Natural Gas Engine Combustion 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

Products of catalytic oxidative coupling of methane to improve thermal efficiency in natural gas engines

Pretreating natural gas using catalytic oxidative coupling of methane (OCM) produces ethylene and ethane, both species that increase fuel reactivity, thus increasing the potential to expand the operability range of highly efficient compression ignition combustion in natural gas engines. This paper presents the first experimental results on the impact of OCM product species on engine thermal efficiency and operability range. In the work, a benchtop experiment was conducted to generate a product species distribution from OCM over a Sr/La2O3 catalyst. A computational study using known chemical mechanisms was then employed to investigate the laminar flame speed and ignition delay of the OCM-modified fuel. Finally, engine experiments in both spark-ignition and compression-ignition combustion modes were carried out using a variable compression ratio single-cylinder engine. Results from the benchtop catalyst experiments showed that practical fuel conversion for single-pass OCM resulted in 18 % methane conversion, 60 % C2 selectivity, and 10.8 % C2 yield at a molar C/O ratio of 6. After determining realistic fuel blending ratios for engine operation, the numerical simulation results showed that fuel reactivity and ignition delay improved compared to with methane alone, while laminar flame speed decreased due to higher dilution from the presence of inert OCM products. Engine experimental results confirmed that OCM products have an advantage in CI mode due to reduced ignition delay time. The CI operating range was widely expanded, and approximately 9.9% thermal efficiency gain was achieved. By contrast, efficiency in SI mode was reduced when using OCM products due to an increase in combustion duration and retarded combustion phasing.

The potential in efficiency improvement of in-cylinder thermochemical fuel reforming in natural gas engines

With decarbonization becoming the primary focus of the automotive industry, high-efficiency and low-emission engine technologies have garnered increasing attention. Among the advanced technologies, fuel reforming is a promising way to reduce emissions and increase efficiency with hydrogen generated onboard. This paper focuses on the potential of improving efficiency using in-cylinder thermochemical fuel reforming combustion in natural gas engines. The experimental results revealed that combustion in the reforming cylinder inevitably deteriorates, and hydrogen generated by fuel reforming could not counteract the slower combustion due to fuel enrichment. However, the presence of hydrogen in the nonreforming cylinder significantly increased the flame propagation speed, hence improving combustion efficiency. After a series of variable-parameter tests, it was found that the appropriate reforming equivalence ratio was 1.15 at low load and ∼1.2–1.3 at medium to high loads, with efficiency increased up to 4–5% and 8–12%, respectively. Dual-fuel reforming combustion experiments were conducted using a rapid compression machine and a single-cylinder engine to further explore the fuel reforming combustion. The results suggested that, at medium load, the combustion instability in the reforming cylinder can be solved by adopting two-stage combustion at an equivalence ratio of 1.3, and the indicated thermal efficiency could reach 43%. Based on the research results, a remarkable potential improvement in efficiency can be achieved if reforming combustion is successfully implemented. Dual-fuel reforming has the most potential to achieve an even higher engine efficiency, and its practical issues such as transient engine control and soot formation deserve further investigation.

Mechanism study of natural gas pre-ignition induced by the auto-ignition of lubricating oil

The lubricating oil auto-ignition induced pre-ignition (LOAP) is the most threatening abnormal combustion in natural gas engines. To reveal the mechanisms of the occurrence and the suppressing methods of abnormal combustion in natural gas/diesel marine engines, influence mechanisms of thermodynamic condition, reagent gas composition and the volume of oil droplet on abnormal combustion were investigated by a newly developed visual rapid compression machine (RCM). Experimental results indicate that the auto-ignition of in-cylinder suspended oil droplet can cause the fuel–air pre-ignition. Furthermore, the heated burning residuals of oil droplet can glow to become the ignition source which can induce the pre-ignition. Two factors, total combustion duration and maximum pressure rise rate (PRRmax), are introduced. Shorter total combustion duration and higher PRRmax represent a stronger occurrence tendency of LOAP in the dual-fuel engine. Under the range of reagent gas composition and thermodynamic condition in the dual-fuel engine, rising equivalence ratio (ϕ) and reducing methane number (MN) of natural gas can reduce the total combustion duration by 57% and 41% respectively, and increase PRRmax by about 30 times and 1.9 times. Rising compression temperature and pressure shortens the total combustion duration by 40.7% and 40.1% respectively and increases PRRmax by about 1.3 times and 0.8 times. Reducing oil droplet volume only slightly advances its ignition moment, and has little effect on subsequent natural gas pre-ignition. Based on the result, the most influential factors on the promotion extent of pre-ignition are ranked as: ϕ, MN, temperature, pressure, droplet volume (strongest: reagent gases, moderate: thermodynamic conditions, weakest: droplet volume). Therefore, to prevent the occurrence of LOAP, the existence of in-cylinder local rich natural gas region should be avoided first. Then the high-MN natural gas ought to be applied. The compression temperature and pressure need to be reduced moderately on the premise of ensuring thermal efficiency. Moreover, the small volume oil mist should be avoided to enter into cylinder by optimizing oil injection strategies and scavenging port structures. These findings are beneficial to prevent abnormal combustions and further improve engine power performance.

