The present work designs an air conditioning system for an urban bus with a 4th generation refrigerant, HFO-1234yf, that has less impact on the environment when compared to the currently used, R-134a. Due to the light flammability of the new fluid, it s proposed a new type of system configuration, an indirect cooling system, where another fluid is used in a secondary cooling cycle that will circulate throughout the car environment. For safety the HFO-1234yf fluid will cycle inside the engine to prevent fire in the inner area of the bus. The secondary fluid will be a nanofluid that will replace the ice water, normally used in indirect cooling systems, aiming at a better thermal conductivity. The design of the air conditioning system will be done by the mobile frontier or multi-zone method, which takes into account the different states that the fluid presents in the heat exchangers.