Heavy-duty Natural Gas Engine Research Articles

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 %.

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%.

Effect of different exhaust parameters on conversion efficiency enhancement of a Pd–Rh three-way catalytic converter for heavy-duty natural gas engines

In this work, the influence of exhaust parameters on the conversion performance of the Pd–Rh three-way catalytic converter (TWC) for heavy-duty natural gas engines is investigated by CFD simulation. A mathematical model adopting an empirical global reaction mechanism for the Pd–Rh TWC is established and validated. The results indicate that the increase of exhaust CO concentration is beneficial to remove the CH4 and NO emissions. As the exhaust CO concentration increases from 0.7 % (stoichiometric condition) to 1.3 % at 623K, the conversion efficiencies of CH4 and NO increase by 8.1 % and 41.8 %, respectively. In GRA, the most important influencing factor on the conversion efficiencies of CH4 and CO is the exhaust O2 concentration. For the NO conversion efficiency, the primary and secondary relationships of the three factors are exhaust CO concentration＞exhaust flow rate＞exhaust O2 concentration. The orthogonal experiment analysis indicates that Case 7 shows better total conversion performance (CH4: 97.6 %; CO: 100 %; NO: 62.2 %) than other cases. Compared with the stoichiometric condition at 623K, the conversion efficiencies of CH4 and NO are improved by 97.8 % and 3.8 %.

Numerical investigation of the effect of swirl number on performance, combustion, and emissions characteristics in a converted heavy-duty natural gas engine

Numerical investigation of the effect of swirl number on performance, combustion, and emissions characteristics in a converted heavy-duty natural gas engine

Fundamental Experiment of Ignition and Flame Development in Turbulent Jet Ignition Ignited Methane Jet Flame

ABSTRACT In heavy-duty natural gas engines, diesel is usually injected before natural gas to initiate methane combustion. However, such an approach requires additional fuel and precise injection strategies. In the present work, an alternative approach of pre-chamber turbulent jet ignition (TJI) ignited methane jet flame is performed experimentally. The underlying mechanism of TJI-HPDI jet combustion is investigated in an optically visualized constant-volume combustion chamber with an active pre-chamber. It is found that with the increase of the injection-ignition delay (ti), the combustion process undergoes several transitions. In addition, the success ignition region and ignition failure region separated by ti and T MI are discerned on the ti-T MI diagram. As ti increases, there is an initial rise in both the mean flame front velocity and peak pressure, followed by a subsequent decrease This behavior highlights the TJI-HPDI system’s notable advantages in expanding the lean combustion limit and enhancing overall combustion characteristics compared to premixed combustion. These advantages can be attributed to the mixture stratification and turbulence. Furthermore, pre-chambers fueled with hydrogen can further improve the intensity and energy of the turbulent hot jet, which further decreases the critical injection-ignition delay to initiate global flame propagation in the TJI-HPDI system. The present work deals with real engineering technologies, and it will provide new insights into the design and control of TJI-HPDI engines, especially for heavy-duty natural gas engines.

Nozzle Design of Plug-and-Play Passive Pre-Chamber Ignition Systems for Natural Gas Engines

To evaluate the significance of the geometrical parameters of a passive pre-chamber on engine performance, this study investigated the design of a plug-and-play passive pre-chamber in a 15 L heavy-duty natural gas engine. Multi-dimensional numerical investigations were conducted for parametric studies involving lateral angle, orifice diameter, and vertical angle. A compressive flow solver was employed for Navier–Stoke equations, coupled with detailed sub-models and a chemical kinetic scheme. The combustion model was calibrated and could well predict the engine combustion and operating performance. Seven pre-chamber schemes were evaluated, and four optimal ones were selected for experimental tests. The characteristics of the scavenging process, turbulent jet ignition, and main-chamber combustion were investigated and analyzed. The results show that, considering the trade-off between the ignition energy and the scavenging efficiency, the ratio of the pre-chamber to clearance volume is recommended to be 0.2~0.7%, and the corresponding area–volume ratio is 0.003~0.006 mm−1. Compared with the original natural gas engine, the pre-chamber retrofit can save up to 13.2% of fuel consumption, which presents a significant improvement in fuel economy.

