Lean Methane-air Mixtures Research Articles

Experimental study of lube oil pre-ignition in an optical gas engine test rig

Lean-premixed gas combustion is an attractive option for meeting future emission standards toward considerably lower particulate matter and emissions while obtaining an efficiency comparable to diesel combustion. However, ignition processes still pose considerable challenges, with pre-ignition being a major issue. The pre-ignition phenomenon might result from the self-ignition of lube oil in hot zones, making it challenging to observe in real engines. To study single or sparsely separated lube oil droplets under engine-like conditions, the “Flex-OeCoS” experimental test facility was used. To be able to induce such pre-ignitions artificially a novel piezo droplet injector was developed and manufactured, which enabled the metering of minor amounts of lubricating oil or even the injection of single droplets. High-speed cameras and specially developed and partially automatable and adaptive evaluation algorithms were used to record and track the behavior of the injected droplet using overlaid schlieren and OH* chemiluminescence. During the measurement campaigns, relevant operating parameters were varied, and optical investigations were carried out with an ignitable mixture on the Flex-OeCoS test facility under engine-relevant operating conditions. Spontaneous ignition of the vaporized lubricating oil was observed, which subsequently led to ignition of the whole air-fuel mixture in the optical combustion chamber. This was especially evident for lean methane-air mixtures up to an air-fuel ratio of 2.0. With this approach, information could be gathered about the necessary internal engine conditions that lead to pre-ignition in lean mixtures and where their limits lie.

Experimental investigations of the influence of radial gas mixing in an inert ceramic foam bed on thermal combustion of lean methane-air mixtures

The paper discusses the application possibilities of ceramic foam in a thermal combustion process of a lean methane-air mixture. The experiments were done in a ceramic foam bed. The foam (Vukopor ® A) was made mainly of Al 2O 3. The foam samples were packed in a tubular reactor symmetrically placed in a laboratory furnace. It was assumed that the tested foam should have a surface close to the monolith surface area which was tested in a previous work (Pawlaczyk and Gosiewski, 2015). Pore density of the tested foam was 10 PPI. The tested air mixture contained 0.51 - 0.76 vol. % of methane. The results show that thermal methane oxidation in foam is possible in the acceptable range of temperatures. The combustion process in foam is characterized by similar ignition temperature to tests carried out in monolith, a more intense course, and better methane conversion at lower temperatures.

Система совместного использования обедненного метана угольных пластов и биогаза для выработки электроэнергии в угольных шахтах

Purpose: Justification and development of new resource-saving technologies aimed at utilization of coal bed methane. The problem of low efficiency of utilization of lean methane-air mixtures is primarily due to the low content of methane itself, which varies significantly depending on the type of coal or concentration. This paper discusses the concept of a co-production system that mixes low-methane methane-air mixtures and biogas to generate power. Methods: Analysis of the possible volumes of utilized methane, biogas requirements and the suitability of the primary feedstock for co-generation of heat and electricity. Selection of appropriate parameters for the processes of utilization of mine methane. Analysis of the merits of the proposed method of utilization of lean coal mine methane. Results: It is recommended that biogas produced by agriculture and forestry in the vicinity of the mines be added to the dry methane stream to obtain the necessary gas concentration for electricity generation. Potential electricity production and greenhouse gas emission reductions have also been evaluated. The result shows that the co-production system can significantly improve the efficiency of poor methane-air mixtures in coal mines. Practical significance: The joint use of lean methane and biogas from straw offers practical advantages both economically, ensuring sufficient energy supply in mines, and as a foundation for broad future prospects. It allows for energy cost reduction and environmental pollution reduction, such as decreasing carbon dioxide emissions.

Large-eddy simulation of dual-fuel spray ignition at varying levels of methane diluted ambient oxidizer using FGM

