Natural Gas Engine Research Articles

Towards optimized excess air ratio and substitution rate for a dual fuel HPDI engine

The diesel-piloted high-pressure direct-injection (HPDI) natural gas engine is a significant application of natural gas in heavy-duty engines. However, there is limited research focusing on quantitative optimization of the excess air coefficient and natural gas substitution rate of HPDI engines. In order to support the design and development of HPDI engines, this article investigates the effects of excess air coefficient and substitution rate on a dual fuel HPDI engine combustion performance and emission characteristics. Results show that increasing the excess air coefficient results in higher cylinder pressure, but it has minimal effect on the heat release rate. A higher excess air coefficient can also lead to higher nitrogen oxides (NOx) emissions, lower soot emissions, methane (CH4) and total hydrocarbon (THC) emissions significantly decrease at 50% loads (Case2 and Case4), but these increase at 70% loads (Case1 and Case3). Furthermore, increasing the substitution rate markedly reduces greenhouse gas emissions, though it comes with the drawback of heightened combustion instability, and there is a trade-off between engine combustion cycle variations and greenhouse gas emissions as the substitution rate changes. The above research findings will provide optimization guidance for the engineering development of HPDI engines.

Evaluating potential of increasing flow intensity and reducing crevice volume to improve thermal efficiency and hydrocarbon emission in spark-ignition natural gas engines

Low thermal efficiency and high hydrocarbon emissions caused by slow combustion rates and high methane emissions hinder the development of natural gas (NG) engines. Engine structure modifications have always been an important means of improving the combustion effects of engines. This study investigates the effects of combustion chamber improvements on the thermal efficiency and hydrocarbon emissions of stoichiometric spark-ignition NG engines through numerical simulations. The improvements aimed to enhance squish flow, organize high tumble flow, and reduce crevice volume. The results indicate that strengthening the squish intensity by increasing the squish area of the piston can accelerate flame propagation and reduce exhaust loss. However, this improvement is hindered by a significant rise in heat transfer loss, which limits further gains in thermal efficiency. Compared to the traditional high swirl combustion chamber, a high tumble combustion chamber (dual straight intake port combined with a hemispherical piston) can promote flame propagation in the early stages of combustion and increase thermal efficiency by 1.43%. However, the accelerated flame propagation rate resulting from increased flow intensity has little potential to reduce hydrocarbon emissions due to the small contribution of flame quenching to unburned hydrocarbon production. The primary source of hydrocarbon emissions from NG engines is unburned methane in the piston crevice. Therefore, when addressing hydrocarbon emissions from NG engines, the first consideration should be to minimize the volume of the piston crevice, which will significantly reduce methane emissions and incomplete combustion loss.

Effects of critical structure parameters on conversion performance enhancement of a Pd–Rh dual-carrier catalytic converter for heavy-duty natural gas engines

In this work, the effects of structure parameters on the conversion performance of the dual-carrier catalytic converter for heavy-duty natural gas engines are investigated. A three-dimensional numerical model adopting an empirical global reaction mechanism for the Pd–Rh dual-carrier catalytic converter is established. The results show that the conversion efficiency slightly increase with the increase of the carrier cell density ratio, the maximum conversion efficiency differences of CH4, CO and NO are 5.7 %, 8.3 % and 7.9 %, respectively. When the carrier wall thickness decreases from 0.24 mm to 0.10 mm, the conversion efficiencies of CH4, CO and NO slightly increase, and the maximum differences are 5.5 %, 8.0 % and 8.0 %, respectively. Additionally, the fuzzy grey relational analysis (FGRA) is employed to make a comprehensive evaluation. For the conversion efficiencies of CH4 and NO, the primary and secondary relationships of the influence factors are carrier diameter > carrier length > carrier cell density ratio > carrier wall thickness. The orthogonal experiment analysis indicates that case 7 shows the best total conversion performance (CH4: 91.9 %; CO: 99.7 %; NO: 83.9 %) at 673K. Compared with the baseline case 2, the conversion efficiencies of CH4 and NO are increased by 14 % and 19.9 %, while the volume increased by 36.1 %.

