Abstract: A waste heat recovery system and control system and method of controlling a waste heat recovery system and control system is provided. The waste heat recovery system and control system comprises a pump, a heat exchanger, an expansion device, a condenser, a plurality of sensors, and a controller. The heat exchanger is in thermal communication with an exhaust of the internal combustion engine. The condenser is in thermal communication with the expansion device and the pump. The plurality of sensors is in communication with the waste heat recovery system. The controller is in communication with the plurality of sensors. In response to information obtained from the plurality of sensors, the controller calculates an efficiency of the waste heat recovery system based on models of the waste heat recovery system and implements a set of control inputs to implement the calculated efficiency on the waste heat recovery system.