BMW Valvetronic (1)
by François Dovat
(© François Dovat)
For the first time, a spark ignition engine (petrol engine) does without an intake manifold throttle.
Diesels engines have none: they suck a quantity of air independent of the load, i.e. their power is regulated only by the volume of fuel injected. It is one of the reasons of their better efficiency. A partially closed intake throttle blade increases the pumping losses by strangling the engine to control its power. Thus a spark ignited engine has its best efficiency (inversely proportional to specific consumption – sfc or bsfc) at almost wide open throttle (WOT), in other words close to full load, and this efficiency drops more with the reduction in load than in the case of a diesel. Many OEMs try to solve this problem by means of charge stratification, a technology which consists to use extremely lean air/fuel mixture in the peripheral zones of the combustion chamber when the engine is under low load, so allowing the throttle to remain wide open. The direct injection of gasoline in the combustion chamber at the end of compression stroke makes it possible to concentrate a mixture in flammable proportions close to the spark plug. But it is a difficult way because the production of nitrogen oxides is increased due to the combustion in excess air, and also in consequence of an increased thermal load – in particular on the top ring and land. Moreover, the engines using the stratified charge process require sulfur free gasoline, or else their advantage in fuel economy becomes illusory.
BMW chose a different technology which resolves the problem at its source, while also solving another.
The mass of charge sucked in the cylinder is adjusted by the infinitely variable intake valves lift and duration, so that the manifold throttle could simply be removed. Since very long ago, researchers, engineers and inventors sought a system able to vary the valves opening durations and lifts in order to optimize it according to the revs. In spite of considerable sums of ingeniousness, only some systems varying these parameters either slightly or by steps were marketed. In addition, devices which modify the intake camshaft timing are current nowadays but they produce only some marginal efficiency improvement and they neither change the valve lift nor opening duration. By the way, BMW was the first OEM to use such a mechanism on the exhaust camshaft also (Double Vanos). From now on this system is used together with Valvetronic, so that most of the parameters of the valves actuation become controllable.
The inlet valves are actuated via two levers. One of them rests on an hydraulic socket whereas the other is shifted by the eccentric of a control shaft. This shaft is controlled by an electric motor and a worm gear; 3/10e of seconds is enough to swing from an extreme position to the other. The transient response of the engine is faster than with a conventional throttle since the atmospheric pressure is not decreased in the whole volume of the intake runners. Under low and part load the pumping losses are greatly reduced because the pistons do not have to suck during the entire intake stroke in a depression created by a partially closed throttle blade, but only during a fraction of this stroke. The atmospheric pressure just behind the valve makes it possible to quickly fill the cylinder wit the desired amount of charge and, moreover, as the said valve can be closed before the BDC instead of after, a good part of the energy spent in sucking is recovered when the piston moves up again.