In-cylinder Soot Formation Research Articles

An experimental study on in-cylinder soot formation and flame oscillation of renewable fuel blends in an optical engine

Methanol has been widely recognized as a promising alternative fuel for mitigating greenhouse gas emissions. However, its application in internal combustion engines often necessitates blending with a high-reactivity fuel to address challenges related to ignitability and combustion stability. In this study, a two-color method was employed in a single-cylinder optical engine to investigate the characteristics of soot formation and flame oscillation of methanol blended with a high-reactivity fuel, hydrogenated catalytic biodiesel (HCB), under partially premixed combustion (PPC) modes. The blended fuels include M15 (15% methanol, 60% HCB, and 25% n-octanol) and M25 (25% methanol, 60% HCB, and 25% n-octanol), with pure hydrogenated catalytic biodiesel M0 (100% HCB) serving as the reference fuel. N-octanol was utilized as the co-solvent to enhance the stability of the mixture. The results indicate that, employing double-injection strategies, as the methanol content increases in the blended composition, leading to a postponed peak cylinder pressure and heat release rate during combustion. Thanks to the high-level premixed combustion and higher oxygen content, the increases of alcohol blending significantly reduced in-cylinder soot formation. Double-injection strategies reduced soot production effectively and resulted in a more uniform soot distribution compared to single-injection strategies. In terms of flame oscillation, its amplitude primarily correlates with the rate of cylinder pressure rise, while the methanol content within the blends exerts a minor impact on flame oscillation. Double-injection strategies contributes to a more uniform fuel distribution, ensuring a smoother heat release process and significantly reducing flame oscillation amplitudes compared to single-injection strategies.

A Synergic Application of High-Oxygenated E-Fuels and New Bowl Designs for Low Soot Emissions: An Optical Analysis

Synthetic fuels significantly reduce pollutant emissions and the carbon footprint of ICE applications. Among these fuels, oxymethylene dimethyl ethers (OMEX) are an excellent candidate to entirely or partially replace conventional fuels in compression ignition (CI) engines due to their attractive properties. The very low soot particle formation tendency allows the decoupling of the soot-NOX trade-off in CI engines. In addition, innovative piston geometries have the potential to reduce soot formation inside the cylinder in the late combustion stage. This work aims to analyze the potential of combining OMEX with an innovative piston geometry to reduce soot formation inside the cylinder. In this way, several blends of OMEX-Diesel were tested using a radial-lips bowl geometry and a conventional reentrant bowl. Tests were conducted in an optically accessible engine under simulated EGR conditions, reducing the in-cylinder oxygen content. For this purpose, 2-colour pyrometry and high-speed excited state hydroxyl chemiluminescence techniques were applied to trace the in-cylinder soot formation and oxidation processes. The results confirm that increasing OMEX in Diesel improves the in-cylinder soot reduction under low oxygen conditions for both piston geometries. Moreover, using radial lips bowl geometry significantly improves the soot reduction, from 17% using neat Diesel to 70% less at the highest OMEX quantity studied in this paper.

Influence of the radial-lip concept design to achieve ultra-low soot emission reductions: An optical analysis

Nowadays, internal combustion engines must reduce pollutant emissions to achieve the upcoming strict regulations in the transport sector. In this context, new hardware designs have shown great potential to reduce pollutant emissions and comply with current and future regulations. This paper focuses on innovative bowl designs with radial lip protrusions, which are radially distributed inside the bowl. The potential of using these protrusions in order to reduce soot formation and improve soot oxidation has been demonstrated by different studies. For this reason, the present work addresses the impact of using two different radial-lip based geometries on soot formation and combustion behaviour in an optical single-cylinder compression ignition engine. High-speed OH* chemiluminescence imaging and 2-Colour pyrometry were applied simultaneously, in combination with a thermodynamic analysis of the in-cylinder pressure signal. The results show that more pronounced radial-lip geometry drive the flames closer to the bowl centre, where more oxygen is available to oxidize the soot, resulting in in-cylinder soot formation reduction. In addition, it has been observed that the improvement achieved by the more pronounced radial-lip geometry is more noticeable under high EGR conditions. This opens a path to reduce the soot-NOX trade-off in compression ignition engines.

