Why Do Valves Open Early and Close Late?
1 Answers
The reason valves open early and close late is that after combustion in the cylinder, both pressure and temperature are very high. Even a small gap allows gases to escape, so even if the piston is moving toward the bottom dead center, gases continue to be expelled. Early valve opening helps with scavenging, allowing exhaust gases to be expelled more thoroughly while fresh gases cool the cylinder block, preventing fatigue. Valves are a critical component of an engine, responsible for introducing air into the engine and expelling post-combustion exhaust gases. Structurally, engine valves are divided into intake valves (inlet valve) and exhaust valves (exhaust valve). The intake valve's role is to draw air into the engine for mixing with fuel and combustion, while the exhaust valve expels combustion gases and dissipates heat. The purposes of early valve opening and late closing: Early intake valve opening aims to provide a larger opening or cross-sectional area at the start of intake, reducing intake resistance for smoother airflow. Late intake valve closing leverages the inertia of the airflow to continue intake during the intake lag angle, increasing the intake volume. Reduced intake resistance not only boosts intake volume but also lowers power consumption during the intake process. Early exhaust valve opening ensures high cylinder pressure when the valve opens, allowing exhaust gases to escape rapidly and in large quantities within a short time. By the time the piston begins the exhaust stroke, cylinder pressure has significantly dropped, and the exhaust valve opening or cross-sectional area increases noticeably, reducing forced exhaust resistance and power consumption. Late exhaust valve closing utilizes the inertia of exhaust flow to continue expelling gases during the exhaust lag angle, minimizing residual exhaust gas in the cylinder.
