When the air conditioning (A/C) is turned on, the intake air to the HVAC is cooled at the evaporator. This is not only used for cooling the air temperature but also to dehumidify. Therefore, for a typical automatic climate control system, A/C will automatically operate even in winter (cold ambient temperature conditions) in order to prevent the windows from fogging despite its effect on fuel economy. In some applications, a humidity sensor is installed on top of the windshield and when the probability of fogging is low the A/C operation is disabled automatically to prevent unnecessary compressor operation which can increase fuel consumption. However, humidity sensor is not widely adopted as it requires some space to be installed and the cost is relatively expensive compared with other HVAC equipped sensors. In this study, a system was invented that disables the compressor operation when the fogging probability is low without using the conventional humidity sensor. This system uses real-time weather information from out of car database with a data communication module (DCM) which is standardly equipped in many Toyota vehicles. Control logic was created that actively judges if the current probability of fogging is high or low by using weather information. In real-world driving test in Aichi, Japan, and Michigan, US, the system reduced 60 ~ 70 % of compressor operation in winter which demonstrated 1.5 ~ 1.6 % improvement in fuel consumption.