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

Lean-Burn Natural Gas Engines: Challenges and Concepts for an Efficient Exhaust Gas Aftertreatment System

High engine efficiency, comparably low pollutant emissions, and advantageous carbon dioxide emissions make lean-burn natural gas engines an attractive alternative compared to conventional diesel or gasoline engines. However, incomplete combustion in natural gas engines results in emission of small amounts of methane, which has a strong global warming potential and consequently makes an efficient exhaust gas aftertreatment system imperative. Palladium-based catalysts are considered as most effective in low temperature methane conversion, but they suffer from inhibition by the combustion product water and from poisoning by sulfur species that are typically present in the gas stream. Rational design of the catalytic converter combined with recent advances in catalyst operation and process control, particularly short rich periods for catalyst regeneration, allow optimism that these hurdles can be overcome. The availability of a durable and highly efficient exhaust gas aftertreatment system can promote the widespread use of lean-burn natural gas engines, which could be a key step towards reducing mankind’s carbon footprint.

High-Efficiency and Clean Combustion Natural Gas Engines for Vehicles

Natural gas engines have become increasingly important in transportation applications, especially in the commercial vehicle sector. With increasing demand for high efficiency and low emissions, new technologies must be explored to overcome the performance limitations of natural gas engines such as limits on lean or dilute combustion, unstable combustion, low burning velocity, and high emissions of CH4 and NOx. This paper reviews the progress of research on natural gas engines over recent decades, concentrating on ignition and combustion systems, mixture preparation, the development of different combustion modes, and after-treatment strategies. First, the features, advantages, and disadvantages of natural gas engines are introduced, following which the development of advanced ignition systems, organization of highly turbulent flows, and the preparation of high-reactivity mixtures in spark ignition engines are discussed with a focus on pre-chamber jet ignition, combustion chamber design, and H2-enriched natural gas combustion. Third, the progress in natural gas dual-fuel engines is highlighted, including the exploration of new combustion modes, the development of novel pilot fuels, and the optimization of combustion control strategies. The fourth section discusses after-treatment systems for natural gas engines operating in different combustion modes. Finally, conclusions and future trends in the development of high-efficiency and clean combustion in natural gas engines are summarized.

Experimental study of pre-chamber jet ignition in a rapid compression machine and single-cylinder natural gas engine

Pre-chamber jet ignition is a promising combustion technology to achieve fast combustion in natural gas engines. First, the ignition and combustion characteristics of mixtures in a pre-chamber system with different diameter orifices were studied under engine-relevant pressures and temperatures in a rapid compression machine. The tested fuels, CH4/air stoichiometric mixtures, were diluted by different proportions of CO2/N2 to simulate the corresponding exhaust gas recirculation conditions in engines. High-speed photography was applied to visualize the jet ignition and combustion processes. The experimental results revealed that two ignition patterns existed in the pre-chambers depending on the diameter of orifices. Pre-chamber jet flame ignition pattern appeared when the orifice diameter of the pre-chamber exceeded a critical value, which produced jet flame and ignited the mixtures in the main chamber directly. Pre-chamber jet auto-ignition pattern produced jet which promoted the auto-ignition of mixture in the main chamber when the orifice diameter was smaller and presented much shorter combustion durations. Based on the experimental results in the rapid compression machine, a practical pre-chamber system was designed in a single-cylinder natural gas engine to investigate the combustion performance and emission characteristics. The experimental results indicated appropriate 0.8%–1.4% increases of indicated thermal efficiency were achieved by pre-chamber jet ignition due to the higher combustion efficiency and shorter combustion duration compared to conventional spark ignition. Lower total hydrocarbon and CO emissions but higher NO x emissions were produced by pre-chamber jet ignition due to the faster burning velocity and higher combustion temperature.

