Low Temperature Diesel Combustion Research Articles

Time frequency analysis for the evaluation of ignition delay in conventional and PCCI combustion modes

• Time frequency analysis can detect both start of injection and start of combustion. • Time frequency analysis can provide an accurate evaluation of ignition delay in diesel engines. • Combustion development for PCCI and conventional diesel modes is investigated. In-cylinder pressure signal has been widely adopted to obtain simple offline and online diagnosis of the combustion process through one-zone analysis, which provides the apparent heat release of the mixture. The present paper also starts from the in-cylinder pressure measurement to apply time frequency analysis for an alternative evaluation of the combustion events. A comparison with traditional one-zone analysis is then performed. In particular, time frequency analysis is here applied for the investigation of all the characteristic events that define the fuel ignition delay, i.e. hydraulic start of injection, first, along with the corresponding start of combustion, next. The study here reported is relative to single-injection patterns and has been applied to both conventional and premixed low temperature diesel combustion modes. For the considered injection pattern, time frequency analysis, compared to single-zone combustion model supplemented by measurements of injection rate at the hydraulic rig and by engine measurements of fuel injection pressure in the injector supply pipe, provided precise evaluation of the relevant combustion events.

Characteristics of Low Temperature Diesel Combustion with Paraffinic Hydrocarbon Fuels

Characteristics of Low Temperature Diesel Combustion with Paraffinic Hydrocarbon Fuels

Prospective fuels for diesel low temperature combustion engine applications: A critical review

Low Temperature Combustion (LTC) strategies are most promising to simultaneously reduce oxides of nitrogen (NOx) and soot emissions from diesel engines along with offering higher thermal efficiency. Commercial wide spread implementation of diesel LTC strategies requires several challenges to be addressed, including lack of precise ignition timing control, widening the narrow operating load ranges and reducing high unburned fuel emissions. These challenges can be addressed through modifications in the engine or fuel design or both. The timing and rate of combustion in several LTC strategies are controlled primarily by the chemical kinetics of the fuel. Since, diesel fuel reactivity and volatility are tailor-made to perform well under conventional diesel combustion conditions, its application in LTC poses several problems, as highlighted in this paper. Hence, it is important to identify suitable alternative fuels for the different diesel LTC strategies. The published literature on LTC over the past 25 years is critically analyzed to discuss the evolution of the different diesel LTC strategies, their operability limits, the challenges and the controlling parameters for each strategy. This is followed by in-depth analysis of the role of the fuel and the fuel requirements for each strategy. Further, the importance of adopting a hybrid surrogate modeling approach to enable numerical simulation of diesel LTC is highlighted. A novel attempt of relating various diesel low temperature combustion (LTC) strategies based on the approach followed to achieve positive ignition dwell through different injection strategies, utilizing high exhaust gas recirculation (EGR), and dual fuels is presented. The need for replacing diesel with alternative liquid fuels in LTC strategies is presented by highlighting the fundamental problems associated with diesel fuel characteristics. The review concludes by suggesting potential alternative fuels for various diesel LTC strategies and provides directions for future work to address the challenges facing compression ignition LTC operation.

In-depth comparison between pure diesel and diesel methanol dual fuel combustion mode

Diesel Methanol Dual Fuel (DMDF) could achieve high efficiency and low emissions simultaneously at part of engine conditions, but the reason of this has not been clarified. In order to find the reason behind this phenomenon, Pressure-Volume (P-V) map, equivalence ratio (φ)-temperature (T) map and three-dimensional simulation software were used to compare diesel and DMDF mode, and two test conditions were selected to fulfill in-depth comparison of two combustion modes. Based on the analysis of P-V map, the reason of higher thermal efficiency at DMDF mode is as follows: lower cylinder pressure in compression stroke contributes to compression work reduction. Meanwhile, higher combustion rate of DMDF mode could reduce cylinder temperature and pressure in the late power stroke, which would further decrease exhaust loss and heat loss. Based on the analysis of φ-T map, higher methanol substitution percent could reduce the intersection area between cylinder φ-T map and nitric oxide & soot generation region. And after analyzing in-cylinder temperature, hydroxy mole fraction together with the equivalent ratio three-dimensional cloud map, it is found that the inhibitory effect of methanol on diesel low-temperature combustion prolongs diesel ignition delay, which helps to form the homogeneous charge in the cylinder. Under DMDF mode, fuel rich areas are significantly reduced, so is the maximum in-cylinder temperature. This is the key to achieve ultra-low nitrogen oxides and soot emissions for DMDF mode.

