Toluene laser-induced fluorescence for in-cylinder temperature imaging in internal combustion engines

A single-laser single-camera imaging technique was demonstrated for in-cylinder temperature distribution measurements in a direct-injection internal combustion engine. The single excitation wavelength two-color detection technique is based on toluene laser-induced fluorescence (LIF). Toluene-LIF emission spectra show a red-shift with increasing temperature. Temperature can thus be determined from the ratio of the signal measured in two separate wavelength ranges independent of the local tracer concentration, laser pulse energy, and the intensity distribution. An image doubling and filtering system is used for the simultaneous imaging of two wavelength ranges of toluene LIF onto the chip of a single camera upon excitation at 248 nm. The measurements were performed in a spark-ignition engine with homogeneous charge and yielded temperature images with a single-shot precision of approximately ±?6%.     

Experimental and numerical investigation of temperature fluctuations in the near-wall region of an optical reciprocating engine

Accurate prediction of in-cylinder heat transfer processes within internal combustion engines (ICEs) requires a comprehensive understanding of the boundary layer effects in the near-wall region (NWR). This study investigates near-wall temperature fluctuations of an optical reciprocating engine using a combined approach of planar laser-induced fluorescence (PLIF) thermometry and numerical conjugate heat transfer modeling. Single-line excitation of toluene and subsequent one-color emission detection is employed for PLIF thermometry, while large-eddy simulations (LES) using commercial CFD software (CONVERGE v2.4.18) is utilized for modeling. The PLIF signal is calibrated to predicted in-cylinder temperatures from a GT-POWER simulation, and precision uncertainty of temperature is found to be ±1.5 K within the calibration region. Near-wall temperature fluctuations are determined about the multi-cycle mean, and the development of thermal stratification is captured in the NWR under motored and fired conditions during the compression stroke. Regions of the largest cycle-to-cycle temperature fluctuations are identified closer to the in-cylinder head surface indicating the unsteadiness of the thermal boundary layer. Analysis includes an assessment of cyclic variability of near-wall temperature fluctuation, and the effects of compression on temperature fluctuations. Additionally, thermal stratification is found to be similar under motored and fired conditions before ignition timing. Lastly, spatial correlation analysis of temperature fluctuations is performed in the wall-normal direction, and it reveals higher correlations under fired conditions. Spatial correlations experience an initial drop outside of the buffer layer in the NWR, and the location of the drop is well captured in the simulations. Analysis of fluctuating temperatures needs to be extended to fluctuations about the spatial average temperature which directly affects the spatial thermal gradients relevant to engine heat transfer.     

Endoscopic fuel film,chemiluminescence, and soot incandescence imaging in a direct-injection spark-ignition engine

In direct-injection spark-ignition engines, fuel films formed on the piston surface due to impinging sprays are a major source of soot. Previous studies investigating the fuel films and their correlation to soot production were mostly performed in model experiments or optical engines. These experiments have different operating conditions compared to commercial engines. In this work, fuel films and soot are visualized in an all-metal engine with endoscopic access via laser-induced fluorescence (LIF) and natural incandescence, respectively. Gasoline and a mixture of isooctane/toluene were used as fuel for the experiments. The fuel films were excited by 266 nm laser pulses and visualized by an intensified CCD camera through a modular UV endoscope. Gasoline yielded much higher signal-to-noise ratio, and this fuel typically took an order of magnitude longer to evaporate than isooctane/toluene. The effects of injection time, injection pressure, engine temperature, and combustion on the fuel-film evaporation time were investigated. This film survival time was reduced with higher engine temperature, higher injection pressure, and later injection time, with engine temperature being the most significant parameter, whereas skip-fired combustion had very little effect on the film survival time. In complementary experiments, LIF from fuel films and soot incandescence were simultaneously visualized by an intensified double-frame CCD camera. At lower engine temperatures the fuel films remained distinct, and soot formation was limited to regions above the films, whereas at higher temperatures, fuel films, and hence the soot, appeared to be spread over the whole piston surface. Finally, high-speed imaging showed the spray, chemiluminescence, and soot incandescence, with results broadly consistent with fuel-film LIF and soot incandescence imaging.     

