Variable valve timing (Diesel)
Variable adjustment of the camshafts in the diesel, indeed you begin to wonder. You're right, so far it was only an issue for petrol engines. But there is even more surprise because the two camshafts in the picture
above are not always assigned respectively the inlet and exhaust valves.
It depends on how the apparently absolutely necessary swirl of the inflowing air is generated. Are there, for example, flaps in the intake system, these can be used for this purpose. If not, then one turns, e.g. here in the
VW group, sometimes even the individual cylinder head, initially hesitant by 45°. Since the assignment of the valve function to both waves was correct yet. Meanwhile, the quarter turn is perfect and this assignment a
thing of past.
But what is the sense of twisting a camshaft in such a way to its drive, that one equally changes the valve timing at the inlet and outlet? Stop, please note that there is still respectively one intake and exhaust valve with
always the same valve timing. Incidentally, the maximum adjustment angle of the camshaft is 25°.
In a first approximation, one can only realize that it is the aim of the whole action, to grant the engine something less filling, which is achieved by an in direction 'late' adjusted input as well as outlet valve. However, one
relies on the Miller-process and this means that even with ignition retard enough filling and thus compression including charging exists.
So with a cold engine, a filling/compression can be adjusted which would harm a warm engine and the engine comes more quickly from the exhaust harmful situation. Of course, all is controlled by engine
management, including an electrically adjustable boost pressure. It is also and primarily to ensure the compliance of the opposite EU 5 very stringent EU 6 standard for nitrogen oxides. 02/16
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