An Investigation of Premixed and Lean Combustion in Engines

Spark ignited internal combustion engines are expected to continue to be the mainstay for the passenger cars and light duty trucks for the next few decades. It is understood that to conform to the stringent fuel efficiency legislations as well as meet the regulated exhaust emission limits, combustion technology must evolve significantly. It is imperative to develop a deeper understanding of the fundamental engine processes such as air intake, fuel-air interaction, and ignition so that avenues for incremental improvements may be explored. With this broad objective, the present study focuses on spark ignition engines in which premixed and lean (air in excess) charge of fuel and air can be burned efficiently. Studies have indicated that under these conditions, it is possible to simultaneously reduce the oxides of nitrogen (NOx), while keeping the carbon monoxide (CO) and unburned hydrocarbons (UHCs) at low levels. The in-cylinder turbulence plays a major role in the fuel-air mixture preparation. When this mixture ignites, the combustion may propagate through what is known as a premixed turbulent flame. Turbulence is beneficial since it enhances the mass burning rate. This is particularly critical in lean burn engines in which it is difficult to complete the combustion within the extremely short time scales typical of modern engines. Excess turbulence however, may lead to flame quenching. In order to investigate the conditions leading up to and the propagation of the turbulent flame itself, analytical and empirical studies are performed. Tests are conducted on a constant volume combustion chamber with optical access to provide insight into the combustion characteristics of lean mixtures subject to turbulence. Fundamental studies on premixed flame propagation are performed with a variety of fuels at different equivalence ratios with different fuels. Impacts of engine operating conditions such as air-fuel ratio, exhaust gas recirculation, engine load, fuels, and ignition strategies on the flame initiation and development are investigated in detail on a research engine test setup. Chemical simulation and computational fluid dynamics (CFD) tools are used to supplement the understanding of the results. Finally, an attempt is made to comprehensively understand the combined effects of in-cylinder flow and fuel reactivity on premixed and lean combustion

Influence of plasma-assisted ignition on flame propagation and performance in a spark-ignition engine

Lean-burn is an attractive concept for reasons of high thermal efficiency and low nitrogen oxide (NOx) emissions, however, successful implementation in spark-ignition (SI) engines turned out to be challenging because of misfire or partial burn caused by attenuated flame propagation. In order to overcome this issue, microwave-assisted plasma ignition system (MAPIS) has been applied in combustion systems. The MAPIS consists of a conventional ignition coil, a non-resistor spark plug, a mixing unit, a waveguide, and a magnetron (2.45GHz, 3kW). A series of experiments was carried out to understand discharge characteristics and to validate its performance in a constant volume vessel as well as in a single-cylinder spark-ignition engine. The fundamental investigation based on optical emission spectroscopy and flame imaging showed that the ejection of the microwave was beneficial to produce more reactive species such as OH and O radicals thanks to higher electron temperature than conventional spark ignition. The lean limit was able to be extended up to an equivalence ratio of 0.5 based on a larger initial flame kernel size with MAPIS in the vessel test. Meanwhile, in the engine test, combustion stability was noticeably improved showing smaller cycle-to-cycle fluctuations in in-cylinder pressure. Improvement in fuel efficiency up to 6% could be achieved by stable operation under fuel-lean conditions. In terms of emissions, MAPIS was advantageous to reduce carbon monoxide (CO) emissions by promoting more complete combustion

Internal Combustion Engines

This book on internal combustion engines brings out few chapters on the research activities through the wide range of current engine issues. The first section groups combustion-related papers including all research areas from fuel delivery to exhaust emission phenomena. The second one deals with various problems on engine design, modeling, manufacturing, control and testing. Such structure should improve legibility of the book and helps to integrate all singular chapters as a logical whole

