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Effect of pre-chamber volume and nozzle number on turbulent jet combustion characteristics of GDI engines
Abstract
There are few studies on the effect of turbulent jets on the combustion characteristics of passive pre-chamber (PC) gasoline engines, and their mechanism needs to be elucidated. Therefore, a 3-D simulation was performed using CONVERGE software. The results reveal that a modest expansion in the PC volume contributes to combustion due to the increased jet energy, but when it is too large, the above effect is weakened. In this study, at a smaller 0.8 ml volume, the combustion is still similar to traditional spark plug ignition. At middle 1.6 ml volume, the mixture concentration in the PC increases and the uniformity is better, resulting in the highest jet velocity and shortest jet duration, thus achieving the fastest flame propagation and the highest heat release rate (HRR). However, at a larger 1.8–2.0 ml volume, the mixture concentration near the spark-plug is sharply reduced, thus the reduced combustion rate in the PC leads to insufficient jet energy in the subsequent jets and resulting deterioration in combustion. An appropriate increase in nozzle number also improves combustion by introducing more ignition sources. But more nozzles not always improving the combustion. In the eight-nozzle case, three nozzles are close to the exhaust valve so that the intake is inhibited, resulting in a worsening of the ignition. The combustion under six-nozzle shows the highest HRR and shortest combustion duration. However, the combustion deteriorates at eight-nozzle, since the increase in the ignition point is not enough to compensate for the negative impact of the reduced jet intensity on combustion.
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A turbulent jet ignition engine enables operation with lean mixtures, decreasing nitrogen oxide (NOX) emissions up to 92%, while the engine efficiency can be increased compared to conventional spark-ignition engines. The geometry of the pre-chamber and engine operating parameters play the most important role in the performance of turbulent jet ignition engines and, therefore, must be optimized. The initial experimental and 3D CFD results of a single-cylinder engine fueled by gasoline were used for the calibration of a 0D/1D simulation model. The 0D/1D simulation model was upgraded to capture the effects of multiple flame propagations, and the evolution of the turbulence level was described by the new K-k-ε turbulence model, which considers the strong turbulent jets occurring in the main chamber. The optimization of the pre-chamber volume, the orifice diameter, the injected fuel mass in the pre-chamber and the spark timing was made over 9 different operating points covering the variation in engine speed and load with the objective of minimizing the fuel consumption while avoiding knock. Two optimization methods using 0D/1D simulations were presented: an individual optimization method for each operating point and a simultaneous optimization method over 9 operating points. It was found that the optimal pre-chamber volume at each operating point was around 5% of the clearance volume, while the favorable orifice diameters depended on engine load, with optimal values around 2.5 mm and 1.2 mm at stoichiometric mixtures and lean mixtures, respectively. Simultaneous optimization of the pre-chamber geometry for all considered operating points resulted in a pre-chamber volume equal to 5.14% of the clearance volume and an orifice diameter of 1.1 mm.
We studied the relationship between pre-chamber jet and main chamber ignition in the pre-chamber combustion (PCC) of an optical engine, fueled with methane and equipped with an active pre-chamber with two rows of orifices. Acetone planar laser-induced fluorescence (PLIF) and OH* chemiluminescence imaging techniques were simultaneously applied to visualize the pre-chamber jet and the reaction zone in the main chamber, respectively. The pre-chamber fueling was constant and the main chamber fueling was increased to form an ultra-lean case and a lean case with global excess air ratios (λ) of 2.3 and 1.8, respectively. Results indicate that a higher pressure difference between pre-chamber and main chamber (▵P) produces larger pre-chamber jet penetration speed; the maximum pre-chamber jet penetration speed appears at timing around the peak ▵P. Over enrichment of the pre-chamber charge reduces the peak ▵P and thus does not favor a faster pre-chamber jet discharge. In addition to the main pre-chamber jet, a weaker post jet discharge process is visualized; the former is due to the pre-chamber combustion while the latter due to the ▵P fluctuation and the cylinder volume expansion. The post pre-chamber jet is accompanied by a post reaction zone in the ultra-lean case (λ=2.3) and there are two unburned regions in the main chamber: one is around the pre-chamber nozzle and the other between the adjacent reaction zones. These two unburned regions are consumed by flame propagation in the lean case (λ=1.8). The weak pre-chamber jet from the upper-row orifice does not produce any distinct reaction zone, indicating that the pre-chamber orifice location and arrangement on the nozzle also matters in the pre-chamber design. The pre-chamber jet penetration length is longer than that of the reaction zone during pre-chamber discharge; the penetration length difference between the pre-chamber jet and reaction zone decreases with increasing main chamber fueling.
