Heavy-duty diesel engines are asked to perform under extremely harsh conditions and keep running for hundreds of thousands of miles, without complaint. And of all the parts within the engine, none takes as much abuse as the piston.
Consider the conditions under which a typical diesel piston operates. It reaches a peak speed of between 4000 and 5000 ft/min. in normal operation. That translates to about 100 kilometres per hour, which doesn’t sound like a lot until you realize it accelerates from a dead stop to 100 km/h to dead stop twice per revolution, or over 80 times per second at 2500 RPM – that’s a peak acceleration of over 3000 Gs.
Until recently, the material of choice for internal combustion and diesel engine pistons was aluminum. Aluminum offers a number of advantages: it’s light, it manages combustion temperatures well, and pistons made from aluminum are easy to manufacture in large quantities.
However, beginning in 1999, diesel engine emission regulations significantly altered the operating conditions inside the combustion chamber. Temperatures and pressures increased significantly, necessitating a re-evaluation of what material was the optimum choice for diesel pistons.
A better design for today’s engines
Initially what manufacturers came up with was a two-piece, articulated piston with a separate steel crown and an aluminum skirt. The piston pin fit through a free-floating aluminum skirt and attached to a forged steel crown. This worked well, but engineers found that the aluminum skirt actually wasn’t needed at all – an all-steel piston was the optimum solution.
Steel offers many significant advantages compared to aluminum, including its strength. Ring land wear is not a concern with steel, and steel pistons expand at a similar rate to the engine bore. However, steel weighs more than aluminum, and steel pistons are much more difficult to manufacture than ones made from aluminum. One-piece steel pistons have a very distinctive appearance. First, you’ll notice the absence of a skirt. In its place are two “runners” down the sides. Next, you’ll notice the piston is black all over. That is a special phosphate coating that provides lubrication during operation, and rust control before installation. It is undesirable to have a steel pin riding in a steel bore; steel against steel creates wear issues. Usually, you want to have steel running against babbit or tin plating.
Initially the solution was to use a bushing inside the bore to create a wear surface, but the newest generation of pistons replaces the bushing with a high-tech phosphate coating. The phosphate coating enables the piston pin to ride without bushings and acts as a sacrificial lubricant. It provides protection against wear for well over 1.6 million kilometres of operation. Another distinctive feature that is not so visually apparent is that the pin bore is “profiled.” Normally, a piston pin bore is straight. If you view a cutaway of the bore on a steel piston, you would see that it is slightly trumpeted, increasing its diameter as it goes towards the outside of the piston.
This is needed since although you have a massive 60 mm piston pin, it still flexes minutely as it is subjected to the enormous forces acting on it. As the piston begins to rise on the compression stroke, the centre of the pin is pushed up and the ends are cocked down. The trumpet shape allows this flexing to occur without squeezing the lubricating oil film out, allowing for metal-to-metal contact. The profiling retains an oil layer, and compensates for the flexing.
Another feature that is not readily apparent is the “shaker plate” oil cooling design. Aluminum pistons were able to provide adequate temperature control for the piston, with squirters directing cooling lubricant at the open crown. Creating oil cooling passages in a forged piston is not feasible, so the shaker plate acts to retain oil in the crown, helping to cool the top of the piston. Two holes in the plate allow the oil to drain back down into the oil pan. This feature was first used on the earlier, two-piece piston design and adds greatly to engine longevity and endurance. Also adding to piston life is a special Grafal coating on the piston runners that reduces scuffing. This is applied over the phosphate coating, so it isn’t visible to the human eye.
As I said earlier, one of the few disadvantages of steel pistons is weight. With the elimination of the skirt, overall piston weight can be reduced to the level of aluminum pistons. Each piston is very highly balanced, ensuring the rotating components are all equally matched and resulting in a smooth running engine.
So, what does this mean for the aftermarket?
Well, your diesel repair customers will be seeing a lot more of these all-steel pistons when they tear down newer engines. And, you’ll be selling more of them to ensure the rebuilt engine is returned to OE specs. Make sure your customers know to examine used pistons in several critical areas before thinking of returning them to service:
Piston Top. Check for erosion or cracking. While a visual inspection is usually adequate, Magnaflux crack testing is good insurance and, since the pistons are steel, is possible.
Ring Grooves. Rectangular ring grooves are fairly easy to measure and evaluate. Tapered ring grooves are more difficult. Some specialized measuring tools are available. But it is always best to check the OE engine specifications for acceptable tolerances for reuse.
Bore Size. Again, check your OE engine specifications for acceptable tolerances.
Galling and Scuffing. Visually inspect the piston runners for scuffing. Look closely inside the pin bores for any indication of galling. Any scuffing or galling would warrant piston replacement.
This new generation of lightweight, one-piece steel pistons represents a significant engineering accomplishment and allows today’s highly efficient, clean diesel engines to deliver the promise of million-mile service.
Jay Wagner is Senior Heavy Duty Product Specialist for Mahle Aftermarket Inc. He has been involved in the heavy-duty diesel engine industry for over 40 years, including a position in his family diesel engine machine shop where he attained the position of ASE Certified Master Machinist. Wagner joined Mahle Clevite in 1993 and has had several responsibilities including overseeing the Clevite engine bearing line, heavy duty cylinder components, heavy duty gaskets, and brand management for heavy duty marketing.
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