Effect of flash boiling injection on combustion and PN emissions of DISI optical engine fueled with butanol isomers/TPRF blends

Direct injection spark ignition (DISI) engines have been widely used in passenger cars due to their lower fuel consumption, better controllability, and high efficiency. However, DISI engines are suffering from wall wetting, imperfect mixture formation, excess soot emissions, and cyclic variations. Applying a new fuel atomization technique and using biofuels with their distinctive properties can potentially aid in improving DISI engines. In this research, the effects of isobutanol and 2-butanol and their blends with Toluene Primary Reference Fuel (TPRF) on spray characteristics, DISI engine combustion, and particle number (PN) emissions are investigated for conditions with and without flash boiling of the injected fuel. Spray characteristics are investigated using a constant volume chamber. Then, the combustion, flame propagation, and PN emissions are examined using an optical DISI engine. The fuel temperature is set to 298 K and 453 K for liquid injection and flash boiling injection, respectively. The tested blending ratio is 30 vol% butanol isomers and 70 vol% TPRF. The results of the spray test reveal that liquid fuel plumes are distinctly observed, and butanol blends show a slightly wider spray angle with lower penetration length compared to TPRF. However, under flash boiling injection, the sprays collapse towards the injector axis, forming a more extended single central vapor jet due to the plumes' interaction. Meanwhile, butanol blends yield a narrow spray angle with more extended penetration compared to TPRF. The flame visualization test shows that the flash boiling injection reduced yellow flames compared to liquid fuel injection, reflecting the improvements in mixture formation. Thus, improvements were noted in the heat release and PN emissions. Butanol addition reduced the PN emissions by 43% under regular liquid injection. Flash boiling injection provided an additional 25% reduction in PN emissions.     

Advanced gasoline engine development using optical diagnostics and numerical modeling

Twenty years ago, homogeneous-charge spark-ignition gasoline engines (using carburetion, throttle-body-, or port-fuel-injection) were the dominant automotive engines. Advanced automotive engine development remained largely empirical, and stratified-charge direct-injection gasoline-engine production was blocked by lack of robustness in its combustion process [W.G. Agnew, Proc. Combust. Inst. 20 (1984) 1-17]. Today, a wide range of direct-injection gasoline engines are in (or near) production, and combustion science is playing a direct role in advanced gasoline-engine development through the simultaneous application of advanced optical diagnostics, three-dimensional computational fluid dynamics (CFD) modeling, and traditional combustion diagnostics. This paper discusses the use of optical diagnostics and CFD in five gasoline-engine combustion systems: homogeneous spark-ignition port-fuel-injection (PFI), homogeneous spark-ignition direct-injection (DI), stratified wall-guided spark-ignition direct-injection (WG-SIDI), stratified spray-guided spark-ignition direct-injection (SG-SIDI), and homogeneous-charge compression-ignition (HCCI). The emphasis is on WG-SIDI, SG-SIDI, and HCCI engines. Key in-cylinder physical processes (e.g., sprays and vaporization, turbulent fuel-air mixing, wall wetting, ignition and early flame development, turbulent partially premixed flame propagation, and emissions formation) can be visualized, quantified, and optimized through optical engine experiments and CFD-based engine modeling. Outstanding issues for stratified engines include reducing piston wall-wetting, pool fires and smoke in WG-SIDI engines, eliminating intermittent misfires in SG-SIDI engines, and optimizing lean NO_x after-treatment systems. HCCI engines require better control of combustion timing and heat-release rate over wide speed/load operating ranges, smooth transitions between operating modes, and individual cylinder sensors and controls. Future directions in optical diagnostics and modeling are suggested to improve our fundamental understanding of important in-cylinder processes and to enhance CFD modeling capabilities.     

