Large Bore Natural Gas Engine Research Articles

Effect of Passive Pre-chamber Igniting Position on the Large Bore Natural Gas Engine Combustion Characteristics

Effect of Passive Pre-chamber Igniting Position on the Large Bore Natural Gas Engine Combustion Characteristics

Numerical Investigation of the Operating Characteristics of the Passive and Active Prechamber Jet Ignition in a Natural Gas Engine.

Prechamber jet ignition is a promising technology that enables stable ignition and fast combustion by combining thermal effects, chemical kinetics, and turbulent disturbance. The development and application of the prechamber ignition require a comprehensive and in-depth understanding of the operating characteristics of the prechamber ignition in the real engine working cycle. Therefore, numerical simulations are conducted to explore the operating performance of the prechamber ignition applied to a large-bore natural gas engine in this study. The differences between the passive prechamber (PPRE) and active prechamber (APRE) near the lean burn limit are compared. The results show that the jet ignition performance of the PPRE is hampered by the high residual gas coefficient and lean mixture in the prechamber under lean burn conditions. The ignition mode of the PPRE is similar to torch ignition, and the combustion process in the main chamber is mainly turbulent flame propagation. The ignition mechanism of the APRE is flame jet ignition. The main chamber combustion process presents a two-stage heat release characteristic, which can be subdivided into three phases: the initial flame development phase, the rapid combustion phase, and the late combustion phase. The heat release rate during the initial flame development phase depends on the physical and chemical properties of the prechamber jet and the mixture conditions in the main chamber. During the rapid combustion phase, the flame propagation along the radial direction of the jet largely depends on the time scale of the chemical reaction. The heat release rate depends on the coverage area of the jet, the jet residual momentum, and the turbulent flame speed. During the late combustion phase, the flame propagation is mainly affected by the turbulent flame speed. These results provide theoretical guidance for the subsequent application of prechamber ignition systems in various powertrains.

Analysis of Unburned Methane Emission Mechanisms in Large-Bore Natural Gas Engines With Prechamber Ignition

Abstract Although precombustion chambers, or prechambers, have long been employed for improving large-bore two-stroke natural gas engine ignition and combustion stability, their design predates modern analysis techniques. Employing the latest computational fluid dynamics (CFD) modeling techniques, this study investigates the importance of temperature and chemistry for ignition of the main chamber, with an emphasis on eliminating unburned methane. The sensitivity of the ignition and complete combustion to main chamber air/fuel mixture homogeneity was also explored. This study compares the effect of purely thermal ignition, purely chemical ignition, and how their interplay can influence the complete combustion of methane in typical mixtures and in homogeneous distributions of fuel in the combustion chamber. The CFD results demonstrated that temperature and chemistry are equally important in the ignition mechanism, and combining the two phenomena is effective at igniting the main chamber. Reduction of residual methane in the main combustion chamber (MCC) is most effective when chemical intermediates and thermal ignition are combined. A rudimentary analysis of the effect of fuel/air stratification was also conducted, and it demonstrated that a dramatic reduction in methane emissions is observed for homogeneous mixtures. The flow field in the main combustion chamber was shown to create detrimental stratification of the fuel/air mixture, which inhibited complete combustion of the methane in the main chamber. By contrast, in the extreme case of a perfectly homogeneous distribution of both chemical intermediates and fuel in the combustion chamber, it is possible to completely eliminate unburned methane in the main combustion chamber.

Co-optimization of miller degree and geometric compression ratio of a large-bore natural gas generator engine with novel Knock models and machine learning

Large-bore natural gas reciprocating generator engines will play crucial roles in future low-carbon energy systems. Knock, as an abnormal combustion phenomenon, is a bottleneck for long time to restrict the engine performance improvement. Development and application of the reliable knock model can contribute the predictive engine control and optimization to further improve the engine efficiency and performance. The biggest challenge of developing the predictive knock model is the stochastic nature of the knock phenomenon. To bridge the technological gap, this study aims to develop a novel predictive knock model for a large-bore natural gas engine and showcase its application for co-optimization of the engine design and control parameters, together with the machine learning. The knock model, taking the effects of hot spots heat release rate and energy density into account, is calibrated by the statistical phenomenological knock factor. The 1-D engine simulation model embedded with the knock model is built and calibrated. The data-driven model based on machine learning is developed and used to predict the relation between the key parameters and the engine performance. The genetic algorithms are employed to achieve the multi-objectives global optimization of geometric compression ratio, Miller degree and other control parameters of the engine to reduce the exhaust gas temperature and improve the engine performance. The co-optimized results show that the engine exhaust gas temperature can be remarkably lowered and the indicated thermal efficiency is increased by 1–3% depending on the operational conditions, without degrading the engine power output and increasing the exhaust NOx emissions.