Investigation of in-cylinder soot formation in a DISI engine during transient operation by simultaneous endoscopic PIV and flame imaging

A single-cylinder full-metal engine with a real combustion chamber geometry was used to investigate particulate number emissions resulting from transient engine operation. The formation of particulate number emissions depends on mixture formation influenced by the in-cylinder flow and injection, and the formation of fuel films on the in-cylinder walls. For the investigation of this multi-parameter process, simultaneous endoscopic PIV and combustion visualization were applied. Hence, the measurement techniques allowed the investigation of in-cylinder flow, flame propagation, and soot formation. The test rig was modified to apply a generic load step and a realistic tip-in with Miller cycle. The reproducibility of the engine parameters during the transient allowed statistical analysis and the comparison between steady-state operating points. Cause-and-effect chains concerning the formation of soot are concluded by correlation analysis of parameters extracted from the flow field, the flame propagation and the soot luminosity.

Diffuse back-illumination temperature imaging (DBI-TI), a novel soot thermometry technique

To meet stringent emissions regulations on soot emissions, it is critical to further advance the fundamental understanding of in-cylinder soot formation and oxidation processes. Among several optical techniques for soot quantification, diffuse back-illumination extinction imaging (DBI-EI) has recently gained traction mainly due to its ability to compensate for beam steering, which if not addressed, can cause unacceptably high measurement uncertainty. Until now, DBI-EI has only been used to measure the amount of soot along the line of sight, and in this work, we extend the capabilities of a DBI-EI setup to also measure in-cylinder soot temperature. This proof of concept of diffuse back-illumination temperature imaging (DBI-TI) as a soot thermometry technique is presented by implementing DBI-TI in a single cylinder, heavy-duty, optical diesel engine to provide 2-D line-of-sight integrated soot temperature maps. The potential of DBI-TI to be an accurate thermometry technique for use in optical engines is analyzed. The achievable accuracy is due in part to simultaneous measurement of the soot extinction, which circumvents the uncertainty in dispersion coefficients that depend on the optical properties of soot and the wavelength of light utilized. Analysis shows that DBI-TI provides temperature estimates that are closer to the mass-averaged soot temperature when compared to other thermometry techniques that are more sensitive to soot temperature closer to the detector. Furthermore, uncertainty analysis and Monte Carlo (MC) simulations provide estimates of the temperature measurement errors associated with this technique. The MC simulations reveal that for the light intensities and optical densities encountered in these experiments, the accuracy of the DBI-TI technique is comparable or even better than other established optical thermometry techniques. Thus, DBI-TI promises to be an easily implementable extension to the existing DBI-EI technique, thereby extending its ability to provide comprehensive line-of-sight integrated information on soot.

Solid particulate mass and number from ducted fuel injection in an optically accessible diesel engine in skip-fired operation

Ducted fuel injection (DFI) is a novel combustion strategy that has been shown to significantly attenuate soot formation in diesel engines. While previous studies have used optical diagnostics and optical filter smoke number methods to show that DFI reduces in-cylinder soot formation and engine-out soot emissions, respectively, this is the first study to measure solid particle number (PN) emissions in addition to particle mass (PM). Furthermore, this study quantitatively evaluates the use of transient particle instruments for measuring particles from skip-fired operation in an optical single cylinder research engine (SCRE). Engine-out PN was measured using an engine exhaust particle sizer following a catalytic stripper, and PM was measured using a photoacoustic analyzer. The study improves on earlier preliminary emissions studies by clearly showing that DFI reduces overall PM by 76%–79% and PN for particles larger than 23 nm by 77% relative to conventional diesel combustion at a 1200-rpm, 13.3-bar gross indicated mean effective pressure operating condition. The degree of engine-out PM reduction with DFI was similar across both particulate measurement instruments used in the work. Through the use of bimodal distribution fitting, DFI was also shown to reduce the geometric mean diameter of accumulation mode particles by 26%, similar to the effects of increased injection pressure in conventional diesel combustion systems. This work clearly shows the significant solid particulate matter reductions enabled by DFI while also demonstrating that engine-out PN can be accurately measured from an optical SCRE operating in a skip-fired mode. Based on these results, it is believed that DFI has the potential to enable fuel savings when implemented in multi-cylinder engines, both by lowering the required frequency of active diesel particulate filter regeneration, and by reducing the backpressure imposed by exhaust filtration systems.

