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What Is Driving the Reduction in Piston Weight?

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What Is Driving the Reduction in Piston Weight?

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What’s driving the discount in piston weight? That’s the query posed by Sotaro, writing in response to my piece “Piston Speed Versus Piston Acceleration.”

The reply is, many issues. Twenty years in the past I had a quick tour via Jack Roush’s NASCAR engine-building facility and was stunned to see how empty these engines look. As Sotaro notes, pistons used to resemble buckets and be fairly heavy. The pistons I noticed in these Roush engines have been radical “ashtray” designs like these in trendy motorbike engines—consisting of little greater than a flat disc thick sufficient to hold the sealing and oil-scraper rings, with a brief stubby wrist pin as near the rings as attainable. Two very brief skirts undertaking downward, so brief that if you set the piston on a desk it’s the wrist pin bosses that contact first.

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

The primary purpose for lightening pistons is that the upper rpm ranges of contemporary engines would in any other case generate larger bearing friction torque from the excessive inertia of the reciprocating elements. Friction is all the time a no-no in our world of carbon limits and mandated auto gas consumption.

Piston crowns in former instances needed to be thick sufficient to conduct combustion warmth, gathered over the piston crown’s space, radially outward to the cooler cylinder wall. Giant plane engines started to make use of piston cooling oil jets within the mid-Forties and oil-cooled pistons had been a characteristic of slow-turning heavy-duty diesels lengthy earlier than that (diesel pistons typically have an oil gallery behind their piston rings to maintain that space cool sufficient to keep away from oil polymerization and ring sticking). However within the case of high-revving auto and bike engines, oil cooling meant that a whole lot of piston crown thickness might be performed away with as a result of piston temperature management now relied on oil jets.

A Honda paper on improvement of its F1 engines reveals that they’ve employed as many as 20 such jets per piston. A easy strategy to oil-jet piston cooling is to intention a jet up on the hottest a part of the undersurface of the crown: its middle. Barely extra subtle is to chop via two or extra of the reinforcing ribs below the crown, so {that a} single oil jet will be aimed up at one facet of the crown’s underside, and its move is then directed throughout the crown to the far facet, and is then deflected down by the skirt on the far facet.

An awesome lesson of racing is that metallic fatigue is accelerated by temperature. The warmer any a part of your piston runs, the earlier it’s going to develop cracks. When the engine from our race van was overhauled within the mid-Nineteen Seventies, seven of its eight pistons have been discovered to be cracked.

Presumably Honda 1) didn’t wish to weaken piston stiffening ribs by machining “oil home windows” via them, and a couple of) knew from temperature mapping that every one just isn’t uniform. Subsequently plenty of oil jets have been offered to sluggish the method of fatigue sufficient to make pistons go the space.

When pistons are made lighter, crank counterweights turn out to be smaller, easing the issues of torsional vibration (large counterweights, by including mass to the crank, decrease its torsional vibration frequencies—presumably sufficient to return into step with cylinder firing frequency).

The pattern in piston ring design has been to make them axially thinner, in order that inertia forces (which peak at TDC on the finish of the exhaust stroke) can’t yank them up off the bottoms of their grooves, thereby breaking their seal. This in flip permits ring lands to be made much less thick, additional shortening the piston.

Quick pistons afford the designer a alternative of two different attainable advantages. Shorter pistons can both permit the cylinder block’s deck top to be decrease, saving important weight, or permit longer connecting rods for use. Again within the days of placing big-inch truck V-8s into the “supercars” of the late Nineteen Sixties, very brief rods have been used to permit a lowered deck-height Vee engine to suit between the automotive’s entrance suspension particulars.

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

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

The longer the rod (normally given as a a number of of the stroke, as in “a 2.2 rod ratio”), the smaller its most angle with the bore centerline, the decrease the piston’s facet thrust and its friction in opposition to the cylinder wall.

When pistons are lighter, con-rods will be made lighter as nicely. Plain journal bearing losses are roughly dependent upon journal diameter, cubed, so lighter elements can permit use of smaller bearings, with diminished friction loss.

It’s all related.

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