The current world situation of an increasing population is leading to an increase in thefuel consumption, green house gas emissions, electrical power consumption etc.Theseincreases are becoming a serious problem which various organizations are concerned about andare trying to solve.An increase of bus air conditioning systems increases fuel consumption andtherefore produces green house gas emissions as conventional bus air conditioners are vaporcompression systems driven by engine power.In order to reduce the fuel consumption and gasemission of a bus, a two stage ejector cooling system is selected to study because the systemrequires little electrical power or engine power to drive the system.It uses exhaust gasemissions from buses to drive.The two stage ejector cooling system is used to cool condensatefrom the condenser of a bus air conditioner to a subeooling state in order to enhance theperformance of vapor compression system.　　The objective of this research is to study in theoretical and experimental performancesof a two stage ejector cooling system using the waste heat of the engine (exhaust gases).Todevelop a combined air conditioning system with a two stage ejector cooling subsystem and avapor compression subsystem and evaluating the applications of a two stage ejector coolingsystem to enhance the performance of the traditional bus air conditioning (vapor compressionsystem).The cooling capacity obtained from the two stage ejector cooling subsystem is in turnused to cool the condensate of the vapor compression subsystem to a subcooled state toincrease the COP of the vapor compression subsystem.The ejector cooling system uses wateras the working fluid; the design operating conditions have a generating temperature of 150℃, acondensing temperature of 54℃ and an evaporating temperature of 13℃.In this work,variables were tested in order to understand the application of the system and the effect ofvarying operating conditions.In addition, the comparison of the performance between thesingle stage and two stage ejector cooling system has been carried out.　　The results indicate that a two stage ejector cooling system can decrease thetemperature of the condensate from the condenser from 54℃ to 26.5℃ under operatingconditions with a generating temperature of 150℃ and an evaporating temperature of 21.7℃,of which the COPejc is 0.37.If condensate from the condenser of a vapor compression systemis cooled by the two stage ejector cooling system under these conditions, the liquid becomes asubcooled state at 26.5℃, the COPvcs improves from 3.6 to 4.85 (+ 34.86％).Furthermore, inthis study, under high evaporating temperature condition, the two stage ejector system hadlower performance than that of the single stage ejector system.The two stage ejector systemshowed better performance than the single stage ejector when used at a low evaporatingtemperature.　　The expected advantages of the proposed system are reduced fuel consumption,reduced green house gas emissions, less demand for power from the engine, much lessvibration while working and very low noise output, except that of circulating pump.