Active Valve Control System

Active Valve Control System, abbreviated AVCS, is the trade name Subaru uses for its variable valve timing technology. Internal combustion engines breathe through valves opened and closed by a rotating camshaft, and the precise moments at which those valves open relative to piston position determine how well the engine fills and empties its cylinders. With fixed timing, an engineer must choose a single compromise that works adequately across the rev range but is optimal nowhere. AVCS exists to remove that compromise by continuously altering valve timing to suit engine speed and load, allowing one engine to behave as though it were tuned several different ways at once.

Mechanically, AVCS works by rotating the camshaft a few degrees relative to the timing chain or belt that drives it. A vane-type actuator, mounted on the end of the cam and fed with pressurised engine oil, advances or retards the cam under the command of the engine control unit. The ECU reads inputs such as crankshaft and camshaft position, throttle opening and engine speed, then meters oil to one side of the actuator vanes through a solenoid-operated oil control valve. On Subaru's horizontally opposed boxer engines, AVCS may act on the intake camshafts alone, or on both intake and exhaust cams in dual AVCS form, giving control over valve overlap as well as opening points.

The benefits are felt across the entire operating envelope. At low revs the system can reduce valve overlap for a stable idle and strong, immediate torque; at higher revs it advances timing to keep the cylinders filling efficiently for peak power. Better-controlled overlap also lets exhaust gas remain in the cylinder when appropriate, an internal form of exhaust gas recirculation that lowers combustion temperatures and curbs oxides of nitrogen, while more complete combustion reduces unburnt hydrocarbons. The net result is broader, flatter torque, improved fuel economy and lower emissions without sacrificing top-end performance.

AVCS is Subaru's counterpart to a family of systems from other manufacturers: Toyota's VVT-i, Honda's VTEC, BMW's VANOS and the generic industry term variable valve timing. The underlying principle is shared, though implementations differ in whether they vary timing alone, valve lift, or both. Subaru introduced AVCS in the late 1990s and progressively extended it from intake-only to dual-cam control as emissions and efficiency demands grew.

In service, AVCS reliability depends heavily on clean oil at the correct level, because the actuators and solenoids rely on oil pressure and unobstructed passages. Sludge, an overdue oil change or a failing oil control valve can cause rough idling, a check-engine light or a rattle on start-up from a drained actuator. The system is a refinement of the basic valvetrain rather than a replacement for it, and it works in concert with the camshaft, timing drive and engine management to deliver its results.