There exists a strong interaction between the engine cylinder, intake and exhaust gas flow dynamics and the dynamics of mechanical and hydraulic components constituting the valvetrain system, which controls the engine gas flow. Technologies such as turbo-charging and Diesel particulate filtration (DPF) can significantly increase port gas pressure forces acting on the exhaust valve. When such systems are introduced or undergo design modifications, the operation of valvetrain system can be greatly affected and even compromised, which in turn may lead to degradation of performance of the internal combustion engine. Often, the valvetrain system needs to be retuned. Further, predictive analysis of design issues or evaluation of design changes requires highly coupled simulations, combining models of gas pressure forces and the dynamics of all mechanical and hydro-mechanical parts constituting the valvetrain.
This paper presents the application of an integrated numerical model to the study and alleviation of undesired phenomena observed in experimentally measured exhaust valve lift in a 4-cylinder, mid-size Diesel engine, during testing with a Diesel particulate filter. The valve is driven by a finger-follower mechanism with a hydraulic lash adjuster post. The phenomena occurred near the closing segment of the lift across the operating speed range and were thought to be related to the features in the cam design profile as well as to the exhaust port backpressure caused by the DPF cycling and/or aging.
First, the coupled dynamic model of a single exhaust valvetrain (rigid camshaft segment, cam lobe, finger follower, hydraulic post, poppet valve) was employed to obtain a verification and physical explanation to the observed exhaust lift features. Once a satisfactory agreement between experimental and simulation results was reached, the model was further exercised in search of solutions that would eliminate the observed exhaust valve lift behavior while preserving the essential elements of the original valvetrain design. Several such design changes were identified and their contributions to a reduction of undesired lift phenomena were investigated.