This thesis addresses temperature control of an engine bleed air system, as used on typical aircraft, with a control strategy aimed at reducing ram air usage while maintaining fast response. The analytical equations describing the system dynamics are presented, and a state space model of a cross-flow heat exchanger is developed. The heat exchanger model is then utilized in the modeling of the bleed air system under study. Different control strategies and configurations are analyzed and compared. In order to achieve fast temperature regulation while saving ram air usage, a ram-air-plus-bypass control configuration is proposed, along with the control of both load temperature and bypass valve opening. Classical PI control is employed, and a hybrid LQ/PI control method is additionally investigated for the proposed control configuration. Simulations for each control strategy are performed using a nonlinear dynamic model at several sample mission operating points, with consideration of disturbances in engine loading or ambient conditions. Comparisons are made and conclusions are drawn based on these simulations. The proposed configuration and control strategy prove to be more effective in terms of satisfying both low ram air usage and fast temperature control respo