In the current work, the Flamelet Generated Manifold (FGM) method is applied with large-eddy simulation (LES) to investigate the effect of methane on dual-fuel (DF) spray ignition. The diesel surrogate n-dodecane is injected as the so-called pilot fuel into selected lean methane–air mixtures, ranging from ϕCH4=0 to ϕCH4=0.75, at engine relevant conditions. The operating conditions are those of the completely characterized Engine Combustion Network (ECN) Spray A configuration, for which the modeling approach adopted in the present study was extensively validated. The specific purpose of this study is to extend and validate the FGM approach for dual-fuel combustion. In order to understand the interplay of chemistry and mixing, the ignition behavior of selected cases is investigated. It is found that both low and high temperature combustion (LTC and HTC, respectively) are increasingly retarded by higher values of ϕCH4, while the induction time between LTC and HTC is relatively insensitive compared to the ignition delay time (IDT). Analysis reveals a more prominent role of mixing for increased ϕCH4. The development of LTC and HTC are quantitatively analyzed for different cases. The transition from LTC to HTC is found to be highly correlated with the evolution of lift-off length (LOL), which on its turn is seriously affected by ϕCH4. The local flame behavior is analyzed via chemical explosive mode analysis (CEMA), suggesting a clear flame propagation due to diffusion towards lean mixtures after the ignition of the pilot fuel. Besides, it is found that diffusion helps to stabilize the flame in leaner mixtures, which is more important in DF combustion. The results show FGM to be a promising tool in modeling the DF sprays.

Characteristics of rotating gliding arc induced thermal ignition of lean methane-air mixtures

The rotating gliding arc discharge (RGA) induced methane-air ignition process under the lean conditions is investigated by the high-speed videography and the numerical simulation, with the focus on the effects of dynamic discharge characteristics. The experimental and numerical results both indicate that the ignition process in the burner can be divided into two stages, including the waiting stage and the rapid flame propagation stage. The effective ignition time (τburner) including both stages decreases nonlinearly as the effective input ignition power (i.e. Pig/Pcomb) increases, following the exponential law, i.e. τburner=2.98 × 10−9(Pig/Pcomb)−7.299+81.75. It is because of that as the input power is relatively low τburner is governed by the duration of the waiting stage to decrease notably with the increment of the input power. However, as the input power is too large τburner becomes determined by the rapid flame propagation stage to be insensitive to the input power. The rotation frequency of the plasma sources can impact the τburner with that too low or too high frequency is detrimental to the ignition. The flame quenching is less likely to occur with the increase of the plasma volume. Based upon the experimental and numerical results, an RGA-induced ignition mechanism is proposed. It indicates that the two-staged ignition process is largely determined by the waiting stage, where multiple flame kernels are frequently initialized by the RGA to be locally quenched or propagate downstream/upstream with flow. Due to the recirculating flow, the energy released by the plasma and the ignited flame kernels can be accumulated upstream to reduce the local Ka number and enhance the flame propagation at the waiting stage. As Ka is lower than a critical value, the flame kernels can propagate rapidly to complete the ignition.

On the catalytic combustion behavior of lean methane-air mixture over platinum catalyst with the navier-stokes and plug-flow models

On the catalytic combustion behavior of lean methane-air mixture over platinum catalyst with the navier-stokes and plug-flow models

Investigation of the impact of NRP discharge frequency on the ignition of a lean methane-air mixture using fully coupled plasma-combustion numerical simulations

The ignition of an atmospheric pressure laminar premixed methane/air mixture by Nanosecond Repetitively Pulsed (NRP) discharges in a pin-pin configuration is studied using fully coupled plasma-combustion numerical simulations. These simulations are performed using the AVIP code specifically developed for low temperature plasma modeling and coupled to the combustion code AVBP. A reduced chemical scheme for plasma-assisted combustion previously derived and validated is used to investigate the effect of the frequency of NRP discharges and the benefits of their chemical enhancement. It is observed that the induced shock wave produced by strong discharges is of major importance for ignition and can lead to quenching of the ignition kernels through strong induced recirculation of gases. Increasing the frequency of the discharges reduces this effect by depositing less energy at each discharge and accumulating energy more homogeneously between the electrodes, leading to a faster and more stable ignition. The minimum energy necessary to ignite decreases with increasing frequency and at the highest studied frequency (100 kHz) ignition has been achieved with 30% less energy than with a single-pulse discharge.

Large-eddy simulation of diesel pilot spray ignition in lean methane-air and methanol-air mixtures at different ambient temperatures

