Modeling and Control of Atkinson Cycle VVT Engine for Hybrid Electric Vehicles (HEV)
The rational usage of fossil fuels and minimization of greenhouse gas emissions, have been the foremost goals of the automotive research and development activities. The innovation of advanced technologies such as variable valve actuation (VVA), variable compression ratio (VCR) mechanisms, use of over-expansion and downsizing have been promising the minimization of fuel consumption and emission reductions of the spark ignition (SI) engines. The integration of these technologies in the conventional SI engines is an active research area. In automotive applications, most of the time the SI engines have to be operated at part load operating conditions resulting in significant increase in part load losses and decrease in the thermal efficiency. These performance degrading aspects arise as a consequence of the throttling effect as well as declining of the effective compression ratio. Thus, performance of the SI engines has to be improved at part load operating conditions, so as to lower the transportation fuel consumption. However, to accomplish the aforesaid objectives and vehicle drivability, availability of accurate and simple control-oriented engine model incorporated with innovative technologies and engine robust controllers are the key contributors. Whereas, SI engine models available in the public literature either have not been incorporated with advanced mechanisms or they suffer from number of limitations, such as complexity and difficulty in analysis and control design. This research frameworks a novel physically motivated control-oriented extended mean value engine model (EMVEM) of the Atkinson cycle engine, wherein the Atkinson cycle, flexible VVA, VCR and over-expansion characteristics have been incorporated. For this purpose, an intake valve timing (IVT) parameter along with its constraints is introduced, which has a vital role, in modeling the inclusive dynamics of the system and to deal with engine performance degrading aspects. The proposed model is validated against the experimental data of a VVT engine during the medium and higher engine operating conditions, to ensure that the model has proficiency, to capture the dynamics of the Atkinson cycle engine and to handle the engine load through an unconventional control strategy. The model encapsulates air dynamics sub-model in which VVA system is unified to cope with the engine performance degrading aspects at part load and physics-based rotational dynamics sub-model, wherein Atkinson cycle is utilized for analysis. The control-oriented model is anticipated for systematic analysis, simulations, development of control system and estimation strategies to support and enable novel load control strategies, as well as to improve fuel economy of the Atkinson cycle SI engine used in hybrid electric vehicles (HEVs). Besides, a conventional PID control and a robust nonlinear higher order sliding mode (HOSM) control framework for the Atkinson cycle engine with an unconventional flexible intake valve load control strategy instead of the throttle is designed and developed. The control frameworks based on the novel EMVEM model are evaluated by using the notion of VCR in the view of fuel economy for the standard NEDC, FUDS, FHDS and US06 driving cycles. The resulting approaches confirm the significant decline in engine part load losses and improvement in the thermal efficiency and accordingly, exhibits considerable enhancement in the fuel economy of the VCR Atkinson cycle engine over conventional Otto cycle engine during the medium and higher load operating conditions. This research introduces an alternative control scheme resulting in around 6.7% fuel savings.