The EnginesBMW designed two totally new V8s for its flagship car, designated N62, "new" replacing the old M62 V8s. They have all the latest size- and weight-reducing technology such as cast-in-place silicon cylinder liners but the real news is induction, which pairs continuously variable-length intake runners with infinitely variable valve actuation. The intake runners are shaped alternately as a "p" or a "q" to feed the appropriate bank, giving the port a straight shot at the valve. The inner surface of the circular part of the runner is ported to a central plenum via a rotating sleeve. There are two sets of sleeves on counter-rotating shafts, one for each bank. The position of this sleeve can be adjusted to give a resonant length between 8.5 in. and 23.9 in. The principle is the same as opening the valves on a wind instrument to change the resonant frequency. The runners are at maximum length below 3500 rpm. The 236 degrees of movement to reach the shortest length can be swept in less than 1 second.
The real breakthrough is Valvetronic, continuously variable valve actuation. It eliminates the role of the throttle plate-as significant as changing from carburetors to electronic fuel injection. The cam's movement is transmitted to the rocker arm that actuates the valve by a third element, an intermediate follower that floats in the head, captured between its movable fulcrum, the camshaft, and the rocker arm. Rather than pushing up and down conventionally on the primary rocker arm, the cam lobes apply sideways movement to the intermediate follower, which transmits this motion to the rocker arm via a curved ramp. This ramp multiplies the cam's movement by anything from zero to full lift, depending on which portion of the ramp is against the rocker arm. The pivot point of the intermediate follower is at its upper end, where it is pressed by valve-spring pressure against an eccentric shaft. As this shaft rotates, it changes the pivot point of the intermediate follower laterally but not vertically. Thus, the intermediate follower is "rocked" on the camshaft, causing a different portion of the ramp to act on the rocker arm, and hence the valve. The eccentric shaft is actuated by a worm drive, just as a conventional drive-by-wire throttle shaft would be. There are no monster hydraulic pressures nor slapping solenoids. As the intermediate follower rocks around the camshaft, its phasing relative to the camshaft is changed, so VANOS, a well-established BMW technology, is required for Valvetronic to work. These new engines mark the first application of bi-VANOS to a V8.
The foregoing explanation is complicated, because the system is complicated. It took several minutes of watching the system work to understand what was happening, but when I finally did, I achieved a brief moment of Nerdvana.
BMW has accomplished the impossible. Until now, only diesel engines have had no throttle, which has been one of the reasons for their inherent fuel efficiency. Every engineer knows that the throttle decreases efficiency though pumping losses, but until now this has simply been accepted as the cost of doing business with the Otto cycle. By pulling manifold vacuum all the way down the intake stroke, the piston does work that it never gets back. With Valvetronic, the intake valves can be opened to admit just the required charge into the cylinders, then closed. The remainder of the intake stroke, the piston is doing work to expand the gases in the cylinder, but this work is returned to it on the upstroke. The result is a fuel efficiency increase of 10 percent or more under typical driving conditions.
Benefits don't stop there. Because the valves open only 0.5- to 2.0mm under typical light-throttle conditions, the intake charge moves past them at very high speed-the speed of sound, according to Dr. Goschel. This high-speed flow pulverizes the fuel as it enters the chamber, making for a more homogeneous mixture. Valvetronic is superior to existing gasoline direct injection (GDI) technology, because it works with high-sulphur fuels, such as are found in the United States.
Unlike conventional engines, where the intake charge moves past a wide-open valve very slowly, swirl dynamics are maintained at low speeds. Good low-speed combustion dynamics make throttle response with Valvetronic superior even to that of individual throttle butterflies for each cylinder, which eliminate lag that occurs while the intake manifold is filled. The list of wins for Valvetronic is a long one. Torque and efficiency can be maximized at all speeds and load conditions when Valvetronic is combined with variable intake and bi-VANOS, each of which requires very sophisticated controls. Dr. Gfschel calls the present and coming years the "software era" of engine development.
The new 7 Series is not the first application of Valvetronic: It has been available for several months in the 316ti compact in Europe. BMW believes so strongly in the technology that it will eventually be used in every BMW engine. The four-cylinder was redesigned first, so it was the first to receive Valvetronic. We are told that within a year of the first 7 Series reaching customers, a 760Li will be released. Its N73 V12 will be derived from the N62 and will have GDI in addition to Valvetronic-purely for performance, said Dr. Gfschel. We can't hardly wait.
| 735i |
| Capacity | 3600cc |
| Power | 272 bhp @ 6200 rpm |
| Torque | 265 lb-ft @ 3700 rpm |
| 745i |
| Capacity | 4398cc |
| Power | 333 bhp @ 6100 rpm |
| Torque | 332 lb-ft @ 3600 rpm |