Temporary Solutions

Temporary solutions

TDC

Kevin Cameron

WHILE READING ABOUT THE EARLY History of motorcycle racing at Britain’s great Brooklands speedway, I learned it was common for the centers of the domes of thin, cast-iron pistons to be supported against sagging. A spike projected downward from the center of the piston dome’s underside, bearing against the top of the wristpin through a slot in the top of the con-rod’s small end.

1 have encountered piston-dome sag myself, but only as a subtle change-never as a large effect that required actual “propping up.” Back in 1977-78, I was looking over a set of used Yamaha TZ750D race pistons, and the domes felt odd. Comparing them with new pistons, I saw that they were ever-so-slightly sagged. Chronic light detonation from inadequate fuel (those were the days when Federal mandate was removing lead anti-knocks from premium pump gas) had exposed those pistons to unusual thumping while heating them abnormally by blowing away some of the typically quite insulating thin layer of stagnant gas that clings to surfaces.

Thinking about the propped-up iron pistons of 1907, I realized they must have been very hot-much hotter than my TZ’s higher-heat-conductivity aluminum pistons-to have needed support struts. Iron begins the softening process at 1350 degrees F, a temperature that is right at the top of the range of usable sparkplug electrode temperatures. Any hotter and the electrode (or piston!) will itself ignite the fuel-air mixture long before the spark. This is pre-ignition, which destroys engines in a big way within a very few revolutions.

This makes it clear that pre-ignition from overheated iron pistons must have been common in early racing. This also explains why W.O. Bentley saw more power on the dyno when in 1911 he replaced the iron pistons in a French-made DFP auto engine with the aluminum ones he had decided were worth a try. Think of the effect of an iron piston, its dome center glowing dull red with heat at 1200 degrees. Even if it isn’t actually pre-ignited, the intake fuel/air charge is heated so much by contact with the hot metal that it loses considerable density. Therefore less of it can fit into the cylinder, and power drops accordingly.

Bentley then boldly substituted aluminum pistons, whose superior heat conductivity transmits heat from the

dome center to the cooler cylinder wall so fast that dome temperature drops to 350 degrees-850 degrees cooler. Now the incoming charge is much less heated, so it loses less density as it enters the cylinder. As a result, more of it fits into the cylinder, so power increases.

This also provides a simple physical explanation for why early engines were given such small bores and long strokes: The smaller the bore, the shorter the heat path from dome center to cylinder wall, so the cooler the piston ran.

Bike engines running in early Brooklands events weren’t the only ones whose iron piston domes were propped up. Giant-displacement early race car engines had pistons 5 or even 6 inches in diameter, and they were cast iron. As engineers learned to make these monsters breathe better, their pistons ran hotter-eventually enough so to cause pre-ignition. When World War I began, thousands of German aircraft engines also had piston-dome support struts, indicating that piston sag affected them as well.

In the fascinating book, Mercedes and Auto Racing in the Belle Epoque, 1895-1915 by Robert Dick, I learned that in those days, the racer’s way to keep pistons from becoming hot enough to cause pre-ignition was “over-lubrication’-sending a lot more oil to the crankcase than necessary for the proper total-loss lubrication of crank and conrods. One of the duties of the riding mechanic always carried in those days was to pump oil to the engine. Racing bikes had a hand pump for the same function.

This, in turn, explains why auto engineering pioneer Frederick Lanchester proposed before 1910 that piston-cooling oil jets become a part of engine design. Hot-mnning iron pistons desperately needed cooling! As you know, today such oil jets are routinely a part of sports and racing engines, even though their pistons are all made of aluminum. Pistons thin and light enough for high rpm use are too thin to act as effective “heat pipes” that can conduct their heat outward to the cooler cylinder wall. This effect is intensified by the steady growth in bore size as strokes have been made progressively shorter. To keep such large-diameter and thin-domed pistons cool enough to retain the strength they need, oil jets must take over much of the cooling job.

Lanchester was also one of the first to design and use effective oil-scraper rings to stop excess oil from reaching the combustion chambers. He disliked exhaust smoke on principle and knew that oil reaching combustion chambers created heavy deposits that required periodic “de-coking.” Modern car and bike engines have highly effective oil-control rings that leave just enough oil behind them on the cylinder wall to lubricate the top ring(s). Fifty years ago, when oils were less good than at present, oil-control rings were deliberately made to leave more oil on cylinder walls to ensure acceptable service from the top rings. This is why the cars in old movies from the 1930s and ’40s can be seen to smoke.

Today, top rings have to make do with less oil because any reaching the combustion chamber becomes unburned hydrocarbon emissions that cost money to clean up via special technologies. This is part of the reason current piston rings have extremely hard wear surfaces of chromium, molybdenum or ceramic oxides. Acceptable service from rings receiving radically less oil requires super-hard wear surfaces.

I love finding out these things because they reveal that early engineers were neither stupid nor ignorant. They were just responding effectively to the problems in front of them. □