Synthesis and performance of high efficient diesel oxidation catalyst based on active metal species-modified porous zeolite BEA

With the appearance of the low temperature combustion (LTC) diesel engines, diesel oxidation catalyst (DOC) becomes more and more important in removing the CO and hydrocarbons (HC) emitted from LTC diesel engines, especially in cold start-up process. Herein, metal species-modified hierarchically porous zeolites BEA (H-BEA) containing 0.8 wt% Pt have been synthesized by an ion-exchange and subsequent deposition-reduction method. It is found the obtained catalyst PtFeMg/H-BEA shows super catalytic activity, high water and sulfur resistance, i.e., the temperatures T90 are as low as 95 °C and 225 °C in the CO and C3H8 oxidation, respectively. It is believed that the strong interaction between Pt species and metal Fe, Mg-modified zeolite H-BEA is conductive to the CO and C3H8 catalytic oxidation. Further integrated into commercial cordierite honeycomb ceramics substrates, the obtained monolithic DOC still exhibits an excellent catalytic activity and anti-aging performance even under high velocity and multiple reaction atmosphere, indicating its promising prospects in the practical application. Besides, FTIR results indicate that there are two forms of Pt species: Pt atom in the zeolite BEA framework and metal Pt nanoparticles/clusters dispersed on the surface of BEA, which jointly promotes the adsorption and oxidation of CO and C3H8.

Ceria-based nanoflake arrays integrated on 3D cordierite honeycombs for efficient low-temperature diesel oxidation catalyst

A new, surfactant-free hydrothermal method has been developed for the growth of CeO2-based nanoflake arrays onto three-dimensional-channeled cordierite honeycomb substrates. Herein, a leaching-crystallization mechanism was proposed where the hydrothermal reaction, only involving cerium nitrate and water, leached the cordierite surface slightly and induced the formation of CeO2 nanoparticles subsequently. Further continued reaction reincorporated Al and Si atoms leached from cordierite into CeO2, finally recrystallizing Ce-Al-Si composite nanoflake structures. By using atomic layer deposition process, well-dispersed, size-controlled Pt nanoparticles were uniformly decorated on the CeO2-based nanoflakes to form the Pt/ CeO2 nano-array-based monolithic catalyst. Despite 5-50 times reduction in the active material usage compared with the traditional wash-coated catalyst, the Pt/CeO2 nano-array monolithic catalyst exhibited good catalytic oxidation activities over various individual gases, such as propylene, propane, CO, and NO oxidation, with 90% conversion efficiencies at temperatures below 200 °C. Under the simulated exhaust condition of low-temperature diesel combustion (LTC-D) developed by US DRIVE, the monolithic catalyst with low Pt loading (˜1 g/l) exhibits 90% conversion of catalytic oxidation over CO and hydrocarbonsat temperatures as low as ˜180 °C, much superior to the performance of traditional washcoated catalysts.

Modeling diesel spray tip and tail penetrations after end-of-injection

Zero-dimensional (0-D) modeling of spray development plays increasingly significant roles in the model-based optimization of parameter designs and control strategies in diesel engine systems. Partially premixed low temperature diesel combustion that has been deemed essential in simultaneous reductions in the nitrogen oxides (NOx) and soot emissions usually features ignition occurrence after the end of fuel injection (EOI). However, so far modeling of the spray development after the EOI is far from adequate. Particularly, a theoretical model for the spray tail penetration is still absent. The objective of this paper is therefore to develop the 0-D model for both diesel spray tip and tail penetrations. The model of spray tip penetration after the EOI is based on the existing models, and the model constant is further developed. The mathematical expression of spray tail penetration is newly deduced with the discrete control volume method. These models are validated against the experimental spray data with varied ambient densities, injection pressures and nozzle hole diameters using the high-speed shadowgraphy and the constant volume chamber. The calculated results by the models agree very well with the experimental ones. The spray tip penetration exhibits a t dependence at the initial stage of injection, a t1/2 dependence at the later stage, and a (t−ti)1/4 dependence after about two injection durations from the start of injection (SOI). While the spray tail penetration is independent of the fuel injection pressures, it exhibits a (t−ti)1/2 dependence. Since the spray tail penetration rate is rather slower than the spray tip, the spray development after the EOI shows a trend with remarkable length extension, causing the quick fuel-air over mixing. These results are believed to be valuable reference for researchers or engineers who are considering the model-based design and control strategy development for diesel engine systems.

