Spark ignition engine with negative valve-overlap

A valve control system for operating an automotive engine during cold-starts. It is conventional to employ a valve overlap period, defined as total crank-angle movement when both the inlet and exhaust valves are open simultaneously in the TDC region. The invention involves a cold-start "negative" valve-overlap (NVO) system wherein the intake valve opens after the exhaust valve has closed during the piston intake stroke. The crank-angle movement during the intake stroke, when both intake and exhaust valves are closed defines the NVO period, creating a high vacuum in the cylinder. At the end of the NVO period, as the intake valve opens it defines a low-lift narrow gap, wherein the high vacuum in the cylinder induces a pressure difference across the gap causing a high velocity in-flow of the air/fuel charge into the cylinder during its intake stroke. The increased turbulence in the cylinder enhances mixing of the charge improving its combustion stability, thereby allowing increased spark retard for achieving rapid catalyst warm-up, enhancing converter efficiency reducing exhaust emissions during the cold-starts.