Significant Impact of Flash Boiling Spray on In-Cylinder Soot Formation and Oxidation Process

The use of flash boiling atomization has demonstrated potential in improving engine efficiency and reducing particulate matter (PM) emissions because of its stronger phase change process and conseq...

An optical investigation of Fischer-Tropsch diesel and Oxymethylene dimethyl ether impact on combustion process for CI engines

Synthetic fuels (E-fuels) have shown to be an interesting alternative to replace the fossil diesel fuel due to its CO2 reduction potential as well as for their capability to diminish the soot production and therefore for improving the soot-NOX trade-off in Compression Ignition engines. Thus, the main objective of this paper was to better understand the combustion process and the in-cylinder soot formation of two of the most popular E-fuels currently: Fischer-Tropsch (FT) diesel and Oxymethylene dimethyl ether (OMEX). To achieve this aim, a single cylinder optical CI engine with a commercial piston geometry was used. Thee optical techniques (Natural Luminosity–NL, OH* chemiluminescence and 2-color pyrometry) were applied to analyze the combustion evolution and quantify the soot formation at different loads (1.5, 4.5 and 7.5 bar IMEP). OMEX presented the largest injection duration due to the low LHV. For the NL analysis, OMEX showed the lowest light intensity for the three loads tested, indicating a very low soot production. Despite of the low NL intensity, it presented the highest OH* chemiluminescence signal, indicating a higher presence of near-stoichiometric zones due to the high amount of oxygen. Regarding FT diesel, it showed a combustion behavior similar to the commercial diesel. NL, OH* and 2-color technique analysis indicated that for the three conditions tested, FT diesel presented lower soot production and a faster soot oxidation than commercial diesel.

Effect of engine conditions and injection timing on piston-top fuel films for stratified direct-injection spark-ignition operation using E30

Mid-level ethanol/gasoline blends can provide knock resistance benefits for stoichiometric spark-ignition engine operation, but previous studies have identified challenges associated with spray impingement and wall wetting, leading to excessive particulate matter emissions. At the same time, stratified-charge spark-ignition operation can provide increased thermal efficiency, but care has to be exercised to avoid excessive in-cylinder soot formation. In support of the use of mid-level ethanol/gasoline blends in advanced spark-ignition engines, this study presents spray and fuel-film measurements in a direct-injection spark-ignition engine operated with a 30 vol.%/70 vol.% ethanol/gasoline blend (E30). Crank-angle resolved fuel-film measurements at the piston surface are conducted using two different implementations of the refractive index matching technique. A small-angle refractive index matching implementation allows quantification of the wetted area, while a large-angle refractive index matching implementation enables semi-quantitative measurements of fuel-film thickness and volume, in addition to fuel-film area. The fuel-film measurements show that both the amount of fuel deposited on the piston and the shape of the fuel-film patterns are strongly influenced by the injection timing, duration, intake pressure, and coolant temperature. For combinations of high in-cylinder gas density and long injection duration, merging of the individual spray plumes, commonly referred to as spray collapse, can cause a dramatic change to the shape and thickness of the wall fuel films. Overall, the study provides guidance to engine designers aiming at minimizing wall wetting through tailored combinations of injection timings and durations.

Refinement of the two-color pyrometry method for application in a direct injection diesel and natural gas compression-ignition engine

The soot emissions from internal combustion engines have significant health and environmental impacts and, as such, are subject to increasingly stringent regulations. Two-color pyrometry provides the in-cylinder soot cloud temperature and soot volume fraction and can provide insight to the in-cylinder soot formation and oxidation processes to guide research for reducing engine-out soot emissions. This work demonstrates improvements to the two-color pyrometry methodology, with a focus on low-temperature, low-soot regimes such as low-temperature combustion or combustion of direct injected natural gas. Through selection of a fast and robust numerical algorithm, characterizing and increasing the detection envelope, performing static and dynamic perspective adjustments, accounting for non-uniform and non-linear system response, as well as localized signal-to-noise ratio enhancement through image filtering, the performance of the pyrometric method was improved by a 40% increase in the resolved signal fraction. The refined two-color method was evaluated for both direct injected diesel and natural gas fueling strategies using a pilot-ignited direct injected natural gas fuel system and facilitated evaluation of local temperatures and soot concentrations in pilot-ignited direct injected natural gas combustion, despite the generally low soot levels in this combustion strategy.