Numerical investigation on knock characteristics and mechanism of large-bore natural gas dual-fuel marine engine

This work revealed the unique knock characteristics and mechanism of large-bore natural gas dual-fuel Marine engine with flame jet and strong swirl by a novel three-dimensional pressure difference method to further reduce the knock tendency. The knock characteristics were investigated by pressure monitoring points distributed in different spaces. The results indicate that, owing to the large cylinder bore, almost no pressure oscillations were found in the center position, whereas, severe pressure oscillations appeared at the edge. The pressure difference results illustrate that the movement of the high-pressure area and the low-pressure area tends to the low-frequency resonance mode (1,0), which manifests as an axial rotation following the swirl motion direction. Furthermore, the pressure difference method combined with the maximum amplitude of pressure oscillation (MAPO) method and energy distribution slices was adopted to explore the knock mechanism. The results show that the large-bore natural gas engine has a special knock mechanism which can neither be attributed to the end-gas self-ignition theory nor completely flame acceleration theory. The knock mechanism is that the flame jets from the prechambers cause the uneven initial pressure distribution, and the uneven pressure distribution is strengthened by the uneven heat release on the flame surface. This knock mechanism was confirmed by changing the pilot fuel mass and equivalence ratio subsequently. By the way, a knock suppression method by reducing the pilot fuel was proposed based on the knock mechanism.

Experimental Evaluation of Volatile Organic Compound Quantification Methods for Reciprocating Natural Gas Engines

Abstract Compressor stations utilizing large-bore natural gas engines transport natural gas through pipelines worldwide. One emission class regulated by the Environmental Protection Agency (EPA) is volatile organic compounds (VOCs), which are nonmethane, nonethane, nonaldehyde hydrocarbons. The combination of a gas chromatograph (GC) and a flame ionization detector (FID) can measure VOCs, following EPA Method 18/25A. The Fourier transform infrared spectrometer (FTIR) also measures VOCs, following EPA Method 320. Multiple VOC calculation techniques are utilized, some combining measurements from separate analyzers. Two basic methods of extracting exhaust gas are direct extraction and Tedlar bag sampling. In this study, various VOC quantification methods are evaluated. Exhaust gas was sampled from a Cooper-Bessemer GMV lean-burn engine and a Caterpillar G3304 rich-burn engine. The GMV was tested in three configurations: open chamber spark ignition, precombustion chamber (PCC) ignition, and PCC ignition with high-pressure fuel injection. Ignition timing sweeps were performed on both engines, and a fuel variability test was performed on the GMV. Results showed that the Gasmet and MKS FTIRs’ (Method 320) VOC measurements deviate significantly from the HP GC when measuring low molar concentrations, albeit below regulatory limits. A common VOC quantification approach is subtracting the sum of methane and ethane FTIR measurements from a total hydrocarbon measurement utilizing a FID. This method produces uncertainties of 190% and overestimates VOC concentration by an average of 100%. The Tedlar bag sampling method produced VOC measurements within −2% of the direct extraction method.

Experimental and numerical investigations of charge motion and combustion in lean-burn natural gas engines

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.

