Method and apparatus to control hydraulic pressure in an electrically variable transmission

A method and apparatus to control an electrically variable transmission, by dynamically controlling system main hydraulic clutch pressures, based upon required clutch capacity, as determined by output load of the transmission. Included is a method to regulate hydraulic clutch pressure in an electrically variable transmission equipped with at least one clutch. This comprises monitoring magnitude of slippage of the clutches and controlling hydraulic boost pressure based upon the magnitude of clutch slippage. Controlling hydraulic boost pressure based upon the magnitude of clutch slippage comprises monitoring operator inputs, determining a requested operator torque command, and determining a required main boost pressure. The main boost pressure is based upon the requested operator torque command, the monitored operator inputs, parameters of the EVT and clutches. Commanded main boost pressure is then determined based upon the determined required main boost pressure.