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Piston Design, An Evolutionary Tale
By Larry Carley
It’s hard to believe the reciprocating piston engine has been around for 137 years. Nicholaus August Otto invented the first such engine in 1866, one year after the Civil War ended. Given that much time, you’d think the pistons inside today’s engines would be radically different from those of their ancestors.
Piston materials and designs have evolved over the years and will continue to do so until fuel cells, exotic batteries or something else makes the internal combustion engine obsolete. But until that happens, pistons will continue to power the vehicles we drive.
One thing that has not changed over the years is the basic function of a piston. The piston forms the bottom half of the combustion chamber and transmits the force of combustion through the wrist pin and connecting rod to the crankshaft. The basic design of the piston is still pretty much the same, too. It’s a round slug of metal that slides up and down in a cylinder. Rings are still used to seal compression, minimize blowby and control oil.
So what has changed? The operating environment. Today’s engines run cleaner, work harder and run hotter than ever before. At the same time, engines are expected to last longer than ever before, too: up to 150,000 miles or more – and with minimal maintenance. Consequently, heat management is the key to survival of the fittest.
"Piston design used to be a process of trial and error." says Kent Fullerton, an engineer with Zollner Pistons. "You’d make and test a new design three or four times before you got it right. Today, everything is modeled in 3D on a computer, then evaluated with finite element analysis software before anything is made. That speeds up the design and testing process, reduces the lead time to create new piston designs, and produces a better product."
According to a book produced by Mahle Inc. called Pistons for Internal Combustion Engines, engineers use two methods to evaluate new piston designs before they are actually produced for engine dyno testing: finite analysis and photoelastic stress analysis. The idea behind finite analysis is to divide a model piston into a fixed (finite) number of elements. The resulting grid forms lines that intersect and connect. Computer software generates equations for each individual element and predicts the overall stiffness of the entire piston.
Analyzing the data shows how the piston will behave in a real engine and allow the engineer to see where loads and temperatures will be greatest and how the piston will react.
With photoelastic stress analysis, a 3D transparent resin model is cast of a piston. When the model piston is subjected to loads, the refractive properties of the plastic change causing polarized light passing through the piston to change colors. This reveals how the piston deforms under load and the areas where it is experiencing the greatest stress.
The most critical area for heat management is the top ring area. One of the "tricks" engine designers came up with to reduce emissions was to move the top compression ring up closer to the top of the piston. A decade ago, the land width between the top ring groove and piston crown was typically 7.5 to 8.0 mm. Today that distance has decreased to only 3.0 to 3.5 mm in many engines.
The little crevice around the top of the piston between the crown and top ring creates a dead zone for the air/fuel mixture. When ignition occurs, this area often does not burn completely leaving unburned fuel in the combustion chamber. The amount isn’t much, but when you multiply the residual fuel in each cylinder by the number of cylinders in the engine times engine speed, it can add up to a significant portion of the engine’s overall hydrocarbon (HC) emissions.
One of the consequences of relocating the top ring closer to the top of the piston is that it exposes the ring and top ring groove to higher operating temperatures. The top rings on many engines today run at close to 600° F, while the second ring sees temperatures of 300° F or less. These extreme temperatures can soften the metal and increase the danger of ring groove distortion, microwelding and pound-out failure. The reduced thickness of the land area between the top of the piston and top ring also increases the risk of cracking and land failure.
The evolutionary advances that enable today’s pistons to handle this kind of environment include changes in piston geometry, stronger alloys, anodizing the top ring groove and using tougher ring materials. Ordinary cast iron top compression rings that work great in a stock 350 Chevy V8 can’t take the kind of heat that’s common in many late model engines. That’s why ductile iron or steel top rings are used in some of these engines.
Anodizing has become a popular method of improving the durability of the top ring groove and is now used in many late model engines. Anodizing reduces microwelding between the ring and piston to significantly improve durability. But it can’t work miracles: an anodized piston can still fail if it gets too hot.
Anodizing is done by treating the ring groove with sulfuric acid. The acid reacts with the metal to form a tough layer of aluminum oxide, which is very hard and wear-resistant. Part of the layer is below the surface of the metal and part is above. On average, the layer is about 20 microns (.001˝) thick so the piston manufacturer compensates for the added thickness when the top ring groove is machined.
Another approach some piston manufacturers have used to improve top ring durability is to weld nickel alloy into the top ring groove. This was the approach used for the OEM pistons in Saturn 1.9L engines made from 1991 to 2001. The 2002-’03 Saturn engine uses an anodized top ring groove.
Low Tension Rings
To further complicate the problem of heat management, rings have been getting smaller. Starting in the 1980s, "low tension" piston rings began to appear in many engines. Typical ring sizes today are 1.2 mm for the top compression ring, 1.5 mm for the second ring, and 3.0 mm for the oil ring. Some are even thinner. A few engines have top compression rings only 1.0 mm thick, and the current Buick 3800 V6 uses a narrow 2.0 mm thick oil ring.