The Effect of Near-Spark-Plug Flow Field on Spark Discharge Characteristics

Advanced spark ignition (SI) engines can operate under lean conditions in order to improve the efficiency and reduce the emissions. Under extensive lean conditions, the ignition and complete combustion of the charge mixture is a challenge, because of the reduced cylinder charge reactivity. The enhancement of the in-cylinder global motion and local turbulence is an effective way to increase the flame velocity, and consequently shorten the combustion duration. The role of air motion in improving air-fuel mixing and combustion has been researched extensively. However, during the ignition process, the excessive charge motion can hinder the spark discharge, the resulting flame kernel formation, and propagation. Therefore, a combined empirical and simulation study is undertaken to elucidate the flow field around the spark gap, and its effect on the spark discharge. The flow field generated by a steady flow of air across the spark gap of a conventional J-type spark plug is studied under ambient conditions. Optical particle image velocimetry (PIV) measurements and computational fluid dynamics (CFD) simulations are performed alongside the high-speed direct imaging. Voltage and current waveforms of the spark channel have been measured, in order to correlate the spark behavior to the local flow velocity. The flow field near the spark gap in an SI engine under motoring conditions is simulated. The results are compared to the empirical current and voltage measurements taken during engine operation. The results show that the turbulence is generated in the wake of the spark plug and flow velocity in the spark gap is higher than the free stream velocity. The optical and electrical measurements show the spark stretching and restrikes increase, and the discharge duration decreases with an increase in flow velocity. Similar behavior is observed during engine operation as well

A New Methodology to Evaluate Engine Ignition Systems in High Density Conditions

In this research, a new methodology to evaluate the operation of spark ignition systems in high density conditions is presented. New requirements in engines and new combustion modes demand more from these systems. One of the most important new requirements is the increase in density. Thus, a better understanding of the effects of high density and the behavior of the ignition system in these conditions seems necessary. To carry out this work two experimental facilities have been used: a transparent constant volume vessel, and an optical engine to simulate real engine conditions. Thus, the study combines the electrical signals derivate parameters and images obtained with a high speed camera. The methodology has been applied for different cases of pressure, intake temperature, and other parameters that affect the density. Results show that an increase in density causes a decrease in integrated power. Additionally, the dispersion in this integrated power increases too. Finally, the methodology results offer a useful data base for the engineers willing to improve the design of the ignition system. Moreover, it is validated that the results of the ambient transparent constant volume vessel follow the same trends and values as the realistic ones.Payri Marín, R.; Novella Rosa, R.; García Martínez, A.; Domenech Llopis, V. (2014). A New Methodology to Evaluate Engine Ignition Systems in High Density Conditions. Experimental Techniques. 38(3):17-28. doi:10.1111/j.1747-1567.2012.00818.xS1728383Banco , G. An Analysis of the Federal Government's Role in the Research and Development of Clean Diesels in the United States 2004Benajes, J., Novella, R., García, A., & Arthozoul, S. (2009). Partially Premixed Combustion in a Diesel Engine Induced by a Pilot Injection at the Low-pressure Top Dead Center. Energy & Fuels, 23(6), 2891-2902. doi:10.1021/ef900034yBenajes, J. V., López, J. J., Novella, R., & García, A. (2008). ADVANCED METHODOLOGY FOR IMPROVING TESTING EFFICIENCY IN A SINGLE-CYLINDER RESEARCH DIESEL ENGINE. Experimental Techniques, 32(6), 41-47. doi:10.1111/j.1747-1567.2007.00296.xZhao, F., Lai, M.-C., & Harrington, D. . (1999). Automotive spark-ignited direct-injection gasoline engines. Progress in Energy and Combustion Science, 25(5), 437-562. doi:10.1016/s0360-1285(99)00004-0Saitzokoff , A. Reinmann , R. Berglind , T. Glavmo , M. An Ionization Equilibrium Analysis of the Spark Plug an Ionization Sensor 1996Chung , S.S. Ha , J.-Y. Park , W.Y. Lee , M.-J. A Study on a Spark Plug for Charging of Stratified Mixture in a Local Area 2003Kondo , N. Suzuki , T. Sakakura , Y. Yamada , T. Combustion Monitoring by use of the Spark Plug for DI Engine 2001Shimanokami , Y. Matsubara , Y. Suzuki , T. Matsutani , W. Development of High Ignitability with Small Size Spark Plug 2004Lee , M.-J. Hall , M. Ofodike , A.E. Matthews , R. Voltage, and Energy Depositation Characteristics of Spark Ignition SystemsPayri, R., Salvador, F. J., Gimeno, J., & Soare, V. (2005). Determination of diesel sprays characteristics in real engine in-cylinder air density and pressure conditions. Journal of Mechanical Science and Technology, 19(11), 2040-2052. doi:10.1007/bf02916497Lapuerta, M., Armas, O., & Hernández, J. J. (1999). Diagnosis of DI Diesel combustion from in-cylinder pressure signal by estimation of mean thermodynamic properties of the gas. Applied Thermal Engineering, 19(5), 513-529. doi:10.1016/s1359-4311(98)00075-