The operation of the small size cogeneration gas engine (6 cylinders, 122mm bore, 142mm stroke) modified for unscavenged prechamber ignition was experimentally investigated. The objective was to evaluate the potential to reduce the exhaust gas emissions, particulary the CO emissions, below the Swiss limits (NOx and CO emissions: 250 and 650 mg/m3N, 5 % O2, respectively), without exhaust gas after treatment, through variations of the prechamber geometrical configuration (size, number, distribution and orientation of the nozzle orifices and prechamber internal volume and shape). The results indicate that trends which increase the penetration of the gas jets and/or promote an early arrival of the flame front at the piston top land crevice entrance (small total nozzle orifice cross sectional area, limited number of nozzle orifices, an orientation of the nozzle orifices towards the squish region, relatively large prechamber internal volume) are benificial to reduce the CO and THC emissions. Further, these trends mainly result in a slight increase in fuel conversion efficiency.
The effects and the consequences of transferring the ignition point of the conventional combustion chamber into a small unscavenged prechamber were evaluted through a numerical simulation based on the CFD-code KIVA- 3V. The prechamber flow characteristics at the location of the gap between the spark plug electrodes and in the crank angle period where the ignition is expected to occur were compared to the corresponding conditions in the conventional combustion chamber. The influence of the prechamber geometrical configuration was evaluted through variations of the nozzle orifice diameter, number and orientation, as well as prechamber volume and internal shape. The results show that the velocity magnitude is mainly dependent on the prechamber shape, and that it is generally of the same order as with direct ignition. Further, the turbulence intensity varies strongly with the geometrical configuration and reaches, in most cases, a much higher value in the prechamber. The partial dilution of the unburnt mixture with unscavenged prechamber residual gas generally leads to a slightly lower fuel to air equivalence ratio. However, nozzle orifices imparting a swirl motion or a prechamber shape with an almost uniform cross section can result in fuel concentration very close to or beyond the flammability limit. The prechamber charge and thereby the amount of energy for the main chamber ignition depends on the prechamber volume and on the pressure drop across the nozzle orifices.
The pre-chamber spark ignition system is a promising advanced ignition system adopted for lean burn spark ignition engines as it enables stable combustion and enhances engine efficiency. The performance of the PCSI system is governed by the turbulent flame jet ejected from the pre-chamber, which is influenced by the pre-chamber geometrical parameters and the operating conditions. Hence, the current study aims to understand the effects of pre-chamber volume, nozzle hole diameter, equivalence ratio, and initial chamber pressure on the combustion and flame jet characteristics of hydrogen-air mixture in a passive PCSI system. Pre-chamber with different nozzle hole diameters (1 mm, 2 mm, 3 mm, and 4 mm) and volumes (2%, 4%, and 6% of the engine clearance volume) were selected and manufactured in-house. The experimental investigation of these pre-chamber configurations was carried out in a constant-volume combustion chamber with optical access. The flame development process was captured using a high-speed camera at a rate of 20000 fps, and the images were processed in MATLAB to obtain quantitative data. The combustion characteristics of hydrogen-air mixtures with the PCSI system improved when compared to the conventional SI system; however, the improvement was more significant for ultra-lean mixtures. Early start of combustion and shorter combustion duration were observed for PCSI – D2 and PCSI – D3 configurations, respectively and improved combustion and flame jet characteristics were also noted for these configurations. With the increase in pre-chamber volume, ignition energy associated with the flame jet increases, which reduces the combustion duration and the ignition lag.
