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The author has developed a model that can be used to predict build-up of cabin carbon dioxide levels for automobiles based on many variables. There are a number of parameters including number of occupants that dictates generation of CO2 within the control volume, cabin leakage (infiltration or exfiltration) characteristics, cabin volume, blower position or airflow rate; vehicle age, etc. Details of the analysis is presented in the paper. Finally, the developed model has been validated with experimental data. The simulated data follows the same trend and matches fairly well with the experimental data.

Recently the author (Mathur, 2017) had presented a model to predict cabin carbon dioxide concentrations 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. The developed model was validated by the author for mid-sized vehicles (vehicles from D-segment). The simulated data was within ±11.5% of the experimental data. In this paper the author has used the developed model to predict cabin CO2 concentrations for vehicles from B, C & D segments. Or in other words, the effect of the cabin volume will be investigated on the rate of build-up of cabin CO2 concentrations. Experimental tests were conducted on these vehicles and are compared to the simulated data. Detailed results have been presented in the paper.

Field tests were conducted on a late full sized sedan with the HVAC unit operating in both Recirculation and OSA modes to monitor build-up of the CO2 concentration inside the cabin and its influence on occupant’s fatigue and alertness. These tests were conducted during 2015 summer on interstate highways with test durations ranging from 4 to 7 hours. During the above tests, fatigue or tiredness of the occupants (including CO2 levels) was monitored and recorded at 30 min intervals. Based on this investigation it is determined that the measured cabin concentration levels reaches ASHRAE (Standard 62-1999) specified magnitudes (greater than 700 ppm over ambient levels) with three occupants in the vehicle. Further, the occupants did show fatigue when the HVAC unit was operated in recirculation mode in excess of 5 hours. Further details have been presented in the paper.

Experimental tests were conducted to monitor cabin carbon dioxide concentrations by driving the vehicle in Farmington Hills & Detroit area. The number of occupants, vehicle speed, and type of driving (local traffic and highway conditions) are the major variables for this study. The tests were conducted during winter season with HVAC unit operating in foot and defrost modes. For foot and defrost modes, there are some noticeable differences in the magnitudes of the carbon dioxide concentration due to the airflow rates and mixing of air within the cabin. The measured peak cabin carbon dioxide levels in foot and defrost modes were found to be of similar magnitudes. However, the initial build-up rates of cabin carbon dioxide for defrost modes were higher in defrost modes in comparison to the foot mode. This is due to different mechanism of mixing of air within the cabin. This is explained in details in the paper.

The vehicle's AC system should not be operated in recirculation mode for extended periods of time due to build up of CO2 inside the vehicle cabin. This is the CO2 that is exhaled by the occupants of the vehicle. This CO2 is then inhaled by the occupants that goes into their blood streams which results in a negative impact on health. This becomes critical when a number of people are sitting inside the vehicle. Field tests were conducted on a MY 2003 vehicle in recirculation mode to monitor the build-up of the CO2 concentration inside the cabin as a function of number of occupants, vehicle speed and ambient temperatures. The vehicle was driven in Detroit Metro area in city and highway traffic conditions. Based on this investigation it is determined that the cabin concentration levels reaches ASHRAE (Standard 62-1999) specified magnitudes in first 5 minutes of driving with only one occupant in the vehicle.

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.

Field tests were conducted on a MY 2003 vehicle with the HVAC unit in OSA mode to monitor build-up of the CO2 concentration inside the cabin as a function of number of occupants, vehicle speed and ambient temperatures. These tests were conducted in the winter season by driving the vehicle in Detroit Metro area in city (Farmington Hills) and highway traffic conditions. Based on this investigation it is determined that the measured cabin concentration levels reaches ASHRAE (Standard 62-1999) specified magnitudes with four occupants in the vehicle. For this investigation, one to three occupants inside the cabin did not increase the level of cabin carbon dioxide to the levels specified by ASHRAE standard. A maximum concentration with four occupants was measured at 1700 ppm. The cabin concentration level would be higher for vehicles that have lower body leakages compared to this one.