A method includes controlling an engine speed based on: intake manifold air temperature and/or intake manifold pressure one, or more, of the following data parameters: an engine load as a function of a fuel level, a fuel injecting timing, an intake oxygen concentration, a constituent concentration from the exhaust gas flow, an engine power, and an engine torque. The method also recirculates a portion of the exhaust gas flow to the combustion cylinders of the engine via a recirculation channel, as a function of intake manifold temperature and/or intake manifold pressure at which the engine is operated. An engine system, other methods, and a non-transitory computer readable medium encoded with a program, to enable a processor-based control unit to control aspects of the engine are also disclosed.