LO 7-2 Classify pistons according to their application, design, construction features, and material

1.7. Articulating Pistons

Articulating Pistons

Another innovation to piston technology is the use of articulating piston articulating pistons. This two-piece piston has an aluminum skirt but a steel crown. An important feature of the steel crown is to reduce the clearance between the crown and the cylinder wall Figure 14. This region, between the piston crown and the cylinder wall above the top compression ring, is called the crevice volume. The strength of a steel crown enables the top ring to be placed closer to the crown without causing a fracture between the top ring groove and the crown (a fracture that would occur if a thin aluminum piston crown was used). Another name for a piston using this feature is low crevice volume piston. Low crevice volume (LCV) pistons place the compression ring closer to the top of the piston crown. Because fuel can hide in this area and cool, or quench due to low heat, manufacturers’ studies have found that reducing crevice volume improves fuel economy as much as 5%. Eliminating the dead air space in the crevice volume area lowers hydrocarbon (HC) exhaust emissions by as much as 50% Figure 15, compared with older, trunk-type aluminum pistons with high crevice volume. Trunk pistons are also made with reduced crevice volume, but aluminum pistons cannot reduce the crevice volume to the same extent as pistons with steel crowns.

FIGURE 07-14: Articulating pistons use steel crowns to withstand high cylinder temperatures and pressures. These pistons have low crevice volume crowns.

FIGURE 07-15: Crevice volume is the area between the crown and the cylinder wall. A larger crevice volume area quenches fuel combustion and produces more noxious emissions.