When W.O. Bentley tested aluminum pistons in 1911, folks regarded him as foolish. But in fact merely replacing the iron pistons in his test engine with aluminum pistons of the
same compression ratio increased power significantly. Why? When fresh fuel-air mixture entered the cylinder of an engine with iron pistons, their high crown temperature (sometimes glowing dull red in their centers) expanded the mixture, reducing its density and thereby reducing power. Replacing those iron (steel was used in some racing engines) pistons with aluminum of three times the heat conductivity eliminated some of that mixture heating and expansion, so power increased with no other change.
As a result, the use of aluminum pistons has been the norm in gasoline engines and light Diesels for 100 years. Heat has other effects besides reducing engine-mixture density. As mixture is taken into an engine, it is heated by contact with hot metal surfaces. It is further heated by compression, and then is heated again as ignition causes a steep pressure rise in the combustion chamber. If all this cumulative heating brings the last parts of the mixture, out near the cylinder wall, to over 900 F, heat-driven chemical changes in that mixture will cause those last bits of mixture to autoignite before the combustion flame can reach them. Such auto-igniting mixture burns at the local speed of sound, generate shock waves that overheat metal parts and erode them. Parts are damaged or destroyed by this “knocking combustion,” or detonation. Engineers now know the importance of keeping piston crowns and combustion-chamber surfaces cool. Because this is harder to achieve in air-cooled engines, they cannot use the high-compression ratios that are common in liquid-cooled designs. Typical an air-cooled compression ratio is in the range of 8 to 10:1, while in liquid-cooled, numbers as high as 12:1 are common, and 15:1 has been used in race engines. Another temperature problem in engines is keeping the oil in piston ring grooves from rising into the 350 to 400 F range where gumming and ring sticking begin. If the ring seal is lost, combustion gas leaks down the piston skirt, destroying lubrication and bringing prompt failure. These two heat-driven effects–detonation and piston-ring sticking–require that piston crowns and ring grooves be kept to survivable temperatures. Small turbo-Diesel auto engines face special problems. To make such engines competitive in power with spark-ignition engines, they must be highly turbocharged, raising combustion pressure very high and driving heat into piston crowns. To suppress emissions of nitrogen oxides, the air entering such engines must be diluted with exhaust gas that has been cooled through a heat exchanger. The presence of this inert gas
in addition to fresh air raises combustion pressure and piston stress even higher, leading to recent chronic problems with surface erosion or cracking in aluminum pistons. Therefore makers of these engines are developing either steel pistons or pistons with steel upper parts. Because steel conducts heat only 1/3 as well as aluminum, such pistons would quickly stick their rings were they not provided with galleries behind the rings, through which cooling oil is circulated. The extra weight of such complex and rugged construction is not a problem in Diesels. Diesels make their power mainly through pressure at low to moderate revs, while unsupercharged spark-ignition engines must make their power more through high rpm. In the early 1940s, former aircraft engine builder Wright Aeronautical, after having the usual problems of aluminum pistons cracking and scoring at high power, tested with pistons made of steel. These were entirely satisfactory in strength and weight, but the problem of keeping their domes and ring grooves cooled sufficient to prevent detonation or ring-sticking could not be solved before aircraft piston engines were made obsolete by jets, circa 1957. I suspect it will not easily be solved today, either, for while Wright’s engines had roughly equal bore and stroke, today’s motorcycle engines have greatly increased piston area from bores that are 35 to 80 percent bigger than their strokes. That area collects a great deal of heat from combustion, and unless that heat is removed, a steel piston crown will quickly rise to temperatures that both limit power (through charge heat expansion and need for lowered compression ratio) and can lead to engine failure (through destructive detonation and/or seal failure through piston ring sticking). Perhaps these problems can be solved, but they are formidable. Large marine Diesel pistons (some are 48-inches in diameter) and their sealing rings are cooled by continuous circulation of liquid coolant through complex passages behind the rings and under the dome. In many cases this liquid is delivered through telescoping pipes. It is hard to imagine such pipes reliably serving a Ducati Panigale’s 112mm pistons during 11,000-rpm operation, and it is not reasonable to imagine taking enough oil for that job from the already hard-working connecting-rod bearing. Therefore the oil would have to be delivered by oil jets and captured by “scuppers” made as part of the piston, then sent to pass through cooling galleries behind the piston rings. Let’s see how thing go.
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When W.O. Bentley tested aluminum pistons in 1911, folks regarded him as foolish. But in fact merely replacing the iron pistons in his test engine with aluminum pistons of the same compression ratio increased power significantly. Why? When fresh fuel-air mixture entered the cylinder of an engine with iron pistons, their high crown temperature (sometimes glowing dull red in their centers) expanded the mixture, reducing its density and thereby reducing power. Replacing those iron (steel was used in some racing engines) pistons with aluminum of three times the heat conductivity eliminated some of that mixture heating and expansion, so power increased with no other change.