Soot and NO emissions control in a natural gas/diesel fuelled RCCI engine by φ-T map analysis

Soot and NO emissions are considered as major pollutants to the atmosphere from compression ignition engines. Researchers have been dedicated to the reduction of soot and NO emissions. Thus, an advance combustion regime, i.e. reactivity controlled compression ignition (RCCI), was proposed to mitigate the formation of these emissions. In this study, the dynamic ?-T (equivalence ratio vs. temperature) map analysis was applied to visualise the combustion processes associated with the in-cylinder temperature and equivalence ratio in an RCCI engine. Therefore, the soot and NO emissions can be efficiently reduced by controlling the combustion process out of the emissions islands on the ?-T map. This analysis method employs KIVA4-CHEMKIN and SENKIN code to construct ?-T maps under various conditions. To find out the significant parameters of controlling combustion process and emissions formation, four parameters were taken into consideration in a natural gas (NG) and diesel fuelled RCCI engine: NG percentage, the first start of injection (SOI) timing, split fraction of diesel and exhaust gas recirculation (EGR) rate. Each parameter was varied at three levels. Finally, the ?-T maps and final soot and NO emissions were compared among varied conditions for each parameter. It is found that the increased NG percentage can significantly reduce soot because of the absence of C-C bond in NG and the reduced diesel fuel impingement on the surface of the piston or cylinder wall. Increasing EGR can decrease the peak combustion temperature due to the dilution effect and thermal effect, consequently maintaining RCCI at low temperature combustion region. This study also indicates that dynamic ?-T map analysis is efficient at manipulating the combustion process to mitigate the soot and NO emissions formation.     

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.     

Influence of EGR and ethanol blending on soot formation in a DISI engine

Soot formation and in-cylinder soot oxidation in an optically accessible DISI-engine is analysed for gasoline–ethanol mixtures under part load operating conditions. A volumetric extinction measurement technique was used both for the determination of the liquid fuel distribution and quantitative soot measurements. Toliso, a mixture containing isooctane and toluene (65?vol% isooctane and 35?vol% toluene) was utilised as a surrogate gasoline fuel. The EGR (EGR-exhaust gas recirculation) rate-dependence and the effect of ethanol blending (0, 20 and 40% ethanol, E0–E40) were studied at part load operation with two different injection timings. Operating point 1 (OP1) represents an early injection with piston wetting and pool fire, operating point 2 (OP2) represents a late injection timing with reduced time for mixture formation. Soot formation is more pronounced for OP1 and for E20 and E40 as compared to E0. Here the pool fire due to the fuel-dependent liquid wall film formation plays a major role in soot formation and oxidation. Late injection timing leads to increased soot formation for E20 compared to Toliso, while E40 shows lowest soot formation. Here the fuel–wall wetting is less pronounced, but mixture inhomogeneities exist, and the soot cloud covers a larger region in the cylinder. An EGR addition leads to a higher soot formation for Toliso for both operating points. The ethanol-blends show a reduction of the soot formation with EGR, which is explained by reduced combustion temperatures and the chemically bound oxygen for E20 and E40 leading to locally leaner mixtures at constant global air–fuel ratio. The study shows optimisation potentials of injection strategies for ethanol-blended gasoline in combination with EGR in DISI-engine applications.     

Soot particles in piston-top pool fires and exhaust at 5 and 15 MPa injection pressure in a gasoline direct-injection engine

This study shows the structure of soot particles sampled directly from wall wetting-induced pool fires formed on the piston top in a spark-ignition direct-injection (SIDI) engine. Of particular interest is its variation with injection pressure considering the current trend of high-pressure DI system development to reduce engine-out particulate emissions. Thermophoretic particle sampling was performed for transmission electron microscope (TEM) imaging, which was post-processed for statistical analysis of key morphology and internal structure parameters. These include the size distribution of soot aggregates and primary particles as well as carbon-layer fringe-to-fringe gap and concentricity. With the fixed engine speed and load conditions, in-flame soot particles are compared to the exhaust particles sampled simultaneously at selected 5 and 15 MPa injection pressures corresponding to low-speed/low-load and high-speed/high-load in practical engine operation. From the TEM images and statistical analysis, it was found that more concentrated and taller pool fire developed for 5-MPa injection leads to smaller soot aggregates composed of smaller soot primary particles due to suppressed soot growth in fuel-rich flames. However, the soot particles formed in 15-MPa injection-induced pool fires are at a more reactive status evidenced by less defined core-shell boundaries and higher fringe separation. The higher soot reactivity results in enhanced soot oxidation, which explains smaller soot aggregates and primary particles found for the 15-MPa injection in the exhaust sample.     

