F2008-06-125
Development of CVT Hydraulic Control System for HEV
Hybrid Electric Vehicle (HEV) has been developed or under development by several car manufacturers as one of down to earth countermeasures at the moment for environmental and fossil fuel energy depletion issues. A mild type HEV which is composed of engine, motor and Continuously Variable Transmission (CVT) is considered in this paper. Among the mild type HEVs, CVT seems mostly favored because of its merits of continuously variable speed ratios which can operate the engine at its optimal operating point. A CVT in this paper adopts forward clutch/reverse brake, which is for forward/reverse reversal in planetary gear set, as a starting device instead of torque converter. A CVT for HEV using multi disc wet clutch as a starting device requires a new concept of hydraulic control system in order to improve clutch control hydraulic response during restart after idle stop, cooling performance of the starting clutch as well as limp-home functionality which controls clutch pressure according to engine revolution for the case of TCU malfunction. This hydraulic control system composes of 4 functional parts. 1) Belt and pulley control part which provides belt clamping force and speed ratio control. In this part, as a means of supplying clamping force, primary and secondary pulley pressure control valves were adopted which is independently controlled from pump pressure. 2) Starting clutch control part including forward/reverse reversal as well as cooling and lubrication flow control. In this part, for precise starting clutch pressure control, direct clutch solenoid was adopted. 3) Whole CVT cooling and lubrication system including belt and pulley. In this part, cooling and lubrication flow was gathered from exhaust flow from pump pressure regulator valve. Belt and pulley cooling, bearing and gear lubrication and cooling is included. 4) Limp-home function parts. In this part, clutch pressure is controlled according to engine revolution for the case of TCU malfunction. This paper presents a new concept of hydraulic control system for mild HEV application.
Session: Powertrain Moduls