Experimental Results on Natural Gas and Liquefied Petroleum Gas Lean Burning in a Diesel Engine Retrofitted for Spark Ignition

In the European context of a planned transition to a zero-carbon future, the initial question at the origin of this study was to assess the energy and environmental performance of a retrofitted diesel engine converted to stoichiometric spark ignition (SI) operation using two gaseous and sustainable fuels: natural gas (NG) and liquefied petroleum gas (LPG). Further to a parametric study regarding the spark advance and injection timing, this paper delivers experimental results obtained at the engine test bed for different operating points, which are compared with the results of a baseline commercial gasoline engine. The results showed a clear improvement in terms of CO2 for the NG (especially) and LPG with a knock-free operation with respect to gasoline. Cyclic variability also improved. As for the engine-out pollutants, in the case of carbon monoxide (CO) and particle number (PN), the results are in favor of gaseous fuels. Concerning the nitrogen oxides (NOx), as expected, a higher emission was obtained with the retrofitted engine due to its higher compression ratio.

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

<div class="section abstract"><div class="htmlview paragraph">Liquefied Petroleum Gas (LPG), as a common alternative fuel for internal combustion engines is currently widespread in use for fleet vehicles. However, a current majority of the LPG-fueled engines, uses port-fuel injection that offers lower power density when compared to a gasoline engine of equivalent displacement volume. This is due to the lower molecular weight and higher volatility of LPG components that displaces more air in the intake charge due to the larger volume occupied by the gaseous fuel. LPG direct-injection during the closed-valve portion of the cycle can avoid displacement of intake air and can thereby help achieve comparable gasoline-engine power densities. However, under certain engine operating conditions, direct-injection sprays can collapse and lead to sub-optimal fuel-air mixing, wall-wetting, incomplete combustion, and increased pollutant emissions. Direct-injection LPG, owing to its thermo-physical properties is more prone to spray collapse than gasoline sprays. However, the impact of spray collapse for high-volatility LPG on mixture preparation and subsequent combustion is not fully understood. To this end, direct-injection, laser-spark ignition experiments using propane as a surrogate for LPG under lean and stoichiometric engine operating conditions were carried out in an optically accessible, single cylinder, heavy-duty, diesel engine. A quick-switching parallel propane and iso-octane fuel system allows for easy comparison between the two fuels. Fuel temperature, operating equivalence ratio and injection timing are varied for a parametric study. In addition to combustion characterization using conventional cylinder pressure measurements, optical diagnostics are employed. These include infrared (IR) imaging for quantifying fuel-air mixture homogeneity and high-speed natural luminosity imaging for tracking the spatial and temporal progression of combustion. Imaging of infrared emission from compression-heated fuel does not reveal any significant differences in the signal distribution between collapsing and non-collapsing sprays at the spark timing. Irrespective of coolant temperatures, early injection timing resulted in a homogeneous mixture that lead to repeatable flame evolution with minimal cycle-to-cycle variability for both LPG and iso-octane. However, late injection timing resulted in mixture inhomogeneity and non-isotropic turbulence distribution. Under lean operation with late injection timing, LPG combustion is shown to benefit from a more favorable mixture distribution and flow properties induced by spray collapse. On the other hand, identical operating conditions proved to be detrimental for iso-octane combustion most likely caused by distribution of lean mixtures near the spark location that negatively impact initial flame kernel growth leading to increased cycle-to-cycle variability.</div></div>

