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The Valvetronic system is a variable valve timing system to offer continuously variable timing (on both intake and exhaust camshafts) along with continuously variable intake valve lift, from ~0 to 10 mm, and duration on the intake camshaft only. Valvetronic-equipped engines are unique in that they rely on the amount of valve lift to throttle the engine 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.
First introduced by BMW on the 316ti compact in 2001, Valvetronic has since been added to many of BMW's engines. The Valvetronic system is coupled with BMW's proven double-VANOS, to further enhance both power and efficiency across the engine speed range. Valvetronic was not coupled to early BMW's N53, "High Precision Injection" (gasoline direct injection) technology engines due to lack of room in the cylinder head. New generation 'compact' Valvetronic allows for direct injection to utilized in the N55B30 turbo engine, as seen in the 2010 BMW 5-series GT. 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 open or close the intake valves.
Because the intake valves now have the ability to move from fully closed to fully open positions, and everywhere in between, the primary means of engine load control 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 improved. By reducing the valve lift, asymmetrically on 4-valve engines, more swirl is introduced in the cylinder for 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.
Valvetronic has so far been limited to BMW's mass-market engines, with no high-performance M-series car using the technology, which continue to utilise multiple throttle-bodied designs instead.
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