Semiconductor sensors for vehicle cabin air quality monitoring


Semiconductor sensors for vehicle cabin air quality monitoring

Sensor and Actuator Department, Central Glass and Ceramic Research Institute
(Council of Scientific and Industrial Research)
196 Raja S C Mullick Road, Kolkata 32, India

Surprisingly, passengers in a closed vehicle are exposed to a greater amount of polluted air than people in homes and workplaces [1,2]. Clouds of pollutions are encountered when crossing major intersections, passing through tunnels, and following vehicles without catalytic converters, such as old buses and cars. Pollutants find their way into the cabin via the ventilation system, also known as the heating, ventilation and air-conditioning (HVAC) system. It has been observed [1,3] that the vehicle cabins show higher amounts of carbon dioxide (CO), hydrocarbons (HC), volatile organic compounds (VOC) and oxides of nitrogen (NOx) higher than the safety limits. Also the fuel vapour from the tank (patrol/diesel/LPG/CNG) can get inside the cabin. Incidentally, the purging process of the pollutants from the cabin is slow as the induction of fresh air in this situation is similar to cleansing by dilution. Amongst the different pollutants, CO is the most dangerous because it does not smell and it reacts with hemoglobin and prevents its functioning as an oxygen carriers. This can lead to fatigue, drossiness loss of alertness, headache, nausea, poor hand-eye coordination etc. Other pollutants in addition to increased level of CO2 due to exhalation can effectively enhances the toxic effects of CO and it has been found that such conditions are the principle cause of 20% of crashes involving fatality in Australia CO concentration greater than 2000 ppm (acceptable upper limit is 50 ppm) can even lead to unconsciousness and death. The recent upmarket cars are incorporated with vehicle air quality monitoring (AQM) using air quality sensors(AQS) when detecting high levels of pollution , this sensor(s) instruct(s) the climate control system to close the fresh air inlet door to keep poisons gases out. At the system level, AQS are integrated within the automobile management systems with smart hardware(e.g., microcontroller) and software integration.


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In recent times AQS for vechicle cabing AQm are based on primarily three technologies1,4 viz. i) Semiconductor (metal oxide semiconductor) ii) electrochemical and iii)IR
Semiconductor sensors are based on n – type semiconducting oxides and their resistance value drop drastically in the presence of reducing gases owing to the reaction of the latter with the metastable adsorbed oxygen species on the surface of the semiconductor.
Electrochemical sensor produce a characteristic EMF(e.g., Potentiometric sensors ) on a characteristic current ( e.g., amperometric sensors), when a specific gas reacts in a controlled manner at a three phase boundary layer atmosphere , electrode of a catalytically active material and electrolyte)
Infrared sensors exploit the characteristic absorption of the gases ( including monatomic and symmetric diatomic gases) in the infrared region.
For AQM, electrochemical sensors are seldom used because of their low life time(2-3 years) and infrared sensors, though having excellent selectivity, are not in much use because they are expensive and not rugged, The sensors of choice for AQM of vehicle cabin are semiconductor sensors . They are small , Rugged and inexpensive with life time of around ten years.

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The major disadvantage of semiconductor sensors is their poor selectivity. Incidentally, high selectivity is not the critical requirement for AQM applications. The overall aim is to monitor the pollutant gases (CO, HC, VOC etc.) as a whole and consequently almost all AQM system have exploited1 the positive aspects of semiconductor sensors in spite of their non-selective nature.