The OEMs went to thinner, shallower rings to improve fuel economy because the rings account for up to 40 percent of an engine’s internal friction losses. Thinner rings produce less drag and friction against the cylinder walls. But the downside is they also reduce heat transfer between the piston and cylinder because of the smaller area of contact between the two. Consequently, pistons with low tension rings run hotter than pistons with larger rings.
Low tension rings also present another problem. They are less able to handle bore distortion. To maximize compression and minimize blowby, the cylinder must be as round as possible. This often requires the use of a torque plate when honing to simulate the bore distortion that is produced by the cylinder head.
Changes in piston geometry have also been made to improve their ability to survive at higher temperatures. Russ Hayes, an engineer with Federal Mogul/Sealed Power, said piston manufacturers used to grind most pistons with a straight taper profile. When the piston got too hot, it would contact the cylinder along a narrow area producing a thin "wear strip" pattern on the side of the piston. "Now we use CNC machining to do a barrel profile on our pistons. The diameter of the piston in the upper land area is smaller to allow for more thermal expansion and to spread any wall contact over a larger area."
Pistons are getting shorter and lighter. In the 1970s, a typical 350 small block Chevy piston and pin assembly weighed around 750 grams. The same parts in a late model Chevy LS1 engine weigh only about 600 grams.
Part of the weight reduction has been achieved by reducing piston height and using shorter skirts. The distance from center of the wrist pin to the top of the piston (called "compression height") used to be 1.5˝ to 1.7˝ back in the 1970s, said Hayes. Today, wrist pins are located higher up. On Ford 4.6L engines, the compression height is 1.2˝, and it’s 1.3˝ on small block Chevys.
Moving the location of the wrist pin higher up on the piston also allows the use of longer connecting rods, which improve torque and make life easier on the bearings and rings.
Some aftermarket pistons are now available with wrist pins that have been relocated upward slightly to compensate for resurfacing on the block and heads. The other alternative is to shave the top of the piston if the block has been resurfaced, but this reduces the depth of the valve reliefs which may increase the risk of detonation and/or valve damage.
Pistons used to have long tail skirts (which sometimes cracked or broke off). Now most pistons have “mini-skirts.” Instead of a 2.5˝ skirt length, the piston may only have 1.5˝ skirt. Shorter skirts reduce weight but also require a tighter fit between the piston and cylinder bore to minimize piston rocking and noise. Consequently, today’s piston clearances are much less than before (typically .001˝ to .0005˝ or less). Some have a zero clearance fit or even a slight interference fit (made possible by special low friction coatings).
The alloy from which a piston is made not only determines its strength and wear characteristics, but also its thermal expansion characteristics. Hotter engines require more stable alloys to maintain close tolerances without scuffing.
Many pistons used to be made from "hypoeutectic" aluminum alloys like SAE 332 which contains 8-1/2 to 10-1/2 percent silicone. Today we see more "eutectic" alloy pistons which have 11 to 12 percent silicone, and "hypereutectic" alloys that have 12-1/2 to over 16 percent silicone.
Silicone improves high heat strength and reduces the coefficient of expansion so tighter tolerances can be held as temperatures change. Hypereutectic pistons have a coefficient of thermal expansion that is about 15 percent less than that for standard F-132 alloy pistons. Because of this, the pistons can be installed with a much tighter fit – up to .0005˝ less clearance may be needed depending on the application.
Hypereutectic alloys are also slightly lighter (about 2 percent) than standard alloys. But the castings are often made thinner because the alloy is stronger, resulting in a net reduction of up to 10 percent in the piston’s total weight.
Hypereutectic alloys are more difficult to cast because the silicon must be kept evenly dispersed throughout the aluminum as the metal cools. Particle size must also be carefully controlled so the piston does not become brittle or develop hard spots making it difficult to machine. Some pistons also receive a special heat treatment to further modify and improve the grain structure for added strength and durability. A "T-6" heat treatment, which is often used on performance pistons, increases strength up to 30 percent.
Machining hypereutectic pistons is also more difficult because of the harder alloy. Consequently, hypereutectic pistons typically cost several dollars more than standard alloy pistons. That’s why most OEMs (except Ford) have gone back to eutectic alloy pistons in their late model engines. High copper eutectic alloys offer most of the advantages of hypereutectic alloys without as much cost.
Survival of the fittest also requires a high degree of scuff resistance. Cold starts without adequate lubrication can cause piston scuffing. The same thing can happen if the engine overheats. Piston-to-cylinder clearances close up and the piston scuffs against the bore. The initial start-up of a freshly built engine is also a risky time for scuffing and is of special concern to engine builders because that’s when many warranty problems occur.
