In a mechanical apparatus having a diesel engine including a combustion chamber, the mechanical apparatus also having an aftertreatment device configured to treat emissions from the diesel engine and an intake valve for passing air into the combustion chamber, a method of operating the engine is disclosed, wherein the method includes performing at least one combustion in the combustion chamber at a first intake valve closure timing, 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 at a second intake valve closure timing to thereby increase the temperature of exhaust emitted by the diesel engine.