Measurement of concentrations of transient gases using a conventional NDIR analyser

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Abstract
Conventional multi-gas emission analysers based upon the non-dispersive infrared measuring technique are of limited use in transient engine testing as the dynamic response of these analysers causes distortion of the emission signal measured during transient engine operation. This problem is magnified when filters are installed upstream of the analysers for the removal of solid carbon present in diesel exhausts. The capacitive/damping effect of these filters makes the measurement of the concentrations of transient gases unreliable. A combined experimental and phenomenological model is hence proposed which simulates the behaviour of gas transport through an emission analyser by a series of alternately arranged pipes and surge volumes such that the distortion of the emission signal can be physically explained and modelled. Characterization of the emission analyser by means of two physical models, namely diffusion and perfect gas-mixing models, is adopted as the basis for analysis, so that physical interpretation can be made to account for the resulting distortion and variation of the measured emission signal. The actual emission signal before being distorted by the slow-response emission analyser can be inferred by solving these models numerically. A computer program has been developed which provides a continuous signal inference from a series of distorted emission pulses measured during transient engine operation.

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