TECHNICAL FIELD

The present disclosure relates generally to a piston for an internal combustion engine, and more particularly to a piston having surfaces selectively smoothed to inhibit deposit formation and/or adhesion.

BACKGROUND

Internal combustion engines employ one or more pistons positioned in a combustion cylinder and movable by way of a controlled combustion reaction within the cylinder to rotate a crankshaft. A great many different fueling, temperature, and pressure control strategies relating to the combustion process have been proposed over the years. Fuel can be directly injected into the cylinder, port injected, or fumigated into a stream of intake air to name a few examples. In the case of directly injected internal combustion engines, commonly operating on a liquid hydrocarbon fuel such as a diesel distillate fuel, the fuel spray is often directed into a combustion bowl in an effort to confine the combustion process to the combustion bowl, although in some instances injected fuel can be directed or spilled over a rim of the piston and wet the wall of the cylinder, typically formed by a cylinder liner. These and other operating and/or fueling strategies are employed to various ends, including emissions mitigation, efficiency optimization, and still others. A flow of engine oil is provided for distribution between the wall of the cylinder and the piston and piston rings to lubricate the interfacing surfaces.

In virtually all engines, and diesel engines in particular, it is common to experience deposit accumulation upon parts of the piston regardless of the operating and fueling regime. Deposits can be observed notably upon parts of the piston and piston rings that face the cylinder wall. Varying of fuel delivery strategies such as that noted above, or perturbations to desired operating parameters, in the dynamic combustion environment can make deposit formation relatively difficult to predict and mitigate. Moreover, goals relating to emissions and efficiency, for example, often take precedence over deposit mitigation even where the mechanisms of deposit accumulation are understood or suspected. Variations in fuel quality and fuel type can further impact the manner and extent of deposit formation.

Engineers have experimented previously with mechanical deposit management techniques, such as deposit scrapers, in an effort to manage formation of such deposits. Excessive piston deposits can interfere with lubricating oil distribution, increase oil consumption, and cause or exacerbate a phenomenon known as “blow-by” where combustion gases escape the cylinder through a clearance between the piston rings and the cylinder wall. One example of a piston deposit mitigation effort is set forth in EP 3043054 A1 and proposes a carbon scraping cuff ring that apparently assists in scraping off undesired deposits from combustion surfaces.

SUMMARY OF THE INVENTION

In one aspect, a piston for an internal combustion engine includes a piston body having a piston crown defining a piston center axis, and a piston skirt attached to the piston crown. The piston skirt includes a skirt outer surface and a skirt inner surface. The piston crown includes a crown outer surface, and a combustion face formed by an annular outer rim surface and a combustion bowl surface radially inward of the annular outer rim surface. The crown outer surface forms a plurality of piston lands alternating axially with a plurality of piston ring grooves and together defining a deposit-sensitive zone. At least one of the skirt outer surface or the skirt inner surface has a roughness number (Ra) of 0.002 millimeters or greater, and the crown outer surface is smoothed within at least a portion of the deposit-sensitive zone to an Ra of 0.0002 millimeters or less.

In another aspect, a piston for an internal combustion engine includes a piston body having a piston crown defining a piston center axis, and a piston skirt attached to the piston crown. The piston crown includes a crown outer surface, and a combustion face formed by an annular outer rim surface and a combustion bowl surface radially inward of the annular outer rim surface. The crown outer surface forms a plurality of piston lands alternating axially with a plurality of piston ring grooves and together defining a deposit-sensitive zone. The piston crown is formed throughout of a piston body material having an exposed surface smoothness that is varied within the piston crown, and a roughness average (Ra) of 0.0002 millimeters or less within at least a portion of the deposit-sensitive zone.

In still another aspect, a method of preparing a piston for service in an internal combustion engine includes receiving a piston body of the piston including a piston crown defining a piston center axis and having a crown outer surface extending circumferentially around the piston center axis and formed by an exposed piston body material extending throughout the piston crown. The method further includes decreasing a surface roughness of the piston crown by at least one of removal or deformation of the exposed piston body material in a deposit-sensitive zone of the piston body defined by a plurality of piston lands and a plurality of piston ring grooves each formed by the crown outer surface. The method still further includes increasing a smoothness of the exposed piston body material based on the decreasing of the surface roughness to a deposit-inhibiting smoothness that is at least an order of magnitude smoother than a smoothness in a deposit-insensitive zone of the piston body.

