A Comparative Study on Fuel Economy for CVT and 9-speed AT based Vehicles 2017-01-2435

It is well-known that, compared with automatic transmissions (ATs), continuously variable transmission (CVT) shows advantages in fuel saving due to its continuous shift manner, since this feature enables the engine to operate in the efficiency-optimized region. However, as the AT gear number increases and the ratio gap narrows, this advantage of CVT is challenged. In this paper, a comparative study on fuel economy for a CVT based vehicle and a 9-speed automatic transmission (AT) based vehicle is proposed. The features of CVT and AT are analyzed and ratio control strategies for both the CVT and 9-speed AT based vehicles are designed from the view point of vehicle fuel economy, respectively. For the 9-speed AT, an optimal gear shift map is constructed. With this gear shift map, the optimal gear is selected as vehicle velocity and driving condition vary. Vehicle simulators are developed using MATLAB/Simulink, including the core components such as engine, clutch, CVT, 9-speed AT and so on. The performances of both vehicles with different transmission types are analyzed using the developed vehicle simulator. The influences of different transmission types and ratio change manners on the operation efficiency of engine and vehicle fuel economy are investigated. It is found from the simulation results that, although CVT shows an advantage in improving the engine efficiency by 8.1% due to its continuous ratio change, 3.8% improvement of fuel economy is achieved by the 9-speed AT based vehicle for the whole driving cycle of FTP-72.