In dual-fuel compression-ignition engines, replacing common fuels such as methane with renewable and widely available fuels such as methanol is desirable. However, a fine-grained understanding of diesel/methanol ignition compared to diesel/methane is lacking. Here, large-eddy simulation (LES) coupled with finite rate chemistry is utilized to study diesel spray-assisted ignition of methane and methanol. A diesel surrogate fuel ( n-dodecane) spray is injected into ambient methane-air or methanol-air mixtures at a fixed lean equivalence ratio [Formula: see text] = 0.5 at various ambient temperatures ([Formula: see text] = 900, 950, 1000 K). The main objectives are to (1) compare the ignition characteristics of diesel/methanol with diesel/methane at different [Formula: see text], (2) explore the relative importance of low-temperature chemistry (LTC) to high-temperature chemistry (HTC), and (3) identify the key differences between oxidation reactions of n-dodecane with methane or methanol. Results from homogeneous reactor calculations as well as 3 + 3 LES are reported. For both DF configurations, increasing [Formula: see text] leads to earlier first- and second-stage ignition. Methanol/ n-dodecane mixture is observed to have a longer ignition delay time (IDT) compared to methane/ n-dodecane, for example ≈ three times longer IDT at [Formula: see text] = 950 K. While the ignition response of methane to [Formula: see text] is systematic and robust, the [Formula: see text] window for n-dodecane/methanol ignition is very narrow and for the investigated conditions, only at 950 K robust ignition is observed. For methanol at [Formula: see text] = 1000 K, the lean ambient mixture autoignites before spray ignition while at [Formula: see text] = 900 K full ignition is not observed after 3 ms, although the first-stage ignition is reported. For methanol, LTC is considerably weaker than for methane and in fully igniting cases, heat release map analysis demonstrates the dominant contribution of HTC to total heat release rate for methanol. Reaction sensitivity analysis shows that stronger consumption of OH radicals by methanol compared to methane leads to the further delay in the spray ignition of n-dodecane/methanol. Finally, a simple and novel approach is developed to estimate IDT in reacting LES using zero-dimensional IDT calculations weighted by residence time from non-reacting LES data.

Thermoacoustic coupling of premixed flames in mesoscale diameter tubes

In the present experimental work, we investigate the thermoacoustic coupling of methane flames propagating inside 4 and 6 mm diameter tubes (slightly larger than the quenching diameter). Multiple tube’s lengths were selected (between 275 and 900 mm) so typical system’s fundamental and harmonic frequencies ranged between 190 and 600 Hz. Experimental results were obtained for stoichiometric and lean methane-air mixtures. The results show that even in such small diameter tubes, open at both ends, thermoacoustic instability still develops and impacts the flame propagation dynamics. We show that depending on the tube length (acoustics) the flame either fully propagates or extinguishes. At 350 Hz (short tubes) both stoichiometric and lean flames propagated over the entire length of the tube in an oscillatory mode, whereas at 190 Hz (long tubes) the flames always quenched within a short distance from the tube’s entrance, even inside the 6 mm diameter tube. These results thus show that the quenching diameter of methane flames can be highly dependent on the tube’s natural acoustics. Moreover, at 250 Hz (medium length tubes), we identified a transition regime where the flame propagation is a stochastic event: combustion, wall-heat transfer and acoustics are in fierce competition in this regime. We have also observed, in tubes with an aspect ratio of 95, a flame shape inversion resembling the tulip flame. In this transition regime, a mode transfer from the fundamental (250 Hz) to subharmonic frequency (125 Hz) of the peak of CH* emission intensity was also observed. This mode transfer is believed to occur when the tulip-flame cusped structure collapses, promoting the mixture of burnt gases with fresh gases, and the consequent increase of the ignition delay and therefore the delay of the peak of CH* emission intensity.

Effects of the discharge frequency on the dielectric barrier discharge ignition behaviors for lean methane–air mixtures at various pressure values

An experimental study on dielectric barrier discharge (DBD) plasma-assisted ignition was performed through the use of a plug-type DBD electrode at various pressures p up to 500 kPa. The ignition behaviors of lean methane–air mixtures with high-frequency DBD were investigated near the lean limit in a constant volume chamber. Using a high-speed video camera, the plasma formations of the DBD were observed for air and a mixture of the equivalence ratio ϕ=0.3. Plasma spectroscopy was also conducted with an echelle grating and imaging spectrometers. The time-averaged detailed plasma emission spectra from 400 nm to 920 nm and the transient spectrograms in the vicinity of the N2 second positive system (372–382 nm) for ϕ=0.5 were measured. The minimum ignition energy (MIE), minimum discharge duration, and minimum amplitude of the applied voltage were obtained through logistic regression. The results indicated that a plasma emission lifetime became much longer than the discharge time with an application of the high-frequency sinusoidal voltage at high pressure. The longest plasma lifetime was observed at a discharge frequency of 1,180 kHz. The plasma lifetime showed the maximum at a specific pressure. The detailed spectroscopy also showed that continuum spectra at 500 kPa could be fitted to the Planck's curve with temperature above 3000 K at 500 kPa. The profiles of the MIE as a function of pressure exhibited the minimum in the vicinity of 100 kPa. The existence of the optimal frequency for high-efficiency ignition was shown. At the fixed frequency, an increase in the applied voltage amplitude resulted in a decrease in the MIE. In contrast, the effects were saturated over a certain threshold. At 500 kPa, time-resolved plasma spectroscopy showed a rapid local gas heating to the flame temperature over a specific frequency of the applied voltage. This indicated that there was the optimal frequency where the MIE was the minimum.

