This paper describes an optical fiber based system that has been developed for the monitoring of carbon dioxide emissions in situ within engines above 500 kW. Conventional sensors, reviewed here, fail to meet monitoring requirements, such as lifespan, accuracy, and robustness. This paper describes a sensor designed as a single point reflective probe configuration using low cost, compact mid-infrared optical components, making it suitable for insertion in large-scale engines including automotive tailpipes. The response of the sensor to carbon dioxide supplied from a cylinder in the laboratory environment is presented, as well as a number of experimental results taken in situ in an exhaust of an automotive diesel engine (smaller than 500 kW). The sensor is shown to have a long term stable operation over a wide range of concentrations (2%-15% CO2) with a lower detection limit smaller than the lowest value encountered in modern day engines. (C) 2012 Optical Society of America