Influence of ethanol blended and dual fueled with gasoline on soot formation and particulate matter emissions in a small displacement spark ignition engine

The implementation of environmentally friendly internal combustion engines (ICEs) has become an urgent need in recent years due to the strict current legislation. The high pollutant emissions such as particulate matter (PM) exhausted from ICE has driven many researchers to explore the possible advantages of alternative fuels. Ethanol is one of the most suitable alternative fuel for spark-ignition (SI) engines due to its higher heat of evaporation, octane number and larger oxygen content that allow to obtain positive effects on engine performance and particle formation and emissions. The main goal of the experimental research is the study of the effect of the different methods of ethanol fueling on particulate matter by means of the correlation between the in-cylinder soot formation and the exhaust particle emissions. The experimental investigation was carried out in a 4-stroke small displacement optical spark ignition engine operated at 2000 and 4000 rpm under full load conditions. The engine was fueled with gasoline and ethanol pure, blended (E30) and dual fueled (EDF). For both E30 and EDF configurations, 30 vol% of ethanol in gasoline was supplied. An optical technique based on 2D-digital imaging was used. Two-color pyrometry theory was applied to assess the in-cylinder soot formation. Particle emissions were measured at the exhaust by means of a smoke meter. The particle size distribution function (PSDF) was measured in the range from 5.6 to 560 nm with an Engine Exhaust Particle Sizer (EEPS). The research has shown that the effect of ethanol on soot formation and particle emission depends not only on ethanol fuel properties, but also on engine configuration and engine operating conditions.

In-Cylinder GDI Soot via Visualization and Time-Resolved Total Cylinder Sampling

<div class="section abstract"><div class="htmlview paragraph">For better understanding, model development and its validation of in-cylinder soot formation processes of Gasoline Direct Injection (GDI) engines, crank-angle-resolved mass and size distribution of in-cylinder soot during a GDI combustion cycle were investigated via optical measurements and total cylinder sampling technique in an optically accessible Rapid Compression and Expansion Machine (RCEM). A direct-injection, spark-ignited and single-shot combustion event was achieved in the RCEM operated with engine speed 600 rpm, compression ratio 9.0, equivalence ratio 0.9 and natural aspiration. A three-component (iso-octane 65%, n-heptane 10%, toluene 25%) gasoline surrogate fuel and a multi-hole injector shared within the Japanese SIP Innovative Combustion Technology research program were used. As for the optical measurements, two-color method and laser/LED-based Diffused Back Illumination (DBI) high-speed imaging through sapphire windows on the cylinder head and the flat-top piston provided time-sequential in-cylinder soot mass. As for the total cylinder sampling, filter gravimetry and Portable Aerosol Mobility Spectrometer (PAMS) measurements of total-cylinder soot-laden gas provided crank-angle-resolved in-cylinder soot mass and size distribution. The total cylinder sampling was realized by replacing the cylinder head window with a stainless-steel diaphragm, rupturing the diaphragm at an arbitrary crank angle during combustion and rapidly expanding the total-cylinder soot-laden gas to effectively freeze secondary reactions and agglomeration of soot particles. The in-cylinder soot mass obtained by the above mentioned four different methods showed reasonable agreement each other both in increasing trend during combustion and quantitative soot mass. Measured variation of soot size distribution during combustion indicates that formation and agglomeration of soot are simultaneously occurring during combustion. Observation and morphology analysis of sampled soot via High-Resolution Transmission Electron Microscopy (HR-TEM) are also in progress.</div></div>

Endoscopic combustion characterization of Jatropha biodiesel in a compression ignition engine

