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<div class="htmlview paragraph">Chemiluminescence imaging has been applied to a parametric investigation of diesel autoignition. Time-resolved images of the natural light emission were made in an optically accessible DI diesel engine of the heavy-duty size class using an intensified CCD video camera. Measurements were obtained at a base operating condition, corresponding to a motored TDC temperature and density of 992 K and 16.6 kg/m<sup>3</sup>, and for TDC temperatures and densities above and below these values. Data were taken with a 42.5 cetane number blend of the diesel reference fuels for all conditions, and measurements were also made with no. 2 diesel fuel (D2) at the base condition. For each condition, temporal sequences of images were acquired from the time of first detectable chemiluminescence up through fully sooting combustion, and the images were analyzed to obtain quantitative measurements of the average emission intensity. These data are presented and discussed with respect to the liquid-phase fuel penetration and the apparent heat release rates. In addition, a spectrometer equipped with an intensified OMA was used to record both chemiluminescence and sooting combustion spectra.</div> <div class="htmlview paragraph">The data show diesel autoignition to be a progressive process that occurs simultaneously over the downstream region of all the fuel jets. The first chemiluminescence is detected shortly after the start of injection for all the conditions studied, and it grows brighter and shifts downstream as the jet penetrates across the chamber, until soot luminosity dominates the emission. Although this process is qualitatively the same for all conditions, the rate at which it proceeds varies greatly with TDC temperature and density. The temporal evolution of the natural emission is similar for the reference and D2 fuels. The spectral data show that the chemiluminescence arises mainly from formaldehyde and CH emission, with no OH emission detected. In contrast, the sooting-combustion spectrum (taken after the diffusion flame has formed) shows OH and “gray-body” soot emission.</div>