The Lives Of Pistons

Pistons of a four-stroke engine are exposed to combustion flame for approximately 70 crank degrees out of the full cycle of 720 or, in round numbers, 10 percent of the time. During the other 90 percent of the time, heat gained by the piston can flow away to the cooler cylinder wall or to engine oil thrown off the rotating crankshaft and its bearings, possibly assisted by cooling oil jets located in the crankcase and directed upward against the underside of the piston dome.

This heating and cooling are serious business because a piston's temperature is a powerful factor in its durability. The poorer the cooling, the hotter the piston operates, and the faster the processes of crack formation and fatigue failure advance. The engine of the old red van I drove to races in the 1970s, torn down at roughly 100,000 miles, had eight cracked pistons yet none had failed. In 1973, Kawasaki downsized the cooling-fin area of cylinders on its 750cc H2-R race engine and its pistons began to show visible cracking under their wrist-pin bosses in as few as 50 miles.

The heat of combustion is intense. Think of Germany’s famed MG42 machine gun, operating at a cyclic rate of 1,200 rounds per minute, which is a cycle time of 60/1,200 = 0.05 second. If the accelerating bullet spends about 0.0015 second in the barrel, that is a heat exposure of just three percent of the cycle time. Yet during operation, the barrel heats up so rapidly that switching from the hot barrel to a cool replacement is recommended at 250 rounds of steady firing. It’s no fun when a live round is ignited by an overheated barrel before the bolt is locked closed.

When I had the privilege of interviewing Udo Gietl, builder of the Butler & Smith racing BMW flat-twins of the 1970s, he told me that pistons in such air-cooled cylinders needed all the help they could get in ridding themselves of heat. Therefore, the team used full-skirted pistons that were in intimate contact with the cylinder wall (through the extremely thin oil film left by the oil-scraper ring) over a large area. While a "slipper-skirt" piston might be lighter, stronger, and offer a bit less friction, its lesser contact area could not cool it as well as a full skirt.

Modern pistons are just a disc tall enough to accommodate the piston rings, with very short vestigial skirts to keep it roughly square in the bore, joined by webs to a pair of wrist-pin bosses. Such very light, flat-topped, and thin pistons are essential for the super rpm of modern sport and racing engines, but they are normally cooled by oil jets. When builders eager to pump up their classic air-cooled bike engines update to such pistons, they are usually rewarded by early failure because such nearly skirt-less and thin-crowned pistons have little ability to rid themselves of combustion heat. Compare the light weight of such modern “ashtray” pistons with the more substantial heft of an original 1960s and ’70s piston. The original has a thicker dome to act as a “heat pipe” to conduct heat to the cylinder wall, generous skirts of large contact area, and a long wrist pin because its bosses are part of the skirt, not “stalactites” projecting downward from the underside of the dome.

By far, the bulk of military aviation in WWII was powered by air-cooled radial piston engines. When the war began (September 1939, in Europe; December 7, 1941, in the Pacific) most of these engines kept their pistons cool by ducting cooling air through close-pitch finning machined into the ODs of their steel cylinder barrels. This was okay for training duty (“Make these things last, boys, because Congress can’t afford to be generous”), but steel fins on steel cylinder barrels conduct heat poorly. When war began, throttles were pushed to the firewall (hence the expression, “balls to the wall”) and steel cooling fins became a bad joke as oil evaporated off overheated cylinder walls, leading to scored, seized, or burned pistons. In some engines, piston rings wore to half their original radial thickness in 30 hours of operation. Stopped props were frequent.

To improve cooling, finned muffs made of higher-conductivity aluminum were shrunk onto the steel cylinder barrels. Even better was “W-fin,” a process by which a multitude of thin pressed aluminum sheet-metal fins were caulked into dovetail grooves machined in cylinder barrel ODs. By this means it was possible to give each barrel 54 fins. The final refinement, adopted just as the war was ending, was piston-cooling oil jets. Tom Sifton, the great Harley-Davidson tuner and innovator from the 1930s to the ’70s, adopted them as well, knowing that pistons in air-cooled engines needed all the help they could be given.

Back in the two-valve era, many engines had wide valve-included angles and deep combustion chambers. To achieve a torque-building high-compression ratio in such a deep chamber required pistons with high domes. Both the deep chamber and the high dome increased the surface area through which heat flowed from combustion gas into head and piston.

In today’s engines, piston crowns and combustion chambers are nearly flat, giving them minimum surface area. This is somewhat offset in racing and sportbike engines by larger-bore/shorter-stroke design (the larger bore allows plenty of valve area and the shorter stroke reduces inertia stress at high revs). Auto engines, under the pressure of government regulation, have generally gone the other way, toward smaller pistons whose diameter is less than their stroke. This has been done to improve fuel mileage by reducing heat loss from combustion.

Maybe all this complexity and thoughtful design is now made irrelevant by a coming era of electric cars, bikes, trucks, trains, planes, and ships. On a recent flight from Dallas to Los Angeles, I looked down on the sun-drenched badlands of New Mexico and Arizona—thousands of square miles. In my mind’s eye, I could imagine the rail lines, freight terminals, and supporting shops being set out, the service roads being graded and surfaced. Also the many residence communities built for the work crews and engineers installing and maintaining millions of tons of solar-electric generation equipment pouring in from manufacturing centers nationwide, operating around the clock. Zero-emissions war on climate change! When does the great work begin? When the investors see guaranteed profitability.