Numerical investigation on combustion regulation for a stoichiometric heavy-duty natural gas engine with hydrogen addition considering knock limitation

Aiming to further improve the thermal efficiency and reduce NOx emissions in the stoichiometric hydrogen-enriched natural gas (NG) engine, a detailed 3-D simulation model of stoichiometric operation heavy-duty NG engine is built based on the actual boundary conditions from high load bench test. The superimposed methods for knock regulation, combustion and emission control, including Miller valve timing, hydrogen volume fraction and EGR rate were proposed and investigated comprehensively. It reveals that the typically bimodal characteristic of heat release rate (HRR) curve is caused by knock, which seriously restricts the performance improvement of stoichiometric NG engine under high load condition. To predict and control the occurrence of the second peak of HHR accurately, a new parameter BI is defined. Moreover, the Miller timing with 20°CA of the intake valve late closing shows better combustion performance within the knock limit, accompanied by a slight increase in NOx emissions. Additionally, the 5% hydrogen blend, coupled with the Miller cycle, can further enhance the indicated thermal efficiency (ITE) of the NG engine due to the stronger effects on acceleration of laminar flame propagation velocity than the promotion of end-gas auto-ignition. Besides, the great potential of EGR rate for balancing NOx and ITE is also confirmed in the heavy-duty hydrogen-enriched NG engine adopting Miller cycle. Compared to the original indexes, combing with the regulation strategies of intake valve late closing (20°CA), hydrogen addition (5%) and EGR (17%) are proved to increase the indicated thermal efficiency by 1.89% and reduce NOx emissions by 11.47% within the knock limit.

Experimental investigation on particulate filters for heavy-duty natural gas engines: Potentialities toward EURO VII regulation

The urgent need to meet the stringent regulation requirements on sub-23 nm particles emissions is pushing the interest towards efficient strategies for their reduction, involving different propulsive technologies, including the Natural Gas engines.Although considered as particulate matter-free, the growing diffusion of Natural Gas Heavy-Duty engines as a key element in the low-term towards decarbonization, requires their compliance with upcoming regulations.The use of particulate filters, in combination with the Three-Way Catalyst (TWC), could represent a promising and viable solution to achieve high conversion of gas-phase criteria pollutants and high particles filtration efficiency.The present activity arose from a collaboration among research groups of CNR-STEMS, FPT Industrial and NGK Europe GmbH, two industrial companies leaders in the topics here addressed. Target of the work is the evaluation of the potentiality offered by the use of filters in the abatement of particles emitted by a Natural Gas engine. Particles number, mass and size distribution analysis have been performed over the World Harmonized Transient Cycles.The exhaust line was properly designed to foresee the installation of particulate filter downstream a conventional TWC, in a close-coupled configuration. The filtration efficiency of two filters, from hereon termed as CNG Particulate Filters (CPFs), with different wall thickness and cell structures and a filter with catalytic coating, was compared.High particle abatement efficiency was found for all the filters, with values close to 90%, without noticeable increases in backpressures. The CPF with standard porosity showed the best performance, while no further significant benefits were found with the addition of a catalytic coating.The performed analysis places in an important emphasis in view of the forthcoming EURO VII regulations on PN limits (PN10) and sets the basis and direction for further optimization in filter material properties and catalyst coating in meeting stringent PN emission targets.

Numerical investigation of the effect of swirl number on performance, combustion, and emissions characteristics in a converted heavy-duty natural gas engine

Numerical investigation of the effect of swirl number on performance, combustion, and emissions characteristics in a converted heavy-duty natural gas engine

Influence of Intake Port Structure on the Performance of a Spark-Ignited Natural Gas Engine

Spark-ignited natural gas engines have received increasing attention in the heavy-duty market due to their low cost and reliability advantages. However, there are still some issues with natural gas engines retrofitted from 10 to 15 L diesel engines, which is a valuable medium-term goal for the automotive industry. In this work, the effect of intake port structure on the performance of a spark-ignited heavy-duty natural gas engine was investigated by multidimensional numerical simulations. A newly designed intake port was proposed, with strengthened in-cylinder turbulent kinetic energy and homogeneous air-fuel mixtures. Bench tests show that the proposed intake port has impressive thermal efficiency, cycle variation, and acceptable emissions performance. The effective thermal efficiency improves from 41.0% to 41.4%, and the cycle variation is 36% lower than traditional schemes. However, with the accelerated flame propagation, the in-cylinder temperature and NOx emission of the mixed-flow port increase while the CO emission decreases. In summary, a proper balance of in-cylinder swirl and tumble flow can significantly affect the economy and stability of natural gas engines. The proposed structure solves the inherent problems of slow natural gas flame propagation and harmful cyclic variations.

