by Jim Kerr
Engine materials and technology have changed dramatically in the last couple of decades. New alloys, electronic fuel injection, and efficient emissions controls have given us economical, high performance powerplants. In January, BMW introduced the new 7-Series sedan at the North American International Auto Show, and its 4.4 litre engine offers unique technical innovations for increased power and economy.
The spec sheet for this new BMW engine is impressive. Eight cylinders, 32 valves, four camshafts, 325 horsepower, 330 ft. lb. torque, and no throttle plates! Yes that is correct. This engine has no throttle. Drive-by-wire throttle systems are common on many new vehicles, but those systems still use an electric motor operated throttle to control engine speed. Engine speed on this new BMW engine design is also computer-controlled, but now the valves are controlled to change engine speed.
BMW calls it “Valvetronic engine technology”. The concept is simple but the technology isn’t. The speed of gasoline engines is currently controlled by regulating the amount of air entering the engine with the throttle plate (sometimes called a butterfly valve). The BMW Valvetronic system changes the opening height of the intake valves to control airflow.
The Valvetronic system compliments BMW’s established double VANOS system. The VANOS system uses computer control to vary the position of the camshafts so valve opening will be advanced or retarded. By changing when the valves open and close, the engine can operate at optimum efficiency throughout the rpm range. By adding a few parts between the camshafts and the intake valves, BMW now varies the lift of the valve as well.
The key components of the Valvetronic system are a lever, placed between the camshaft and the valve, an eccentric shaft, and an electric motor with worm drive connected to the eccentric shaft. The engine’s computer operates the electric motor, which in turn rotates the eccentric shaft. The eccentric changes the position of the specially shaped lever between the camshaft and valves, causing the amount the valves lift to change. Valve lift can be altered from almost closed to the maximum designed into the camshaft.
At idle, the valves barely open. At high rpm and full load, the valves open fully. The computer can change the lift by rotating the eccentric in just 0.3 seconds.
So why add this complexity just to get rid of the throttle plates? The answer is performance. The intake manifold design is not restricted by having to position a throttle plate. There is no throttle plate or shaft to cause turbulence in the intake airflow, and the Valvetronic operation gives a very fast response, with very little delay of the air entering the combustion chamber. The result is an increase in fuel economy by approximately 14 per cent, with an increase in power also of about 14 per cent.
BMW took advantage of the Valvetronic system to design a special variable length intake manifold. Changing the length of the air intake runners for different engine speeds allows for maximum airflow into the engine and therefore more power. You may have seen tall intake manifolds used at the drag strip to “ram” the air into the engine. These work great, but only in a very limited rpm range. Many manufacturers are producing “staged” intake manifolds, where internal valves are opened or closed to vary the effective length of the intake passages. This works well at several rpm ranges. BMW goes one step further.
BMW’s variable intake manifold uses a rotating internal centre section that can continuously vary the length of the intake passages. With this design, the computer can position the centre section for optimum volumetric efficiency at any engine rpm. Simple but effective.
Three computer controlled systems – VANOS, Valvetronic, and variable length intake manifold combine to give the 2002 BMW 7-Series engine outstanding performance. But this is only part of the new technology in BMW’s flagship sedan. Equally impressive is the car’s driver-to-vehicle interface system, called iDrive. BMW has taken an interesting and impressive direction with their new engine technology. I wonder if others will follow.