clings to it, providing efficient insulation. Also, aluminum conducts heat 3 times faster than iron or steel, while having a little more than one-third of their weight. This means that heat absorbed by the piston crown is quickly conducted away to a cooler part of the piston. Because aluminum is light, more of it can be used to carry away combustion heat.
FAST FACTS: Pistons – The key to fast, efficient combustion is a flat, featureless combustion chamber with at least some squish area (seen outboard of the valve cutouts). – To minimize bearing friction, pistons must be as light as possible. Forging combines light weight and improved resistance to fatigue under stress. – To ensure that gas pressure can keep piston rings always on the bottoms of their grooves, the rings are lightened by being made ever thinner.
As high-performance engines rev higher, with larger bores and shorter strokes, piston weight must be reduced to cut vibration and stresses on bearings and mechanical parts. Traditional pistons conducted their heat away to the cooler cylinder wall through thick crowns, but such weight is impracticable at high revs. Therefore, today’s light, thin, and short-skirted pistons are also cooled by jets of engine oil. People once spoke of a “limiting piston speed” (usually given as 4,000 to 4,500 feet per minute), but what really limits rpm is the extreme stress of piston acceleration. As the pistons in a V-10-era Formula 1 engine reversed direction at TDC, they reached peak accelerations as high as 10,000 G. Pistons in a 15,000-rpm, 600cc sportbike engine peak at 7,000 G. The higher the acceleration, the harder the connecting rod tries to tear the wristpin out of the piston, and the faster cracks form in the material. Because they are closest to the heat, top piston rings must be plated or filled with a high melting-point metal such as chromium (3,405 degrees Fahrenheit) or molybdenum (4,720 degrees Fahrenheit). This retards wear by discouraging local welding and plucking. Top rings are usually barrel-faced to seal even with some piston tilting. Second rings (seldom used in racing engines) are usually taper-faced. The oil scraper ring is made as a pair of thin, flexible rails exerting high specific pressure, pressed outward by a backing spring. Durable oil control is essential to long life of exhaust catalysts. Although some high-duty diesels have steel pistons, the ones used in spark-ignition engines are cast or forged from 2 basic aluminum alloy types: either a low-expansion, wear-resistant aluminum-silicon alloy, or a high-hot-strength aluminum-copper-nickel-magnesium alloy. Both alloy types have a long history. Pistons have evolved from a bucket-like shape with high dome of the 1960s to the present flat-topped “ashtray” proportions, with every detail of their underside given organic grace by refined stress studies. Parts shaped this closely to nature’s requirements have a compelling beauty. They cannot have any other form and function as well.