Investigations on combustion system optimization of a heavy-duty natural gas engine

Nowadays, the carbon-free policies and the crisis of crude oil make natural gas get increasing attention. However, spark-ignited natural gas engines continually suffer from problems of thermal efficiency and NOx emissions. Optimizing intake and combustion systems are effective ways to improve combustion and emission performance, but comprehensive work involving both intake and combustion systems is still lacking, especially for heavy-duty commercial engines. In this work, both intake and combustion systems of spark-ignited heavy-duty natural gas engines were investigated using muti-dimensional numerical simulations. Two intake ports and four combustion chambers were considered. An optimized chemical model was employed to accelerate the computation efficiency of combustion processes. The optimal combination of intake and combustion systems was obtained, with impressive thermal efficiency and acceptable emission performance. The results show that the mixed-flow intake port performs better than the swirl intake port for the natural gas engine with premixed combustion mode, manifesting increased in-cylinder tumble ratio and turbulent kinetic energy. Meanwhile, the eccentric hemispherical combustion chamber (EHCC) combined with mixed-flow intake ports presents the best optimal combustion performance, exhibiting an effective thermal efficiency beyond 41.53%. However, the EHCC scheme shows an increased NOx emission due to fast combustion speed and high combustion temperature while negligible CO and CH4 emissions. Despite this, natural gas engines equipped with three-way catalysis after-treatment can still meet emission regulations under stoichiometric operating conditions.

Experimental study on transient emissions characteristics of a heavy duty natural gas engine under world harmonized transient cycle

In this study, the transient emissions characteristics under WHTC for both cases with and without an aftertreatment system (AS) were investigated. The engine operating parameters were measured to reveal the factors influencing the emissions. The effects of engine thermal state (the oil temperature, coolant temperature and exhaust temperature) on emissions and AS were analyzed. The results show that emissions under WHTC are influenced by engine operating conditions and are higher in the cold start test. The CO emissions are sensitive to changes in the excess air coefficient. The NOx emissions are highly consistent with changes in the engine throttle values. The HC emissions are almost dominated by variations of the engine torque at high speeds. In addition, the exhaust temperature has a significant impact on AS performance. The conversion efficiencies of CO, NOx, and HC in the cold start test are respectively 95.8%, 97.5%, and 88.3%, while those of CO, NOx, and HC in the hot start test are respectively 98.4%, 99%, and 99.1%. Overall, the CNG engine with AS can meet the China VI emission limits and even more stringent emission standards in the future. This study provides a reference for exploring methods of emission reduction for CNG engines.

Sub-23 particle control strategies towards Euro VII HD SI natural gas engines

Approaching Euro VII regulation limits for natural gas engines represents an arduous but not impossible challenge.The technological improvement and research progress in propulsion systems can offer strong tools in the achievement of next emission target. The efforts in this field are justified by the interests in natural gas powertrains that represent an alternative solution mainly in case of Heavy-Duty engines for off-road and transport applications. One of the aspects involving the OEMs effort is the particle number emissions abatement, that, due to the next future stringent limits involving sub-23 nm particles, represents a challenge also in case of natural gas fueled engines.The present paper describes the potentiality offered by different strategies in the control of particles emitted by Heavy-Duty natural gas engines. Three approaches will be described and analyzed using experimental characterization carried out by the authors with the use of an engine test bench. Assuming that the major source of particles derives from the lube oil combustion, firstly the effect of improved piston ring pack design has been assessed, comparing two versions of the same engine.Secondly the quality of lubricant oils in particle formation has been evaluated, testing oils with improved chemical/physical properties.A last aspect analyzed was the evaluation of the particulate filter technology capability when applied to a natural gas engine, a novelty in the scientific literature.All the investigated approaches proofed powerful tools in the control of sub-23 particle, paving the way for natural gas engines compliant with Euro VII PN limits.