Fundamental Aspects of Jet Ignition for Natural Gas Engines

© 2017 SAE International. Large-bore natural gas engines may use pre-chamber ignition. Despite extensive research in engine environments, the exact nature of the jet, as it exits the pre-chamber orifice, is not thoroughly understood and this leads to uncertainty in the design of such systems. In this work, a specially-designed rig comprising a quartz pre-chamber fit with an orifice and a turbulent flowing mixture outside the pre-chamber was used to study the pre-chamber flame, the jet, and the subsequent premixed flame initiation mechanism by OH* and CH* chemiluminescence. Ethylene and methane were used. The experimental results are supplemented by LES and 0D modelling, providing insights into the mass flow rate evolution at the orifice and into the nature of the fluid there. Both LES and experiment suggest that for large orifice diameters, the flow that exits the orifice is composed of a column of hot products surrounded by an annulus of unburnt pre-chamber fluid. At the interface between these layers, a cylindrical reaction zone is formed that propagates in the main chamber in the axial direction assisted by convection in the jet, but with limited propagation in the cross-stream direction. For small orifice diameters, this cylinder is too thin, and the stretch rates are too high, for a vigorous reaction zone to escape the pre-chamber, making the subsequent ignition more difficult. The methane jet flame is much weaker than the one from ethylene, consistent with the lower flame speed of methane that suggests curvature-induced quenching at the nozzle and by turbulent stretch further downstream. The velocity of the jet is too high for the ambient turbulence to influence the jet, although the latter will affect the probability of initiating the main premixed flame. The experimental and modelling results are consistent with ongoing Direct Numerical Simulations at ETH Zurich.

Investigation of Performance and Emission Characteristics on a Large-Bore Spark-Ignition Natural Gas Engine with Scavenged Prechamber and Miller Cycle Attribute

AbstractAdoption of methane scavenged prechamber has been considered as a promising solution to extend the lean limit of large-bore natural gas engines. In this paper, the effects of prechamber ene...

Prediction of air–fuel ratio control of a large-bore natural gas engine using computational fluid dynamic modeling of reed valve dynamics

Air–fuel ratio control of large-bore, two-stroke, natural gas engines, typically used in the oil and gas field, is critically important to maintain stable operation and emission compliance. Many two-stroke applications rely on reed valves to control air and gas induction, which can involve complicated gas flow behavior; standard gas dynamic relationships are typically insufficient to characterize such behavior. Computational fluid dynamic simulations offer the needed complexity, but even so the computational fluid dynamic models, as shown in this work, must also capture the dynamic behavior of the valves themselves. The current work reports on a computational fluid dynamics–based model representing this type of large-bore, two-stroke, natural gas engine using commercially available computational fluid dynamic software. The engine under study is an AJAX E-565 with rated power of 30 kW (40 HP), a bore of 216 mm (8½″), and a stroke of 254 mm (10″). The large engine geometry makes a relatively large solution domain, hence requiring an intense, time-consuming numerical investigation. This large-bore engine works at a rated speed of 525 RPM with a compression ratio of 6 to 1. Two approaches to modeling the reed valve are investigated: (1) a pressure difference–based user-defined function and (2) a fluid–structure interaction user-defined function. The pressure difference–based user-defined function captures reed valve behavior in a simple, binary fashion (i.e. valves are either open or closed based on the pressure difference between the intake pipe and the engine’s stuffing box). The fluid–structure interaction user-defined function, however, predicts the motion of the reed valve strips based on fluid and body motions; although a more complex solution, the fluid–structure interaction user-defined function accurately predicts the engine’s gas exchange process. In this article, the results of each method are presented and validated to show that the added complexity is necessary to properly predict (and thus eventually improve) the engine’s air–fuel ratio control.

Erratum: “Experimental Examination of Prechamber Heat Release in a Large Bore Natural Gas Engine,” Journal of Engineering for Gas Turbines and Power, 2008, 130(5), p. 052802.

Erratum: “Experimental Examination of Prechamber Heat Release in a Large Bore Natural Gas Engine,” Journal of Engineering for Gas Turbines and Power, 2008, 130(5), p. 052802.

The Effect of Retrofit Technologies on Formaldehyde Emissions from a Large Bore Natural Gas Engine

Formaldehyde is an air toxic that is typically emitted from natural gas-fired internal combustion engines as a product of incomplete combustion. The United States Environmental Protection Agency (EPA) regulates air toxic emissions, including formaldehyde, from stationary reciprocating internal combustion engines. National air toxic standards are required under the 1990 Clean Air Act Amendments. This work investigates the effect that hardware modifications, or retrofit technologies, have on formaldehyde emissions from a large bore natural gas engine. The test engine is a Cooper-Bessemer GMV-4TF two stroke cycle engine with a 14” (35.6 cm) bore and a 14” (35.6 cm) stroke. The impact of modifications to the fuel injection and ignition systems are investigated. Data analysis and discussion is performed with reference to possible formaldehyde formation mechanisms and in-cylinder phenomena. The results show that high pressure fuel injection (HPFI) and precombustion chamber (PCC) ignition significantly reduce formaldehyde emissions