Quantitative analysis of the relationship between charge motion and knocking combustion in spark-ignition natural-gas engines under critical knocking conditions

Increasing the in-cylinder flow intensity is considered to have the potential to restrain engine knock and improve thermal efficiency. However, the effect of charge motion on the knocking combustion in natural gas (NG) engines is unclear. Therefore, swirl, tumble, and squish motions of different intensities were organized by changing the shape of the combustion chamber and initializing the in-cylinder flow velocity. Subsequently, the effect of charge motion on the knocking combustion of a spark-ignition NG engine under critical knocking conditions was investigated via numerical simulations. The results indicate that the knock intensity (KI) first increases and then decreases with an increase in swirl and tumble intensities and linearly increases with an increase in squish intensity. Swirl had the smallest impact on KI, followed by tumble, whereas squish had the greatest impact. The differences in flame propagation resulted in differences in KI under different charge motion patterns. The obvious non-uniformity of the radial and axial flame propagation is a potential cause of the significant increase in KI. Therefore, while increasing the flow intensity, matching a suitable combustion chamber to make the flame propagation more uniform helps achieve anti-knock and rapid combustion. Improving the swirl and tumble intensities can help NG engines achieve anti-knock and rapid combustion, whereas improving the squish intensity does not. This study provides an important reference for determining whether increasing flow intensity can improve NG engines’ knock characteristics and combustion performance.

Decelerating catalyst aging of natural gas engines using organic Rankine cycle under road conditions

High exhaust temperature is an intrinsic nature of natural gas engines which underlies power de-rating and thermal aging of after-treatment system; therefore, this study integrates an organic Rankine cycle (ORC) system between engine and it's three-way catalyst (TWC) to address these challenges. ORC facilitates power output enhancement through exhaust energy recovery and alleviates thermal aging by reducing exhaust temperature. To estimate the effectiveness of this hypothesized system, a simulation-based investigation is performed. First, simulation models, including engine, TWC, and vehicle dynamic models, are built and validated by experimental data. According to the temperature characteristics of different TWCs, three scenarios, representing old, current, and prospective TWC technology, are formulated to estimate the ORC performance under Worldwide Harmonized Light Vehicles Test Cycle. Results show that ORC system can substantially alleviate the thermal damage caused by high exhaust temperature and extend TWC lifespan. It is estimated that over 98.5 % of thermal damage can be decreased by proper ORC setting, and the average TWC lifespan extension can be at least 55.4, making a reduced noble metal usage and cost of TWC. Meanwhile, with the decrease of the working temperature of TWC, ORC can recover exhaust energy under more road conditions, further improving the net power and shortening the payback period of extra ORC hardware costs. A reduction in the working temperature of TWC from 770.5 K to 618 K yields a 109 % enhancement in maximum power, coupled with a 62.30 % reduction in the payback period. These findings fully reflect the advantage of ORC-TWC coupling and indicate that ORC is supposed to be used more for the TWC with a low working temperature to maximize economic effectiveness. This study provides a novel pathway for thermal aging alleviation of TWC and a valuable reference for prospective studies on matching ORC with TWC under road conditions.

Performance analysis of combining solid oxide electrolysis cell hydrogen production and marine hydrogen compressed natural gas engine system

Natural gas holds promise as an alternative energy source for ships, and its combustion blended with hydrogen can achieve better beneficial of energy saving and emission reduction. Nevertheless, the current challenges in hydrogen, characterized by its high cost and difficulties in storage and transportation, hinder its application in maritime technology. To address the issue, this study proposes a system that integrates a marine hydrogen compressed natural gas (HCNG) engine, a supercritical carbon dioxide Brayton cycle (SCBC), and a solid oxide electrolysis cell (SOEC). In this system, SCBC is capable of recovering waste heat for electricity generation, while SOEC serves as an energy storage device to adjust engine load and enhance fuel economy. The generated hydrogen can be utilized as a source of hydrogen in HCNG fuel, thereby improving engine performance and reducing pollutant emissions. Various system numerical simulation schemes with different engine hydrogen volume fractions were studied. The results show that the electrical efficiency of SOEC is improved by 17.86% by heating the feed stream with engine exhaust gas. In addition, the hydrogen fraction in which SOEC can act as a complete hydrogen source ranges from 30% to 40%. The results of the sensitivity analysis show that the cycle efficiency reaches a peak of 31.5% at a compressor outlet pressure of 16 MPa, while the efficiency of the whole system reaches a maximum of 50.8% at 21 MPa. The optimal operating temperature for the SOEC falls within the range of 650–700 °C for the 30, 40 HCNG schemes, and between 750 and 800 °C for the 50, 60 HCNG schemes.

