Further, CVT control aims at reducing slip by means of clamping force control. Numerous control methodologies have been developed to achieve the desired speed ratio, while improving the efficiency and drivability of the vehicle by means of minimizing the rate of ratio change. The main aim of a CVT control is to achieve the desired speed ratio in a fast and accurate manner. In practice and under steady-state operation, the regulation of the primary pressure is governed by a 3-D map that relates the primary to secondary pressure ratio with the speed and torque ratios of the CVT. On the other hand, the primary valve regulates the supply line pressure feeding the primary pulley. This supply line pressure is controlled through the secondary valve to modulate the pressure according the required clamping force needed for a given load torque. An example of the hydraulic pressure control circuit is depicted in Figure 2, here the hydraulic pump feeds the supply line pressure directly to the secondary pulley. The displacement of the movable sheaves is controlled hydraulically, and accordingly the hydraulic pressure develops a frictional torque between the pulley sheaves and the belt, which assist the torque transmission from the primary to secondary pulley. However, to change the gear ratio, the movable sheaves have to be displaced equally and oppositely, as shown in Figure 1. If the linear displacement of the movable sheaves in both pulleys is restricted, then a constant gear ratio is maintained. Each pulley has a fixed sheave and a movable sheave. The push-belt CVT, Figure 1, consists of a metal push-belt that connects the primary (driver) with the secondary (driven) pulley. Section V details the control strategy used. Section IV describes the model that has been developed to represent a CVT in an actual vehicle. Section III describes different control approaches available today. Section II gives an overview of the working principle of a CVT and the literature. The paper is divided into the following sections. This paper addresses CVT control problem using a model-based pressure control strategy to achieve the desired speeds. Thus, CVT control had become a major topic of research and control models are being developed to reduce the complexity associated with CVTs. The control of CVTs is complicated as they are usually non-linear systems. Further, the market resistance of CVTs is due to the inherent issue of belt slip, limited torque capacity and cost. However, the overall CVT efficiency is heavily dependent on the parasitic loads needed to actuate and control the pulleys hydraulically.ĭespite the inherent benefits discussed above, CVTs market share showed a large market penetration resistance during 2000s, but after the new CAFE regulations and the advances in CVT controls and designs, National Research Council estimated a 19.3% market share in 2014. Therefore, CVT eliminates the shifting jerks in comparison to conventional automatic transmissions, resulting in an improved driver feel. Ī CVT enables to have continuous range of gear ratios between certain limits that help in increasing the overall powertrain efficiency by running the engine at its optimal operating point while providing a smooth speed-torque curve. Since late 90s, the CVT research has gained prominence, these researches were conducted to understand the mechanics of CVT, the role of friction and the influence of control strategies on the performance and efficiency of the vehicle powertrain, the details of these researches are reviewed by Srivastava and Haque. In this regard, Continuously Variable Transmission (CVT) holds greater promise to improve the fuel economy of vehicles.ĬVTs are available in various forms, including the commonly used push-belt type CVTs, the Toroidal traction drive, the variable diameter elastomer belt, and the variable geometry CVT. All this has led to a lot of research on vehicle technologies, including hybridization and electrification. Automakers have been trying to achieve the maximum fuel economy possible from their vehicle fleet to achieve the strict Corporate Average Fuel Economy (CAFE) targets. Mandates to reduce carbon emissions and increase the fuel economy of vehicles have gained prominence in the recent years with the declining fuel resources and increased effects of global warming.
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