Intake air strategy for low HC and CO emissions in dual-fuel (CNG-diesel) premixed charge compression ignition engine

Single fueled advanced combustion technologies, such as homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI), and low temperature diesel combustion (LTC), could make it possible to escape nitrogen oxides (NOx) and particulate matter (PM) generation. However, single fueled advanced combustion technologies have several challenges to overcome in order to be commercialized, such as combustion phase control, low combustion stability, narrow operating ranges, high level of maximum pressure rise rate (MPRR), and high amounts of unburned hydrocarbon (HC) and carbon monoxide (CO) emissions. Dual-fuel premixed charge compression ignition (DF-PCCI) combustion concepts have suggested many solutions to overcome the drawbacks of single-fueled advanced combustion technologies, such as combustion phase control, combustion stability, and limited operating range. However, high amounts of unburned HC and CO emissions are still regarded as the main hurdles of DF-PCCI combustion. In this study, the effects of global equivalence ratio (ϕglobal) and initial charge temperature, which were controlled by means of intake throttling, charge heating, and exhaust gas recirculation (EGR) strategies, were investigated to overcome the bulk quenching phenomenon under low load conditions of DF-PCCI operation in a heavy-duty (HD) single cylinder engine. The optimized intake charge strategy which used the throttle and hot-EGR, showed the possibility of simultaneous HC and CO reduction, combustion efficiency (ηc) improvement, and combustion stability enhancement while satisfying NOx and PM emissions under EU-VI regulations. The results suggest that controlling the charge air quantity and charge temperature is an effective way to mitigate the bulk quenching phenomenon under low load conditions on DF-PCCI.

Direct numerical simulation of a temporally evolving air/n-dodecane jet at low-temperature diesel-relevant conditions

We present a direct numerical simulation of a temporal jet between n-dodecane and diluted air undergoing spontaneous ignition at conditions relevant to low-temperature diesel combustion. The jet thermochemical conditions were selected to result in two-stage ignition. Reaction rates were computed using a 35-species reduced mechanism which included both the low- and high-temperature reaction pathways. The aim of this study is to elucidate the mechanisms by which low-temperature reactions promote high-temperature ignition under turbulent, non-premixed conditions. We show that low-temperature heat release in slightly rich fuel regions initiates multiple cool flame kernels that propagate towards very rich fuel regions through a reaction-diffusion mechanism. Although low-temperature ignition is delayed by imperfect mixing, the propagation speed of the cool flames is high: as a consequence, high-temperature reactions in fuel-rich regions become active early during the ignition transient. Because of this early start, high-temperature ignition, which occurs in fuel-rich regions, is faster than homogeneous ignition. Following ignition, the high-temperature kernels expand and engulf the stoichiometric mixture-fraction iso-surface which in turn establish edge flames which propagate along the iso-surface. The present results indicate the preponderance of flame folding of existing burning surfaces, and that ignition due to edge-flame propagation is of lesser importance.. Finally, a combustion mode analysis that extends an earlier classification [1] is proposed to conceptualize the multi-stage and multi-mode nature of diesel combustion and to provide a framework for reasoning about the effects of different ambient conditions on diesel combustion.

Effects of intake-port throttling on combustion behaviour in diesel low-temperature combustion

This article describes the effects of intake-port throttling on diesel low-temperature combustion at a low and medium load condition. These conditions were known for their characteristically high hydrocarbon emissions predominantly from over-mixed and under-mixed mixture zones, respectively. The investigation was carried out to supplement current findings in the literature with valuable information on the formation of high hydrocarbon emissions with increasing swirl levels generated by intake-port throttling. This was achieved through the use of cycle-resolved high hydrocarbon measurements in addition to cycle averaged emissions and in-cylinder pressure-derived metrics. While there was negligible overall effect at the moderately dilute low-load conditions, increasing swirl has been shown to be beneficial to premixing efficacy under highly dilute conditions with extended ignition delay. This potential advantage was found to be nullified by the swirl-induced confinement of fuel and combustion products to the central region of the cylinder leading to poor late cycle burn rates and increased smoke emissions. High hydrocarbon emissions from the squish and head quench regions were reduced by an increase in swirl ratio.