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.

Design of an innovative system for hydrogen production by electrolysis using waste heat recovery technology in natural gas engines

This research proposes designing and implementing a system to produce hydrogen, utilizing the thermal energy from the exhaust gases in a natural gas engine. For the construction of the system, a thermoelectric generator was used to convert the thermal energy from the exhaust gases into electrical power and an electrolyzer bank to produce hydrogen. The system was evaluated using a natural gas engine, which operated at a constant speed (2400 rpm) and six load conditions (20 %, 40 %, 60 %, 80 %, and 100 %). The effect of hydrogen on the engine was evaluated with fuel mixtures (NG + 10 % HEF and NG + 15 % HEF). The results demonstrate that the NG + 10 % HEF and NG + 15 % HEF mixtures allow for a decrease of 1.84 % and 2.33 % in BSFC and an increase of 1.88 % and 2.38 % in BTE. Through the NG + 15 % HEF mixture, the engine achieved an energy efficiency of 34.15 % and an exergetic efficiency of 32.84 %. Additionally, the NG + 15 % HEF mixture reduces annual CO, CO2, and HC emissions by 9.52 %, 15.48 %, and 13.39 %, respectively. The addition of hydrogen positively impacts the engine's economic cost, allowing for a decrease of 1.56 % in the cost of useful work and a reduction of 3.32 % in the cost of exergy loss. In general, the proposed system for hydrogen production represents an alternative for utilizing the residual energy from exhaust gases, resulting in better performance parameters, reduced annual pollutant emissions, and lower economic costs.

Design and performance analysis of hybrid electric class 8 heavy-duty regional-haul trucks with a micro-pilot natural gas engine in real-world highway driving conditions

Diesel-fueled heavy-duty (HD) trucks are a key component of global freight transport, but consume 25% of the transportation industry's energy and emitting substantial greenhouse gas (GHG) emissions. Utilizing low-carbon fuels like natural gas (NG), combined with hybridization, presents an excellent opportunity for reducing GHG emissions and fuel consumption. Micro-pilot dual fuel diesel/NG engines offer high engine brake thermal efficiency (BTE) with an efficient three-way catalyst (TWC) to limit pollutant emissions like CO2 without the need for costly exhaust aftertreatment systems. This engine is particularly well-suited for hybridization, as it features high-efficiency under stable, mid-, and high-load conditions, but with reduced performance at low loads. This paper validates two conventional baseline models (X15 Cummins diesel engine and micro-pilot NG engine) based on a tested class 8 regional-haul truck on highway. The study investigates the combined effect of low-carbon fuels and hybridization on improving the fuel economy and GHG emissions of class 8 regional-haul trucks. This involves designing pre-transmission parallel and plug-in hybrid configurations with downsized 6.7-liter micro-pilot NG engines. The cargo load and performance requirements are maintained equivalent to the conventional diesel baseline. Results show comparable fuel economy and engine BTE between conventional diesel baseline and NG models on highways with full cargo load. The parallel and series plug-in hybrid NG designs exhibit fuel economy improvements of 2.9% and 26%, respectively, compared to the micro-pilot NG over highway with full cargo load. In the same conditions, the CO2 emissions reductions for parallel and series plug-in hybrid NG designs are 15% and 51%, respectively, relative to the conventional diesel baseline. In addition, this article provides a detailed comparison and analysis of non-plug-in and plug-in hybrids based on component energy losses and internal combustion engine (ICE) operating points.