Pre-chamber combustion (PCC) has the potential to extend the lean-burn limit in spark-ignition engines, which can promote engine efficiency and relieve the concern of emissions of nitrogen oxides. This work assessed the effects of pre-chamber (PC) and piston geometries on the combustion characteristics of an optical methane PCC engine with both experimental and computational approaches. Five active-type PCs with different volumes and nozzle diameters (12 nozzles distributed evenly in two layers) and two pistons (bowl and flat) were tested under lean-burn conditions. Multi-cycle pressure and heat release profiles, natural flame luminosity images, and OH* chemiluminescence images were measured and employed for CFD modeling validations. The PCs with the smaller nozzle diameter yielded more intensely-reacting jets from the upper layer of nozzles compared to the other PCs, attributed to the stronger gas choke there, which dramatically affected the flow fields. A larger PC volume allowed more air-fuel mixture to be trapped within the PC whose combustion then resulted in faster pressure buildup, which, however, led to higher heat transfer loss. Compared to the bowl piston, the flat piston generated a higher heat release rate during the late combustion period owing to the relatively longer jet propagation within the squish region.
The use of advanced ignition concepts and enhanced combustion strategies is being widely studied for modern engine applications. In this framework, the passive pre-chamber ignition system is gaining popularity for its mechanical simplicity. However, in order to implement this ignition system in passenger car vehicles, the concept must be able to operate in the whole engine map. Numerous experimental investigations in the literature have found that the concept operates suitably at high load/speed conditions but has several problems operating at low engine load/speeds. However, not many investigation conducted to date have focused on comparing and understanding the underlying physics of this ignition strategy in different engine load/speed conditions. Therefore, in this investigation, a numerical study is carried out using a state of the art 3D-CFD model, to analyze the performance of the passive pre-chamber ignition system in three relevant operating conditions of the engine map. A novel methodology is developed to analyze the fundamentals of the passive pre-chamber concept in terms of internal flow field characteristics, combustion and energy conversion. The model is validated with experiments performed previously by the authors. In particular, the low load/speed operating point, representative of idle conditions, was deeply analyzed to asses the issues reported in the literature. Results have shown that the inherent deterioration of the flow field properties inside the pre-chamber as the engine load/speed decreases, compromises the operation at low load/speed conditions. The energy conversion in the pre-chamber is also worsened when operating at lower load/speeds, hindering the generation of suitable jets for igniting the main chamber charge. Moreover, the position of the pre-chamber in the cylinder head has a significant impact on the energy distribution in the main chamber. The results also revealed that a higher amount of non-reacting/cold flow is ejected from the pre-chamber as the spark is pushed from MBT conditions towards the top dead center (TDC), and the gap between the triggering of the spark and the onset of main chamber combustion increases. The findings of this article have allowed to close the knowledge gap between the physical characteristics of the passive pre-chamber concept and the experimental trends found in other researches on this topic.
The effects of narrow-throat pre-chamber geometry on the main chamber combustion were investigated on a heavy-duty optical engine fueled with methane. Simultaneous dual formaldehyde PLIF imaging and OH* chemiluminescence imaging were applied to characterize the early-stage formaldehyde jet and flame jet discharge process. The formaldehyde jet velocity on the jet boundary was quantified by dual formaldehyde PLIF for the first time. Three narrow-throat pre-chambers with two rows of orifices on the nozzle and with different pre-chamber volume and inner throat diameter were studied under the same pre-chamber to main chamber fuel ratio and global excess air ratio of 2.0. The results indicate that the narrow-throat pre-chamber performance is not very sensitive to the pre-chamber volume. The pre-chamber with a larger volume produces only slightly higher pressure buildup in the pre-chamber (ΔP) and similar main chamber combustion phasing and engine efficiency. The inner throat diameter is the key dimension in the narrow-throat pre-chamber design. A larger inner throat diameter produces a smaller peak ΔP and a slower jet penetration, resulting in longer combustion duration and lower engine efficiency. The formaldehyde PLIF shows that the main chamber combustion can be generally classified into two stages: the initial flame ignition and the following flame propagation. The maximum local formaldehyde jet boundary velocity of the narrow-throat pre-chamber with an inner throat diameter of 3.3 mm is up to about 280 m/s and it decreases dramatically when the inner throat diameter is increased to 5.3 mm. The flame jet characteristics of lower and upper orifices can be significantly different due to the local pressure difference between them in the pre-chamber throat. A smaller inner throat diameter results in a larger pressure difference between the lower and upper row orifices and could lead to weak upper-row flame jets. The narrow-throat pre-chamber design with an optimized inner throat diameter can produce highΔP, fast jet penetration, and short combustion duration, which favor the lean limit operation and high engine efficiency.
