An engine is provided with two spark plugs in each combustion chamber to ignite stoichiometric air-fuel mixture mixed with a large proportion of inert gases to lower NOx emission level. The engine is equipped with hydraulic valve lifters which are mechanically connected to intake and exhaust valves to control the valve overlap of the intake and exhaust valves. By the action of the valve lifters, the valve overlaps are increased in the normal driving operation range of the engine to prevent discharge at the terminal stage of exhaust valve opening exhaust gases containing a considerably large amount of hydrocarbons.