Abstract 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. Keywords CVT, 9-speed AT, fuel economy, ratio controlIntroduction For an internal combustion engine based vehicle, transmissions are installed between the engine and the driveshaft, which adapts the performance of the engine to the vehicle wheels in terms of speed and torque, considering the characteristics of the engine and the demand of the driver [ 1]. Torque transmission of the CVT is performed by the friction between the belt and the pulley [ 2]. The torque transmission capacity of CVT is related to the primary and secondary clamping forces, in other words, line pressure. To increase the torque transmission capacity, high line pressure needs to be supplied, which causes high hydraulic loss and mechanical loss within the CVT. The maximum CVT line pressure can climb up to 50 bar while the maximum pressure for AT is less than half of that [ 3]. Thus, the transmission efficiency of CVT is 10%-20% lower than that of AT due to its higher friction loss and hydraulic loss [ 4]. For continuously variable transmission (CVT), the ratio changes continuously between its maximum and minimum values, which decouples the engine speed from the vehicle velocity [ 5]. This feature enables the engine an additional degree of freedom to follow the optimal operation line (OOL) to operate in a higher efficiency region [6]. However, for an automatic transmission (AT), since the transmission ratio is stepped, the gear shift can only help to make the engine operation points locate in areas near its optimal region. This difference enables 5%-15% improvement of fuel economy for a CVT based vehicle when compared with a stepped AT based vehicle [ 1]. Especially when the maximum vehicle velocity is relatively low, the improvement of fuel economy is more significant [ 1]. As the AT gear number increases and ratio gap narrows, such as the ZF 9-speed AT [7], the influences of different ratio change manner (stepped and continuous) on engine operation efficiency is weakened. Furthermore, the improvement of vehicle fuel economy by CVT becomes unclear, since the AT efficiency is higher due to its lower hydraulic loss.A Comparative Study on Fuel Economy for CVT and 9-Speed AT based Vehicles2017-01-2435 Published 10/08/2017 Jian Ji, BoZhou He, and Lei Yuan Wuhan University of Technology CITATION: Ji, J., He, B., and Yuan, L., "A Comparative Study on Fuel Economy for CVT and 9-speed AT based Vehicles," SA