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Lean fueling of spark ignited (SI) engines is a valid method for increasing efficiency and reducing nitric oxide (NOx) emissions. Gasoline direct injection (GDI) allows better fuel economy with respect to the port-fuel injection configuration, through greater flexibility to load changes, reduced tendency to abnormal combustion, and reduction of pumping and heat losses. During homogenous charge operation with lean mixtures, flame development is prolonged and incomplete combustion can even occur, causing a decrease in stability and engine efficiency. On the other hand, charge stratification results in fuel impingement on the combustion chamber walls and high particle emissions. Therefore, lean operation requires a fundamentally new understanding of in-cylinder processes for developing the next generation of direct-injection (DI) SI engines. In this paper, combustion was investigated in an optically accessible DISI single cylinder research engine fueled with gasoline. Stoichiometric and lean operations were studied in detail through a combined thermodynamic and optical approach. The engine was operated at a fixed rotational speed (1000 rpm), with a wide open throttle, and at the start of the injection during the intake stroke. The excess air ratio was raised from 1 to values close to the flammability limit, and spark timing was adopted according to the maximum brake torque setting for each case. Cycle resolved digital imaging and spectroscopy were applied; the optical data were correlated to in-cylinder pressure traces and exhaust gas emission measurements. Flame front propagation speed, flame morphology parameters, and centroid motion were evaluated through image processing. Chemical kinetics were characterized based on spectroscopy data. Lean burn operation demonstrated increased flame distortion and center movement from the location of the spark plug compared to the stoichiometric case; engine stability decreased as the lean flammability limit was approached.

Combustion a lean air-fuel mixture in a spark ignited (SI) engine is one way to reduce nitrogen oxide (NOx) emissions that results in an increase of engine efficiency by decreasing a peak combustion temperature. An effective concept to lean mixture combustion can be a two-stage combustion system of stratified burn mixture in the engine with pre-chamber, in which combustion starts in a pre-chamber (1 stage) and, further, the flame jet from a pre-chamber initiate lean mixture combustion in the engine cylinder (2 stage). The paper presents the results of the laboratory research of the SI stationary engine with two-stage combustion system powered by LPG gas. The results were compared to the results of the dual-fuel engine with two-stage combustion system and the conventional engine with one-stage combustion process. Air-fuel mixture stratification method in the test engine, by using two-stage combustion system with pre-chamber, allowed burning of lean mixture with the overall excess air ratio (») up to 2.0, and thus led to lower emissions of nitrogen oxides in the exhaust gases of the engine. The test engine implementing a conventional, single-stage combustion process has allowed a correct burning of air-fuel mixtures of excess air ratio not exceeding 1.5. Of the value λ > 1.5, an increase of coefficient of variation indicated mean effective pressure (COVIMEP), and it decreased the engine thermal efficiency (ITE), which became virtually impossible to operate. The engine implementing a two-stage combustion process, working with λ = 2.0 allowed to reduce the NOx content in exhaust gases to a level of about 0.02 g/kWh (for gas engine) and 1.15 g/kWh (for dual-fuel engine). These values are significantly less than the values obtained in the conventional engine, which indicated thermal efficiency (34%) characterized by the emission of NOx — 26.25 g/kWh.

NOx reduction from a large bore natural gas engine via reformed natural gas prechamber fueling optimization

Natural gas is widely used in sequentially port fuel injection engine to meet stringent emission regulation. Lean burn operation is one of the ways to improve spark-ignition engine fuel economy. The instability in the combustion process of the lean burn engine is one of the major challenges for engine research. In this study, the performance and combustion characteristics of a lean burn sequential injection compressed natural gas (CNG) engine were investigated numerically using computational fluid dynamics (CFD) modeling over a wide range of air/fuel equivalence ratio. A detailed chemical kinetic mechanism was used for natural gas combustion along with laminar flame speed model to capture lean burn operating condition within the combustion chamber. Combustion pressure, indicated mean effective pressure (IMEP), and heat release were analyzed for performance analysis, whereas flame development angle (CA 10), combustion duration, thermal efficiency were taken for combustion analysis. The results show that on increasing air/fuel equivalence ratio at a given spark timing, IMEP decreases as the lean burn mixture produces less amount of gross power output due to insufficient available energy. Moreover, lower burning velocity characteristic of natural gas extends the combustion duration, where a substantial amount of total energy released after top dead center. It is also seen that optimum spark timing (MBT) for maximum IMEP advances with an increase in air/fuel equivalence ratio due to late ignition timing under lean burn condition. CFD model successfully captures the effect of dilution to illustrate the considerations to design future combustion engine for spark ignited natural gas engine.