Air Separation Membranes: An Alternative to EGR in Large Bore Natural Gas Engines

Air separation membranes (ASMs) could potentially replace exhaust gas recirculation (EGR) technology in engines due to the proven benefits in NOx reduction but without the drawbacks of EGR. Previous investigations of nitrogen-enriched air (NEA) combustion using nitrogen bottles showed up to 70% NOx reduction with modest 2% nitrogen enrichment. The investigation in this paper was performed with an ASM capable of delivering at least 3.5% NEA to a single-cylinder spark-ignited natural gas engine. Low temperature combustion is one of the pathways to meet the mandatory ultra low NOx emissions levels set by regulatory agencies. In this study, a comparative assessment is made between natural gas combustion in standard air and 2% NEA. Enrichment beyond this level degraded engine performance in terms of power density, brake thermal efficiency (BTE), and unburned hydrocarbon emissions for a given equivalence ratio. The ignition timing was optimized to yield maximum brake torque for standard air and NEA. Subsequently, conventional spark ignition was replaced by laser ignition (LI) to extend lean ignition limit. Both ignition systems were studied under a wide operating range from ψ:1.0 to the lean misfire limit. It was observed that with 2% NEA, for a similar fuel quantity, the equivalence ratio (Ψ) increases by 0.1 relative to standard air conditions. Analysis showed that lean burn operation along with NEA and alternative ignition source, such as LI, could pave the pathway for realizing lower NOx emissions with a slight penalty in BTE.

Experimental Examination of Prechamber Heat Release in a Large Bore Natural Gas Engine

A common solution in reducing NOx emissions to meet new emission regulations has been lean burn combustion. However, with very lean air∕fuel (A∕F) ratios, both carbon monoxide and hydrocarbon emissions become unacceptably high due to the spark misfiring and combustion instabilities. In order to mitigate this, a prechamber ignition system is often used to stabilize combustion at very lean A∕F ratios. In this paper, the heat release in a retrofit prechamber system installed on a large bore natural gas engine is examined. The heat release analysis is based on dynamic pressure measurements both in the main chamber and prechamber. The Woschni correlation is utilized to model heat transfer. Based on heat release modeling and test data analysis, the following observations are made. Main chamber heat release rates are much more rapid for prechamber ignition compared to spark ignition. During combustion in the prechamber, much of the fuel flows into the main chamber unreacted. About 52% of the mass in the prechamber, at ignition, flows into the main chamber during prechamber combustion. Prechamber total heat release, pressure rise, and maximum jet velocity all increase with increasing prechamber equivalence ratio. Prechamber combustion duration and coefficient of variation of peak pressure are minimized at a prechamber equivalence ratio of about 1.09.

Cycle-Resolved NO Measurements in a Two-Stroke Large-Bore Natural Gas Engine

Cycle-resolved NO data were acquired from a Cooper-Bessemer GMV 4TF two-stroke engine to better understand and quantify large bore natural gas engine NOx emission. The cycle resolved NO data were extracted separately from the engine’s cylinder two and four exhaust ports and taken simultaneously with cycle resolved pressure traces, conventional steady-state emission measurements and a variety of additional performance and diagnostic data. The test variables were intake manifold boost pressure, ignition method and ignition timing. Relationships between individual cycle pressure traces and the NO produced by that cycle were investigated. Furthermore, mass-averaged NO values were calculated and integrated in order to compare with average exhaust emissions from a steady-state analyzer and combustion pressure characteristics. The steady measurements revealed that NO and NO2 emissions respond differently to the test variables. The mass averaged cycle-resolved NO values compare well with the steady exhaust emission measurements and exhibit strong correlations with peak pressure and crank angle location of peak pressure.

Mass integration of fast-response NO measurements from a two-stroke large-bore natural gas engine

A thermodynamic two-stroke-cycle engine simulation with a quasi-steady scavenging model was developed and used to mass-integrate cycle-resolved NO measurements made with a fast-response NO analyser. The engine tested was a Cooper-Bessemer GMV-4TF large-bore natural gas engine and the fast NO measurements were made in cylinders 2 and 4 using a Cambustion fNOx400 two-channel fast-response analyser. The engine simulation was deemed to provide a good representation of the cycle-resolved scavenging mass flow. The mass-integrated NO results were compared to Fourier transform infrared (FTIR) steady measurements taken downstream of the cylinders 2 and 4 exhaust-manifold junction. The correlation between the two techniques was linear to within 2 per cent. A strong correlation was exhibited between mass-integrated cycle-to-cycle NO and measured peak and crank angle location of peak combustion pressure. The correlation with peak pressure was slightly better than the location of peak pressure.

