Abstract: Inventive embodiments are directed to components, subassemblies, systems, and/or methods to improve combustion efficiency in compression ignition, indirect injection, diesel engines, and in particularly for diesel engines having a removable pre-chamber. In all embodiments, a combustion system is fitted with a pre-chamber adapted to cooperate with a piston in a manner that produces a highly efficient combustion process. In some embodiments, the pre-chamber has passages that have a variable cross-section and a variable angular orientation with respect to a centerline of the pre-chamber body. In one embodiment, the piston is provided with a number of surfaces that improve combustion of the fuel/air mixture within the combustion chamber. In some embodiments, the piston surfaces are generally aligned with angles of the combustion chamber such as the angle of the intake and exhaust valves. In other embodiments, the piston has surfaces that are adapted to cooperate with a tip of the pre-chamber.