Evaluation of the use of ammonia/hydrogen blends in turbocharged gas engines

The potential for ammonia/hydrogen (NH3/H2) blends as fuel in modern, turbocharged, gas engines is evaluated by comparing the combustion properties of these blends with those of methane. To ensure proper combustion phasing in existing spark-ignited (SI) engines, the laminar burning velocity (LBV) is used, while the ignition delay times (IDT) of the mixtures are compared to assess the risk of engine knock. The chemical mechanisms used for the computation of IDT and/or LBV of NH3/H2 mixtures are selected from 8 chemical mechanisms based on their ability to predict experimental data. The application of a turbocharger is shown to “boost” the temperature in the cylinder after compression to values that admit the use of NH3/H2 mixtures in natural-gas SI engines without increasing the compression ratio (CR). The analysis also identifies regions of temperature and H2 fraction that increase the risk of knock. Using the conditions in a practical turbocharged SI engine as an example, 40–50% H2 in NH3 yields combustion phasing equivalent to that obtained with methane, while avoiding the risk of knock. Analysis of the IDT for the conditions in a high-pressure direct-injection natural-gas engine also shows that the use of a turbocharger in compression-ignition (diesel-type) engines strongly reduces the demands on CR for achieving the conditions necessary for reliable ignition of NH3/H2 mixtures. Increasing the temperature by only a few tens of degrees above the conditions studied will yield an IDT for ∼20% H2 in NH3 that satisfies the ignitibility requirements for use in a turbocharged diesel engine.

Effect of Entrainment on the Liquid Film Behavior in Pipe Elbows

Multiphase flow entrainment in natural gas engineering significantly influences the safety and efficiency of oil companies since it affects both the flow and the heat transfer process, but its mechanisms are not fully understood. Additionally, current computational fluid dynamics (CFD) methodologies seldom consider entrainment behavioral changes in pipe elbows. In this article, a verified CFD method is used to study the entrainment behavior, mechanism, and changes in an elbow. The results show that droplet diameter in a developed annular flow follows a negative skewness distribution; as the radial distance (from the wall) increases, the fluctuation in the droplets becomes stronger, and the velocity difference between the gas and the droplets increases linearly. Turbulence bursts and vortices sucking near the wall jointly contribute to droplet entrainment. As the annular flow enters the elbow, the secondary flow promotes the film expansion to the upper and lower parts of the pipe. Droplets re-occur near the elbow exit intrados, and their size is much smaller than those in the upstream pipe. Vortices sucking under low gas velocity play an important role in this process. These findings provide guidelines for safety and flow assurance issues in natural gas production and transportation and bridge the gap between multiphase flow theory and natural gas engineering.

Effect of Variable-Nozzle-Turbocharger-Coupled Exhaust Gas Recirculation on Natural Gas Engine Emissions and Collaborative Optimization

Equivalent combustion natural gas engines typically utilize exhaust gas recirculation (EGR) systems to tackle their high thermal burden and NOx emissions. Variable nozzle turbochargers (VNT) can increase the engine intake and EGR rate simultaneously, resulting in NOx reduction while ensuring robust power performance. Using a VNT along with engine bench testing, the impact of VNT- and EGR-coordinated control on the performance and emissions of equivalent combustion natural gas engines was investigated under different operating conditions. Subsequently, multi-objective optimization was performed using a support vector machine. The results demonstrated that the use of VNTs in equivalent combustion natural gas engines could bolster the capacity to introduce EGR under several operative conditions and extend the scope of EGR regulation, thereby decreasing the engine’s thermal burden, improving fuel efficiency, and curbing emissions. Owing to the implementation of a multi-objective optimization method based on a support vector regression model and NSGA-II genetic algorithm, VNT and EGR control parameters could be optimized to slightly improve the economy and significantly reduce NOx emissions while maintaining the original engine power performance. At 20 operating points optimized for validation, brake-specific fuel consumption (BSFC) and NOx decreased by 0.94% and 47.0%, respectively, and CH4 increased by 3.7%, on average.