3D modeling framework and investigation of pollutant formation in a condensing gas boiler

Condensing gas boilers exhibit complex global flow-field patterns, which lead to noteworthy inhomogeneities of the burnt gas temperature distribution and levels of local pollutant formation in the combustion chamber. To enable numerical simulations of realistic heating systems, a modeling framework was developed. Challenges like high computational times and the combined multi-physics aspects that occur in quite complex geometries were addressed. The flamelet progress variable (FPV) model was applied, which tabulates detailed reaction kinetics as a function of progress variable and enthalpy. As the larger timescales of CO and NO formation inhibit a direct lookup of local CO and NO mass fractions from the chemistry table, the so called NOMANI model was utilized for NO and a rescaling model was applied for CO. All models were validated against resolved simulation results and experimental measurements. The multi-hole burner was modeled as a porous medium, because it was computationally unfeasible to resolve each burner hole. While local gas acceleration and preheating was considered, the exact flame structure as well as the recirculation zone behind the flame front could not be represented. To still capture the burnt gas temperature, the missing heat losses from the recirculation zone were modeled and coupled to the conjugate heat transfer (CHT) model. The modeling framework was applied to a commercial condensing gas boiler device at nominal load utilizing a lean methane-air mixture. The simulation shows reasonable agreement with experimentally measured emission levels and critical regions with CO and NOx emissions twice as high as in other regions were identified. Temperature variations of 200 K in the flue gas throughout the combustion chamber and 450 K in the solid region of the multi-hole burner were found. Based on these results, design aspects are correlated to CO and NO formation and suggestions on possible design modifications are discussed to further reduce pollutant emissions of such condensing gas boilers.

Thermal effect of hollow spheres in a filtration combustion process

Filtration combustion, or porous media combustion, deals with the partial or total oxidation of a fuel in presence of a solid phase, achieving a more dynamic heat transfer process than conventional technologies. The thermal performance of the solid media has a direct effect on the reaction process, and commonly features a packed bed structure composed of solid spheres. The main objective of this manuscript is to investigate the thermal effect of hollow spheres as an inert solid medium in a filtration combustion process. A heat transfer analysis was developed considering the internal radiation of the hollow spheres and its density reduction in comparison with solid spheres. Then a one-dimensional model based on a two temperatures (gas and solid) approximation was presented using a one-step combustion chemistry for lean methane-air mixtures. Numerical and experimental results with hollow spheres as packed bed are in good agreement and demonstrate higher combustion wave temperatures and propagation rates than solid spheres. Upstream and downstream wave propagations were observed in the range of air–fuel ratios studied. It is shown that hollow spheres as packed bed is a good option for new designs of porous media burners.

Investigation of laminar combustion characteristics of ozonized methane-air mixture in a constant volume combustion bomb

The effect of ozone on the combustion performance of methane-air mixture was studied with a constant volume combustion bomb test bench under different initial pressures and equivalence ratios. The laminar combustion characteristic parameters were obtained according to the flame propagation images. The comparison of the laminar combustion characteristic parameters showed that the O-atom decomposed from ozone can promote the reaction activity of the mixture. Based on the Markstein length, it can be concluded that ozone can increase the flame instability of the mixture, also the flame thickness decreased with ozone addition due to the acceleration of chemical reaction. Both the laminar flame speed and laminar burning velocity increased with ozone addition, and the maximum relative increment of the parameters all appeared at the equivalence ratio of 0.6, which showed the ozone had a more notable enhancement effect for lean methane-air mixture. It was feasible to enhance the combustion performance for natural gas engine using lean burn combustion in principle.

