Measurement strategies for in-cylinder water imaging in a homogeneous charge compression ignition engine

Engine applications of Chemical Species Tomography (CST) are envisaged for a variety of target species important for combustion and emissions control. In-cylinder CST imaging of long-chain hydrocarbon fuel has been previously demonstrated using fixed-wavelength diode lasers, exploiting the relatively broad and unstructured absorption feature at 1700nm. In-cylinder imaging of water distribution is a high-priority objective because of its value as a marker for the distribution of residual combustion gases, for the study of Homogeneous Charge Compression Ignition (HCCI) engines. However, the Near-IR spectroscopy of water is characterised by strong, but narrow absorption lines, whose strength and width are functions of pressure and temperature. Using the HITRAN spectroscopic database, this paper presents a simulation study of water absorption at various pressures and temperatures relevant to the HCCI case. The full-width at half-maximum of the candidate absorption lines ranges from the order of 0.05 nm at intake pressure, to around 0.4 nm at 10 bars. Hence, it is necessary to measure in-cylinder pressure simultaneously with spectroscopic measurements. To account for the temperature sensitivity of the absorption features, it is essential to measure the ratio of absorption over at least two spectral lines; the rationale for the choice of these lines is discussed in detail.