Feature October 1, 2011 by
Robert Jacobs, Identifix European Master Technician, Certified Infiniti Master, ASE Master + L1
BMW Intake Air Strategy
How difficult can it be to control the amount of fresh air entering an internal combustion engine? Traditionally, the intake air control for internal combustion automobile engines utilizes a throttle-controlled butterfly valve. Engines with...
How difficult can it be to control the amount of fresh air entering an internal combustion engine? Traditionally, the intake air control for internal combustion automobile engines utilizes a throttle-controlled butterfly valve. Engines with carburetors utilized 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 they control fuel by reading numerous input parameters and then injecting the proper amount of fuel via fuel injectors. Some manufacturers like to add a great deal more engineering to the process than a simple throttle body with a butterfly valve for intake air control. While the butterfly style throttle valve has been a successful means of controlling intake air, it also has downfalls. At idle or under partial load, the throttle butterfly valve is only slightly open, which 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, but it actually reduces the amount of pressure above the intake valves. To achieve 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 effect on cylinder filling, mixture control and fuel economy. This particular problem can be addressed in either a super-charged or turbo-charged 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 they do not combat the main downfall of the throttle butterfly valve at idle and partial load. Starting in 2002, BMW introduced the Valvetronic control system for intake management on its 7-Series vehicles. There are several items which come into play to make the Valvetronic system a more efficient system, but the general principle that it has is 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-in case you’re wondering, this comes from the German words: Variable Nockenwellen-Steuerung) and a conventional butterfly style 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 butterfly valve 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 butterfly throttle valve when specified by the engine control module. The Valvetronic valve lift control has a few more components than a typical overhead camshaft configuration: 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: 1 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. 2 The intermediate levers connect the camshaft and the eccentric shaft. 3 The camshaft contacts the side of the intermediate lever. 4 The top of the intermediate lever is in contact with the eccentric shaft. 5 At the bottom of the intermediate lever is a rocker arm device which makes contact with the top of the valve on one end and with a hydraulic lash adjuster on the other end. 6 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. 7 An additional feature which enhances the level of intake air control is the 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. 8 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. 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.