Modelling biodiesel–diesel spray combustion using multicomponent vaporization coupled with detailed fuel chemistry and soot models

A multicomponent vaporization model is integrated with detailed fuel chemistry and soot models for simulating biodiesel–diesel spray combustion. Biodiesel, a fuel mixture comprised of fatty-acid methyl esters, is an attractive alternative to diesel fuel for use in compression-ignition engines. Accurately modelling of the spray, vaporization, and combustion of the fuel mixture is critical to predicting engine performance using biodiesel. In this study, a discrete-component vaporization model was developed to simulate the vaporization of biodiesel drops. The model can predict differences in the vaporization rates of different fuel components. The model was validated by use of experimental data of the measured biodiesel drop size history and spray penetration data obtained from a constant-volume chamber. Gas phase chemical reactions were simulated using a detailed reaction mechanism that also includes PAH reactions leading to the production of soot precursors. A phenomenological multi-step soot model was utilized to predict soot emissions from biodiesel–diesel combustion. The soot model considered various steps of soot formation and destruction, such as soot inception, surface growth, coagulation, and PAH condensation, as well as oxidation by oxygen and hydroxyl-containing molecules. The overall numerical model was validated with experimental data on flame structure and soot distributions obtained from a constant-volume chamber. The model was also applied to predict combustion, soot and NOx emissions from a diesel engine using different biodiesel–diesel blends. The engine simulation results were further analysed to determine the soot emissions characteristics by use of biodiesel–diesel fuels.     

小尺度薄油池火沸腾燃烧特性研究

通过对小尺度薄油池火燃烧特性进行实验研究,分析油池不同燃烧阶段的特点,探讨沸腾燃烧对油池燃烧特性的影响。测量了直径分别为0.10 m、0.14 m、0.20 m和0.30 m正庚烷油池火的燃烧速率以及温度分布随时间变化。分析燃烧过程中燃油液面温度和池壁温度的变化规律,研究池壁沸腾传热对油池沸腾燃烧的影响。结果表明:油池沸腾燃烧阶段的燃烧速率明显大于稳定燃烧阶段;燃油液面温度在油池燃烧初期迅速上升至沸点,随后基本保持不变;池壁温度达到并超过燃料的沸点,从而在油池壁面上发生沸腾现象,是油池发生沸腾燃烧的条件。     

Vaporisation characteristics of methanol,ethanol and heptane droplets in opposed stagnation flow at low temperature and pressure

A computational model is developed and applied to study the vaporisation behaviour of three liquid fuels. This fundamental study is motivated by a need to understand how the performance of direct-injection-spark-ignition (DISI) engines may be affected by changes in fuel composition, especially alcohols. Currently, most DISI engines are designed for homogeneous-charge combustion, where the in-cylinder fuel injection, vaporisation and mixing is accomplished during the intake and early in the compression process. Thus the temperature and pressure are low, compared to post-compression conditions. The two-phase axisymmetric model is based upon an ideal opposed stagnation flow field. Liquid droplets are carried in one air stream that is met by an opposed air flow. Because of stagnation-flow similarity, the mathematical model can be represented as a one-dimensional boundary-value problem. Results show significant differences between methanol, ethanol and heptane fuels, which have potentially important impacts on the design and modification of fuel-injection systems for direct-injection engines with alternative fuels.     

LB simulation on soot combustion in porous media

Although diesel engines have an advantage of low fuel consumption in comparison with gasoline engines, several problems must be solved. One of the major concerns is that diesel exhaust gas has more particle matters (PM) including soot, which are suspected to be linked to human carcinogen. As one of the key technologies, a diesel particulate filter (DPF) has been developed to reduce PM in the after-treatment of exhaust gas. In this study, we conduct lattice Boltzmann (LB) simulation on combustion in porous media. Results show that the combustion reaction is well simulated to observe the decrease of soot attached to the porous wall. This information is indispensable for the better design of DPF, and LB method can be a good tool for combustion simulation in porous media.     