The pre-chamber combustion concept (PCC) concept shows promises for lean combustion to achieve improved combustion stability and engine efficiency. The KAUST narrow-throat pre-chamber design, which can readily fit into the diesel injector pocket of a heavy-duty engine, has demonstrated an increased lean limit extension compared to conventional pre-chamber designs without a distinct throat. This study examines the effect of pre-chamber volume and nozzle opening area on the PCC concept by employing five different pre-chambers with fixed throat diameter. The engine was fueled with methane, and the combustion characteristics of each pre-chamber were assessed at different operating conditions. A 1-D GT-Power pre-chamber engine model was utilized to estimate the temperature and mixture composition inside the pre-chamber and main chamber. A multi-chamber heat release analysis method was applied to determine the response of the main chamber heat release process with different pre-chamber geometries. Engine-out emissions were also measured to compare the emission performance between the different pre-chambers. It was found that an increased pre-chamber volume promoted earlier ignition in the main chamber, and the throat area was a critical limiting factor in determining the engine performance for the pre-chambers with different nozzle opening areas at a given pre-chamber volume.
Turbulent jet ignition is an advanced pre-chamber approach for improving combustion rates and extending lean burn limitations. The hot flows jetted from the pre-chamber through jet hole act as a distributed ignition source, igniting the mixture in the main chamber. In this paper, the effects of structures on flame propagation and engine performance are systematically studied based on an optical constant volume combustion chamber and a single cylinder engine. Firstly, the visualization of the effects of pre-chamber hole apertures and jet hole numbers on flame propagation is realized. Then, the effects of pre-chamber hole structures on engine performance are studied. The results show that small jet hole aperture could lead to flame quenching while too large aperture may cause low flame speed, both of which will decrease combustion rates. Moreover, the disturbance zone formed by cold jet flow could help maintain the jet flame speed induced by hot jet. Meanwhile, adopting multi jet hole structure could improve combustion in main chamber while modifying jet hole structure considering combustion chamber of real engine helps to achieve lower fuel consumption and emissions. TJI engine has great potential for achieving low fuel consumption and low NOx emissions without severe engine knock when the excess air coefficient ranges from 1.4 to 1.7.
Turbulent jet ignition (TJI) has been proven to effectively improve the burning rate and expand the lean-burn limit of gasoline engines. In this work, based on a single-cylinder engine, injection parameters in the pre-chamber were investigated, combustion characteristics were studied, and pollutant emissions were optimized. Results show that the injection timing in the pre-chamber should be carefully controlled to generate a well-mixed air–fuel mixture and to avoid the diffusion of fuel from the pre-chamber to the main chamber. Therefore, the injection event should be set at the early stage of the compression stroke. Besides, excessive fuel supply in the pre-chamber should be avoided to get optimal engine performance and low pollutant emissions. The lowest indicated specific fuel consumption (ISFC) is achieved when the λ is between 1.5 and 1.6, because lean combustion reduces the heat dissipation but too lean combustion causes more fuel to be burned incompletely. Moreover, to further improve the engine performance and reduce the pollutant emissions, three types of optimized pre-chamber are tested, and results show that with lower pre-chamber volume, better engine performance and lower pollutant emissions are both realized because smaller pre-chamber volume leads to less heat dissipation and less fuel burning around stoichiometric equivalence ratio in the pre-chamber. Furthermore, the pre-chamber with one jet hole is effective to improve the burning rate and to expand the lean-burn limit compared with the 7-hole pre-chamber, and the lean-burn limit of λ is expanded to 2.2 by the optimized pre-chamber, when the minimum NOx of 0.15 g/kWh is achieved.
