Forged Steel Diesel Pistons Lead Performance Into The Future

Forged steel pistons own the industrial diesel market. In these applications, steel pistons are favored because they best handle the severity of the combustion environment and the associated challenges it presents to optimum performance and long-term durability. Steel pistons are significantly stronger than aluminum pistons, and they offer potential combustion and thermodynamic advantages that favor fuel consumption and reduced emissions levels. Improved ring land wear characteristics and the ability to cool the piston through design features are two of the major advantages attributed to steel pistons.

As the OEMs continue to push their engines to higher and higher power levels, they have reached the limits of what cast factory pistons can withstand. Then, zealous consumers decide they can do even more with different tuning strategies and add-on parts. Failure lurks like the grim reaper in these scenarios. The need for tougher pistons in extreme applications leads us back to a steel piston configuration that is more suited to these severe operating environments.

These conditions are particularly hard on piston rings. A stable ring environment is crucial to ring performance under the high cylinder pressures that diesel engines create. If ring performance deteriorates, combustion efficiency is compromised, and emissions tend to rise. A thermodynamically stable piston reinforces ring performance and reduces friction. Steel pistons also have the advantage of large internal cooling galleys to help stabilize piston temperature and thus ring performance within the ring grooves and against the cylinder walls. In addition, steel pistons expand and stabilize at close to the same rate as the cast iron engine block, further contributing to improved ring seal and overall emission characteristics.

Steel pistons are a staple of the heavy-duty commercial diesel industry. They are tougher than nails and allow some commercial trucks to deliver more than one-million miles of service without distress.

Why Use Steel?

The benefits of steel pistons in severe-duty applications are difficult to ignore. The weight difference would seem substantial when compared to an aluminum piston, but that concern is mitigated by the hollow spaces for the cooling galleries and the elimination of mass in areas where high strength is unnecessary. And it isn’t a major factor in commercial applications, because most powerplants are relatively low-speed engines operating well below 4,500 rpm. Since the coefficient of thermal expansion in steel pistons is equivalent to that of the cast iron engine blocks, maintaining consistent component clearances is easier and more reliable.

On the downside, steel pistons are heavier, which increases parasitic drag. They are also more difficult to machine, particularly in large numbers, as in—for example—the domestic pickup truck market. That’s a primary reason for the use of cast aluminum pistons with steel ring groove inserts in diesel pickup engines. Factory cast aluminum pistons are manufactured with steel ring groove inserts to resist the severe operating environment. But, as truck owners modify their vehicles for more power, higher cylinder pressures can cause factory pistons to fail.

(Left) The large oil cooling galleries seen here in the skirt help promote longevity by allowing engine oil to pull heat off the piston and return it to the pan where it can be circulated through a cooler. (Right) Another view of oil cooling galleries on the underside of a steel piston. The cooling effect of these galleries is a substantial contributor to the longevity of the pistons and rings in heavy-duty diesel applications.

It’s not possible to add steel ring inserts to forged pistons, but Diamond Pistons addresses ring groove deterioration by forging its factory replacement pistons out of billet 2618 aluminum with the addition of its exclusive Type III Hard Anodize Coating. The superior strength of the billet material with coating reinforcement makes these pistons plenty tough for the performance builds on Duramax, Power Stroke and Cummins diesel engines.

High cylinder pressures, heat, and erosion from high injection pressures are the core contributors to diesel engine distress. Diesels operate with compression ratios anywhere from 16:1 to 20:1; using such high compression ratios helps to improve their thermal efficiency, but it also creates massive cylinder pressures that can range from 4,000 to peaks of over 8,000 psi. That kind of pressure creates abnormally high temperatures. It requires oil cooling to dissipate the heat and control component thermal expansion. Supplemental cooling is also necessary to cool the rings and prevent them from losing shape and tension.

(Left) This is a cast OEM 2008 Ford Power Stroke piston. The top ring land is a steel insert cast into the piston to provide the durable ring support necessary for the diesel. Steel pistons are more durable, but more expensive to manufacture, hence the OEMs use cast pistons with ring land inserts. (Right) Underview of the same OEM cast piston reveals the lack of cooling galleries as found in steel pistons shown above.

Special Features

Steel pistons have oil cooling galleries in the top of the piston to draw heat off the rings. Oil is sprayed onto the bottom of the pistons and directed to these cooling galleries to remove heat. Cooling galleries typically reduce the top ring temperature by up to 100°F, which leads to better sealing and longer ring life. Heavy-duty, high-output commercial and truck engines often use two-piece “articulated” steel pistons with steel crowns and aluminum skirts or one-piece weldment pistons. Articulated pistons are two-piece pistons held together by the wrist pin. They allow the piston to handle higher loads than one-piece cast pistons and typically carry a very hefty wrist pin to mate the articulating crown and skirt components. One-piece steel pistons also offer advantages over one-piece cast aluminum and forged aluminum pistons and two-piece aluminum/steel pistons.

As a rule, steel pistons are more expensive and more difficult to make, but their practical application offers unparalleled strength and durability. The performance diesel marketplace has taken of note of this and has been retrofitting agriculture and industrial pistons into performance builds in the hopes of pushing their competition engines beyond the limits of what engines equipped with traditional pistons can handle—all in the hopes of making more power with longer life.

The main issue with this is that steel pistons are few and far between and don’t offer any room for customization in popular performance build configurations. Diamond’s unique design and manufacturing capabilities offers such customization for the performance diesel market. Diamond can design, engineer, and manufacture custom steel pistons for those applications.

Diamond Pistons makes these custom forged Cummins, Duramax and Power Stroke pistons as replacement pistons. They are specified for moderate competition use. To resist high temperatures and pressures Diamond applies its hard-anodizing process to the complete piston along with a molybdenum coating to its skirts.

As the diesel market continues to emerge into the performance realm, the need for durable performance steel pistons is sure to expand. Manufacturing steel pistons isn’t anything new for Diamond, however it is relatively new for the performance market, and the company is positioned to assist OEM and prototype development outfits with rapid piston design and manufacturing capabilities. The company uses state-of-the-art CNC equipment, and proprietary manufacturing methodology to construct lighter-weight performance diesel pistons that will take the abuse of the tremendous heat and cylinder pressures from these applications.

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