Rankine cycle waste heat recovery for a heavy-duty natural gas engine meeting China VI emission standards

The China VI natural gas engine is based on the theory of the air–fuel ratio method, leading to a high exhaust temperature, large heat load and difficult cooling. The paper carried out a thermal balance experiment of the China VI natural gas engine under different working conditions. A dual-loop organic Rankine cycle system is designed to reduce the difficulty of cooling system and improve the thermal efficiency. The high-temperature loop uses water as working fluid to recover the heat of engine exhaust gas and recirculation exhaust gas. The low-temperature loop recovers the heat from cooling water and waste heat of the high-temperature loop. The high-temperature and low-temperature loops are coupled through a shared heat exchanger. The results show that the China VI natural gas engine has a higher and more stable waste heat temperature than traditional engines and is more adaptable to waste heat recovery under variable operating conditions. For fixed evaporation temperature, with the increase of inlet temperature of the expander, the net output power of high-temperature loop increases first and then decreases, reaching the maximum value at about 800 K. In the low-temperature loop working fluids R601a, R601b, R600, R600a and R12333zd, The maximum net output power of the waste heat recovery system based on the combination of water and R12333zd is approximately 75.4 kW under the rated condition, which is 22.24% of the rated power of the engine.

Improvement of a Heavy-Duty Natural Gas Engine's Combustion System: Cfd Modeling and Experimental Research

Improvement of a Heavy-Duty Natural Gas Engine's Combustion System: Cfd Modeling and Experimental Research

Combustion and emission characteristics of natural gas engine with partial-catalytic oxidation of the fuel

Combustion, emission characteristics and the economic performance of a heavy-duty natural gas engine blended with different ratios of reformed gas were investigated, and the feasibility of applying the fuel partial oxidation catalytic reforming system on a heavy-duty natural gas engine was discussed. Experimental results showed that the use of Rh/γ-Al2O3 as a catalyst of partial oxidation reaction of natural gas fuel requires low initial reaction inlet gas temperature, and it has good catalytic performance and durability, which makes it feasible to be applied on vehicles. Blending with reformed gas from partial oxidation reaction significantly reduces NOX and CH4 emission and effectively reduces coefficient of variance of peak pressure. In additions, no significant change in fuel consumption was observed after blending with reformed gas, while CO emissions increased. In general, the combination of on-board fuel partial oxidation catalytic reforming system to improve the performance of natural gas engines has good prospects.

Experimental analysis of the EGR rate and temperature impact on combustion and emissions characteristics in a heavy-duty NG engine

Cooled exhaust gas recirculation (EGR) is one of the most effective methods in spark ignition (SI) natural gas (NG) engines to improve the fuel economy and reduce the nitrogen oxides (NOx) emissions. The objective of this study is to anatomize the EGR rate and temperature impact on the combustion and emissions characteristics by the experiments in a heavy duty (HD) NG engine under various spark timings. The results showed that the increasing EGR rate reduced the burning rate and extended the combustion duration. Spark timing should be optimized to copy with the retarded combustion phase. Under high EGR rate, the combustion performance could be optimized with the increase of EGR temperature. There exists a proper EGR rate in real application for the fuel consumption of the engine. Higher EGR temperature plays positive effects on fuel economy, owing to the higher combustion efficiency and lower throttle losses, and the decreasing amplitude of BSFC is 1.25% between the EGR temperatures of 333 K and 373 K. With the increase of EGR rate from 5% to 25%, NOx emissions are reduced by up to 76%, the unburned hydrocarbon (HC) and carbon monoxide (CO) emissions are increased by 34.6% and 12.9% respectively. Higher EGR temperature could alleviate the increase of HC and CO emissions, but sacrificing the NOx emissions, and the increase extent of NOx emissions is 35.3%. The results provide an insight into the quantitative comparisons of EGR rate and temperature on the NG engine performance and emissions, which promotes the utilizing EGR strategy in SI NG engines.

Fundamental insights on ignition and combustion of natural gas in an active fueled pre-chamber spark-ignition system