Bowl piston geometry as an alternative to enlarged crevice pistons for rapid compression machines

Thermal inhomogeneity and physical processes like fluid dynamics reduce the utility of rapid compression machine (RCM) facilities to accurately study fuel combustion phenomenon relevant to internal combustion engines. Most current RCMs incorporate a large crevice volume in the piston to capture roll-up vortices that encroach into the combustion zone during compression. In this work, a bowl piston design similar to those used in diesel engines is proposed as an alternative to enlarged creviced pistons for creating a sufficiently thermally homogenous gas mixture prior to ignition without undesirable fluid motion found in flat piston configurations. The bowl piston also eliminates the possibility of cold unreacted gases entering the combustion chamber when the piston is retracted in rapid compression and expansion machines (RCEMs) like in creviced piston designs. In the work, a bowl piston was compared to creviced piston and flat piston configurations numerically and experimentally. Through non-reacting computational fluid dynamics simulations, the bowl piston reduced the roll-up vortex found for the flat piston and led to a higher temperature and more thermally uniform core of gas at peak compression compared to the enlarged crevice piston. Experimentally, three pistons were studied in a RCM facility with ethanol and n-butane as fuels. Results showed that the bowl piston yielded benefits over conventional piston geometries including: reduced heat loss due to lower surface area, higher turbulent Reynolds Number, stronger ignition, and higher heat release rate and combustion efficiency as estimated using heat release analysis. Based on the findings presented here, we conclude that bowl piston geometries are a promising alternative to creviced pistons for conducting fuel ignition studies in RCM and RCEM facilities.     

Analysis of combustion chamber insulation effects on the performance and exhaust emissions of a DI diesel engine using a multi-zone model

A comprehensive two-dimensional multi-zone model of a diesel engine cycle is presented in this study, in order to examine the influence of insulating the combustion chamber on the performance and exhaust pollutants emissions of a naturally-aspirated, direct injection (DI), four-stroke, water-cooled diesel engine. The heat insulation is taken into account by the corresponding rise of wall temperature, since this is the final result of insulation useful for the study. It is found that there is no remarkable improvement of engine efficiency, since the decrease of volumetric efficiency has a greater influence on it than the decrease of heat loss to the coolant, which is converted mainly to exhaust gas enthalpy (significant rise of the exhaust gas temperature). As far as the concentration of exhaust pollutant emissions is concerned, it is found that the rising heat insulation leads to a significant increase of the exhaust nitric oxide (NO) and to a moderate increase of the exhaust soot concentration. Plots of temperature, equivalence ratio, NO and soot distributions at various instants of time inside the combustion chamber, emanating from the application of the multi-zone model, aid the correct interpretation of the insulation effects gaining insight into the underlying mechanisms involved.     

Ability of Particulate Matter Index to describe sooting tendency of various gasoline formulations in a stratified-charge spark-ignition engine

This study investigates the ability of Particulate Matter Index (PMI) to describe the sooting behavior of various gasoline formulations in a stratified-charge (SC) spark-ignition engine. The engine was operated at 2000 rpm with an intake pressure of 130 kPa where soot formation is known to primarily occur in the bulk gases. Exhaust soot emissions were measured for nine test fuels at various exhaust gas recirculation levels. A comparison between measured soot levels and PMI shows that PMI is a relatively poor predictor of the sooting tendency of the tested fuels under lean SC combustion. Among the fuels, three fuels, namely the di-isobutylene blend, High Olefin, and E30 fuels exhibit measured soot behavior opposite of that predicted by PMI. Optical diagnostics were utilized to further investigate the in-cylinder phenomena for these three fuels. Analysis of natural luminosity and diffused back-illumination extinction imaging suggests that fuel-induced differences in the amount of soot formed are responsible for a majority of the discrepancy in measured versus predicted sooting tendency. Fuel-induced differences in soot oxidation and spray development seem to play minor roles. Because the combustion and air-fuel mixing processes for lean SC combustion are different from conventional stoichiometric operation it is hypothesized that the PMI correlation needs to be modified to account for differences in stoichiometric air-fuel ratio and level of oxygenation between fuels. Furthermore, the role of fuel volatility in PMI possibly needs to be de-emphasized for SC operation with fuel injection into compression-heated gases.     

Modelling soot emission from a direct injection diesel engine fuelled with diesel–methanol dual fuel

The objective of this paper is to develop a soot model for multi-dimensional simulations of diesel–methanol dual-fuel engines to predict engine-out soot emissions. To the two-step soot model a special term, based on experimental study and analysis, is appended to the soot formation rate to account for the effect of methanol. The results of engine-out soot emissions predicted by the models were compared with experimental data and it is shown that the existing model predicts well for diesel engines, whereas the proposed model predicts well for both diesel and dual-fuel engines especially for the large fractional methanol flow rates. The results suggest that the soot model must be modified for the dual-fuel combustion mode.     

