A piston may have an annular body including a crown portion defining a longitudinal axis, a radial direction perpendicular to the longitudinal axis, a plane containing the longitudinal axis and the radial direction, and a contoured combustion bowl. In the plane containing the longitudinal axis and the radial direction, the crown portion includes a radially outer squish surface, and a swirl pocket having a reentrant surface that extends axially downwardly and radially outwardly from the squish surface defining a tangent that forms a reentrant angle with the squish surface that ranges from 53.0 degrees to 57.0 degrees.

A combustion chamber for an opposed-piston engine has a rotationally skewed shape in a longitudinal section that is orthogonal to a chamber centerline, between diametrically-opposed openings of the combustion chamber through which fuel is injected. The rotationally skewed shape interacts with swirl to generate a tumble bulk charge air motion structure that increases turbulence.

A method for injecting nitrogen into an internal combustion engine cylinder includes moving a piston back and forth in a cylinder and injecting pure nitrogen into a combustion chamber during the compression stroke. The method includes injecting the fuel and nitrogen into different regions to create a stratified gas environment, igniting the fuel, and releasing exhaust emissions. An engine system for injecting nitrogen into an internal combustion engine includes a cylinder, a first intake line, one or more nitrogen injectors, and a fuel injector. An engine system for injecting nitrogen into an internal combustion engine includes a cylinder, an intake line, and a multi-nozzle injector with a fuel nozzle and a nitrogen nozzle.