Pistons have a very hard life, and as such, are a focal point for many research and development efforts across the industry. But, in order to truly understand and appreciate what’s being developed, we must have a solid understanding of the basics. United Engine & Machine — UEM Pistons — understands this, and recently held a webinar in conjunction with the AERA Engine Builder’s Association, where Pim van den Bergh, Director of Sales for UEM, covers the basics of pistons, and a whole lot more (so be sure to carve out some time to watch the above recording in its entirety).
Casting Or Forging
When it comes to manufacturing pistons, there are two primary methods of creating a piston: casting and forging. As most of you reading this likely know, in manufacturing, whether it’s a piston or a wrench, a forged part is stronger than its cast counterpart. While that holds true for pistons, cast pistons are not only “good enough” in some circumstances, but can actually offer benefits over a forged piston.
A cast piston is created by pouring molten metal into a mold, where it assumes its general size and shape as the metal cools and the mold is removed. A forged piston starts off as a piece of bar stock that is then formed into its general size and shape under intense pressure from a forging machine, which ensures the molecules in the piston structure are all lined up in the most beneficial way. Both types of pistons will undergo final machining to bring the piston into final design specs.
However, the very first consideration in designing a piston is the application. That will determine whether the additional strength of a forged piston is required. “We have a compromise that we have to make when we start making a piston,” says van den Bergh. “First, we look at the application it will have to face. We determine whether or not we will be using a cast piston or a forged piston.”Once that determination is made, then engineers can move onto the material the piston will be made out of.
Choosing An Alloy
“Once we’ve decided whether the piston needs to be forged or cast, I have to decide whether I want high ductility at the cost of a higher thermal conductivity, or something harder, with better wear properties and lower thermal expansion,” explains van den Bergh. Most pistons within our scope — automotive engines — will be made from some form of aluminum alloy. The key difference in the range of materials is the amount of silicon in the alloy. “We start with an aluminum-silicon base. Silicon is a key element used by piston manufacturers to enhance the features that aluminum alone wouldn’t have.”
You might have heard the term “eutectic” in some form in reference to pistons. To make it simple, hypereutectic basically means the alloy has a high percentage of silicon — up to 20-percent. Hypoeutectic means the opposite, as there is very little silicon in the alloy, and eutectic is the middle point, at about 12.2-percent silicon content in the alloy.
If you are looking at a cast piston UEM’s offerings range from about 8- to 10-percent silicon on the low side to upwards of 16-percent in its hypereutectic 390 aluminum cast pistons. “The 390 is something commonly used by OEs in gasoline engines. You can run tighter clearances and it works well at higher temperatures,” van den Bergh explains. “This is where most engine rebuilds will live as well. The 390 is just a good, solid material, which will make for an all-around good piston.”
Moving to the forged side, the two most popular alloys — 4032 and 2618 — sit in the eutectic and hypoeutectic zones, respectively. At 12.2-percent silicon, the 4032 has proven itself as a solid, all-around performance piston material, with solid wear resistance and enough resistance to thermal expansion to be quiet enough to be tolerable on the street.
Conversely, the 2618 alloy only has 0.23-percent silicon, which makes it more thermally expansive, requiring more piston-to-wall clearance and its associated noise. Its higher malleability makes it a great choice for race engines which will see a lot of boost and cylinder pressure, but the tradeoff for that, is that it’s not as wear resistant, so, in theory, makes less sense in a high-use, daily driven application.
Coatings Can Bridge The Gap
One of the options in order to expand a given alloy’s operational envelope lies in today’s modern piston coatings. “Piston coatings used to just be used for engine break-in, but that is no longer the case,” van den Bergh explains. “Coatings can overcome alloy limitations, deal with the challenges of diverse fuels and address performance applications.”
By enhancing the benefits of a given material, and supplementing its shortcomings, you have piston designs operating and thriving in environments where previously it would have been almost unheard of (like 2618 being used in a “street” application — but more on that in a future article). There are a number of coatings used by UEM to address specific concerns.
UEM’s piston skirt coating, known as M42, reduces friction, skirt-abrasion, as well as piston noise. “Our M42 is a mixture of graphite, Teflon, and molybdenum,” says van den Bergh. “Reducing the noise from piston slap is critical in engines that use knock sensors. Once you activate a knock sensor with piston slap, it can really screw with your electronics and fueling, and make the engine run poorly.”
UEM also offers a ceramic-metallic crown coating, which forms a reflective barrier that prevents heat from seeping through the crown and into the rest of the system. That also keeps the heat in the chamber, where it is used for work. When applied to something like a forged 2618 piston in a very high chamber temperature environment, you can mitigate the alloy’s inherent high thermal transfer properties.
Additionally, an abradable coating from Line2Line coatings is offered, in order to allow for a zero-clearance assembly, and then having the coating lap itself against the cylinder walls to create exactly the amount of piston-to-wall clearance the piston needs. And finally, hard-anodized ring lands, which is more of a metal treatment than a coating, can help pistons live in harsh-fuel environments. “[Hard anodizing] resists abrasive wear we see with some of the exotic fuels. It also helps reduce micro-welding of the piston ring in extreme situations,” says van den Bergh.
While this article only scratches the surface, the full webinar goes a little deeper. But, really, this is all just a high-level overview of performance pistons as a whole. We can (and will) dedicate entire articles to focusing on just a single facet of piston design and engineering that was touched on here. It’s obvious, though, that with technology marching on, the parameters for the use of each alloy are starting to blur and overlap.