Study of Alternative Valvetrains for Heavy-Duty CI Engines

Abstract

Current heavy-duty diesel engines rely on traditional camshaft technology for reliable valvetrain operation. The fixed valve lift profiles limit the engines’ opportunities to adapt to varying operating conditions for improved performance, fuel saving and emissions control. As variable valve actuation (VVA) technology matures it has the potential to become a vital system to extend efficiency and functional management of the combustion engine. Independent valve control has been a design goal of engine operation for many decades. An initial discussion describes the effects of changing valve lift and timing on engine performance and emissions. The discussion encompasses the various operating modes and features available with diesel combustion. This work includes a background investigation of the development of VVA systems. Cam based systems are explained as predecessors to camless, fully variable valve actuation (FVVA) systems. FVVA technology in its infancy has led to a multitude of various actuator designs. Examples of several systems are explained highlighting the method of valve actuation as well as benefits and limitations of the systems. Prior to conducting simulations a complete engine model was built in GT-Power and calibrated in conjunction with ongoing EU emissions reduction research. The simulations conducted focused on representing ideal valve lift profiles to demonstrate the potential of FVVA systems in contrast to traditional systems. Simulations were conducted varying valve lift parameters looking for trends associated with emissions, performance and efficiency. The valve lift profiles used in the simulation activities were artificially created to resemble the idealized profiles reported in literature. The performed simulations show significant potential for FVVA implementation. Improvements up to 4% in fuel consumption were predicted by FVVA implementation, without optimization. In addition to these several valve actuation strategies were investigated and further benefits predicted. Valve train energy consumption can be reduced by lowering valve lift height without compromising engine performance and emissions. Fuel consumption and nitrogen oxide (NOx) emission reduction were seen with Miller cycle operation. Various internal EGR strategies were evaluated and showed potential to reduce NOx by up to 25%. Promising engine braking performance was observed from idealized cycle analysis. A limited failure mode scenario was investigated and a discussion of potential solutions is presented.