Investigation of evaporation and auto-ignition of isolated lubricating oil droplets in natural gas engine in-cylinder conditions

The auto-ignition of isolated lubricating oil droplets in cylinder conditions is one of main causes of abnormal combustion of natural gas engines. The evaporation behavior of less volatile lubricating oil droplets is the key parameter controlling the auto-ignition event. The suspended droplet technique was used and a fully transient multi-component model was developed to study the evaporation and auto-ignition behaviors of oil droplets. The transient model is closed against mass, species, and energy conservation, and the global one-step chemistry is used to estimate the chemical reaction source term. The boundary conditions at the gas-liquid interface are based on mass, species, and energy flux continuity, as well as the fugacity equilibrium. The model predictions were validated against experimental results, including the evaporation rate of binary-component droplets (n-heptane/n-decane), as well as the overall ignition delay of single-component (n-dodecane) and base oil droplets. The results show that the overall ignition delay of based oil droplets is very sensitive to the initial size and ambient temperature. The ignitable diameter limit is decreased with an increase in ambient pressure and temperature, and the ignitable temperature limit decreases as the ambient pressure increases. The dependence of ignition delay on pressure is determined by the ambient temperature. The ignition radius is reduced as the ambient temperature, pressure, and the initial droplet size increase. The involved methane concentration in natural gas engine in-cylinder conditions shows moderate effects on the ignition delay time of oil droplets depending on the initial size.

Potential of direct-injection for the improvement of homogeneous-charge combustion in spark-ignition natural gas engines

The direct-injection has been considered as a way to improve the performance of conventional spark-ignition natural gas engines with the port-fuel-injection by increasing the engine volumetric efficiency. However, insufficient attention has been paid to the potential of direct-injection for the combustion and thermal efficiency improvement of homogeneous-charge combustion in natural gas engines that can be achieved by the spray-induced turbulence enhancement of the in-cylinder fuel–air mixture. The current study investigates the engine thermal efficiency, combustion speed, combustion stability and hydrocarbon emissions of a direct-injection natural gas engine under varied injection timings, and compares the combustion characteristics of the direct-injection with those of port-fuel-injection. Then, a particular engine operation scheme (port-fuel-injection of natural gas and direct-injection of nitrogen) is applied to evaluate the potential of spray-induced turbulence on the thermal efficiency improvement in the homogeneous-charge combustion mode. The results showed that the direct-injection could improve the engine combustion speed, combustion stability and thermal efficiency in low-load conditions as a result of enhanced in-cylinder turbulence at retarded injection timings. Meanwhile, the direct-injection decreased the engine thermal efficiency in high-load conditions and increased the hydrocarbon emissions regardless of the engine load condition due to the deteriorated mixture homogeneity. When the in-cylinder turbulence and mixture homogeneity were both achieved, a few percent of thermal efficiency improvement was obtained by the direct-injection, and the improvement rate increased as the engine load decreased.

Effects of oil and water contamination on natural gas engine combustion processes

Abundant availability and potential for lower emissions are drivers for increased utilization of natural gas in automotive engines for transportation applications. A novel bimodal engine has been developed that allows on-board refueling of natural gas by utilizing the engine as a compressor. Engine compression however, results in altering the initial state of the natural gas. Increase in temperature and addition of oil are two key effects attributed to the onboard refueling process. A secondary effect is the presence of water in the natural gas supply line. This study investigates the effect of upstream conditions of natural gas on three parameters - autoignition temperature, ignition delay, and laminar flame speed. These parameters play key roles in the engine combustion process. Parametric studies are conducted by varying the initial mixture temperature, water, and oil content in the fuel. The studies utilize numerical simulations conducted with detailed chemistry for natural gas with n-heptane used as a surrogate for oil. Water addition to natural gas at 1–5% by volume did not result in any major changes in the combustion processes, other than a slight reduction in laminar flame speeds. Oil addition of 1–5% by volume reduced autoignition temperature by 5–10% and ignition delay by 27–95% depending on the initial temperature. Sensitivity analysis showed that this was likely due to decrease in the sensitivity of two recombination reactions with oil addition. Evolution profiles of key radical species also showed increasing mole fraction of the hydroperoxy radical at lower temperature that likely aids in reducing the ignition delay. Oil addition resulted in a relatively small increase in the laminar flame speed of 1.7% along with an increase in the adiabatic flame temperature. These results help inform the combustion process and performance to be expected from the bimodal engine.

An experimental and numerical study of the evaporation and pyrolysis characteristics of lubricating oil droplets in the natural gas engine conditions

The auto-ignition of lubricating oil droplets in cylinder has been confirmed to be one of the major reasons of abnormal combustion in natural gas engines. A serial of fundamental experiments were carried out to elucidate the lubricating oil evaporation and pyrolysis behaviors under different ambient temperatures. An enhanced multi-component evaporation and pyrolysis model for lubricating oil droplets was presented. This model takes into account all key processes during the droplet lifetime, including the finite heat conduction and mass diffusion in the liquid phase, multi-component diffusion in the gas phase, as well as the pyrolysis and polymerization of high molecular-weight hydrocarbons in high temperatures. By comparing with the measurements of the droplet behaviors of lubricating oil and heavy oil, the present model shows good predictions of the droplet behaviors. Furthermore, this model was used to discuss the detailed behaviors of lubricating oil droplets in natural gas engine conditions. It is found that the lubricating oil droplets have significantly different behaviors from those of diesel and gasoline. They show slight pyrolysis and polymerization behaviors due to the high molecular-weight components. The heating period is crucial for the lubricating oil droplets, and the highest ratio of the heating period to the total droplet lifetime is up to 80%. The smaller droplet takes shorter time to get the high concentration of oil vapor surrounding the droplet surface than the larger droplet.

