LO 7-3 Identify and explain the purpose and functions of major piston construction features

1.1. Piston Construction Materials

Piston Construction Materials

Due to the high speed of a reciprocating engine and the tremendous inertial forces of the piston at the end of each stroke, easily machined aluminum alloys are traditionally the material of choice for pistons. The lower inertial force of aluminum alloy pistons translates into less stress on other engine components, including connecting rods and crankshafts, which extend engine life. Engine vibration and harshness are also minimized with the use of aluminum alloy pistons. In addition to being one-third lighter than steel or iron, aluminum alloy conducts heat approximately four times faster than iron. This means more heat will be transferred from the piston to the cylinder wall and engine oil. While aluminum pistons appear to have all the important advantages, alloyed steel pistons are now the most common piston material used in medium-duty (MD) and heavy-duty (HD) diesels. These pistons can withstand higher cylinder temperatures and pressures, and steel pistons can support features that resist the formation of noxious and GHG emissions. Before examining the reasons for the widespread adoption of steel alloy pistons, it is important to first understand a few disadvantages of aluminum pistons.

Disadvantages of Aluminum Pistons
Advances in material and manufacturing technology have enabled the use of more rugged alloyed steel pistons, which were once unthinkable to use. Alloyed steel pistons have not completely displaced the use of aluminum but are more widely used now due to several disadvantages of aluminum. These disadvantages include:

• Low melting point: Aluminum has a melting temperature of approximately 1250° F (677° C), but the flame temperature of diesel fuel is much higher at 3900° F (2149° C). High compression pressures and combustion temperatures would easily melt aluminum pistons if fuel was burned at stoichiometric ratio and did not have excess combustion air Figure 18. To overcome this problem, aluminum is alloyed with approximately 10% of other materials, including silicon, copper, zinc, or chromium. Silicon, which can be found in quartz or beach sand, gives the alloy higher temperature resistance and minimizes thermal expansion. A hypereutectic piston, used by almost all diesel engines with aluminum pistons, has high silicon content (approximately 16–20% silicon). Other silicon pistons are 8–11% silicon.
• Poor wear characteristics: Because aluminum is soft, it does not stand up well to wear. This problem can be partially overcome by alloying aluminum. One of the areas on a piston that encounters the greatest wear is the top ring groove. The high combustion temperatures, combined with ring movement in the top ring groove, can cause rapid wear to this area. To prevent this, a band of special stainless steel–nickel alloy is cast into the top ring groove or even the top two ring grooves. The band is machined with a groove to accept the piston ring. This band is commonly known as a Ni-Resist insert. A Ni-Resist insert further reinforces the top ring piston land, which will prevent the top of the weaker aluminum of the ring land from fracturing and breaking away Figure 19. Installing this nickel insert has the disadvantage of adding complexity and cost to the manufacturing process.
• High expansion coefficient: Aluminum, when heated, will expand four to seven times more than ferrous metal. If this expansion is not controlled, the piston will expand and score the cylinder and piston or even seize in the cylinder. Using excessive clearances is not a good alternative because this will result in accelerated piston wear and piston slap, a noise most often heard when the engine is cold. This noise is the result of large operating clearances between a piston and a cylinder wall. Collapsed skirts, poor sealing, excessive cylinder and piston wear, and poor oil control are other undesirable results of running excessive clearances.

FIGURE 07-18: This aluminum piston crown melted due to the failure of a common rail injector. After the crown overheated, cylinder pressure pushed a hole through the piston.

FIGURE 07-19: A. A nickel insert is located within the compression ring grooves of this trunk piston. B. The steel crown of this articulating piston does not need ring groove inserts.