1990-09-01

The New Camshaft Adjustment System by Mercedes-Benz--Design and Application in 4-Valve Engines 901727

The valve timing of internal-combustion engines usually represents a compromise with regard to the requirements placed on power output and torque.Extensive operation process simulation and measurements on real engines have shown that it is quite possible to achieve considerable improvements with constant valve lift and differing intake timing. Varying valve lift with engine speed is not necessary, since in this way only minor torque improvements can be gained. The requirements are met sufficiently with two timing settings. Extensive system analyses and concept studies led to the mechanically/hydraulically actuated camshaft adjuster which is described below in detail, and which is used in the new four-valve Mercedes-Benz engines. The camshaft adjuster is controlled in accordance with the requirements of engine and vehicle by the gasoline injection control unit, depending on engine speed and load. A full-load comparison with the predecessor model shows an average torque increase of 8 % in the most frequently used range between 2000 and 6000 rpm.AS IS GENERALLY KNOWN, conventional engine designs with 4 valves per cylinder possess a high power-to-weight ratio. With the previous configuration, however, the idling quality and particularly the torque curve in the lower engine-speed range are unsatisfactory. A specific improvement in these variables can theoretically be achieved by adjusting the lift and timing of the valves. However, variable valve lift can only be attained at present at unacceptably high cost. For this reason, this is currently implemented only in test-bench units for basic tests.

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