Valvetronic

The Valvetronic system is a variable valve timing system to offer continuous and precise control over variable intake valve lift, from 0.3 to 9.7 mm (0.18mm to 9.9mm, Valvetronic II and III), and duration. It typically works in conjunction with the independent Double VANOS system that continuously varies the timing (on both intake and exhaust camshafts). Valvetronic-equipped engines rely on the amount of valve lift for load control, rather than a butterfly valve in the intake tract. In other words, in normal driving, the "gas pedal" controls the Valvetronic hardware rather than the throttle plate.

Cylinder heads with Valvetronic use an extra set of rocker arms, called intermediate arms (lift scaler), positioned between the valve stem and the camshaft. These intermediate arms are able to pivot on a central point, by means of an extra, electronically actuated camshaft. This movement alone, without any movement of the intake camshaft, can vary the intake valves' lift from fully open, or maximum power, to almost closed, or idle.

Because the intake valves lift now have the ability to move from almost closed to fully open positions, and everywhere in between, the primary means of controlling engine output is transferred from the throttle plate to the intake valvetrain. By shortening the duration of the intake instead of throttling, pumping losses are reduced and fuel economy is improved. By reducing the valve lift, asymmetrically on 4-valve engines, swirl is generated in the cylinder, leading to a better air/fuel mixture. By avoiding a large air reservoir between the throttle and the engine responsiveness can be improved, though it depends on the speed of the electric motor actuating the second camshaft.

However, the throttle plate is not removed, but rather defaults to a fully open position once the engine is running. The throttle will partially close when the engine is first started, to create the initial vacuum needed for certain engine functions, such as emissions control. Once the engine reaches operating speed, a vacuum pump run off the passenger side exhaust camshaft (on the N62 V8, exhaust cam on the N52/K) provides a vacuum source, much as a diesel engine would, and the throttle plate once again goes to the fully open position.

The throttle plate also doubles as an emergency backup, should the Valvetronic system fail. In this case, the engine would enter a "limp home" program, and engine speed would once again be controlled by the throttle plate.