Marcis Jansons

COMPARISON OF SOOT EVOLUTION USING HIGH-SPEED CMOS COLOR CAMERA AND TWO-COLOR THERMOMETRY IN AN OPTICAL DIESEL ENGINE FUELED WITH B20 BIODIESEL BLEND AND ULTRA-LOW SULFUR DIESEL

Biodiesel is a desirable alternative fuel for the diesel engine due to its low engine-out soot emission tendency. When blended with petroleum-based diesel fuels, soot emissions generally decrease in proportion to the volume fraction of biodiesel in the mixture. While comparisons of engine-out soot measurements between biodiesel blends and petroleum-based diesel have been widely reported, in-cylinder soot evolution has not been experimentally explored to the same extent. To elucidate the soot emission reduction mechanism of biodiesel, a single-cylinder optically-accessible diesel engine was used to compare the in-cylinder soot evolution when fueled with ultra-low sulfur diesel (ULSD) to that using a B20 biodiesel blend (20% vol/vol biodiesel ASTM D6751-03A). Soot temperature and KL factors are simultaneously determined using a novel two-color optical thermometry technique implemented with a high-speed CMOS color camera having wide-band Bayer filters. The crank-angle resolved data allows quantitative comparison of the rate of in-cylinder soot formation. High-speed spray images show that B20 has more splashing during spray wall impingement than ULSD, distributing rebounding fuel droplets over a thicker annular ring interior to the piston bowl periphery. The subsequent soot luminescence is observed by high-speed combustion imaging and soot temperature and KL factor measurements. B20 forms soot both at low KL magnitudes over large areas between fuel jets, and at high values among remnants of the fuel spray, along its axis and away from the bowl edge. In contrast, ULSD soot luminescence is observed exclusively as pool burning on the piston bowl surfaces resulting from fuel wall impingement. The soot KL factor evolution during B20 combustion indicates earlier and significantly greater soot formation than with ULSD. B20 combustion is also observed to have a greater soot oxidation rate which results in lower engine-out soot emissions. Measured soot temperatures near 1875K were similar for the two fuels for the duration of combustion. For both fuels, higher fuel injection pressure led to lower late-cycle soot KL levels. The trends of soot natural luminosity correlated well with the trends of soot KL factor, suggesting that relatively simple measurements of combustion luminosity may provide somewhat quantitative information about in-cylinder soot formation and oxidation. The apparent rate of heat release (ARHR) analysis under steady skip-fire conditions indicates that B20 combustion is less sensitive to wall temperature than that observed with ULSD due to a lesser degree of pool burning. B20 was found to have both a shorter ignition delay and shorter combustion duration than ULSD.Copyright

Chemiluminescence imaging of pre-injection reactions during engine starting

The thermal and chemical state of residual gas is known to influence the likelihood of autoignition, ignition delay and combustion phasing of the subsequent diesel engine cycle. To elucidate the role of residual gases in these processes, ultraviolet chemiluminescent reactions and their spectra are observed during the pre-injection, compression period in a dynamometer-driven, optically-accessible, diesel engine operated with a single fuel injection event. During a cold start sequence, while the engine is motored and fuel is injected without firing, the pre-injection chemiluminescence (PIC) intensity increases from cycle to cycle. This leads to a second mode of intermittent firing cycles which are observed to follow a higher intensity of PIC. In the third mode, decreased PIC intensity is measured in firing cycles that are preceded by partial misfires. In the fourth mode, firing is continuous, but with a high IMEP coefficient of variation (COV). Here, PIC intensity is found to strongly correlate with advanced combustion phasing. As firing continues, it is observed that COV, PIC intensity and the phasing correlation decrease. Upon fuel shutoff, PIC intensity decays with time. Spectral measurements confirm that reactions of low temperature combustion intermediates, including chemiluminescent formaldehyde (HCHO*) and CHO* comprise the observed PIC.

Investigation of Low-Temperature Combustion in an Optical Engine Fueled with Low Cetane Sasol JP-8 Fuel Using OH-PLIF and HCHO Chemiluminescence Imaging

