valvetiming

Valve timing refers to the scheduling of when engine intake and exhaust valves open and close in relation to the position of the piston in each cylinder. It determines how air enters the cylinder and how exhaust leaves, affecting volumetric efficiency, torque, and emissions. Modern engines use camshafts and driven timing parts to coordinate these events with the crankshaft.

In a four-stroke engine, four timing events govern valve operation: intake valve opening (IVO), intake valve closing (IVC), exhaust valve opening (EVO), and exhaust valve closing (EVC). These events occur at specific crank angles and are tuned to optimize engine performance. The exact timing varies by design, but common goals include maximizing low- and mid-range torque, improving scavenging of exhaust, and preventing valve-to-piston interference in certain configurations.

Variable valve timing (VVT) systems adjust valve timing during operation to optimize performance across RPM ranges. Techniques include cam phasing (changing the relative position of the camshaft to the crankshaft), variable lift, and switching cam profiles. VVT can advance timing at lower engine speeds to boost torque and ease starting, and delay timing at higher speeds to enhance top-end power and efficiency. Some systems employ multiple strategies, and many modern engines combine VVT with other technologies to reduce fuel consumption and emissions.

Maintenance and failure modes: valve timing is set during assembly and relies on timing chains or belts, gears, and tensioners. Wear or failure of these components can cause timing to drift, reducing power, throttle response, and efficiency, increasing emissions, or, in interference engines, risking valvecontact with the piston if timing severely jumps.