Effects of Natural Gas Compositions on Engine In-Cylinder Process

Natural gas (NG) is one of the most promising alternative fuels of diesel and petrol because of its economics and environmental protection. Generally the NG engine share the similar structure profile with diesel or petrol engine but the combustion characteristics of NG is varied from the fuels, so the investigation of NG engine combustion process receive more attentions from the researchers. In this paper, a zero-dimensional model on the basis of Vibe function is built in the MATLAB/SIMULINK environment. The model provides the prediction of combustion process in natural gas engines, which has been verified by the experimental data in the NG test bed. Furthermore, the influence of NG composition on engine performance is investigated, in which the in-cylinder maximum pressure and temperature and mean indicated pressure are compared using different type NG. It is shown in the results that NG with higher composition of methane results in lower maximum temperature and mean indicated pressure as well as higher maximum pressure.

Experimental Study on Comparison of Flame Propagation Velocity for the Performance Improvement of Natural Gas Engine

Natural gas possesses several characteristics that make it desirable as an engine fuel; 1)lower production cost, 2)abundant commodity and 3)cleaner energy source than gasoline. Due to the physics characteristics of natural gas, the volumetric efficiency and flame speed of a natural gas engine are lower than those of a gasoline engine, which results in a power loss of <TEX>$10-20{\%}$</TEX> when compared to a normal gasoline engine. This paper describes the results of a research to improve the performance of a natural gas engine through the modification and controls of compression ratio, air/fuel ratio, spark advance and supercharging and method of measuring flame propagation velocity. It emphasizes how to improve the power characteristics of a natural gas engine. Combustion characteristics are also studied using an ion probe. The ion probe is applied to measure flame speed of gasoline and methane fuels to confirm the performance improvement of natural gas engine combustion characteristics.

Early Stages of Combustion in Internal Combustion Engines Using Linked CFD and Chemical Kinetics Computations and its Application to Natural Gas Burning Engines

The paper describes a numerical model for the early stages of combustion in Natural Gas engines using linked CFD and detailed chemical kinetics. The importance of such a combustion device stems from the characteristics of natural gas which make it an attractive near-term olution for the automotive emissions problem. The 3-D simulation, which incorporates a chemical model, turbulence model and ignition model is taken under engine-like conditions. This is achieved by coupling the numerical codes KIVA II, developed to solve the transient equations of conservation of turbulent chemically reacting mixture of ideal gases, CHEMKIN II, designed to facilitate simulations of elementary chemical reactions in flowing systems, TRANSPORT, used for the evaluation of gas-phase multicomponent transport properties, and LIOR - a linking code. The numerical tool has predictive capability for combustion behaviour under various conditions, an ability to interpret observed combustion phenomena and ability to guide the design of better natural gas engines. A justification of the validity of the approach is discussed in terms of the asymptotic structure of the methane-air flame. The early states of the combustion event are described in some detail and two specific studies are briefly reported, that of the effects of various ignition source types for natural gas burning domestic vehicle engines, and that of the effects of variation in composition of the natural gas.

A Natural Gas Engine Combustion Rig With High-Speed Photography

Engines today must satisfy stringent emission requirements but must at the same time have low fuel consumption. One method of approaching both of these goals in spark-ignited natural gas engines is with lean combustion. The use of as much as 80 percent excess air significantly reduces the peak combustion temperature and, as compared to a stoichiometric engine, reduces the NOx emissions by up to 90 percent and the fuel consumption by up to 15 percent. One limitation on lean combustion, however, is the high energy needed for ignition. In larger engines, a small prechamber containing an easily ignitable near-stoichiometric mixture has proved to be both successful and popular as one method of producing the necessary high ignition energy. Although this form of stratified charge combustion has been known for many years, its development has largely been the result of “cut and try” procedures. Lack of access for suitable instrumentation, combined with the difficulty of isolating the individual variables which affect performance, has limited the fundamental understanding of the mechanism of prechamber combustion. This paper summarizes results from a research program where a constant-volume combustion rig is used to simulate engine operation. Emphasis is placed on high-speed photography of the prechamber combustion. A second program on a single-cylinder prechamber spark-ignited gas engine and a third on a multiple-cylinder engine will be reported in subsequent papers.