Comparisons of particle size distribution from conventional and advanced compression ignition combustion strategies

Particulate size distribution measurements are of importance in engine research as stricter regulations on particulate matter emissions (both mass and number based) are being implemented. Particulate size distribution measurements can be very sensitive to the laboratory environment or experimental setup, making it difficult to compare results for different combustion strategies acquired in different labs. In this study, a comparison of particulate size distribution measurements over a wide variety of conventional and advanced combustion strategies was conducted using a four-stroke single-cylinder diesel engine test setup to eliminate lab-to-lab variations and enable direct comparison of particulate size distribution results for different combustion strategies. Eight combustion strategies are included in the comparison: conventional diesel combustion, diesel/gasoline reactivity controlled compression ignition, homogeneous charge compression ignition, two types of gasoline compression ignition (early injection and late injection), diesel low temperature combustion, natural gas combustion with diesel pilot injection, and diesel/natural gas reactivity controlled compression ignition. Measurements were performed at four different load-speed points with matched combustion phasing when possible; for several strategies, it was necessary to operate with slightly different combustion phasing. Particle size distributions were measured using a scanning mobility particle sizer. To study the influence of volatile particles, measurements were performed with and without a volatile particle remover (thermodenuder) at low and high dilution ratios. The results show that non-uniformity in the fuel distribution caused by direct injection results in increased accumulation-mode particle concentrations compared to premixed strategies even for low particulate mass advanced combustion strategies. Premixed combustion strategies (homogeneous charge compression ignition) and early injection gasoline compression ignition show higher nuclei-mode particle concentrations. Overall particle number and mass concentrations vary significantly between engine operating conditions and between combustion strategies.

Evaluation of Mn and Sn-Modified Pd-Ce-Based Catalysts for Low-Temperature Diesel Exhaust Oxidation

Pd-impregnated Ce-based catalysts were tested for carbon monoxide (CO) and hydrocarbon (HC) oxidation under challenging low-temperature diesel combustion conditions. The results indicate that the light-off temperatures for CO over Pd/CeO2, Pd/MnO x -CeO2 (Pd/MC), and Pd/SnO2-MnO x -CeO2 (Pd/SMC) catalysts shift to higher temperatures in the presence of simulated diesel exhaust gas. The lowest T 50 for CO is observed over Pd/MC at 173 °C, whereas Pd/CeO2 is shown to oxidize most of the HCs at temperatures below 400 °C. In all catalysts, the oxidation of HCs starts right after the onset of CO oxidation, revealing that the competitive adsorption of CO, NO, and alkenes controls the catalytic activity. Further evaluation of the catalytic activity in the presence of only CO and C3H6 reveals the immediate inhibiting effect of C3H6 at catalyst temperatures below 150 °C. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments performed over Pd/CeO2, Pd/MC, and Pd/SMC show that C3H6 inhibits the formation of carbonyl species on Pd n+ sites, which limits the catalytic activity for CO. Such inhibition is observed on all supports, implying that the activity is independent of oxygen storage capacity (OSC) or lattice oxygen reducibility of the supports in the presence of C3H6.

Control oriented modeling and analysis of gas exchange and combustion processes for LTC diesel engine

Diesel low temperature combustion (LTC) has great potential for the realization of ultra low emission of internal combustion engine, which has been widely studied in recent years. Precise control of the LTC combustion is essential for the real applications of LTC concept on diesel engine. Constructing the model which can accurately describe the dynamic characteristics of diesel LTC is the foundation of the model-based controller design. In this paper, a dynamic physics based control oriented model of low temperature combustion diesel engine is presented. This model is a three-input-three-output physics-based control oriented model and consists of gas exchange sub-model and combustion sub-model. The inputs of the model are fuel rate, EGR opening rate and VGT position. The outputs are intake oxygen mass fraction, CA50 and IMEP. The model has eight states and all the states are able to be measured easily. The model is validated by a large number of steady-state and transient diesel experimental test data. The steady-state and transient validation results show that the model can accurately describe the dynamic characteristics of LTC diesel engine with high accuracy for control applications. Which lays a foundation for model-based controller design and verification of different control algorithms in LTC diesel engine.