ABSTRACTThe present study deals with the performance and emission characteristics of a multi-point fuel injection (MPFI) spark ignition (SI) engine in gasoline–liquefied petroleum gas (LPG) dual fuel mode of operation. The LPG–gasoline ratio varied from 0 to 100% by controlling the injector signals at various speed and load conditions. Experiments show that the power output decreases with increase in speed and LPG content at lower load marginally due to lower volumetric efficiency. At higher load and lower speed conditions as the percentage of LPG increases there is not much difference in the power output. Results also reveal that 50% LPG flow gives maximum efficiency at full load condition and 4000 rpm due to lower fuel consumption. With 50% usage of LPG, the average increase in brake thermal efficiency (BTE) is 2% till the engine speed of 4000 rpm at full load (100%) and half load (50%) conditions. As the LPG ratio increases the engine will work in the lean region for all speed and load conditions. For all load and speed conditions, results reveal that 100% LPG will give minimum hydrocarbon (HC) and carbon monoxide (CO) emissions. Oxide of nitrogen (NOX) emissions are higher for 100% LPG. However 50% LPG flow gives good agreement of NOX, HC and CO emissions when compared with gasoline operation.

Low temperature and dilute homogenous charge compression ignition (HCCI) and spark-assisted compression ignition (SACI) can improve fuel efficiency and reduce engine-out NOx emissions, especially during lean operation. However, under lean operation, these combustion modes are unable to achieve Environmental Protection Agency (EPA) Tier 3 emissions standards without the use of lean aftertreatment. The three way catalyst (TWC)-SCR lean aftertreatment concept investigated in this work uses periodic-rich operation to produce NH3 over a TWC to be stored on a selective catalytic reduction (SCR) catalyst for NOx conversion during subsequent lean operation. Experiments were performed with a modified 2.0 L gasoline engine that was cycled between lean HCCI and rich SACI operation and between lean and rich spark-ignited (SI) combustion to evaluate NOx conversion and fuel efficiency benefits. Different lambda values during rich operation and different times held in rich operation were investigated. Results are compared to a baseline case in which the engine is always operated at stoichiometric conditions. SCR system calculations are also presented to allow for comparisons of system performance for different levels of stored NH3. With the configuration used in this study, lean/rich HCCI/SACI operation resulted in a maximum NOx conversion efficiency of only 10%, while lean/rich SI operation resulted in a maximum NOx conversion efficiency of 60%. If the low conversion efficiency of HCCI/SACI operation could be improved through higher brick temperatures or additional SCR bricks, calculations indicate that TWC-SCR aftertreatment has the potential to provide attractive fuel efficiency benefits and near-zero tailpipe NOx. Calculated potential fuel efficiency improvement relative to stoichiometric SI is 7–17% for lean/rich HCCI/SACI with zero tailpipe NOx and −1 to 5% for lean/rich SI with zero tailpipe NOx emissions. Although the previous work indicated that the use of HCCI/SACI increases the time for NH3 to start forming over the TWC during rich operation, reduces NH3 production over the TWC per fuel amount, and increases NH3 slip over the SCR catalyst, if NOx conversion efficiency could be enhanced, improvements in fuel efficiency could be realized while meeting stringent tailpipe NOx standards.