ABSTRACT Low cetane JP-8 fuels have been identified as being difficult to use under conventional diesel operation. However, recent focus on low-temperature combustion (LTC) modes has led to an interest in distillate hydrocarbon fuels having high volatility and low autoignition tendency. An experimental study is performed to evaluate low-temperature combustion processes in a small-bore optically-accessible diesel engine operated in a partially-premixed combustion mode using low-cetane Sasol JP-8 fuel. This particular fuel has a cetane number of 25. Both single and dual injection strategies are tested. Since long ignition delay is a consequence of strong autoignition resistance, under the conditions examined, low cetane Sasol JP-8 combustion can only take place with a double injection strategy: one pilot injection event in the vicinity of exhaust TDC and one main injection event near firing TDC. In this work, the effects of autoignition properties are examined by comparing the behavior of a high cetane number JP-8 fuel with that of a low CN Sasol JP-8. The double injection strategy also served to reduce pressure rise rates during operation at light load (2 bar IMEP) conditions. Dual injection timing is optimized for peak IMEP, at which point simultaneous OH Planar Laser-Induced Fluorescence (OH-PLIF) and high-speed crank-angle-resolved HCHO chemiluminescence imaging are performed to analyze the partially-premixed combustion process. Fuel efficiency and engine-out emissions performance are also presented. In terms of IMEP, low CN Sasol JP-8 fuel is shown to be a satisfactory fuel for low-temperature combustion under light-load condition using the proper dual injection strategy. However, partially-premixed combustion operation in this work results in higher UHC emissions and lower fuel efficiency when compared with the high cetane JP-8 fuel.

Soot Evolution With Cyclic Crank-Angle-Resolved Two-Color Thermometry in an Optical Diesel Engine Fueled With Biodiesel Blend and ULSD

Biodiesel is a desirable alternative fuel for the diesel engine due to its low engine-out soot emission tendency. When blended with petroleum-based diesel fuels, soot emissions generally decrease in proportion to the volume fraction of biodiesel in the mixture. While comparisons of engine-out soot measurements between biodiesel blends and petroleum-based diesel have been widely reported, in-cylinder soot evolution has not been experimentally explored to the same extent. To elucidate the soot emission reduction mechanism of biodiesel, a single-cylinder optically-accessible diesel engine was used to compare the in-cylinder soot evolution when fueled with ultra-low sulfur diesel (ULSD) to that using a B20 biodiesel blend (20% vol./vol. biodiesel ASTM D6751-03A). Soot temperature and KL factors are simultaneously determined using a novel two-color optical thermometry technique implemented with a high-speed CMOS color camera having wide-band Bayer filters. The crank-angle resolved data allows quantitative comparison of the rate of in-cylinder soot formation. High-speed spray images show that B20 has more splashing during spray wall impingement than ULSD, distributing rebounding fuel droplets over a thicker annular ring interior to the piston bowl periphery. The subsequent soot luminescence is observed by high-speed combustion imaging and soot temperature and KL factor measurements. B20 forms soot both at low KL magnitudes over large areas between fuel jets, and at high values among remnants of the fuel spray, along its axis and away from the bowl edge. In contrast, ULSD soot luminescence is observed exclusively as pool burning on the piston bowl surfaces resulting from spray wall impingement. The soot KL factor evolution during B20 combustion indicates earlier and significantly greater soot formation than with ULSD. B20 combustion is also observed to have a greater soot oxidation rate, which results in lower late-cycle soot emissions. For both fuels, higher fuel injection pressure led to lower late-cycle soot KL levels. The apparent rate of heat release (ARHR) analysis under steady skip-fire conditions indicates that B20 combustion is less sensitive to wall temperature than that observed with ULSD due to a lesser degree of pool burning. B20 was found to have both a shorter ignition delay and shorter combustion duration than ULSD.

RCCI of synthetic kerosene with PFI of N-butanol-combustion and emissions characteristics

In this study, the combustion and emissions characteristics of Reactivity Controlled Compression Ignition (RCCI) obtained by direct injection (DI) of S8 and port fuel injection (PFI) of n-butanol were compared with RCCI of ultra-low sulfur diesel #2 (ULSD#2) and PFI of n-butanol at 6 bar indicated mean effective pressure (IMEP) and 1500 rpm. S8 is a synthetic paraffinic kerosene (C6–C18) developed by Syntroleum and is derived from natural gas. S8 is a Fischer-Tropsch fuel that contains a low aromatic percentage (0.5 vol. %) and has a cetane number of 63 versus 47 of ULSD#2. Baselines of DI conventional diesel combustion (CDC), with 100% ULSD#2 and also DI of S8 were conducted. For both RCCI cases, the mass ratio of DI to PFI was set at 1:1. The ignition delay for the ULSD#2 baseline was found to be 10.9 CAD (1.21 ms) and for S8 was shorter at 10.1 CAD (1.12 ms). In RCCI, the premixed charge combustion has been split into two regions of high temperature heat release, an early one BTDC from ignition of ULSD#2 or S8, and a second stage, ATDC from n-butanol combustion. RCCI with n-butanol increased the NOx because the n-butanol contains 21% oxygen, while S8 alone produced 30% less NOx emissions when compared to the ULSD#2 baseline. The RCCI reduced soot by 80–90% (more efficient for S8). However, S8 alone showed a considerable increase in soot emissions compared with ULSD#2. The indicated thermal efficiency was the highest for the ULSD#2 and S8 baseline at 44%. The RCCI strategies showed a decrease in indicated thermal efficiency at 40% ULSD#2-RCCI and 42% and for S8-RCCI, respectively.S8 as a single fuel proved to be a very capable alternative to ULSD#2 in terms of combustion performance nevertheless, exhibited higher soot emissions that have been mitigated with the RCCI strategy without penalty in engine performance.Copyright

