The first pistons for internal combustion engines appeared way back in 1866 when Nicholaus August Otto invented the first such engine. Given that much time, you would 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 most of 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 is a round slug of metal that slides up and down in a cylinder. Rings are still used to seal compression, minimize combustion blowby and control oil. So what has changed? The operating environment. Engines today 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 chilometri or more, and with minimal maintenance and extended oil change intervals. Consequently, heat management is the key to survival of the fittest. Piston design used to be a process of trial and error. A piston engineer would make and test a new design three or four times before he 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. Piston Heat Management 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. In the 1990s, the distance or "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 or less 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 is not 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 degrees F, while the second ring sees temperatures of 300 degrees 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 worked great in a stock 350 Chevy V8 cannot take the kind of heat that is common in many late model engines. That is why ductile iron or steel top rings are used in many late model engines as well as performance engines. Steel rings outperform cast iron rings several ways: reduced oil consumption, better sealing for less blowby, reduced wear (up to 50 percent less!), less risk of breakage, and reduced friction. 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 cannot 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 use 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 used an anodized top ring groove. Low Tension Piston 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. On Chevy LS series engines, the first and second rings are both 1.5mm, while the oil ring is 3.0mm. Some are even thinner. A few engines have top compression rings only 1.0 mm thick, and Buick used a 2.0 mm oil ring in their 3800 V6. The OEMs went to thinner, shallower rings to improve fuel economy because the rings account for up to 40 percent of an engines 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 cylinder 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. Piston Geometry Changes in piston geometry have also been made to improve their ability to survive at higher temperatures. 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 they use CNC machining to create a barrel profile on the piston. 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 LS 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. 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 Chevy. 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 feature 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, piston to bore clearances are now tighter, typically .001˝ to .0005˝ or less. Some have a zero clearance fit made possible by a low friction anti-scuff skirt coatings. Piston Materials 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 silicon. Today we see more "eutectic" alloy pistons which have 11 to 12 percent silicon, and "hypereutectic" alloys that have 12-1/2 to over 16 percent silicon. Silicon …

Fonte: AA1Car.com