Valve overlap duration is determined by comparing the closing and opening events of the intake and exhaust valves, respectively. Specifically, it’s the period (measured in crankshaft degrees) during which both valves are simultaneously open. This period is calculated by adding the intake valve closing angle (after bottom dead center) and the exhaust valve opening angle (before bottom dead center). For instance, an intake valve closing at 60 ABDC and an exhaust valve opening at 50 BBDC results in a 110 overlap.
This seemingly brief period plays a significant role in engine performance, particularly at higher RPMs. By allowing a portion of the exhaust pulse to help draw in the fresh intake charge, it effectively increases volumetric efficiency. This scavenging effect promotes more complete cylinder filling, leading to increased power output. Conversely, excessive overlap can diminish performance at lower engine speeds due to reduced cylinder pressure. Historically, adjusting valve timing and overlap was a complex mechanical process. Modern engines with variable valve timing systems, however, offer dynamic control over these parameters, optimizing performance across a wider RPM range.