Integrated 1D Simulation of Aftertreatment System and Chemistry-Based Multizone RCCI Combustion for Optimal Performance with Methane Oxidation Catalyst

This paper presents a comprehensive investigation into the design of a methane oxidation catalyst aftertreatment system specifically tailored for the Wärtsilä W31DF natural gas engine which has been converted to a reactivity-controlled compression ignition NG/Diesel engine. A GT-Power model was coupled with a predictive physical base chemical kinetic multizone model (MZM) as a combustion object. In this MZM simulation, a set of 54 species and 269 reactions as chemical kinetic mechanism were used for modelling combustion and emissions. Aftertreatment simulations were conducted using a 1D air-path model in the same GT-Power model, integrated with a chemical kinetic model featuring 15 catalytic reactions, based on activation energy and species concentrations from combustion outputs. The latter offered detailed exhaust composition and exhaust thermodynamic data under specific operating conditions, effectively capturing the intricate interactions between the investigated aftertreatment system, combustion, and exhaust composition. Special emphasis was placed on the formation of intermediate hydrocarbons such as C2H4 and C2H6, despite their concentrations being lower than that of CH4. The analysis of catalytic conversion focused on key species, including H2O, CO2, CO, CH4, C2H4, and C2H6, examining their interactions. After consideration of thermal management and pressure drop, a practical choice of a 400 mm long catalyst with a density of 10 cells per cm2 was selected. Investigations of this catalyst’s specification revealed complete CO conversion and a minimum of 89% hydrocarbon conversion efficiency. Integrating the exhaust aftertreatment system into the air path resulted in a reduction in engine-indicated efficiency by up to 2.65% but did not affect in-cylinder combustion.

Thermal efficiency and emissions improvement of the lean burn high compression ratio HPDI NG engine at different combustion modes

High pressure direct injection (HPDI) technology has great potential in achieving high thermal efficiency and is currently one of the most advanced combustion technologies for natural gas (NG) engines. However, with the tightening of emission regulations, it is essential to further explore how HPDI NG engines can limit pollutant emissions while still achieving high thermal efficiency. In this study, based on NG mixture-limited combustion (NMLC) and NG slightly premixed combustion (NSPC) modes, the potential of the change in excess air ratio (EAR) in achieving high indicated thermal efficiency (ITE) and low pollutant emissions at different compression ratios (CR) is systematically investigated by constructing a three-dimensional (3D) model. The conclusion of the investigation shows that: (1) in the NMLC mode, the ITE reaches its maximum of 45.9 % after increasing the EAR to 3.0 at medium CR (CR = 17), and the CO and soot emissions can be kept at lower values. (2) In the NSPC mode, at medium–high CR, an increase in EAR to 2.5 increases the high ITE limit further (up to 47.8 %), while the very low CO and soot emissions are maintained but NOx emissions increased. (3) Overall, in both modes, CO, CO2 and soot emissions can be reduced to a greater extent by adjusting EAR and CR. In comparison, the NMLC mode has greater potential to reduce NOx emissions, while the NSPC mode has more advantages in terms of improving ITE limits and achieving near-zero CO and soot emissions.

Tuning Pd species via electronic metal-support interaction for methane combustion

Utilizing catalytic combustion to convert methane (CH4) into CO2 and H2O stands as one of the most effective approaches for mitigating unburnt CH4 emissions from natural gas engines. Supported Pd catalysts have been extensively researched for their role in low-temperature CH4 combustion, with their catalytic activity greatly influenced by metal-support interactions. Surface interaction Pd phases, as a special type of Pd species originating from metal-support interactions on supported Pd catalysts, show controversial catalytic performance in CH4 combustion. Moreover, the impact of electronic metal-support interactions (EMSI, which refers to metal-support interactions associated with electron transfer) remains unclear. Hence, we opted for Ce-Zr solid solutions with different Ce:Zr molar ratios as supports and synthesized a range of supported Pd catalysts with varying EMSI intensities. Characterization revealed that as the oxygen vacancy concentration on the support increased, electron transfer weakened, leading to a higher Pd-O-Ce content, resulting in a lower CH4 activation barrier and better catalytic performance. This study offers a promising approach for regulating EMSI and active Pd species on supported catalysts in practical applications.