Technology Focus In 2019, the US experienced the lowest natural gas prices since 2016. This was despite natural gas consumption increasing in the residential and commercial sector by 2% (between October and December) according to the US Energy Information Administration (EIA). In July and August, the electric generation sector also experienced an increase in natural gas consumption because of above-average humidity levels in the Midwest and Northeast. Natural gas inventories at the end of March recorded the lowest levels since 2014 during withdrawal season, whereas the injection season recorded the second-highest levels from sustained growth in natural gas production. This trend of low gas prices was because of natural gas production growth, as prices continued to decline through the rest of 2019. The EIA forecasts an increase in natural gas consumption by 1.4 Bcf/D (1.7%) in 2020. The average US natural gas price is projected to fall by 9% to $2.33/million British thermal units (MMBtu) in 2020. The EIA said it expects continued growth in natural gas production in 2020 in response to improved drilling, increased associated gas production, and increased Appalachian and Permian pipeline take-away capacity. As of 3 February, Henry Hub spot price closed at $1.82/MMBtu, $0.87 lower than the same time last year. Weather temperatures were warmer than expected, except in the Northeast and Midwest where it was colder than expected, hence affecting winter demand. Globally, the Paris-based International Energy Agency’s 5-year forecast shows natural gas demand driven by Asia-Pacific growth, with the region accounting for approximately 60% of the total gas consumption forecasted increase through 2024. China is expected to be at the forefront of this demand growth because it is expected to constitute approximately 40% of the total gas demand increase through 2024. The global liquefied petroleum gas (LPG) market has continued to grow reasonably in the past decade, and the US, as a result of its strong domestic LPG production, has ascended in recent years as the largest LPG producer and exporter in the world. The Asia-Pacific region, because of its growing population, constitutes the highest market share of the global LPG consumption. The global LPG market trend is predominantly supply-driven by natural gas and crude oil but not so much from refining. According to IHS Markit, LPG demand in Asia-Pacific grew approximately 3.5% in 2018 and has continued to maintain its steady growth in 2019. Global supplies of LPG are expected to rise approximately 5% to 325 million tonnes in 2020. By region, approximately 28% of the global supplies will be from the US and approximately 20% from the Middle East. Asia-Pacific will continue to be the main driver for LPG demand growth, accounting for approximately 45% (4.6 million B/D) of the global demand. The prominent increase in US LPG exports has resulted in significant changes in the LPG global trade flow patterns in recent years, demonstrating a shift from a dependency on exports from the Middle East. More can be learned by attending the SPE Annual Technical Conference and Exhibition in Denver on 5–7 October and the SPE Asia Pacific Oil and Gas Conference and Exhibition in Perth, Australia, on 8–11 September. Recommended additional reading at OnePetro: www.onepetro.org. SPE 195759 Solutions for Seal Gas Leakage Recovery in Methane Compression: Integration Into Processing Lines or Gas Valorization Systems by Filippo Conforti, Baker Hughes, et al. SPE 197582 Running Sour Hydrocarbon Assets: Eni’s Story of Experience by Luciano Scataglini, Eni, et al. SPE 195555 Experimental Investigation of Integrity Issues of Underground Gas Storage Containing Hydrogen by Erik Clemens Boersheim, Clausthal University of Technology, et al.

Low temperature and dilute Homogenous Charge Compression Ignition (HCCI) and Spark Assisted Compression Ignition (SACI) can improve fuel economy and reduce engine-out NOx emissions to very low values, often less than 30 ppm. However, these combustion modes are unable to achieve stringent future regulations such as SULEV 30 without the use of lean aftertreatment. Though active selective catalytic reduction (SCR) with urea injection and lean NOx traps (LNT) have been investigated as options for lean gasoline engines, a passive TWC-SCR system is investigated in this work because it avoids the urea storage and dosing hardware of a urea SCR system, and the high precious metal cost of an LNT. The TWC-SCR concept uses periodic rich operation to produce NH3 over a TWC to be stored on an SCR catalyst for subsequent NOx conversion during lean operation. In this work a laboratory study was performed with a modified 2.0 L gasoline engine that was cycled between lean HCCI and rich SACI operation, or between lean and rich SI (spark ignited) combustion, to evaluate NOx conversion and reduced fuel consumption. Different lambda values during rich operation and different times held in rich operation were investigated. Results are compared to a baseline case in which the engine is always operated at stoichiometric conditions. SCR system simulations are also presented that compare system performance for different levels of stored NH3. With the configuration used in this study, lean/rich HCCI/SACI operation showed a maximum NOx conversion efficiency of 10%, while lean/rich SI operation showed a maximum NOx conversion efficiency of 60%. However, if the low conversion efficiency of lean/rich HCCI/SACI operation could be improved through higher brick temperatures or additional SCR bricks, simulation results indicate TWC-SCR aftertreatment has the potential to provide near-zero SCR-out NOx concentration and increased system fuel efficiency. In these simulations, fuel efficiency improvement relative to stoichiometric SI were 7 to15% for lean/rich HCCI/SACI with zero tailpipe NOx and −1 to 5% for lean/rich SI with zero tailpipe NOx emissions. Although previous work indicated increased time for NH3 to start forming over the TWC during rich operation, less NH3 production over the TWC per fuel amount, and increased NH3 slip over the SCR catalyst for advanced combustion systems, if NOx conversion efficiency could be enhanced, improvements in fuel economy and low engine-out NOx from advanced combustion modes would more than make up for these disadvantages.

