Automobile heat pump technology is an effective method to save energy consumption and extend driving ranges in cold weather for electrical vehicles. Due to the reverse thermal cycle and changed operation conditions, automobile heat pump cycle characteristics are different from conventional air conditioning system. This paper presents KULI-based models to simulate steady-state operation of both direct expansion (DX) and secondary loop (SL) automobile heat pump systems using HFC-R134A or HFO-R1234yf refrigerant. The DX model includes a compressor, an indoor condenser, an expansion device and an outdoor evaporator. The condenser in DX model is replaced by a secondary coolant loop in SL model, which consists of a coolant plate type heat exchanger, an electrical pump and a coolant to air heat exchanger. The developed R134A DX and SL models were verified against steady-state experimental data. Furthermore, the verified models were employed to investigate the DX and SL heat pump cycle characteristics. The effects of ambient temperature, compressor speed, airflow rate and coolant flow rate on the heating performance and COP were revealed, and the heating performance comparison between R134A and R1234yf refrigerant was also illustrated. In order to study the impact of secondary loop on system performance, the heat transfer effectiveness of condenser, plate type heat exchanger and coolant to air heat exchanger were analyzed. The variation of effectiveness with the change of refrigerant and coolant mass flow rate were also discussed. In addition, some significant cycle characteristics were presented and concluded, such as discharge temperature, sub-cooling, superheat and heat transfer temperature difference. Based on above discussion, the optimization orientation of R134A/R1234yf heat pump system were further investigated, like the design of indoor condenser and outdoor evaporator.