Decline in global resources of petroleum based fuels such as diesel and gasoline have attracted attention of researchers towards alternative fuels which are capable of providing both, energy security and protection against environmental degradation due to emission of hazardous pollutants. As an oxygenated fuel, biodiesel has emerged as a potential alternative fuel, which can be used in diesel engines on large-scale to reduce CO, HC and PM emissions. In this experimental study, Jatropha curcas derived biodiesel (JB100) and its 20% (v/v) blend with mineral diesel (JB20) was tested in a production-grade single cylinder diesel engine for evaluating its combustion characteristics. Since soot and NOx emissions are major concerns in CI engine combustion, it is very important to investigate their in-situ spatial distribution, so that effective control measures can be devised. For this, endoscopic visualization technique was implemented on a modified prototype engine to evaluate the effect of biodiesel blending percentage and engine load on in-situ spatial distribution of in-cylinder soot and flame temperature distributions. Inherent oxygen content of biodiesel helped in achieving superior combustion; as a result, in-cylinder soot formation was lower than baseline mineral diesel. Spatial flame temperature distribution was also relatively lower in biodiesel. Endoscopic visualization technique was found to be effective in evaluating different combustion characteristics in a modified prototype engine.

Analysis of combustion phenomena and pollutant formation in a small compression ignition engine fuelled with blended and pure rapeseed methyl ester

This paper deals with an investigation on the combustion process and pollutant formation of a small diesel engine fuelled with blended and pure biodiesel. The engine is a three-cylinder, 1028 cc, equipped with a common rail injection system. Endoscope based optical setup was used to observe in the cylinder without significant interference to the combustion process. Combustion images were post-processed by two-colour pyrometry method to evaluate the flame temperature and the in-cylinder soot concentration. Optical data were correlated to the nitrogen oxides and the particulate matter emissions measured at exhaust. Experiments were carried out at different operating conditions. It was found out that without EGR (exhaust gas recirculation), blended and pure RME (rapeseed methyl ester) were characterized by higher flame temperature and also by higher NOx emissions. In presence of EGR, lower flame temperature was detected for biodiesel; in this case higher NOx emissions were measured with biodiesel but the difference with NOx emissions from diesel fuel were reduced. Moreover, blended and pure biodiesel combustion was characterized by lower in-cylinder soot formation and also lower PM (particulate matters) at exhaust.

A comprehensive analysis of the impact of biofuels on the performance and emissions from compression and spark-ignition engines

The compression ignition and small displacement spark-ignition engines play an important role in the urban air pollution. In particular, vehicles equipped with compression ignition engines are widely used because of their higher performance and fuel efficiency with respect to the spark-ignition ones. Nevertheless, spark-ignition engines with low displacements are even more wide-spreading because of the lower fuel consumption and emissions. They are also used for two-wheeled vehicles, whose easier navigation makes them widely used in heavily congested areas. Their contribution on urban pollution is worsened by the fact that these vehicles have to comply with the Euro 3 standard; light vehicles have, instead, to fulfill the more restrictive Euro 6, which for the compression ignition and gasoline direct injection engines indicates for particle emissions also a number-based regulation. This article aims to characterize the effects of biofuels on engine emissions and performance of compression ignition and spark-ignition engines. The investigation was carried out on different class of engines. Direct injection and a port fuel injection spark-ignition engines fueled with ethanol and its blends, 10 v/v%, 50 v/v% and 85 v/v% of ethanol in gasoline. The compression ignition engine was equipped with a common rail injection system and was fueled with pure rapeseed methyl ester, representative of fatty acid methyl ester, and its blends in diesel, 20 v/v% and 50 v/v%. The gaseous emissions and the particle concentration were measured at the exhaust by means of conventional instruments. Particle size distribution function was measured in the range from 5.6 to 560 nm by means of an engine exhaust particle sizer. A comprehensive characterization of the particulate carbon was performed by means of optical diagnostics in the combustion chamber. In particular, two-dimensional images of flame evolution were detected and processed by two-color pyrometry technique to assess the in-cylinder soot formation and oxidation processes. For both the investigated spark-ignition and compression ignition engines, the use of biofuels shows a partial increase in the specific fuel consumption and a reduction of the soot particles emission. Nevertheless, a further effort on engine technology should be paid to balance the mass with the size and number of the particles.

