A robust optical fibre based CO<sub>2</sub> exhaust gas sensor operating in the mid infrared spectral range is described. It is
capable of detecting on board carbon dioxide (CO<sub>2</sub>) emissions from both diesel and petrol engines. The optical fibre
sensor is not cross sensitive to other gaseous species in the exhaust such as water vapour (H<sub>2</sub>O), carbon monoxide (CO),
oxides of nitrogen (NO<sub>x</sub>) or oxides of sulphur (SO<sub>x</sub>).The response of the sensor to carbon dioxide present in the exhaust
of Fiat Croma diesel engine are presented.
Online detection of harmful exhaust gases is necessary for optimal engine control to reduce the polluting emissions of diesel cars. Optical detection methods in the UV-VIS range enable the simultaneous characterisation of various gases such as NO, NO<sub>2</sub> and SO<sub>2</sub>. Additionally this technique is fast and has a low cross-sensitivity to other exhaust components. First results show the advantages for the optical approach compared with standard electrochemical gas sensors.
In order to meet increasingly stringent emission control laws it is necessary to develop a sensor that can accurately monitor the level of pollutants entering the atmosphere from land transport vehicles. These pollutants are generally a mixture of hot gases and particulates. An optical fibre sensor is particularly well suited to this task. Due to it's small size and weight it is minimally invasive making it suitable for insertion into the vehicle's exhaust system. Optical fibres are immune from poisoning by the analyte gases, although they do require shielding from airborne particulates. As they do not transmit electricity they are also highly safe and furthermore they are immune from electromagnetic interference. To detect the presence of the gases it is proposed to use an optical absorption technique. The majority of gases of industrial and environmental importance have their fundamental absorption line in the mid-infrared region of the electromagnetic spectrum, with weaker overtones in the near infrared. Due to the greater availability of components, optimised for communications, most optical fibre gas sensing has taken place in the near-infrared region of the spectrum. In this paper mid-infrared optical fibre gas sensing techniques are investigated and the results of the investigation are presented. Due to the inhomogeneous state of the gas flow it is necessary to measure temperature especially just upstream of the after-treatment section where this can rise to as high as 650oC with large temporal gradients. Measurements of temperature of hot gases from a full size test engine using an optical fibre probe based on fluorescence decay time measurements are also presented.