Z63R said:
Flash!I'd like to know exactly how that works so that I'm not guessing at it.
BMW's Valvetronic engine is the world's first engine getting rid of throttle butterfly. What's the advantage ? BMW claimed it saves around 10% fuel compare with conventional engines.
To understand how throttleless engine save fuel, we must first know how our conventional engines work. When you floor down throttle pedal, you do not control the amount of fuel injection (which would have led to change of air-fuel mixture ratio hence misfire) but the amount of air running into the cylinders. Fuel injection system monitor the amount of air passing the throttle butterfly and then determine the corresponding amount of fuel needed. The more the throttle butterfly opens, the more air runs into the combustion chamber. This is how your throttle pedal control the engine. The below picture shows a conventional engine with throttle butterfly in the intake manifold.
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However, as you can see in the picture as well, at light throttle (when the car is running leisurely, down hill or at light load) the throttle butterfly partially or even nearly closes. Simultaneously, the pistons are still running, sucking air from the partially closed intake manifold. Needless to explain, the intake manifold between throttle and combustion chamber will become partial vacuum, resisting the sucking / pumping action of pistons. This waste energy. Engineers called this energy loss as "Pumping Loss". The slower the engine runs, the more the throttle butterfly closes, thus the more energy loss.
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Valvetronic mechanism get rid of throttle butterfly thus saves fuel, especially at lower rev. According to the EU combined test cycle consist of high speed and low speed running, the 1.8-litre Valvetronic engine used by 316ti Compact saves 10% fuel (at 53.3 mpg or 5.3 litre / 100 km). For those who regularly drive in city, the saving could be bigger. Moreover, unlike direct-injection (the most popular fuel-saving technology right now), it need not to drink low-sulphur gasoline.
Without throttle butterfly, how does it control the amount of air entering cylinders ? by controlling the lift of valve. Valvetronic is essentially a variable valve lift system acting on intake valves. Compare with conventional twin-cam engines with finger followers, it employs an additional eccentric shaft, an electric motor and several (one per valve) intermediate rocker arms (see picture). The motor control the angle of eccentric shaft, which rotate the angle hence depth of the intermediate rocker arms acting on finger followers, which in turn activate opening and closing of valves. If the rocker arms push deeper towards finger followers, the intake valves will have higher lift, vice-versa.
Now you might think about Honda's VTEC. Can Valvetronic use its variable lift to increase power like VTEC ? Sadly, Valvetronic is actually less efficient at high rev than conventional engines, let alone VTEC. As you can see, the camshaft drives the long intermediate rocker arms, in turn drive finger followers, this generates quite a lot of friction. Therefore the efficiency and refinement of Valvetronic engines drop rapidly at over 6,000 rpm. No wonder in the foreseeing future BMW will not equip its M-power engines with Valvetronic.