Visualization of soot formation from evaporating fuel films by laser-induced fluorescence and incandescence

Late-evaporating liquid fuel wall-films are considered a major source of soot in spark-ignition direct-injection (SIDI) engines. In this study, a direct-injection model experiment was developed to visualize soot formation in the vicinity of evaporating fuel films. Isooctane is injected by a multi-hole injector into the optically accessible part of a constant-flow facility at atmospheric pressure. Some of the liquid fuel impinges on the quartz-glass windows and forms fuel films. After spark ignition, a turbulent flame front propagates through the chamber, and subsequently sooting combustion is detected near the fuel films. Overlapping laser light sheets at 532 and 1064 nm excite laser-induced fluorescence (LIF) of polycyclic aromatic hydrocarbons (PAH) -potential soot precursors- and laser-induced incandescence (LII) of soot, respectively. The 532 nm light sheet has low fluence to avoid the excitation of LII. The LII and LIF signals are detected simultaneously and spectrally separated on two cameras. In complementary line-of-sight imaging, the fuel spray, chemiluminescence, and soot incandescence are captured with a high-speed color camera. In separate experiments, toluene is added to the isooctane as a fluorescent tracer and excited by pulsed 266 nm flood illumination. From images of the LIF signal, the fuel-films’ thickness and mass evolutions are evaluated. The films survive the entire combustion event. PAH LIF is found in close vicinity of the evaporating fuel films. Soot is found spatially separated from, but adjacent to the PAH, both with high spatial intermittency. Average images additionally indicate that soot is formed with a much higher spatial intermittency than PAH. Images from the color camera show soot incandescence earlier and in a similar region compared to soot LII. Chemiluminescence downstream of the soot-forming region is thought to indicate the subsequent oxidation of fuel, soot, and PAH.     

Investigations of soot formation in an optically accessible gasoline direct injection engine by means of laser-induced incandescence (LII)

This study presents the results of laser-induced incandescence (LII) measurements in an optically accessible gasoline direct injection engine. The focus was to evaluate LII as a particle measurement technique which is able to provide a deeper understanding of the underlying reaction and formation processes of soot in order to optimize the injection system to reduce exhaust gas emissions. A comparison of time-resolved LII, based on the model described by Michelsen, with an Engine Exhaust Particle Sizer (EEPS) was performed. In this context, the air–fuel ratio, the injection pressure and the injection timing have been varied while applying the measurement techniques in the exhaust system. In case of a variation of the air–fuel ratio, two-dimensional LII has been performed in the combustion chamber additionally. For each measurement, the Filter Smoke Number (FSN) was taken into account as well. Finally, a good agreement of the different techniques was achieved. Moreover, we found that by combining time-resolved LII and EEPS a differentiation of primary particles and agglomerates is possible. Consequently, a determination of the processes in the combustion chamber and agglomeration in the exhaust gas is feasible.     

蒸发性对燃油瞬态喷雾撞击壁面的动态传热影响

直喷发动机燃油喷雾撞击壁面形成油膜,导致燃烧效率降低,颗粒物排放增加.伴随撞壁的动态传热过程对油膜蒸发具有重要影响.本文针对正戊烷、甲醇、甲醇-汽油混合燃料瞬态喷雾撞击壁面,研究了不同条件下蒸发性对燃油瞬态喷雾撞击壁面动态传热影响.结果表明,提高喷油温度可促进燃油雾化,增大喷油压力或降低喷油距离可提高液滴撞壁强度,缩短...     

基于光谱分析的油料池火内部传热特性研究

油料池火焰内部分为不同燃烧区域,目前对油池火内部传热特性研究较少。针对油池火内部传热特性研究不足的现状,构建了红外火焰光谱测试系统,研究分析了92#汽油、95#汽油及润滑油池火焰红外光谱特性,对油池火焰不同燃烧区域的光谱信息进行了提取分析,结果表明：三种油料池火焰光谱特征相似,存在多个CO₂,H₂O及炭黑颗粒等燃烧产物的特征发射波段,3.4 μm处C—H伸缩振动峰明显;火焰烟气区主要光谱特征为4～4.5 μm波段范围内高温CO₂发射峰,该区域火焰与空气换热剧烈,温度变化不稳定,火焰脉动频率较高;火焰间歇区的光谱特征是4～4.5 μm波段范围内高温CO₂发射峰,与烟气区相比,火焰间歇区脉动频率相对较低;与烟气区及间歇区相比,火焰连续区燃烧较为稳定,该区域的光谱特征明显,在2.5～3 μm波段范围内炭黑粒子发射光谱强度较高,且在3.4 μm处存在C—H伸缩振动峰,表明油料池火焰光谱3.4 μm处的特征峰由高温油蒸汽产生。油池火焰不同燃烧区域光谱特征分析表明,油池火焰液态油表面的“富燃料层”吸收火焰传热,引起3.4 μm附近油蒸汽分子能级的改变。油池火焰不同燃烧区域发射光谱强度计算表明,火焰连续区的强度最大,其次为间歇区,火焰烟气区与空气对流强烈,测得的发射光谱强度最低。研究结果为火焰—油料传热模型的修正提供了参考。     