Large-eddy simulation of spray assisted dual-fuel ignition under reactivity-controlled dynamic conditions

Here, a large-eddy simulation and a finite-rate chemistry solver (see Kahila et al. Combustion and Flame, 2019) is utilized to investigate diesel spray assisted ignition of a lean methane-air mixture. A compression heating model is utilized to emulate the ambient temperature and pressure increase in a compression ignition (CI) system. The key parameter is the start of injection (SOI) relative to a virtual top dead center (TDC), where the peak adiabatic compression pressure/temperature would be achieved. Altogether, five different cases are investigated by advancing the SOI further away from the TDC with constant injection duration. The main findings of the paper are as follows: 1) Advancing the SOI advances the ignition timing of the spray with respect to the TDC from 0.91 to 7.08 CAD. However, beyond a critical point, the ignition time starts retarding towards the TDC to 4.46 CAD due to the excessively diluted diesel spray. 2) Advancing the SOI increases the contribution of leaner mixtures to the heat release rate (HRR). Consequently, the low-temperature combustion HRR mode becomes more pronounced (from 33.9% to 76.7%) while the total HRR is reduced by a factor of 4. 3) Ignition is observed for all investigated SOI’s. However, the numerical findings indicate that advancing the SOI decreases the ignition kernel size, resulting in weaker ignition. 4) An ignition index analysis with frozen flow assumption indicates that for the SOI’s close to the TDC the HRR mode appears as spray mixing controlled, while for advanced SOI it becomes reactivity controlled, dominated by fuel stratification.

Emission Characteristics for Swirl Methane–Air Premixed Flames with Ammonia Addition

This paper details the experimental and numerical analysis of a combustion process for atmospheric swirl burners using methane with added ammonia as fuel. The research was carried out for lean methane–air mixtures, which were doped with ammonia up to 5% and preheated up to 473 K. A flow with internal recirculation was induced by burners with different outflow angles from swirling blades, 30° and 50°, where tested equivalence ratio was 0.71. The NO and CO distribution profiles on specified axial positions of the combustor and the overall emission levels at the combustor outlet were measured and compared to a modelled outcome. The highest values of the NO emissions were collected for 5% NH3 and 50° (1950 ppmv), while a reduction to 1585 ppmv was observed at 30°. The doubling of the firing rates from 15 kW up to 30 kW did not have any great influence on the overall emissions. The emission trend lines were not proportional to the raising share of the ammonia in the fuel. 3D numerical tests and a kinetic study with a reactor network showed that the NO outlet concentration for swirl flame depended on the recirculation ratio, residence time, wall temperature, and the mechanism used. Those parameters need to be carefully defined in order to get highly accurate NO predictions—both for 3D simulations and simplified reactor-based models.

Comparison of laser ignition and spark plug by thermodynamic simulation of multi-zone combustion for lean methane-air mixtures in the internal combustion engine

Usage of the laser ignition system can potentially solve most of the combustion issues, for instance, low flame velocity, imperfect combustion, and improve the lean-burn limit. In the present study, the ultimate purpose is to investigate the combustion of methane and lean air mixture in an internal combustion engine by laser ignition. Using quasi-dimensional and multi-zone approach, the engine thermal performance parameters for laser ignition and spark plug have been compared. One of the main results obtained is that the use of laser ignition in comparison with the spark plug for methane and air mixture reduces combustion duration at different equivalence ratios. The use of laser ignition improved thermal efficiency by 8.13%, the average pressure by 10%, specific fuel consumption by 27.44%, and the maximum pressure by 37.83% compared to compression spark plug for an equivalence ratio of 0.7. Also, the result of the use of spark plug for the mixture of methane and air with an equivalence ratio of 0.6 was part of the total mixture remaining unburnt, and as the piston approached the BDC position, the unburned mixture remained in the cylinder, while the usage of laser ignition solved this challenge.

Experimental Study on Combustion Characteristics of Lean Methane-air Mixture in a Combustor Filled with Staggered Alumina Cylinders

ABSTRACT The premixed combustion of lean methane-air mixture in a structured porous combustor filled with alumina cylinders was experimentally studied. The alumina cylinders with a diameter of 6.5 mm and a height of 19.5 mm were regularly staggered placed in the combustor and the average porosity of the packed bed was approximately 0.43. The characteristics of interest were the temperature distribution, the peak combustion temperature, and the combustion wave propagation velocity. The parameters investigated that affect these characteristics included gas inlet velocity and equivalence ratio. Results indicated that the combustion wave propagation was stable and super-adiabatic combustion phenomena were observed under the test conditions of the equivalence ratios in the range of 0.162–0.324 and inlet velocities in the range of 0.43–0.86 m/s. The lean flammability limit was extended to an equivalence ratio of 0.162 at the inlet velocity of 0.43 m/s. The larger equivalence ratio and inlet velocity can supply more fuel to be sufficient combustion, resulting in the higher peak combustion temperature. The combustion wave propagation velocity was inversely correlated with the equivalence ratio. With increasing inlet velocity, the combustion wave velocity increased. The order of magnitude of the combustion wave velocity was 0.1 mm/s. The pressure drop across the structured porous packed bed was remarkably smaller than that of the random one.