Comparative analysis of different heat transfer models, energy and exergy analysis for hydrogen-enriched internal combustion engine under different operation conditions

The implementation of hydrogen as a fuel in internal combustion engines is widely considered a favorable optimal due to its zero-carbon emissions and high energy content. Heat transfer from engine cylinder walls has a major role on engine combustion, performance and emission characteristics. In this study, heat transfer analysis conducted by different heat transfer models and identify the best model to analyse the energy and exergy analysis of hydrogen enriched internal combustion engine. A series of experiments were conducted on a compressed natural gas internal combustion engine across varying hydrogen fractions, EGR ratios, engine speeds and load conditions to capture in-cylinder pressure data and heat transfer rate. The authors compared six distinct models for heat transfer based on absolute percentage errors of maximum pressure and indicated mean effective pressure. These models systematically incorporated into a quasi-dimensional combustion model (QDCM) on MATLAB for hydrogen-compressed natural gas engine, accounting for diverse operating conditions. Convective heat transfer models, including Woschni, Nusselt, Hohenberg, Han, Eichelberg, and Assanis correlations were employed. Comparative analyses were undertaken to identify the most precise correlation for predicting engine in-cylinder pressure and heat transfer rates with experimental results across a broad spectrum of operational conditions. The Woschni model, as presented, is most suitable for the QDCM of the hydrogen-compressed natural gas fuel blend. This suitability is indicated by the close alignment of Taylor length and turbulent intensity coefficients to 1, as well as the minimal absolute percentage error observed for indicated mean effective pressure. It is also observed that heat transfer rate is increased by increasing the engine load 25 %, 50 %, 75 % and 100 % as 49.57 J/deg, 129.26 J/deg, 245.35 J/deg and 250.62 J/deg respectively and heat transfer rate is decreased by increasing the speed of engine 1100 rpm, 1200 rpm, 1500 rpm and 1700 rpm as 137.34 J/deg, 134.21 J/deg, 95.33 J/deg and 48.57 J/deg respectively. The energy and exergy analyses revealed that elevating the engine load corresponds to a reduction in exergy destruction. Brake energy increased 15 % by increasing the load conditions from 25 % to 100 % and decreased 7 % by increasing the speed from 1100 rpm to 1700 rpm. Another significant finding from this study is that the values of both energy and exergy efficiencies were 38.42 % and 40.12 % respectively obtained by thermodynamic analyses.

A comprehensive phenomenological model for unburned fuel emission simulation applied to natural gas engines

The development of lean-burn concepts for internal combustion engines, utilizing a homogeneous air/fuel charge, aims to improve multiple aspects simultaneously, including thermal efficiency, fuel consumption, nitric oxide, and carbon monoxide emissions. However, the adoption of this technology may result in a notable increase in the emission of unburned hydrocarbons (uHC) compared to stoichiometric engines. The sources of uHC are diverse and their relative significance depends on fuel characteristics, engine operating conditions, and geometric features of the combustion chamber. This concern becomes more pronounced when considering engines fueled with natural gas, as methane has a high global warming potential compared to other pollutant emissions. This research proposes an enhancement to an existing simulation model that describes the primary mechanisms involved in uHC formation, which are filling/emptying of the piston top-land crevice volume and the flame wall quenching, with subsequent uHC post-oxidation. The improvements include the calculation of flame penetration into crevice volumes, the description of uHC production from non-piston crevices, and the introduction of a novel scavenging model. The uHC model is integrated into commercial software (GT-Power) as a “user routine”. Validation of the model is conducted with reference to three large bore engines (W31DF, W31SG, and W46SG) with bore sizes of 31 cm and 46 cm, fueled with natural gas, and operated under lean mixtures (λ > 2), employing different ignition methods (dual fuel mode or pre-chamber device). The uHC model is validated against engine-out measurements, exhibiting an average error of 10.0 % for W31DF, 19.4 % for W31SG, and 14.0 % for W46SG engines, respectively. The reliability of the uHC model is demonstrated by the absence of case-specific adjustments to its tuning constants, even when there are significant variations in engine load and geometric design of the crevice volumes.