Quantitative, dynamic fuel distribution measurements in combustion-related devices using laser-induced fluorescence imaging of biacetyl in iso-octane

Knowledge of in-situ fuel distributions in practical combustion devices, such as internal combustion engines, is crucial for research and devlopment purposes. Numerous imaging techniques, mostly based on laser-induced fluorescence (LIF), have been developed and yield high levels of 2-D spatial information, but generally lack the temporal resolution (frame rates) necessary to resolve important timescales at sub-millisecond levels for sustained times. A planar LIF technique for quantitatively visualizing fuel distribution is presented which gives not only high spatial resolution, but also high temporal resolution. Using a high-speed CMOS camera, a lens-coupled image intensifier, and frequency-tripled diode-pumped Nd:YAG laser allows for capturing LIF images of biacetyl that is used as a fluorescence tracer at 12 kHz (one crank-angle resolution at 2000 RPM) for hundreds of consecutive engine cycles. The LIF signal strength of biacetyl doped in iso-octane is shown to vary substantially over a wide range of temperatures and pressures. The low absorption coefficient at 355 nm and a longpass filter in the detection path exclude bias errors due to laser beam attenuation and fluorescence trapping. An intensifier gate time of 350 ns is shown to suppress the detection of phosphorescence signals under practical conditions. An example for a quantitative high-speed measurement of fuel concentration at varying pressure and temperature conditions is presented. Quantitative equivalence ratio maps are shown for the fuel injection event within a single cycle in a spark-ignition direct-injected engine, showing the ability of the technique to not only reveal static fuel concentration maps, but also the motion of the fuel cloud along with very steep gradients. Spray velocities determined from the moving fuel cloud are in agreement with previous particle image velocimetry measurements.     

Flame kernel characterization of laser ignition of natural gas–air mixture in a constant volume combustion chamber

In this paper, laser-induced ignition was investigated for compressed natural gas–air mixtures. Experiments were performed in a constant volume combustion chamber, which simulate end of the compression stroke conditions of a SI engine. This chamber simulates the engine combustion chamber conditions except turbulence of air–fuel mixture. It has four optical windows at diametrically opposite locations, which are used for laser ignition and optical diagnostics simultaneously. All experiments were conducted at 10 bar chamber pressure and 373 K chamber temperature. Initial stage of combustion phenomena was visualized by employing Shadowgraphy technique using a high speed CMOS camera. Flame kernel development of the combustible fuel–air mixture was investigated under different relative air–fuel ratios (λ=1.2?1.7) and the images were interrogated for temporal propagation of flame front. Pressure-time history inside the combustion chamber was recorded and analyzed. This data is useful in characterizing the laser ignition of natural gas–air mixture and can be used in developing an appropriate laser ignition system for commercial use in SI engines.     

LIF-based imaging of preferential evaporation of a multi-component gasoline surrogate in a direct-injection engine

In direct-injection engines, fuel components of different volatility can segregate in the transient evaporating spray. The resulting spatial mixture inhomogeneities may impact ignition and combustion. The technique presented here images the effect of preferential evaporation of a multi-component gasoline surrogate fuel in an optically-accessible direct-injection engine motored on nitrogen. It is based on laser-induced fluorescence (LIF) of two aromatic tracers with different volatilities, 1,4-difluorobenzene (representing light to medium components) and 1-methylnaphthalene (heavy component) added to a base-fuel of n-pentane, iso-octane, and n-undecane. LIF from the two tracers is spectrally separated and detected on two cameras, with channel crosstalk corrected in post-processing. Consistent with previous measurements in a high-pressure vessel, the light components are preferentially found downstream, towards the front of the evaporated fuel jet. Throughout large regions of the field of view, about 20% surplus of 1-methylnaphthalene is found, and throughout smaller ones about 40% of 1,4-difluorobenzene. To better assess the impact of the (unknown) local temperature on the measurement accuracy, two-color thermometry based on LIF of anisole (methoxybenzene) is performed in separate experiments. In the relevant range of crank-angles the local temperature is found to be 25 K lower in regions of high fuel concentration than in the rest of the charge, implying a systematic temperature-induced error in the fuel-tracer ratio of 0.11.     