Flame front analysis of ethanol, butanol, iso-octane and gasoline in a spark-ignition engine using laser tomography and integral length scale measurements

Direct-injection spark-ignition engines have become popular due to their flexibility in injection strategies and higher efficiency; however, the high-pressure in-cylinder injection process can alter the airflow field by momentum exchange, with different effects for fuels of diverse properties. The current paper presents results from optical studies of stoichiometric combustion of ethanol, butanol, iso-octane and gasoline in a direct-injection spark-ignition engine run at 1500 RPM with 0.5 bar intake plenum pressure and early intake stroke fuel injection for homogeneous mixture preparation. The analysis initially involved particle image velocimetry measurements of the flow field at ignition timing with and without fuelling for comparison. Flame chemiluminescence imaging was used to characterise the global flame behaviour and double-pulsed Laser-sheet flame tomography by Mie scattering to quantify the local topology of the flame front. The flow measurements with fuel injection showed integral length scales of the same order to those of air only on the tumble plane, but larger regions with scales up to 9 mm on the horizontal plane. Averaged length scales over both measurement planes were between 4 and 6 mm, with ethanol exhibiting the largest and butanol the smallest. In non-dimensional form, the integral length scales were up to 20% of the clearance height and 5–12% of the cylinder bore. Flame tomography showed that at radii between 8 and 12 mm, ethanol was burning the fastest, followed by butanol, iso-octane and gasoline. The associated turbulent burning velocities were 4.6–6.5 times greater than the laminar burning velocities and about 13–20% lower than those obtained by flame chemiluminescence imaging. Flame roundness was 10–15% on the tomography plane, with largest values for ethanol, followed by butanol, gasoline and iso-octane; chemiluminescence imaging showed larger roundness (18–25%), albeit with the same order amongst fuels. The standard deviation of the displacement of the instantaneous flame contour from one filtered by its equivalent radius was obtained as a measure of flame brush thickness and correlated strongly with the equivalent flame radius; when normalised by the radius, it was 4–6% for all fuels. The number of crossing points between instantaneous and filtered flame contour showed a strong negative correlation with flame radius, independent of fuel type. The crossing point frequency was 0.5–1.6 mm−1. The flame brush thickness was about 1/10th of the integral length scale. A positive correlation was found between integral length scale and flame brush thickness and a negative correlation with crossing frequency

Impact of Spark Assistance and Multiple Injections on Gasoline PPC Light Load

Along the last years, engine researchers are more and more focusing their efforts on the advanced low temperature combustion (LTC) concepts with the aim of achieving the stringent limits of the current emission legislations. In this regard, several studies based on highly premixed combustion concepts such as HCCI has been confirmed as a promising way to decrease drastically the most relevant CI diesel engine-out emissions, NOx and soot. However, the major HCCI drawbacks are the narrow load range, bounded by either misfiring (low load, low speed) or hardware limitations (higher load, higher speeds) and the combustion control (cycle-to-cylce control and combustion phasing). Although several techniques have been widely investigated in order to overcome these drawbacks, the high chemical reactivity of the diesel fuel remains as the main limitation for the combustion control. The attempts of the researchers to overcome these disadvantages are shifting to the use of fuels with different reactivity. In this sense, gasoline PPC has been able to reduce emissions and improve efficiency simultaneously, but some drawbacks regarding controllability and stability at low load operating conditions still need solution. In this field, previous researches have been demonstrate the multiple injection strategy as an appropriate technique to enhance the combustion stability. However, PPC combustion has been found limited to engine loads higher than 5 bar BMEP when using fuels with octane number greater than 90. In this regard, previous work from the authors showed the capability of the spark plug to provide combustion control in engine loads below this limit even using 98 ON gasoline. The main objective of the present work is to couple the control capability of the spark assistance together with an appropriate mixture distribution by using double injection strategies with the aim of evaluating performance and engine-out emissions at low load PPC range using a high octane number gasoline. For this purpose the optical and metal version of a compression ignition single-cylinder engine, to allow high compression ratio, has been used during the research. A common rail injection system enabling high injection pressures has been utilized to supply the 98 octane number gasoline. An analysis of the in-cylinder pressure signal derived parameters, hydroxyl radical (OH*) and natural luminosity images acquired from the transparent engine as well as a detailed analysis of the air/fuel mixing process by means of a 1-D in-house developed spray model (DICOM) has been conducted. Results from both analysis methods, suggest the spark assistance as a proper technique to improve the spatial and temporal control over the low load gasoline PPC combustion process. A noticeable increase in the cycle to cycle repeatability (5% versus 15.1% CoV IMEP at 2 bar load) as well as a reduction in the knocking level (20.5 versus 33.6 MW/m2 at 7 bar load) is observed. In addition, the combination of the spark assistance with the use of the double injection strategy provides a great improvement in terms of combustion efficiency (93% versus 88% for a single injection strategy) with a benefit around 18% in the IMEPBenajes Calvo, JV.; Tormos Martínez, BV.; García Martínez, A.; Monsalve Serrano, J. (2014). Impact of Spark Assistance and Multiple Injections on Gasoline PPC Light Load. SAE International Journal of Engines. 7(4):1875-1887. doi:10.4271/2014-01-2669S187518877

