Tech Tip Fix: BMW Intake Air Strategy

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Tech Tip Fix: BMW Intake Air Strategy

by Robert Jacobs, WORLDPAC Assist European Team Member

How difficult can it be to control the amount of fresh air entering an internal combustion engine? The traditional method of intake air control for internal combustion automobile engines has been through the throttle controlled butterfly valve. Engines with carburetors utilize the butterfly valve to control the amount of air, and at the same time, the amount of fuel being drawn into the engine. Most engines with modern fuel injection also operate with the butterfly valve for intake air control but typically control fuel by reading numerous input parameters and then injecting the proper amount of fuel for the given input parameters.

Some manufacturers like to add a great deal more engineering to the process than a simple throttle body with butterfly valve for intake air control. While the butterfly throttle valve has been a successful means of controlling intake air, it also has downfalls. When at idle or under partial load, the throttle plate is only slightly open. This leads to the engine creating vacuum in the intake manifold. This may sound like a normal condition of engine operation and something that should be expected. What this actually does is reduce the amount of pressure above the intake valves. For the best filling of combustion chambers, ambient pressure should be available directly at the intake valves. The reduction of pressure at the intake valves has a negative affect on cylinder filling, mixture control and fuel economy. This particular problem can be addressed in either a supercharged or turbocharged engine but at greater expense and design complexity. Turbochargers and superchargers are also designed to provide boost pressure at high loads, which will produce more power, but don’t combat the main downfall of the throttle valve at idle and partial load.

Enter BMW and their Valvetronic control system for intake air management. Starting in 2002 with the BMW 7-series vehicles, Valvetronic has come into play as a more efficient means of intake air control. There are several items which come into play with the Valvetronic system, but the general principle is in the ability to control valve lift to manage intake air. Besides valve lift control, there are variable length intake runners, variable camshaft timing control (VANOS) and a conventional throttle body which is used as a backup system in the event of Valvetronic system failures and in other specific situations. During normal Valvetronic operation, the throttle body is in the wide open position. In the event of a failure, it can be operated in the same fashion as an electronically controlled throttle body. There is no actual mechanical linkage to the throttle body itself. An electric motor is used to operate the throttle valve when specified by the engine control module.

Valvetronic valve lift control has a few additional components when compared to a typical overhead camshaft configuration. They are the Valvetronic motors, eccentric shafts, eccentric shaft position sensors and intermediate levers. Using these components, Valvetronic can adjust valve lift from 0.3mm to 9.85mm in 300 milliseconds. Smaller changes in valve lift obviously require less time.

The physical placement and function of the components are as follows (see Figure A for reference). The eccentric shaft is set slightly above and off to the side of the camshaft. Think of the eccentric shaft as a small second intake camshaft. The intermediate levers are the means by which the camshaft and eccentric shaft are connected. The camshaft contacts the side of the intermediate lever. The top of the intermediate lever is in contact with the eccentric shaft. At the bottom of the intermediate lever is a rocker arm device which makes contact with the top of the valve on one side and with a hydraulic lash adjuster on the other side. In the middle of the eccentric shaft is a gear which meshes with a Valvetronic motor. The motor is capable of turning the eccentric shaft either clockwise or counterclockwise. The motor is a high amperage motor, which at times can flow as much as 100 amps of current. The eccentric shaft position sensor is mounted on the back of the eccentric shaft and translates information to the Valvetronic control unit for precise feedback of eccentric shaft positioning. As the motor adjusts the eccentric shaft, due to the shape of the eccentric shaft, intermediate lever and rocker arm, the lift of the valve is changed. The basic camshaft profile is adhered to, but the lift of the valve is the only thing affected by the Valvetronic system.

An additional feature which enhances the level of intake air control is the variable camshaft timing control (VANOS) system. Aside from adjusting valve lift, the overlap and timing of when the valves are opened can also be adjusted. The VANOS assembly is located on the front of each camshaft and oil pressure is used to change the timing of the camshafts. This allows for greater overlap at high engine speed to produce more power and less overlap at idle for smoother engine idle characteristics and greater emission control.

Finally, a variable length intake runner is utilized to provide optimum low end torque without penalty at high engine speeds. The basic design of the intake manifold is such that a scroll shaped component inside the intake manifold can be turned to increase or decrease the distance intake air must travel before reaching the valves. A motor with an integrated position sensor is located on the back of the intake manifold to control the scroll component within the manifold (see Figure B for reference).

With all of the above components working in unison, there is an increase in power and fuel economy and all-around better engine running characteristics. I haven’t heard of any advances to this extent for the exhaust side of the internal combustion engine, but I’m sure it’s only a matter of time before something of similar performance benefit will be discovered.

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