What Is Driving the Reduction in Piston Weight?

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What is driving the reduction in piston weight? That is the question posed by Sotaro, writing in response to my piece “Piston Speed Versus Piston Acceleration.”

The answer is, many things. Twenty years ago I had a brief tour through Jack Roush’s NASCAR engine-building facility and was surprised to see how empty those engines look. As Sotaro notes, pistons used to resemble buckets and be quite heavy. The pistons I saw in those Roush engines were radical “ashtray” designs like those in modern motorcycle engines—consisting of little more than a flat disc thick enough to carry the sealing and oil-scraper rings, with a short stubby wrist pin as close to the rings as possible. Two very short skirts project downward, so short that when you set the piston on a table it is the wrist pin bosses that touch first.

When pistons are made lighter, crank counterweights can be smaller, contributing to the “empty engine look” I saw at Roush.

The first reason for lightening pistons is that the higher rpm levels of modern engines would otherwise generate higher bearing friction torque from the high inertia of the reciprocating parts. Friction is always a no-no in our world of carbon limits and mandated auto fuel consumption.

Piston crowns in former times had to be thick enough to conduct combustion heat, gathered over the piston crown’s area, radially outward to the cooler cylinder wall. Large aircraft engines began to employ piston cooling oil jets in the mid-1940s and oil-cooled pistons had been a feature of slow-turning heavy-duty diesels long before that (diesel pistons often have an oil gallery behind their piston rings to keep that area cool enough to avoid oil polymerization and ring sticking). But in the case of high-revving auto and bike engines, oil cooling meant that a lot of piston crown thickness could be done away with because piston temperature control now depended on oil jets.

A Honda paper on development of its F1 engines reveals that they have employed as many as 20 such jets per piston. A simple approach to oil-jet piston cooling is to aim a jet up at the hottest part of the undersurface of the crown: its center. Slightly more sophisticated is to cut through two or more of the reinforcing ribs under the crown, so that a single oil jet can be aimed up at one side of the crown’s underside, and its flow is then directed across the crown to the far side, and is then deflected down by the skirt on the far side.

A great lesson of racing is that metal fatigue is accelerated by temperature. The hotter any part of your piston runs, the sooner it will develop cracks. When the engine from our race van was overhauled in the mid-1970s, seven of its eight pistons were found to be cracked.

Presumably Honda 1) didn’t want to weaken piston stiffening ribs by machining “oil windows” through them, and 2) knew from temperature mapping that all is not uniform. Therefore lots of oil jets were provided to slow the process of fatigue enough to make pistons go the distance.

When pistons are made lighter, crank counterweights become smaller, easing the problems of torsional vibration (big counterweights, by adding mass to the crank, lower its torsional vibration frequencies—possibly enough to come into step with cylinder firing frequency).

The trend in piston ring design has been to make them axially thinner, so that inertia forces (which peak at TDC at the end of the exhaust stroke) cannot yank them up off the bottoms of their grooves, thereby breaking their seal. This in turn allows ring lands to be made less thick, further shortening the piston.

Short pistons afford the designer a choice of two other possible benefits. Shorter pistons can either allow the cylinder block’s deck height to be lower, saving significant weight, or allow longer connecting rods to be used. Back in the days of putting big-inch truck V-8s into the “supercars” of the late 1960s, very short rods were used to allow a lowered deck-height Vee engine to fit between the car’s front suspension details.

Notice that the wrist pin boss sits below the piston skirts on this KTM 250cc motocross piston. (KTM/)

The longer the rod (usually given as a multiple of the stroke, as in “a 2.2 rod ratio”), the smaller its maximum angle with the bore centerline, the lower the piston’s side thrust and its friction against the cylinder wall.

When pistons are lighter, con-rods can be made lighter as well. Plain journal bearing losses are roughly dependent upon journal diameter, cubed, so lighter parts can allow use of smaller bearings, with reduced friction loss.

It’s all connected.

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