Influence of Partial Recirculation on the Build-Up of Cabin Carbon Dioxide Concentrations 2019-01-0908

Carbon dioxide exhaled by occupants remains within the cabin during operation of HVAC unit in recirculation mode. The CO₂ inhaled by the occupants goes into their blood stream that negatively affects occupant’s health. ASHRAE Standard 62 specifies safe levels of carbon dioxide in conditioned space for humans. The CO₂ concentration limit per ASHRAE is 700 ppm over ambient conditions on a continuous basis. In a recent investigation the author had developed a model to predict cabin carbon dioxide concentrations for recirculation mode as a function of time, number of occupants, vehicle speed, body leakage characteristics, occupant lung capacities and concentrations of the carbon dioxide coming out from occupant’s mouth, blower position and vehicle age. This developed model has been modified to simulate cabin airflows from 100% recirculation mode to 100% outside air mode, i.e., for any percentage of partial recirculation.

In this paper, the author has presented simulated results for a vehicle from D segment (mid-sized sedan). The effect of partial recirculation is investigated to maintain good air quality per ASHRAE standard 62. Detailed analysis has been presented as a function of partial recirculation. Using partial recirculation in the HVAC control logic is a viable option for reducing build-up of cabin CO₂ concentrations resulting in an improved safety and comfort for the driver and the occupants.