It is important to understand the lean-burn combustion process of large-bore natural gas engines and influences on it in order to improve next generations of gas engines to meet the increasing requirements for high efficiencies and low emissions. The investigations in this study focus on the ignition and early flame propagation phase using optical experiments on a single-cylinder research engine, since both phases highly influence the subsequent main-combustion. Scavenged prechambers with different operating conditions and a tangential and radial nozzle alignment as well as unscavenged prechamber and direct spark plug ignition are compared. Beside the phenomenology of the ignition system itself, the interaction of the main-chamber charge motion and the ignition system is important to understand. Therefore, different valve timings (conventional timings and Miller cycle) as well as a low and high turbulence setup are subject of the study. Numerical simulations of the cold flow are used to understand the charge motion and mixture formation in the prechamber as well as in the main-chamber. The experiments depict an enhancement of the first flame propagation phase using a scavenged prechamber due to hot turbulent jets emerging from the nozzles. Furthermore, an influence of the nozzle geometry and the boundary conditions on the jet development and on the early flame propagation is observed. It is seen by the optical measurements that cycle-to-cycle variations can originate from the hot turbulent jets and its influence on the ignition of the main-chamber charge. Further, the optical measurements show that a low in-cylinder swirl and turbulence due to Miller cycle has an impact on the interaction between the in-cylinder charge motion and the jets. A comparison between high turbulence and low turbulence in-cylinder flows on the combustion is carried out. It is shown that the scavenged prechamber can compensate the lack of turbulence in the main-chamber. Hence, the scavenged prechamber enhances the flame propagation due to induced turbulence in the main-chamber and larger flame front surfaces generated by penetrating flame torches.
This work investigates the effects of premixed combustion kinematics in pre-chamber volumes on the development of emitted hot jets from the igniter. The effects of fuel type, orifice diameter, and ignition location are evaluated experimentally, with high-speed OH∗ and CH∗ chemiluminescence imaging, and computationally with Large-Eddy Simulations (LES). The imaging experiments allowed for simultaneous viewing of combustion processes within a quartz chamber and of the developing jet flow. Results from these experiments provided insight on the temporal evolution of the jet relative to the growth of an ignited kernel within the chamber, as well as information on the emission or lack of emission of radical species from the chamber. Computational results provided data on the temporal behavior of the pressure within the chamber and profiles of the high velocity flow through the orifice. These results, combined, have shown that dependent on the strain rate and effective orifice size, local quenching of radical species at the orifice occurs which fundamentally change whether hot products, reactive layers, or both are present in the turbulent jet emission. The dynamic structure and composition of the turbulent jet controls its relevance as an effective ignition source.
Use of lean or ultra-lean air-fuel ratios is an efficient and proven strategy to reduce fuel consumption and pollutant emissions. Previous works indicate that lean burn mixtures improves engine thermal efficiency by improving combustion quality, reducing heat transfer losses and increasing the possibility of apply higher compression ratios. However, lower fuel concentration in cylinder hinders mixture ignition, requiring greater energy to start combustion. To favor the ignition process, several high energy providers methods have been studied. Between them, prechamber ignition system presents potential reductions in emission levels and fuel consumption, operating with lean burn mixtures and expressive combustion stability. In this paper, a literature review has been made about prechamber ignition systems as lean combustion technology, focusing in the several investigations regarding combustion and emissions characteristics and presenting the key advantages and challenges in prechamber ignition technology application. From this review can be observed that the pre-chamber ignition system is an important way to increase thermal efficiency, reduce fuel consumption and emissions in spark ignition engines.
The prechamber combustion characteristics were studied using a rapid compression and expansion machine (RCEM) to improve the efficiency of cogeneration natural gas engines. The torch flames generated by a prechamber were used to investigate the effect that a prechamber has on the main combustion. In this study, we focused on observing the correlation between the torch flame and the main flame (which is a so-called “prechamber combustion”) as well as the knocking phenomena for various prechamber configurations.
It has previously been shown by the authors that the pre-chamber ignition technique operating with fuel-rich pre-chamber combustion strategy is a very effective means of extending the lean limit of combustion with excess air in heavy duty natural gas engines in order to improve indicated efficiency and reduce emissions. This article presents a study of the influence of pre-chamber volume and nozzle diameter on the resultant ignition characteristics. The two parameters varied are the ratio of pre-chamber volume to engine's clearance volume and the ratio of total area of connecting nozzle to the pre-chamber volume. Each parameter is varied in 3 steps hence forming a 3 by 3 test matrix. The experiments are performed on a single cylinder 2L engine fitted with a custom made pre-chamber capable of spark ignition, fuel injection and pressure measurement. The measured main and pre-chamber pressure data is then used to perform heat release analysis to understand combustion phenomenon in pre-chamber and the ignition and combustion of fuel-lean charge in main chamber that follows. Within the span of parameter variation, it has been observed that a larger pre-chamber provides higher ignition energy which results in shortened flame development angle and combustion duration. It is also observed that at a given pre-chamber volume, nozzle diameter mainly affects the combustion duration which may be due to difference in penetration depths of pre-chamber jets. The varied parameters seemed to have minor effect on NOx emissions.