Effects of six-carbon alcohols, ethers and ketones with chain or ring molecular structures on diesel low temperature combustion

The effect of oxygenated additives with different oxygen functional groups and different carbon chain structures on the combustion and emissions was investigated on a single-cylinder diesel engine. Three different oxygenated hydrocarbons with ring molecular structure including cyclohexanone, cyclohexanol and 2,5-dimethylfuran and other three oxygenated hydrocarbons with chain molecular structure including 2-hexanone, n-hexanol, and isopropyl ether were blended with diesel fuel separately to formulate 6 blended fuels with the same oxygen content (4wt%). A wide range of exhaust gas recirculation rates (from 0 to misfire) were employed to cover both conventional diesel combustion and low temperature combustion. The results indicate that the effect of oxygenated fuels on combustion and emission get larger at high rates of exhaust gas recirculation. For chain structure oxygenated additives, although n-hexanol presents much longer ignition delay at high rates of exhaust gas recirculation, it exhibits the highest soot peak due to its high boiling point. And isopropyl ether has the lowest soot emission attributing to its better volatility. The ring structure oxygenated additives present more significant difference in combustion than chain structure ones, and the sequence of ignition delay is 2,5-dimethylfuran>cyclohexanol>cyclohexanone>diesel. The soot emissions show a close relation with the ignition delay of the fuel, i.e., the fuel with longer ignition delay always exhibits lower soot emissions. And the lowest soot emission can be obtained with 2,5-dimethylfuran companied with apparent improvement in combustion efficiency and thermal efficiency. For the soot reduction ability of different oxygen functional groups, the sequence is alcohols<ketones<ethers. The oxygenated fuels with ring molecular structure tend to produce more soot compared with the ones with chain structure if only the oxygenated effect is considered, while the soot emissions is also influenced by the position of oxygen functional group. Taking all the effects together, 2,5-dimethylfuran is more effective in realizing high efficiency and clean low temperature combustion.

A parametric study for enabling reactivity controlled compression ignition (RCCI) operation in diesel engines at various engine loads

Reactivity controlled compression ignition (RCCI) is demonstrated as a promising combustion strategy to achieve high efficiency and clean combustion. In this work, an extensive experimental investigations are performed focusing on investigating the effect of operational parameters (intake pressure, exhaust gas recirculation rate, gasoline proportion, diesel injection timing, etc.) on the combustion performance and emission characteristics at various engine loads. Then, the key limiting factors for extension of the upper and lower limits of RCCI operations are analyzed. Results suggest that ultra-low NOx and soot emissions as well as high thermal efficiency can be achieved simultaneously with gasoline/diesel dual-fuel RCCI regime at moderate-to-high loads, while diesel low temperature combustion (LTC) with single-shot fuel injection can be more suitable for low-load operations. Moreover, to extend the RCCI operating range to lower loads, the proportion of diesel injection should be raised to increase the global reactivity of the in-cylinder charge. While increasing EGR rate and gasoline proportion as well as advancing diesel injection timing can lower the global reactivity of the cylinder charge and avoid premature combustion, excessive pressure rise rate and high peak in-cylinder pressure, thereby in favor of extending RCCI operation to higher loads. However, ultra-high EGR rate or too early fuel injection can lead to unstable combustion, making combustion phasing control extremely difficult.

Volatility characterization of nanoparticles from single and dual-fuel low temperature combustion in compression ignition engines

ABSTRACTThis work explores the volatility of particles produced from two diesel low temperature combustion (LTC) modes proposed for high-efficiency compression ignition engines. It also explores mechanisms of particulate formation and growth upon dilution in the near-tailpipe environment. The number distribution of exhaust particles from low- and mid-load dual-fuel reactivity controlled compression ignition (RCCI) and single-fuel premixed charge compression ignition (PPCI) modes were experimentally studied over a gradient of dilution temperature. Particle volatility of select particle diameters was investigated using volatility tandem differential mobility analysis (V-TDMA). Evaporation rates for exhaust particles were compared with V-TDMA results for candidate pure n-alkanes to identify species with similar volatility characteristics. The results show that LTC particles are mostly comprised of material with volatility similar to engine oil alkanes. V-TDMA results were used as inputs to an aerosol condens...

Exploring the benefits of multiple injections in low temperature combustion using a diesel surrogate model

Advanced low temperature combustion (LTC) strategies are promising to achieve high efficiency and low emissions of nitric oxides (NOx) and Particulate Matter (soot). The thermal efficiency advantages are obtained through lower temperatures with reduced heat losses. A lower charge temperature and the sufficiently longer time available for mixing help to avoid NOx and soot formation zones. However, important shortcomings of LTC strategies include higher unburned hydrocarbon emissions and higher combustion noise. The present work investigates the use of multiple injection strategies to combat the above shortcomings in low temperature diesel combustion. Numerical investigations were carried out using a three dimensional computational fluid dynamic (CFD) code KIVA-ERC-CHEMKIN to explore the benefits of multiple injections over a single injection. The physical and chemical properties of a typical American diesel fuel used in the present work were modeled using suitable surrogate components. The results show that significant benefits are possible with the use of multiple injections, including lower unburned emissions and lower combustion noise. The CFD investigations provide a better understanding of the mechanisms behind the benefits and are useful to suggest guidelines for further improvements.