Temperature dependence of intensities of laser-induced fluorescences of ethylbenzene and naphthalene seeded in gas flow at atmospheric pressure

The present study has been carried out with the aim of developing a technique for measuring two-dimensional gas temperature profiles based on two-color fluorescence induced by a one-color laser. The laser sheet of the fourth harmonic (266 nm) from a Nd:YAG laser induced fluorescence in species doped in a nitrogen gas flow. The LIF spectra of seven fluorescent species, namely acetone, methylethylketone, acetaldehyde ethylbenzene, anisole, aniline, and naphthalene, were measured to select the best prospective pair of fluorescent species for this technique. Ethylbenzene and naphthalene show relatively high LIF intensities and their LIF spectra overlap less with each other than with other species. Also, ethylbenzene has a high temperature dependence while naphthalene has a low temperature dependence. Thus by selecting one portion of wavelengths in the range where ethylbenzene or naphthalene is dominant, the temperature of the gas can be determined using the ratio of LIF intensities of the mixture at the two wavelengths with good temperature sensitivity. In addition, a general principle is presented for finding out an optimum pair of wavelengths to obtain a good temperature sensitivity in those LIF spectra.     

NO and PO photofragments as trace analyte indicators of nitrocompounds and organophosphonates

Laser photolysis that releases characteristic photofragments, combined with laser-induced fluorescence (LIF) that subsequently monitors them, facilitate detection of trace-vapor analytes in air. The feasibility of the technique is demonstrated for nitrocompounds and organophosphonates detection. Using one-color and two-color lasers, respectively, the representative NO and PO photofragments were tagged. Employment of longer wavelengths for photodissociation, and where possible also for LIF, than for fluorescence collection, obstructs background fluorescence and in the former compounds also prevents interference of ambient ground state NO. Received: 1 February 2000 / Revised version: 27 April 2000 / Published online: 20 September 2000     

Simultaneous 36 kHz PLIF/chemiluminescence imaging of fuel,CH2O and combustion in a PPC engine

The requirements on high efficiency and low emissions of internal combustion engines (ICEs) raise the research focus on advanced combustion concepts, e.g., premixed-charge compression ignition (PCCI), partially premixed compression ignition (PPCI), reactivity controlled compression ignition (RCCI), partially premixed combustion (PPC), gasoline compression ignition (GCI) etc. In the present study, an optically accessible engine is operated in PPC mode, featuring compression ignition of a diluted, stratified charge of gasoline-like fuel injected directly into the cylinder. A high-speed, high-power burst-mode laser system in combination with a high-speed CMOS camera is employed for diagnostics of the autoignition process which is critical for the combustion phasing and efficiency of the engine. To the authors’ best knowledge, this work demonstrates for the first time the application of the burst-system for simultaneous fuel tracer planar laser induced fluorescence (PLIF) and chemiluminescence imaging in an optical engine, at 36?kHz repetition rate. In addition, high-speed formaldehyde PLIF and chemiluminescence imaging are employed for investigation of autoignition events with a high temporal resolution (5 frames/CAD). The development of autoignition together with fuel or CH₂O distribution are simultaneously visualized using a large number of consecutive images. Prior to the onset of combustion the majority of both fuel and CH₂O are located in the recirculation zone, where the first autoignition also occurs. The ability to record, in excess of 100 PLIF images, in a single cycle brings unique possibilities to follow the in-cylinder processes without the averaging effects caused by cycle-to-cycle variations.     

Temperature measurements of the gas-phase during surrogate diesel injection using two-color toluene LIF

Planar laser-induced fluorescence (LIF) of toluene has been applied in an optical engine and a high-pressure cell, to determine temperatures of fuel sprays and in-cylinder vapors. The method relies on a redshift of the toluene LIF emission spectrum with increasing temperature. Toluene fluorescence is recorded simultaneously in two disjunct wavelength bands by a two-camera setup. After calibration, the pixel-by-pixel LIF signal ratio is a proxy for the local temperature. A detailed measurement procedure is presented to minimize measurement inaccuracies and to improve precision. n-Heptane is used as the base fuel and 10 % of toluene is added as a tracer. The toluene LIF method is capable of measuring temperatures up to 700 K; above that the signal becomes too weak. The precision of the spray temperature measurements is 4 % and the spatial resolution 1.3 mm. We pay particular attention to the construction of the calibration curve that is required to translate LIF signal ratios into temperature, and to possible limitations in the portability of this curve between different setups. The engine results are compared to those obtained in a constant-volume high-pressure cell, and the fuel spray results obtained in the high-pressure cell are also compared to LES simulations. We find that the hot ambient gas entrained by the head vortex gives rise to a hot zone on the spray axis.