Infrared spectral analysis of engine preflame emission

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 ...

Infrared high-speed thermography of combustion chamber wall impinged by diesel spray flame

“…The crux of the combustion diagnostics is separation and spectroscopic identification of the emitters, including H 2 O, CO 2 , hydrocarbons, radical and intermediate species, soot, chamber wall and windows. 41,42 In contrast, the crux of the high-speed chamber wall thermography is separation of targeted wall surface radiation from flame soot luminosity and emissions of combustion products, for which the coating on the window is crucial as shown in the present study.…”

High-frequency wall heat flux measurement during wall impingement of a diffusion flame

“…Multiple diagnostics have been used to measure in-cycle thermal information on the surface of the combustion chamber, in particular thermocouples, 8,9 resistance temperature detectors (RTD), 10,11 thermopiles, 12–14 laser-induced phosphorescence (LIP) 15,16 and infrared thermometry. 15,17 The most common is thin-junction thermocouple and is the one used here; as with every other method but thermopiles, a surface temperature is measured, which allows us to recover heat flux via a mathematical model. Thermopiles provide direct heat flux information with minimal data processing, but require a large surface area (causing significant spatial averaging) and have larger response times than other methods.…”

“…10 LIP and infrared thermometers use emissions from heated body or activated phosphors and are difficult to apply in compression-ignition (CI) conditions due to interference between the optical signal and soot radiation. 17–19 Further comparisons of the methods of heat flux measurement can be found in the literature. 11,20…”