Immaturity of soot particles in exhaust gas for low temperature diesel combustion in a direct injection compression ignition engine

The particulate matter sampled in exhaust gas from low temperature diesel combustion (LTC) with an exhaust gas recirculation rate of 60% in a direct injection compression ignition engine was comprehensively assessed and compared to conventional diesel combustion using thermogravimetric analysis and elemental analysis. With LTC, relatively significant mass reduction occurred due to the oxidation and desorption of hydrocarbons. 95% of mass was reduced at around 520°C in LTC, which is lower than for conventional combustion (595°C). The weight fractions of carbon and hydrogen species found by the elemental analysis after thermogravimetry with inert gas up to 400°C revealed that the change of weight fractions into higher carbon and lower hydrogen contents were larger for soot particles from LTC. The atomic ratios of carbon to hydrogen (C/H) as a measure of soot maturity were 6.74 and 3.84 for conventional combustion and LTC, respectively. The C/H ratio for LTC is close to that of the precursor particles (1.80) from a normal diffusion flame at low height or young soot particles (2.2) from an inverse diffusion flame, which demonstrates the immaturity of the soot particles from LTC, while the range of the C/H ratio in the conventional combustion was suggestive of highly matured soot particles. Transmission electron microscopy revealed the primary particle and agglomerates from LTC appeared small without distinguishable boundaries. The LTC mode also exhibited clumps of amorphous materials and translucent precursor particles, which were attributed to intermediate substances from the low combustion temperature and abundant hydrocarbon species.

Efficiency improvements with low heat rejection concepts applied to diesel low temperature combustion

There is a strong motivation to decrease the production and release of harmful substances such as oxides of nitrogen (NOX) and particulate matter from internal combustion engines. Simultaneously, there are on-going efforts to increase fuel efficiency to curb usage of natural resources and emission of carbon. In general, improvements in one of these areas come at the cost of the other; however, the results of a previous computational study have indicated that emissions can be decreased while simultaneously increasing efficiency through the application of low heat rejection techniques to low temperature combustion. The goal of this study is to experimentally confirm these findings using a light-duty, multi-cylinder diesel engine. Low temperature combustion is realized through high levels of exhaust gas recirculation and retarded injection timings while different degrees of low heat rejection are achieved by means of higher coolant temperatures which should serve to decrease the temperature gradients across the cylinder walls. By applying low heat rejection techniques to diesel low temperature combustion operation, the undesirable side effects of low temperature combustion such as lower combustion and energy conversion efficiencies were found to be mitigated. Specifically, the emissions of carbon monoxide and unburned hydrocarbons were reduced and the loss in fuel conversion efficiency was also diminished. NOX and smoke (an indicator of particulate matter) emissions did increase but they remained at acceptably low levels and below those of conventional combustion. While the full potential of improvements in low temperature combustion was not explored, these results point to the viability of further research into low heat rejection–low temperature combustion concepts.

Effect of oxygen concentration on highly diluted charge compression ignition combustion in an optical engine

To implement low temperature diesel combustion, the charge dilution with nitrogen was used in an optical engine to simulate exhaust gas recirculation of a metal engine. In-cylinder visualization is focused on a regime of the simultaneous reduction in soot and nitrogen oxides (NOx) emissions, which is demonstrated by the metal engine experiment. Injection timing was varied from 30 to 20° crank angle (CA) before top dead center (BTDC) by 5° increment, and the oxygen concentration was between approximately 8 and 11%. For an injection timing of a 20° CA BTDC, approximately 10% oxygen concentration showed two peaks of natural luminosity: one corresponds to the OH chemiluminescence from high temperature heat release and the other coincides with soot luminescence in a diffusive flame. Both peaks are reduced as the oxygen concentration at the intake stream decrease; in addition, the luminosity corresponding to the diffusive flame is not apparent below oxygen concentration of approximately 9% in this experiment. For an injection timing of 30° CA BTDC with the oxygen concentration being similar to that of 20° CA BTDC, the luminosity is weak for OH chemiluminescence and not apparent for soot luminescence. The earlier injection timing allows a prolonged mixing period; thus, a relatively well-premixed charge resulted in reduction of soot particles emitting soot incandescence. This result is also supported by the longer ignition delay calculated from the metal engine test.