The new Mercedes-Benz A-Class: CAMTRONIC valve lift adjustment - Using the camshaft to accelerate
When the steeper half of the cam is active, the valve lift is increased and the valves remain open for longer. Switching to the flatter half of the cam shortens the valve lift and the valves close sooner.
"Accelerating" with the camshaft
Load control with the smaller valve lift is realised using various components. At very low engine torque the load control is conventional, using the position of the throttle flap, at medium torque levels using the position of the intake camshaft and at high torque levels using the charging level of the turbocharger.
As the torque increases the valve lift is switched to the larger level, load control once again being conventional via the throttle flap or, in the charged operating range, via the charging level of the turbocharger. In popular terms one might also say that the new Mercedes-Benz A‑ Class also accelerates with its camshaft.
Mercedes-Benz development engineers took numerous measures to ensure the most efficient combustion even with the smaller valve lift. Owing to the smaller valve lift and early intake valve closure, the turbulence in the combustion chamber is reduced at the spark plug. This turbulence decisively influences the combustion speed and full combustion of the fuel/air mixture. To compensate this apparent disadvantage, the turbulence is increased in the lower partial load range by using a multiple injection strategy with injection ignition, while multi-spark ignition ensures reliable combustion.
The switchover from the smaller to the larger valve lift goes unnoticed by the driver. As cylinders 1 and 2 as well as 3 and 4 are coupled in pairs with one cam-piece each, it is possible to adjust the valve lift of all four cylinders within one camshaft revolution using just one double actuator. A correspondingly large effort was required to develop the synchronisation for the switching process and ensure the long-term durability of the components.
The variable, hydraulic vane-type camshaft adjusters on the intake and exhaust sides have a wide adjustment range of 40 degrees with reference to the crankshaft. This new development excels by virtue of its significantly smaller dimensions. The installation space along the engine's longitudinal and vertical axes can therefore be made very compact.
Credits: Daimler AG
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