Numerical investigation on knock intensity, combustion, and emissions of a heavy-duty natural gas engine with different hydrogen mixing strategies

With the continuous development of hydrogen energy technology, mixing hydrogen has become a potential method to enhance the performance of natural gas engines and reduce emissions. Aiming to offer guidance for optimizing strategies for mixing hydrogen from the perspectives of knock control and thermal efficiency enhancement, a three-dimensional model of a commercial heavy-duty natural gas engine is constructed based on the actual boundary conditions from a high load bench test. In this study, simulations were conducted under conditions where the hydrogen volume fraction ranged from 0% to 40%. The study explored the effects of two injection strategies, port fuel injection of hydrogen-enriched natural gas (PFI) and port fuel injection of natural gas combined with direct injection of hydrogen into the cylinder (PFI + DI), on engine knocking, performance, and emissions. Results indicate that for various injection strategies, there is an increasing trend in knock intensity (KI) with the higher hydrogen mixing ratio. When the hydrogen volume fraction exceeds 20%, the timing of direct hydrogen injection significantly affects KI. This effect is mainly caused by the distribution of unburned hydrogen at the knock onset crank angle (KOCA). Compared to other injection strategies, the PFI + DI strategy when the direct hydrogen injection time at 120°CA BTDC results in a high concentration of hydrogen near the spark plug at the ignition moment, combined with good mixture uniformity in the cylinder. This leads to the shortest CA0-10 and CA10-90, and achieves a maximum indicated thermal efficiency (ITE) of 44.68% under 20% hydrogen volume fraction. Compared to the original natural gas engine, the ITE increased by 2.9% and the NOx emissions increased by 166%, while the HC and CO emissions decreased by 88% and 53%.

Experimental study on the impact of Miller cycle coupled EGR on a natural gas engine

The Miller cycle has significant advantages of suppressing knocking and improving nitrogen oxide (NOX) emissions without the penalties of reducing engine fuel consumption and power output. In the study, the effect of non-Miller cycle and Miller cycle on the exhaust gas recirculation (EGR) introduction capability and the knocking boundary were evaluated on a stoichiometric combustion natural gas engine at three different loads. Additionally, the effect of Miller cycle coupled EGR and ignition timing on engine performance and combustion process was discussed. The results indicate that, for the same condition, the Miller cycle could reduce peak temperature and pressure, increased combustion duration, and delayed combustion phase, which makes for effective knocking suppression. Therefore, the Miller cycle has less dependence on the EGR strategy to control knock. Moreover, the Miller cycle can also reduce the ability to introduce EGR, which could up to 3.8%. By matching the ignition timing and EGR rate, the BSFC of the Miller cycle were decreased by 0.6g/kW·h, 2.4 g/kW·h and 2.9 g/kW·h respectively for three test conditions compared with non-Miller cycle. In terms of emissions, the Miller cycle and EGR can both suppress NOX emissions, while the EGR being more effective. The study will provide valid information for the application of the Miller cycle to achieve efficiency clean combustion of natural gas engine.

Research on Dual Fuel Combustion Characteristics of PFI Natural Gas +DI Gasoline in Large Cylinder Diameter Engine

A 3D simulation model of a single cylinder of a natural gas engine was established. PFI natural gas DI gasoline was used to control the time matching between injection and ignition, so that gasoline could form a small active stratified area near the spark plug. The combustion characteristics under different matching conditions were studied, and finally compared with pure natural gas. It is found that gasoline can be used in large cylinder diameter natural gas engine in a small range, which can accelerate the combustion of natural gas fuel, shorten the time used by CA0-CA5 by 77.6%, shorten the time used by CA5-CA90 by 26.5%, and increase the indicated thermal efficiency of natural gas engine by 1.4%.