What Is Valvetronic? BMW’s Innovative Throttle System [Engineering Explained]
What is Valvetronic? Why don’t BMWs use Throttle valves?
In most gasoline cars, when you press the gas pedal you’re changing how
much the throttle valve opens, a butterfly valve regulating the amount
of air that enters the engine. In BMW engines with Valvetronic, however,
a throttle valve is not necessary, and when you press the gas pedal
you’re altering how much the intake valves open.
There is indeed a throttle butterfly valve in these engines, but it is simply used as a failsafe. Under normal operating conditions the valve will always remain fully open.
There’s a major efficiency benefit of regulating airflow this way, as it reduces the pumping losses of the engine, particularly at low load operations, or any time you’re not flooring it. With throttle bodies, you have close to atmospheric pressure on the filter side, and a vacuum on the intake manifold side. With Valvetronic, you’ll have near atmospheric pressure directly outside the intake valves, and thus the engine does not have as much resistance to pulling in air, making it more efficient. BMW claims this valve train design improves fuel consumption by at least 10% under practical driving conditions.
There are additional benefits to this system as well. With throttle bodies, once you floor it, you have to wait for the air to travel from the throttle body to the intake valve. By using the valves to regulate airflow, and already having nearly atmospheric pressure right outside the cylinder, the intake is immediate and thus the response is improved.
Another benefit at low load operations; the valves will only open about .5 mm to about 2 mm, and this small cross sectional area means the air fuel mixture will move rapidly, improving fuel atomization. This also helps with cold starts.