Pre-chamber spark-ignition (PCSI), either fueled or non-fueled, is a leading concept with the potential to enable diesel-like efficiency in medium-duty (MD) and heavy-duty (HD) natural gas (NG) engines. However, the inadequate scientific base and simulation tools to describe/predict the underlying processes governing PCSI systems is one of the key barriers to market penetration of PCSI for MD/HD NG engines. To this end, experiments were performed in a heavy-duty, optical, single-cylinder engine fitted with an active fueled PCSI module. The spatial and temporal progress of ignition and subsequent combustion of lean-burn natural gas using PCSI system were studied using optical diagnostic imaging and heat release analysis based on main-chamber and pre-chamber pressure measurements.Optical diagnostics involving simultaneous infrared (IR) and high-speed (30 kfps) broadband and filtered OH* chemiluminescence imaging are used to probe the combustion process. Following the early pressure rise in the pre-chamber, IR imaging reveals initial ejection of unburnt fuel-air mixture from the pre-chamber into the main-chamber. Following this, the pre-chamber gas jets exhibit chemical activity in the vicinity of the pre-chamber region followed by a delayed spread in OH* chemiluminescence, as they continue to penetrate further into the main-chamber. The OH* signal progress radially until the pre-chamber jets merge, which sets up the limit to a first stage, jet-momentum driven, mixing-controlled (temperature field) premixed combustion. This is then followed by the subsequent deceleration of the pre-chamber jets, caused by the decrease in the driving pressure difference (ΔP) as well as charge entrainment, resulting in a flame front evolution, where mixing is not the only driver. Chemical-kinetic calculations probe the possibility of flame propagation or sequential auto-ignition in the second stage of combustion. Finally, key phenomenological features are then summarized so as to provide fundamental insights on the complex underlying fluid-mechanical and chemical-kinetic processes that govern the ignition and subsequent combustion of natural gas near lean-limits in high-efficiency lean-burn natural gas engines employing PCSI system.

Combustion sensitivity to the nozzle hole size in an active pre-chamber ultra-lean heavy-duty natural gas engine

Active pre-chamber configurations are considered a valuable solution for improving the operation of the internal combustion engine, in the view to overcome the many challenges it has been made to face. Combining this technology with the use of natural gas, a fuel that has increasing availability and interest in the market share, it is possible to burn ultra-lean mixtures (with air-to-fuel ratios, λ greater than 1.5) delivering reduced emissions and fuel consumption, without compromising efficiency and stability requirements. In this work three pre-chamber nozzles differing for the orifice diameter were tested in a stationary heavy-duty 6-cylinder engine (originally a compression ignition) converted to work with a single cylinder and spark ignition operated. An extensive test matrix was carried to perform spark timing, global lambda and load target sweeps in order to assess the behaviour of the three nozzles with respect to the changing operating conditions. Analysis of in-cylinder pressure traces and heat release rate have allowed to unveil the characteristics of the combustion phasing starting from the pre-chamber to the development in the main chamber, relating these to the performance of the engine in terms of emissions, efficiency and stability.

Experimental investigation of a heavy-duty natural gas engine performance operated at stoichiometric and lean operations

There is a need for decarbonization in power generation and transportation. Natural gas can replace conventional petroleum fuels due to its low carbon-to-hydrogen ratio, especially in existing diesel engines. This paper was based on the hypothesis that “ideal” natural gas engines would operate both stoichiometric and lean, as needed. Therefore, the need to identify the effect of leaning the mixture on several parameters not usually shown in the literature. This paper compared the performance and emissions of a heavy-duty diesel engine converted to natural gas spark ignition operation under stoichiometric (equivalence ratio, ϕ = 1.0) and lean (ϕ = 0.8) operation. The original cylinder head and piston were maintained, and no exhaust gas recirculation was used. The results showed that lean operation decreased peak cylinder pressure and maximum pressure rise rate by 10–15% and 1 bar/°CA, respectively, and increased the volumetric efficiency from ~72% to ~74%. While it also increased the ignition lag, which translated in up to 5°CA delay in the location of peak pressure and crank angle associated with 50% of energy release at the same spark timing and a ~ 5°CA advance in the location of maximum indicated mean effective pressure, it had a negligible effect on the combustion duration due to distinguishing characteristics of gas and flame motion in the bowl-in-piston chamber. Lean operation increased unburned hydrocarbon and nitrogen oxides emissions by up to 15% and 300%, respectively, and carbon monoxide emissions were ~ 20× lower. Lean operation improved indicated thermal efficiency by two percentage points due to a 15% decrease in the heat losses but decreased the exhaust temperature by ~ 50 °C, which would affect the aftertreatment performance. Stoichiometric operation reduced the combustion fluctuations. However, the increased turbulence inside the “fast-burn” bowl-in-piston chamber compensated for the lower natural gas flame speed, with a variation of the indicated mean effective pressure below 2.5% even at ϕ = 0.8. Finally, important differences were observed compared to a traditional spark ignition engine. For example, diesel engine conversation increased nitrogen oxides emissions under lean burn but lowered unburned hydrocarbon and carbon oxide emissions when retarding spark timing from the optimum value. These findings suggest that more studies are needed to better understand the optimization of dedicated natural gas engines converted from diesel, under both lean and stoichiometric operations.