Development of the Tracer Gas Method for Large Bore Natural Gas Engines—Part II: Measurement of Scavenging Parameters

In this work the tracer gas method using nitrous oxide as the tracer gas is implemented on a stationary two-stroke cycle, four-cylinder, fuel-injected large-bore natural gas engine. The engine is manufactured by Cooper-Bessemer, model number GMV-4TF. It is representative of the large bore natural gas stationary engine fleet currently in use by the natural gas industry for natural gas compression and power generation. Trapping efficiency measurements are carried out with the tracer gas method at various engine operating conditions, and used to evaluate the scavenging efficiency and trapped A/F ratio. Scavenging efficiency directly affects engine power and trapped A/F ratio has a dramatic impact on pollutant emissions. Engine operating conditions are altered through variations in boost pressure, speed, back pressure, and intake port restriction.

Development of the Tracer Gas Method for Large Bore Natural Gas Engines—Part I: Method Validation

The tracer gas method is investigated as a means to study scavenging in fuel-injected large-bore two-stroke cycle engines. The investigation is performed on a Cooper-Bessemer GMV-4TF natural gas engine, with a 36-cm bore and a 36-cm stroke. Two important parameters are evaluated from the tracer gas measurements, which are scavenging efficiency and trapped A/F ratio. Measurements with the tracer gas method are compared with in-cylinder sampling techniques to evaluate the accuracy of the method. Two different tracers are evaluated, monomethylamine and nitrous oxide. Monomethylamine is investigated because of its common use historically as a tracer gas. Nitrous oxide is a new tracer gas that overcomes many of the difficulties experienced with monomethylamine. The tracer gas method with nitrous oxide is determined to be accurate for evaluating scavenging efficiency and trapped A/F ratio in comparison to the in-cylinder sampling techniques implemented.

Formaldehyde Characterization Utilizing In-Cylinder Sampling in a Large Bore Natural Gas Engine

This research addresses the growing need to better understand the mechanisms through which engine-out formaldehyde is formed in two-stroke cycle large bore natural gas engines. The investigation is performed using a number of different in-cylinder sampling techniques implemented on a Cooper-Bessemer GMV-4TF four-cylinder two-stroke cycle large bore natural gas engine with a 36-cm (14-in.) bore and a 36-cm (14-in.) stroke. The development and application of various in-cylinder sampling techniques is described. Three different types of valves are utilized, (1) a large sample valve for extracting a significant fraction of the cylinder mass, (2) a fast sample valve for crank angle resolution, and (3) check valves. Formaldehyde in-cylinder sampling data are presented that show formaldehyde mole fractions at different times during the engine cycle and at different locations in the engine cylinder. The test results indicate that the latter part of the expansion process is a critical time for engine-out formaldehyde formation. The data show that significant levels of formaldehyde form during piston and end-gas compression. Additionally, formaldehyde is measured during the combustion process at mole fractions five to ten times higher than engine-out formaldehyde mole fractions. Formaldehyde is nearly completely destroyed during the final part of the combustion process. The test results provide insights that advance the current understanding and help direct future work on formaldehyde formation.

Formaldehyde Formation in Large Bore Natural Gas Engines Part 1: Formation Mechanisms

Recent testing of exhaust emissions from large bore natural gas engines has indicated that formaldehyde CH2O is present in amounts that are significant relative to hazardous air pollutant standards. In consequence, a detailed literature review has been carried out at Colorado State University to assess the current state of knowledge about formaldehyde formation mechanisms and evaluate its applicability to gas engines. In this paper the following topics from that review, which bear directly on formaldehyde formation in natural gas engines, are discussed: (1) post combustion equilibrium concentrations; (2) chemical kinetics; (3) flame propagation and structure; (4) partial oxidation possibilities; and (5) potential paths for engine out formaldehyde. Relevant data taken from the literature on equilibrium concentrations and in-flame temperatures and concentrations are presented in graphical form. A map of possible paths for engine out formaldehyde is used to summarize results of the review, and conclusions relative to formation and destruction mechanisms are presented. [S0742-4795(00)00904-2]