Turbulent Jet Ignition is a pre-chamber initiated combustion system that can replace the spark plug in a standard spark ignition engine. The nozzle orifice is critical in a turbulent jet ignition system as it determines the shape and structure of the jet which acts as a distributed ignition source. In this paper, the effect of nozzle diameter and number was studied by performing combustion visualization and characterization for combustion of a premixed propane/air mixture initiated by a turbulent jet ignition system in a Rapid Compression Machine. Color images of the jet ignition process and visualization of the emission of the chemiluminesence of OH* and CH* radicals was performed. Several nozzle configurations were tested which expanded on the limited experimental results that were available in the literature. The performance of the turbulent jet ignition system based on the nozzle orifice diameter was characterized by considering the 0-10% and 10-90% burn durations of the pressure due to combustion. In general it was found that for near stoichiometric air to fuel ratios, a nozzle that produced more spatially distributed jets would result in faster combustion progression. However, at leaner conditions a smaller diameter nozzle that produced a faster and more vigorous jet was required to initiate combustion. The Reynolds number of the discharging jet for the single orifice cases were calculated where it was found that the nozzle diameter increased the Reynolds number, and thus the turbulence for λ=1. The Reynolds number was not found to be sensitive to orifice diameter at the leaner condition of λ=1.25. Further characterization of the Jet development lead to the conclusion that the jet was considered to be in the intermediate flow field, where the transient jet does not have enough time to become fully developed before it reaches the combustion chamber wall.
Precombustion chambers (PCCs) are an ignition technology for large bore, natural gas engines, which can extend the lean operating limit through improved combustion stability. Previous research indicates that the PCC is responsible for a significant portion of engine-out emissions, especially near the lean limit of engine operation. In this work, six concept PCC designs are developed with the objective of reducing engine-out emissions, focusing on oxides of nitrogen (NO x). The design variables include chamber geometry, chamber volume, fuel delivery, nozzle geometry, and material thermal conductivity. The concepts are tested on a single cylinder of a large bore, two-stroke cycle, lean burn, natural gas compressor engine, and the results are compared with stock PCC performance. The pollutants of interest include NO x, carbon monoxide, total hydrocarbons, and volatile organic compounds (VOCs). The results indicate that PCC volume has the largest effect on the overall NO x-CO tradeoff. Multiple nozzles and electronic PCC fuel control were found to enhance main chamber combustion stability, particularly at partial load conditions. The PCC influence on VOCs was insignificant; rather, VOCs were found to be heavily dependent on fuel composition.
A semidecoupling methodology for developing skeletal chemical kinetic models is presented and applied to construct an enhanced skeletal model for PRF (primary reference fuel) oxidation, which consists of 41 species and 124 reactions. The basic idea and the semidecoupling methodology are to consider the oxidation mechanism of alkane as two parts: a comprehensive part to describe detailed reaction processes of C0–C1 radicals and molecules as the ‘core’ and a skeletal part that couples the ‘core’ to control the ignition characteristics. Accounting for the major weakness in the existing skeletal models for PRF oxidation, the enhancement on the new skeletal model mainly focuses on the laminar flame speed and important species evolution while maintaining the precise ignition delay prediction of the previous models. The new PRF skeletal model is validated against various experimental data including shock tube, jet-stirred reactor, flow reactor, premixed laminar flame speed, and internal combustion engines over a wide range of temperatures, pressures, and equivalence ratios. The results show good agreement with the experimental data, indicating that the semidecoupling methodology and the new skeletal model are promising for various reactors and engine applications incorporated with a multidimensional computational fluid dynamics (CFD) model.
Effects of pre-chamber jet flame ignition on performance of a gasoline direct injection engine
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