Technologies for Variable Valve Trains; a Contribution to Modern S.I. Engines
Published October 20, 2004 by Transilvania University of Brasov in Romania
The article refers to innovative variable valve train systems for spark ignition engines. First of all, the development history of valve trains beginning with the non-variable actuation leading up to different possibilities of variable valve train is described.
Different existing variable valve train systems will be compared with regard to benefits and costs, as well as to compliance with legal requirements.
Following this, we will concentrate on two particular systems, namely the Variable Cam Timing (VCT) and Cam Profile Switching (CPS) mechanisms, which, in combination, cover approximately 80% of the benefits of fully variable lift and timing systems.
The function and the components of Variable Cam Timing mechanisms for chain and belt actuation will be described. The energy balance necessary to actuate the mechanism is analyzed in detail, since it is a characteristic value for the shifting velocity.
For the Cam Profile Switching mechanism, again, the function and components will be described using a Switchable Tappet and a Switchable Finger Follower as examples. The defining parameters for switching operations will be analyzed.
Furthermore, the application of VCT in combination with CPS on the intake camshaft, as it is used in different Porsche engines under the name 'VarioCam Plus,' will be presented. In this context, the focus is on emission reduction, fuel economy, as well as on the specific characteristics of a sports car engine like power increase.
Finally we will give an outlook on the future development of both systems, as well as on the perspectives of using more complex systems with higher degrees of flexibility.