Simulation Study of Advanced Variable Displacement Engine Coupled to Power-split Hydraulic Hybrid Powertrain

The simulation-based investigation of the variable displacement engine is motivated by a desire to enable unthrottled operation at part load, and hence eliminate pumping losses. The mechanism modeled in this work is derived from a Hefley engine concept. Other salient features of the proposed engine are turbocharging and cylinder deactivation. The cylinder deactivation combined with variable displacement further expands the range of unthrottled operation, while turbocharging increases the power density of the engine and allows downsizing without the loss of performance. While the proposed variable displacement turbocharged engine (VDTCE) concept enables operations in a very wide range, running near idle is impractical. Therefore, the VDTCE is integrated with a hybrid powertrain allowing flexibility in operating the engine, elimination of idling and mitigation of possible issues with engine transients and mode transitions. The engine model is developed in AMESim using physical principles and 1-D gas dynamics. A predictive model of the power-split hydraulic hybrid driveline is created in SIMULINK, thus facilitating integration with the engine. The integrated simulation tool is utilized to address design and control issues, before determining the fuel economy potential of the powertrain comprising a VDTCE engine and a hydraulic hybrid driveline. INTRODUCTION Development of modern vehicles is driven by the need to address the energy security and climate change with increased fuel economy, while simultaneously meeting strict exhaust emission regulations. Hybrid technologies are critical for reducing emission and vehicle fuel consumption. This is due to the possibility of (i) downsizing the engine, (ii) recovering energy during regeneration, and (iii) optimizing engine operation. The latter has traditionally been very important for hybrids equipped with an SI engine. Pumping losses (throttled operation) in an SI engine is its Achilles heel and is the main reason for its poor fuel economy at part load. Therefore, hybrid system design and control typically attempts to avoid extended low load operation, thus improving the average fuel conversion efficiency over the driving cycle. Various concepts to reduce pumping loss like variable valve timing and variable cylinder displacement have been proposed over time, therefore offering pathways for further improvement of overall powertrain efficiency. Our intention is to explore the potential of combining one such system for varying engine displacement with a hybrid driveline. In this case the role of hybridization is to enable the application of an advanced engine concept that might be viable only within a certain operating range. Concept of variable displacement has been addressed in many technical publications over the last few decades, but it has not yet been demonstrated on a production engine. Several authors [1, 2, 3] have proposed different mechanisms to achieve variable in-cylinder displacement. Pouliot et al. [4], proposed, constructed and studied a 5-cylinder, four-bar linkage engine. Wong et al. [5] presented and analyzed a four cylinder engine with Alvar cycle that utilizes secondary pistons and auxiliary chambers. Independent of the actual means of achieving variable displacement several authors explored the theoretical aspects of applying such a concept. Early work by Siegla and Siewert [6] estimated that the Variable Stroke Engine could improve fuel economy by up to 20%, depending on allowable NOx emissions and vehicle power-to-weight ratio. In a followup experimental study Siewert [7] uncovered penalties associated with combustion deterioration and increased heat losses at very short strokes. Alsterfalk et al. [8] studied the