Pore scale numerical simulation of heat transfer in propagating thermal wave during filtration combustion of rich and lean methane-air mixtures

The porous media combustion phenomenon was numerically studied with focus on the heat transfer effect in propagating combustion wave for lean and ultra-rich mixture compositions using the approach when the three-dimensional porous structure and interstitial flow are simulated directly at pore scale with explicit consideration of the thermal interaction between fluid and solid phases including the detailed chemical kinetics model and solid-to-solid radiation. The results demonstrate that irregularity of the porous structure leads to a large spatial variation of the process parameters and the flow inhomogeneity. The most intensive heat sources placed within the cavities where the flow is developed. During the process, the interface heat transfer and radiation contribute to heat recuperation mechanism that leads to thermal non-equilibrium in the combustion wave. The heat is transferred through the bed via radiation layer-by-layer due to restricted visibility of the particles. The numerical data about a local variation of the process parameters were presented. The data of this type can be used for modification of the volume-averaged models within the context of spatial variation consideration. It was shown that there is a correlation between the heat release rate, the interface heat flux, the radiative heat flux, and its root mean square values.

Three-dimensional pore-scale numerical simulation of methane-air combustion in inert porous media under the conditions of upstream and downstream combustion wave propagation through the media

Abstracts Porous media combustion of lean and ultra-rich methane-air mixtures was numerically studied using the approach when the process is simulated at the pore scale with explicit consideration of the three-dimensional porous structure and interstitial flow with thermal interaction between fluid and solid phases including the detailed chemical kinetics model and solid-to-solid radiation. The geometrical model of the packed bed was generated synthetically by simulation of particles falling under gravity force. A novel workflow for a randomly packed bed meshing at the pore scale was used to avoid the point contact problem. The results demonstrate that the model predicts a thermal nonequilibrium and heat recuperation mechanism in the combustion wave in the low-velocity regime correctly. The inhomogeneous structure of the porous media leads to large spatial variation of the process parameters such as velocity and temperature. It was shown that radiation makes a significant contribution to heat transfer in the region of a large temperature gradient. Radiative heat flux is transferred through the bed via radiation layer-by-layer due to restricted mutual visibility of the opaque particles. The mixture composition has a significant impact on the heat release rate and wave velocity and direction as well as thermal flame thickness and exhaust gas composition. At the macro scale, the mode of combustion wave propagation is quasi-steady whereas at the pore scale with little time steps the unsteady effects can be observed. Different subregions of the flame front propagate with not equal velocity, stand or accelerate depending on the local flow and porous matrix inhomogeneity. For the lean mixture the oscillatory regime of propagation was detected and analyzed.

A large-eddy simulation study on the influence of diesel pilot spray quantity on methane-air flame initiation

The present study is a continuation of the previous work by Kahila et al. (2019), in which a dual-fuel (DF) ignition process was numerically investigated by modeling liquid diesel-surrogate injection into a lean methane-air mixture in engine relevant conditions. Earlier, the injection duration (tinj) of diesel-surrogate exceeded substantially the characteristic autoignition time scale. Here, such a pilot spray ignition problem is studied at a fixed mass flow rate but with a varying tinj. The focus is on understanding the influence of pilot quantity on spray dilution process and low- and high-temperature chemistry. In total, ten cases are computed with multiple diesel pilot quantities by utilizing a newly developed large-eddy simulation/finite-rate chemistry solver. The baseline spray setup corresponds to the Engine Combustion Network (ECN) Spray A configuration, enabling an extensive validation of the present numerical models and providing a reference case for the DF computations. Additionally, experimental results from a single-cylinder laboratory engine are provided to discuss the ignition characteristics in the context of a real application. The main results of the present study are: (1) reducing tinj introduces excessive dilution of the DF mixture, (2) dilution lowers the reactivity of the DF mixture, leading to delayed high-temperature ignition and slow overall methane consumption, (3) low enough pilot quantity (tinj < 0.3 ms) may lead to very long ignition delay times, (4) cumulative heat release is dominated by low/high-temperature chemistry at low/high tinj values, (5) analysis of the underlying chemistry manifold implies that the sensitivity of ignition chemistry on mixing is time-dependent and connected to the end of injection time, and 6) long ignition delay times at very low tinj values can be decreased by decreasing injection pressure.