The expediency and advantages of using gas motor fuels, in particular, liquefied petroleum gas with respect to traditional liquid motor fuels, are shown. Technical solutions for the use of liquefied petroleum gas by diesel engines are presented and analysed. The expediency and advantages of converting diesel engines to gas spark ignition internal combustion engines with respect to conversion to gas diesel engines. Developed by the Ukrainian synthesis technology Avenir Gaz has for converting diesel engines to gas internal combustion engines with spark ignition. According to the synthesis technology of Avenir Gaz, re-equipment of diesel engines of vehicles is carried out on the basis of the universal electronic control system for gas internal combustion engines, which is based on the multifunctional electronic microprocessor control unit Avenir Gaz 37. The developed electronic microprocessor control system for gas internal combustion engines with forced ignition has a modular structure and consists of two main and a number of additional subsystems. A schematic diagram of a universal electronic control system of a gas internal combustion engine with spark ignition for operation on liquefied petroleum gas is presented. The principle of operation of the main subsystems, which include the subsystem of power management and injection of liquefied petroleum gas by gas electromagnetic injectors into the intake manifold of a gas engine, and the principle of operation of the control subsystem of the ignition with two-spark ignition coils are described. A multifunctional electronic control unit Avenir Gaz 37 has been designed and manufactured. Non-motorized tests of the electronic control unit confirmed its performance. Based on the synthesis technology of Avenir Gaz using the universal electronic control system for gas internal combustion engines with the Avenir Gaz 37 ECU, the D-240 diesel engine was converted into a gas spark ignition internal combustion engine of the D-240-LPG model. Keywords: gas internal combustion engine with forced ignition, liquefied petroleum gas (LPG), electronic microprocessor control system for gas internal combustion engines, vehicles operating on LPG.

<div class="section abstract"><div class="htmlview paragraph">The combustion process in spark-ignition engines can vary considerably cycle by cycle, which may result in unstable engine operation. The phenomena amplify in natural gas (NG) spark-ignition (SI) engines due to the lower NG laminar flame speed compared to gasoline, and more so under lean burn conditions. The main goal of this study was to investigate the main sources and the characteristics of the cycle-by-cycle variation in heavy-duty compression ignition (CI) engines converted to NG SI operation. The experiments were conducted in a single-cylinder optically-accessible CI engine with a flat bowl-in piston that was converted to NG SI. The engine was operated at medium load under lean operating conditions, using pure methane as a natural gas surrogate. The CI to SI conversion was made through the addition of a low-pressure NG injector in the intake manifold and of a NG spark plug in place of the diesel injector. Flame luminosity images of the whole combustion event inside the piston bowl were used to analyze the major sources of cyclic variation. The optical measurements were combined with in-cylinder pressure measurements to infer the characteristics of the cycle-by-cycle variation. The results suggested that the spark intensity, arc continuity, and arc location affected the flame kernel inception. The gas motion and the mixture equivalence ratio around the spark location also influenced it. Then, the intake swirl and the turbulence during the compression stroke determined the flame propagation speed and direction. The variation in the fast burning between individual cycles compounded the cyclic variation caused by the ignition event. In addition, the reduction in flame propagation near the bowl wall decreased the cyclic variation. Moreover, the complex phenomena at the entrance of the squish region increased the cycle-by-cycle variations but it seems to not have a strong influence on the power output difference between cycles. Furthermore, the large surface-to-volume ratio in the squish region resulted in a large variation in the heat loss, then producing large differences in the flame development in the squish, which in turn affected the heat loss variation, and so on. But the COV<sub>IMEP</sub> was less than 4%, despite the extremely lean burn operation (ϕ = 0.66). It was probably due to the high turbulence intensity inside the bowl that helped with the rapid burning process inside the bowl. The strong turbulence was generated by the squish during the compression stoke. The reasonable COV<sub>IMEP</sub> suggest that the significant cycle-by-cycle variation in the burn inside the squish region had little impact on the COV<sub>IMEP</sub>. However, a large cycle-by-cycle variation in the squish burn would cause unstable CO and HC emissions, which is a concern for efficient engine operation.</div></div>

Combustion characteristics of a variable compression ratio laser-plasma ignited compressed natural gas engine