Comparison of the Lift-Off Lengths Obtained by Simultaneous OH-LIF and OH* Chemiluminescence Imaging in an Optical Heavy-Duty Diesel Engine

The presence of OH radicals as a marker of the high temperature reaction region usually has been used to determine the lift-off length (LOL) in diesel engines. Both OH Laser Induced Fluorescence (LIF) and OH∗ chemiluminescence diagnostics have been widely used in optical engines for measuring the LOL. OH∗ chemiluminescence is radiation from OH being formed in the exited states (OH∗). As a consequence OH∗ chemiluminescence imaging provides line-of-sight information across the imaged volume. In contrast, OH-LIF provides information on the distribution of radicals present in the energy ground state. The OH-LIF images only show OH distribution in the thin cross-section illuminated by the laser. When both these techniques have been applied in earlier work, it has often been reported that the chemiluminescence measurements result in shorter lift-off lengths than the LIF approach. In order to investigate this discrepancy this work presents a dedicated comparison of the LOL obtained from these two diagnostic techniques. In diesel engines, the cycle-to-cycle variations in lift-off region are usually significant. To avoid misinterpretations caused by these variations simultaneous measurements are needed. The statistical analysis based on our simultaneous data can conclude that the OH-LIF method yields longer LOL than the OH∗ chemiluminescence method by a smaller sample size and more precisely than non-simultaneous data. This can be partially explained by the 3D geometry and flame axis asymmetry effects. A numerical simulation with OH and OH∗ distribution was performed for the comparison. It shows a great agreement with the experimental results in this study.

Simultaneous High-Speed Two-Color Thermometry and Laser-Induced Incandescence Soot Measurement in a Small-Bore Optical Engine Fueled With JP-8

In-cylinder soot measurements obtained with a high-speed two-color method are compared to those simultaneously determined by the laser-induced incandescence (LII) technique in a single-cylinder, optically-accessible diesel engine fueled with JP-8. A double injection strategy was chosen to reduce pressure rise rates during operation at light load (2 bar IMEP) conditions. Injection timing was optimized for peak efficiency, at which point sufficient soot was produced to provide ample signal for both optical diagnostic techniques. Application of the two-color method to a highspeed CMOS camera allows the crank-angle-resolved observation of soot temperature and soot optical depth (KL) evolution, while LII provides soot volume fraction distribution at a known axial location in the cylinder independent of combustion gas temperature. Comparison of soot KL and LII signal at various stages of combustion shows high spatiallyaveraged correlation of the two signals near TDC. The degree of correlation decreases as the piston bowl descends and the line-of-sight soot KL value increasingly includes soot volumes not in the path of the laser sheet, the location of which is fixed 6.5 mm below the fire deck. The correlation between the two parameters again increases during the late cycle, indicating that in the later phases of combustion soot occurs in the squish zone above the piston bowl. Spatial cross-correlation of the two signals is weak, but increases in the highly luminous period immediately following heat release and illustrating a high degree of soot stratification. Soot KL and temperature evolution over a cycle are presented, which show no indication of being affected by the LII laser fluence.

A CFD Study of the Effect of HCHO Addition on Autoignition and Combustion

Experimental and theoretical research [1] targeted towards the effect of formaldehyde on combustion has identified its OH-scavenging role and recent data suggests it plays an important role in combustion instability observed during engine cold-starting. This effect was further studied using a CFD approach and the kinetic inhibiting effect of formaldehyde on the combustion process has been found to be enhanced by thermal/diffusion effects.Copyright

Effect of injection parameters and strategy on the noise from a single cylinder direct injection diesel engine

Acoustic noise emitted by a diesel engine generally exceeds that produced by its spark-ignited equivalent and may hinder the acceptance of this more efficient engine type in the passenger car market (1). This work characterizes the combustion noise from a single-cylinder direct-injection diesel engine and examines the degree to which it may be minimized by optimal choice of injection parameters. The relative contribution of motoring, combustion and resonance components to overall engine noise are determined by decomposition of in-cylinder pressure traces over a range of load, injection pressure and start of injection. The frequency spectra of microphone signals recorded external to the engine are correlated with those of in-cylinder pressure traces. Short Time Fourier Transformation (STFT) is applied to cylinder pressure traces to reveal the occurrence of motoring, combustion noise and resonance in the frequency domain over the course of the engine cycle. Loudness is found to increase with enhanced resonance, in proportion to the rate of cylinder pressure rise and under conditions of high injection pressure, load and advanced injection timing.Copyright