Applying a permanent low friction coating to the sides of the pistons provides a layer of protection against scuffing. Many rebuilders have found that using coated pistons has virtually eliminated warranty problems due to scuffing.
Many late model OEM engines including Ford 4.6L V8, Chrysler 3.2L, 3.5L, 3.8L and 4.0L, and GM 3.1L use pistons with graphite moly-disulfide coatings on the piston skirt to improve scuff resistance. Most aftermarket piston manufacturers also offer some type of coated replacement pistons to rebuilders who want them. Coatings typically add about a buck to the price of a replacement piston, but the added scuff protection and reduction in warranty claims more than offsets the higher cost say many engine builders who use them.
"Thermal barrier" ceramic-metallic coatings for the tops of pistons are another type of coating that have been used on some diesel pistons and performance pistons. Improving heat retention in the combustion chamber improves thermal efficiency and makes more power. It also helps the piston run cooler. But too much heat in the combustion chamber also increases the risk of detonation and preignition, which is not a problem with diesels but is with gasoline engines. So when a coating is used, ignition timing must usually be retarded several degrees to reduce the risk of detonation.
The shape and finish on the tops of pistons has also been changing. Flat top pistons have been replaced by dished pistons, domed pistons and pistons with intricate contours to swirl the fuel mixture and promote better fuel atomization.
Some piston crown designs can be very complex because they are designed to produce the lowest possible emissions with the best overall fuel efficiency. The shape of the crown controls the movement of air and fuel as the piston comes up on the compression stroke. This, in turn, affects the burn rate and what happens inside the combustion chamber. Replacement pistons for stock engines with complex piston designs should be the same as the original to maintain the same emissions and performance characteristics.
With performance pistons, designs can be even more specialized. Manufacturers have developed special "fast burn" configurations that allow engines to safely handle more compression without detonating.
John Erb of United Engine & Machine (Silvolite and KB Pistons) said an "Attenuator-Groove" is used on some KB pistons to enhance the valve reliefs. The groove removes two potential hot spots in the combustion chamber and improves airflow and wet flow atomization.
Another unique design feature, said Erb, is the "Mini-Grooves" machined into the top ring land on KB performance pistons. If the piston gets too hot, the top of the piston swells causing the Mini-Grooves to contact the cylinder. This momentary contact helps cool the piston to reduce the danger of detonation and piston destruction.
Zollner’s Fullerton says piston pin holes have also been changing. "Rather than being round and straight, pin bores are taking on new shapes. Some are oval and some are trumpet-shaped, flaring out toward the inside edges of the pin bosses. The reason for these shapes is to accommodate wrist pin bending and ovalization. These variances from straight and round are quite small, measured in tenths of a thousandth, but have proven to extend piston life."
Down The Road
Pistons may continue to get shorter and lighter, but most engineers believe rings can’t get much smaller than they are today. Some do think, though, that the two ring piston may not be too far away. Some Indy racing motors are already running two ring pistons quite successfully.
Other design innovations that may shape the direction of future piston development include lightweight alloy wrist pins, more anodizing and/or the use of ceramic coatings on the tops of pistons and upper ring groove to improve heat resistance and wear, and maybe top rings with no end gaps.
One engineer mentioned a new piston design he’s working on for an undisclosed performance application that has only a one-inch compression height.
The best indication of what’s coming down the road is to look at today’s state-of-the-art racing pistons: super lightweight designs with almost no skirts, holes machined into the sides to reduce weight, and various design tricks to control thermal expansion and detonation under high load.
We may see some exotic graphite reinforced pistons for certain high output engines similar to ones that are now being used in diesel engines. The development of direct injection gasoline engines in the U.S. market will likely require complex fuel bowls in the tops of pistons similar to those now used in many diesel engines. Direct injection, which is starting to come on strong in Europe, allows extremely lean air/fuel mixtures (up to 40:1) and much better fuel economy. But it also requires precise control of airflow in the combustion chamber for reliable ignition and complete combustion.
If hybrid gasoline/electric or diesel/electric vehicles become more common in the not-too-distant future (which many predict will happen), no big changes in piston design will be needed because most such systems use the same basic engine designs as today.
The biggest change in piston design will occur if and when fuel cells become a competitive power source for automotive applications. In that case, there will be no need for pistons and they’ll be on the endangered species list.
Most experts believe fuel cell technology is still years away. And when it does go into production, volumes will be very limited because of high costs. Eventually the cost will come down.
But even if fuel cells do eventually take over, many experts believe piston engines will continue to be produced for smaller, economy vehicles as well as heavy-duty vehicles.
There will also be an ongoing replacement market for pistons as long as piston-powered vehicles remain on the road.