Modelling of a Discrete Variable Compression Ratio (VCR) System for Fuel Consumption Evaluation - Part 2: Modelling Results 2019-01-0472

Variable Compression Ratio systems are an increasingly attractive solution for car manufacturers in order to reduce vehicle fuel consumption. By having the capability to operate with a range of compression ratios, engine efficiency can be significantly increased by operating with a high compression ratio at low loads, where the engine is normally not knock-limited, and with a low compression ratio at high load, where the engine is more prone to knock. In this way, engine efficiency can be maximized without sacrificing performance. This study aims to analyze how the effectiveness of a VCR system is affected by various powertrain and vehicle parameters. By using a Matlab model of a VCR system developed in Part 1 of this work, the influence of the vehicle characteristics, the drive cycle, and of the number of stages used in the VCR system was studied. This model takes as inputs: a switching time for the VCR system, the vehicle characteristics, the engine performance maps corresponding to the distinct compression ratios utilized and a vehicle drive cycle. The impact of the vehicle characteristics were evaluated by analyzing four vehicles, each belonging to a different segment, but all equipped with the same base engine. Separately, the influence of vehicle mass and final drive ratio were also analyzed. Simulations were run on five different drive cycles from the EPA and European Commission including the FTP-75, HWFET, US06, NEDC, and WLTP, in order to assess the role of the drive cycle characteristics on VCR performance. Finally, the possible advantages coming from the usage of a 3-stage VCR system were evaluated. Results showed that effectiveness of the VCR system was increased under lower power demand conditions where a higher compression ratio could be utilized over a wider range of the drive cycle. Thus, lighter vehicles and drive cycles with milder accelerations and lower vehicle speeds are more suitable for this type of system. Additionally, a 3-stage VCR system is more effective in terms of increasing fuel economy than a 2-stage VCR system, and the VCR system should be designed with a large enough range in compression ratio to maximize its effectiveness.