Challenges for turbulent combustion

Turbulent combustion will remain central to the next generation of combustion devices that are likely to employ blends of renewable and fossil fuels, transitioning eventually to electrofuels (also referred to as e-fuels, powerfuels, power-to-x, or synthetics). This paper starts by projecting that the decarbonization process is likely to be very slow as guided by history and by the sheer extent of the current network for fossil fuels, and the cost of its replacement. This transition to renewables will be moderated by the advent of cleaner engines that operate on increasingly cleaner fuel blends. A brief outline of recent developments in combustion modes, such as gasoline compression ignition for reciprocating engines and sequential combustion for gas turbines, is presented. The next two sections of the paper identify two essential areas of development for advancing knowledge of turbulent combustion, namely multi-mode or mixed-mode combustion and soot formation. Multi-mode combustion is common in practical devices and spans the entire range of processes from transient ignition to stable combustion and the formation of pollutants. A range of burners developed to study highly turbulent premixed flames and mixed-mode flames, is presented along with samples of data and an outline of outstanding research issues. Soot formation relevant to electrofuels, such as blends of diesel-oxymethylene ethers, hydrogen-methane or ethylene-ammonia, is also discussed. Mechanisms of soot formation, while significantly improved, remain lacking particularly for heavy fuels and their blends. Other important areas of research, such as spray atomization, turbulent dense spray flames, turbulent fires, and the effects of high pressure, are briefly mentioned. The paper concludes by highlighting the continued need for research in these areas of turbulent combustion to bring predictive capabilities to a level of comprehensive fidelity that enables them to become standard reliable tools for the design and monitoring of future combustors.     

Influence of pilot-fuel mixing on the spatio-temporal progression of two-stage autoignition of diesel-sprays in low-reactivity ambient fuel-air mixture

The spatial and temporal locations of autoignition for direct-injection compression-ignition engines depend on fuel chemistry, temperature, pressure, and mixing trajectories in the fuel jets. Dual-fuel systems can provide insight into both fuel-chemistry and physical effects by varying fuel reactivities and engine operating conditions. In this context, the spatial and temporal progression of two-stage autoignition of a diesel-fuel surrogate, n-heptane, in a lean-premixed charge of synthetic natural-gas (NG) and air is imaged in an optically accessible heavy-duty diesel engine. The lean-premixed charge of NG is prepared by fumigation upstream of the engine intake manifold. Optical diagnostics include high-speed (15kfps) cool-flame chemiluminescence-imaging as an indicator of low-temperature heat-release (LTHR) and OH* chemiluminescence-imaging as an indicator high-temperature heat-release (HTHR). NG prolongs the ignition delay of the pilot fuel and increases the combustion duration. Zero-dimensional chemical-kinetics simulations provide further understanding by replicating a Lagrangian perspective for mixtures evolving along streamlines originating either at the fuel nozzle or in the ambient gas, for which the pilot-fuel concentration is either decreasing or increasing, respectively. The zero-dimensional simulations predict that LTHR initiates most likely on the air streamlines before transitioning to HTHR, either on fuel-streamlines or on air-streamlines in regions of near-constant ?. Due to the relatively short pilot-fuel injection-durations, the transient increase in entrainment near the end of injection (entrainment wave) is important for quickly creating auto-ignitable mixtures. To achieve desired combustion characteristics, e.g., multiple ignition-kernels and favorable combustion phasing and location (e.g., for reducing wall heat-transfer or optimizing charge stratification), adjusting injection parameters could tailor mixing trajectories to offset changes in fuel ignition chemistry.