Effects of Ethanol on In-Cylinder and Exhaust Gas Particulate Emissions of a Gasoline Direct Injection Spark Ignition Engine

The effects of ethanol on reducing the particulate emissions of a direct injection spark ignition (DISI) engine were investigated using a single-cylinder, optically accessible engine. Neat anhydrous ethanol was compared with a baseline fuel of reference grade gasoline. A high speed camera was used to record crank-angle resolved in-cylinder images of the fuel spray, combustion, and thermal radiation from the soot formed for the engine operating conditions studied. Particulate emissions in the engine exhaust gas were also measured using opacity measurements (i.e., using a smoke meter). All experiments were conducted at the same load conditions with a net indicated mean effective pressure of IMEPnet ≈ 5.5 bar and an intake manifold absolute pressure of 76 kPa. The engine speed was fixed at 1500 rpm, and the fuel injection duration was controlled to achieve stoichiometric combustion. Spark timing was adjusted to target combustion phasing (CA50) of 8° aTDC. The effects of engine coolant temperature and fuel injection timing on fuel spray characteristics and soot formation were studied for each fuel. The imaging data indicate that soot formation is a strong function of liquid fuel impingement on the piston surface, consistent with previous in-cylinder imaging studies of soot formation in DISI engines using different engine hardware and imaging orientation. A quantitative metric was applied to the imaging data of this work, and the in-cylinder soot formation based on the imaging data were in good agreement with the engine-out smoke measurements, indicating the optical imaging results are representative of engine-out particulate emissions. Higher coolant temperatures and later fuel injection timing significantly mitigated the in-cylinder soot emissions for both fuels by altering the fuel spray interactions with the piston surface and cylinder liner. As expected based on previous studies, ethanol systematically produced less soot than gasoline for each operating condition. Features of the fuel spray roll-up were identified as important indicators of pool fires on the piston surface for the earliest fuel injection timings.

Optical investigation of fuel and in-cylinder air-swirl effects in a high-speed direct-injection engine

The primary aim of this study is to investigate current issues of combustion in high-speed direct-injection diesel engines in detail by optical diagnostics. Both fuel and engine design are considered. Recently, measurements of engine-out emissions demonstrated that approximately soot-free combustion can be achieved using a newly designed two-component fuel named BLT. It was composed of 70% butyl levulinate and 30% n-tetradecane (by volume). In this work, the underlying mechanism is clarified by in-cylinder visualization of OH* radicals and soot. In particular, it turns out that in-cylinder soot formation is avoided almost completely, that is, the soot oxidation process is much less important. This is basically achieved by both the oxygen content (about 21%) and the low cetane number (approximately 33) of BLT. The latter leads to enhanced pre-mixing of fuel and air. Consequently, soot formation can be greatly reduced because it depends on the air–fuel ratio of the mixture shortly before high-temperature combustion. The influence of in-cylinder air swirl on combustion and soot formation is also studied for both BLT and conventional diesel fuel, respectively, using the same optical diagnostic. The measurements show that the combustion zone strongly depends on the local gas-flow velocity for diesel fuel. Soot formation decreases with increasing swirl because of enhanced air entrainment and pre-mixing of fuel and air. In addition, the results indicate that soot oxidation is improved under high-swirl conditions for diesel fuel. In contrast, the influence of swirl on the combustion of BLT is overall found to be weak, suggesting that swirl could be reduced in future high-speed direct-injection engines for BLT-like fuels.

Forensics of soot: C5-related nanostructure as a diagnostic of in-cylinder chemistry

We report observations of nanoscale microstructural changes in soot from an experimental light-duty diesel engine, produced with varying levels of biodiesel fuel blending. Based on these observations and current information in the literature, we propose a mechanistic hypothesis to explain the effects of biodiesel blending. Our underlying assumption is that particulate nanostructure is closely coupled to the local chemistry at the time the soot is formed. In the context of in-cylinder soot formation, this implies that changes in nanostructure may aid in diagnosing important changes in fuel–air mixing.

Simulation and Analysis of In-Cylinder Soot Formation in a Low Temperature Combustion Diesel Engine Using a Detailed Reaction Mechanism

Simulation and Analysis of In-Cylinder Soot Formation in a Low Temperature Combustion Diesel Engine Using a Detailed Reaction Mechanism

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.