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Fuel economy is a major driver in the development of new passenger car engines. One requirement for a good fuel economy is to achieve a low level of mechanical friction. The valvetrain can contribute a substantial part to this mechanical friction, especially at low speeds where fuel economy is most important. As a result of this, friction optimized engines often have a roller follower type valvetrain design despite its disadvantages versus bucket tappet concepts in other aspects. This paper presents a study on modern amorphous carbon coatings at the cam / tappet interface aimed to investigate the potential for bucket tappet valvetrains to overcome the friction disadvantage. The work presented is divided into 1) a pin-on-disk pre-study to identify coatings with the most promising properties and 2) a cylinder head friction study to validate the findings in the realistic environment. The paper demonstrates that the coatings have a substantial potential for friction and wear reduction.

A model for valvetrain dynamics is presented, which aims at both accuracy and efficiency. Important aspects in achieving these aims were (i) to formulate the dynamical equations in terms of deviations from the kinematic displacement. (ii) a tappet stiffness which varies with contact point. (iii) a model of the hydraulic lash adjuster. (iv) a model of valve spring dynamics in terms of spring surge modes. The model results show good agreement with measured accelerations for a DOHC engine with direct acting bucket tappets. Moreover the model allows one to quickly gain an insight into the dynamics of valvetrains.