Characterisation of flame development with ethanol, butanol, iso-octane, gasoline and methane in a direct-injection spark-ignition engine

Research into novel internal combustion engines requires consideration of the diversity in future fuels that may contain significant quantities of bio-components in an attempt to reduce CO2 emissions from vehicles and contribute to energy sustainability. However, most biofuels have different chemical and physical properties to those of typical hydrocarbons; these can lead to different mechanisms of mixture preparation and combustion. The current paper presents results from an optical study of combustion in a direct-injection spark-ignition research engine with gasoline, iso-octane, ethanol and butanol fuels injected from a centrally located multi-hole injector. Methane was also employed by injecting it into the inlet plenum of the engine to provide a benchmark case for well-mixed ‘homogeneous’ charge preparation. Crank-angle resolved flame chemiluminescence images were acquired and post-processed for a series of consecutive cycles for each fuel, in order to calculate in-cylinder rates of flame growth and motion. In-cylinder pressure traces were used for heat release analysis and for comparison with the image-processing results. All tests were performed at 1500 RPM with 0.5 bar intake plenum pressure. Stoichiometric (ϕ = 1.0) and lean (ϕ = 0.83) conditions were considered. The combustion characteristics were analysed with respect to laminar and turbulent burning velocities obtained from combustion bombs in the literature and from traditional combustion diagrams in order to bring all data into the context of current theories and allow insights by making comparisons were appropriate

An investigation of partially premixed compression ignition combustion using gasoline and spark assistance

Nowadays the automotive scientific community and companies are focusing part of their efforts on the investigation of new combustion modes in Compression Ignition (Cl) engines, mainly based on the use of locally lean air fuel mixtures. This characteristic, combined with exhaust gas recirculation, provides low combustion temperatures that reduce pollutant formation. However these combustion concepts have some shortcomings, related to combustion phasing control and combustion stability under the light load engine operating range which must be overcome. The aim of this work is focused on the study of the integration of phasing and cycle-to-cycle repeatability control by means of an ignition spark plug system in a CI engine working under partially premixed charge (PPC) in order to overcome the lack of combustion stability in light load conditions when very low fuel reactivity is used. To achieve this objective, experimental tests have been carried out in a single cylinder optical engine combining broadband luminosity images with cylinder pressure derived heat release rate analysis. Research results reveals the spark assistance as a proper methodology to provide temporal and spatial control over the combustion process solving the lack of cycle to cycle control on the highly premixed compression ignition modes overall in light loads with high octane number (ON) fuels. Additionally, different stages have been identified in the combustion mode. The process starts with the spark discharge, which produces a flame kernel around the spark plug that later evolves to a premixed flame front. This premixed flame front heats unburned mixture and progresses into an auto-ignition combustion that burns out the rest of the charge with higher light intensity and finally an extinction of combustion process. Finally, the effect of injected fuel mass on the combustion mode has been also tested. An increase in the injected fuel mass has a positive effect on the assistance of the spark in the combustion process for both, combustion stability and cycle to cycle control. (C) 2013 Elsevier Ltd. All rights reserved.The authors would like to thank General Motors for supporting this research.Benajes Calvo, JV.; García Martínez, A.; Doménech Llopis, V.; Durret, R. (2013). An investigation of partially premixed compression ignition combustion using gasoline and spark assistance. Applied Thermal Engineering. 52(2):468-477. doi:10.1016/j.applthermaleng.2012.12.025S46847752