In a mechanical apparatus having a diesel engine including a combustion chamber, an intake valve for passing air into the combustion chamber, and an exhaust valve for passing exhaust from the chamber, the mechanical apparatus also having an aftertreatment device configured to treat emissions from the diesel engine, a method of operating the engine is disclosed, wherein the method includes performing at least one combustion in the combustion chamber with a first quantity of fuel, determining a temperature of the aftertreatment device, and if the temperature of the aftertreatment device is equal to or below a preselected temperature threshold, then performing at least one combustion in the combustion chamber with a second, greater quantity of fuel and varying at least one of an intake valve timing and an exhaust valve timing relative to a default valve timing to cause at least one of a compression loss and a pumping loss.