Infrared spectral analysis of engine preflame emission

Abstract

Abstract

An investigation has been conducted to examine the previously observed infrared emission occurring at the end of an engine's compression period. The infrared preflame spectrum is measured and quantitatively compared with a radiation emission model incorporating wall effects. The likelihood and degree of contributions to the observed emission by intermediate species under non-local thermodynamic equilibrium conditions are examined, and the viability of applying high-speed infrared imaging as a diagnostic technique for observing the distribution of fuel vapour is demonstrated. A Fastie—Ebert-type grating spectrophotometer was constructed to employ a 64×64 pixel platinum-silicide charge-coupled device imager. This instrument was integrated with a four-camera infrared spatial imaging system. The combined apparatus allows the simultaneous recording of the infrared spectra and four spatial images, each recorded through separate narrow-band filters. One filter is centred at 2.47 μm and another centred at 3.43 μm, covering strong H₂O and hydrocarbon bands. Images are recorded at a framing rate of 1880 frames/s in 64-frame sequences from consecutive engine cycles and triggered at a chosen crank-angle position determined by a crankshaft-mounted encoder wheel. To interpret the measured spectrum, a line-by-line radiation model was created utilizing the high-resolution transmission (HITRAN) database of molecular parameters. This database includes spectral line data for radical and intermediate species including hydroxyl (OH) and formaldehyde (HCHO), and the alkyl compounds methane (CH₄), ethylene (C₂H₄), and up to ethane (C₂H₆). The model predicts the spectra of an isothermal, homogeneous gas mixture consisting of fuel, residual gases H₂O and CO₂ in air at a given temperature, pressure and concentration, and considers the effect of interaction with an emitting combustion chamber wall. Preflame images and spectra were recorded from an optically accessible spark-ignition engine using four fuels representing minimal (CH₄, C₂H₄), marginal (C₂H₆), and significant (propane, C₃H₈) cool-flame chemistry. Results show the measured preflame spectra of the four fuels to have similar large-scale features and magnitude, including the propane fuel known for chemiluminescence during low-temperature reactions. The preflame spectrum of the latter fuel mixture is notable for its lack of spectral features expected of HCHO or OH, suggesting that emission by intermediate species produced by low-temperature mechanisms is not readily evident in the infrared preflame spectrum. There is a high degree of correlation between the measured preflame spectra of CH₄ and C₂H₄ mixtures and the spectrum modelled using parameter values reasonably representative of engine conditions. The prominent features of the observed infrared preflame spectra are explained by thermal emission of the species considered. Since reported absorption cross-sections of n-heptane, n-dodecane, and gasoline at 3.39 μm are greater than that of CH₄, these fuels are expected to have thermal emissions of similar if not greater magnitude near this wavelength. Infrared images of fuel vapour during a diesel combustion cycle that have been recorded with the four-camera system are presented. Significantly, it has been shown that mid-infrared imaging can be used as a technique for observing in-cylinder fuel vapour distributions.