Author: Rahul JhavarPublisher:
ISBN:
Category :
Languages : en
Pages : 292
Book Description
A Computational Study of the Impact of Mixing on Homogeneous Charge Compression Ignition
Author: Rahul JhavarReaction-space Analysis of Homogeneous Charge Compression Ignition Combustion with Varying Levels of Fuel Stratification Under Positive and Negative Valve Overlap Conditions
Author: Publisher:
ISBN:
Category :
Languages : en
Pages : 19
Book Description
Full-cycle computational fluid dynamics simulations with gasoline chemical kinetics were performed to determine the impact of breathing and fuel injection strategies on thermal and compositional stratification, combustion and emissions during homogeneous charge compression ignition combustion. The simulations examined positive valve overlap and negative valve overlap strategies, along with fueling by port fuel injection and direct injection. The resulting charge mass distributions were analyzed prior to ignition using ignition delay as a reactivity metric. The reactivity stratification arising from differences in the distributions of fuel-oxygen equivalence ratio ([Phi]FO), oxygen molar fraction ([chi]O2) and temperature (T) was determined for three parametric studies. In the first study, the reactivity stratification and burn duration for positive valve overlap valve events with port fuel injection and early direct injection were nearly identical and were dominated by wall-driven thermal stratification. nitrogen oxide (NO) and carbon monoxide (CO) emissions were negligible for both injection strategies. In the second study, which examined negative valve overlap valve events with direct injection and port fuel injection, reactivity stratification increased for direct injection as the [Phi]FO and T distributions associated with direct fuel injection into the hot residual gas were positively correlated; however, the latent heat absorbed from the hot residual gas by the evaporating direct injection fuel jet reduced the overall thermal and reactivity stratification. These stratification effects were offsetting, resulting in similar reactivity stratification and burn durations for the two injection strategies. The higher local burned gas temperatures with direct injection resulted in an order of magnitude increase in NO, while incomplete combustion of locally over-lean regions led to a sevenfold increase in CO emissions compared to port fuel injection. The final study evaluated positive valve overlap and negative valve overlap valve events with direct injection. Furthermore, relative to positive valve overlap, the negative valve overlap condition had a wider reactivity stratification, a longer burn duration and higher NO and CO emissions associated with reduced fuel-air mixing.
A Computational Study of Auto-ignition and Flame Propagation in Stratified Mixtures Relevant to Modern Engines
Author: Ramanan SankaranComputational Studies of Homogeneous Charge Compression Ignition, Spark Ignition and Opposed Piston Single Cylinder Engines
Author: Ali Mubark AlqahtaniComputational Modeling to Study the Effect of Fuel Pre-treatment on IC Engine Combustion Control
Author: Venkateswara Raju DantuluriPublisher:
ISBN:
Category :
Languages : en
Pages : 556
Book Description
Conventional internal combustion (IC) engine combustion strategies such as homogeneous charge spark ignition (HCSI) and stratified charge compression ignition (SCCI) engines have nearly reached their maximum performance and emission reduction capabilities. New low-temperature combustion (LTC) strategies such as homogeneous charge compression ignition (HCCI) and derivitives have the potential to reduce engine-out emissions while maintaining high efficiency; however, combustion phasing challenges must be solved before their widespread use is adopted. The present work studies the potential of two strategies to control combustion phasing of LTC systems: (1) using intra-cycle re-circulated partial oxidation products (RePOx) and (2) internal fuel reformation by residuals during negative valve overlap (NVO). Both systems were studied using chemical kinetic modeling assuming n-heptane as the fuel. A detailed kinetic mechanism was constructed by combining existing n-heptane and nitrogen mechanisms and validated using HCCI experimental data available from the literature. The RePOx strategy was newly conceived as part of this work. The partial oxidation products are created by extracting a portion of the lean charge products during the expansion stroke and mixing these with the fuel in an auxiliary chamber (RePOx prechamber). The equivalence ratio of the recirculated reactants is controlled by varying the amount of mass extracted. The re-circulated partially-oxidized products are then reintroduced into the main chamber and mixed with compressed air to facilitate the main chamber reaction. This process is modeled using a complex reactor network in the CHEMKIN-PRO software package combined with an external program to balance mass and energy for the RePOx system. The study of this concept was performed in two phases. In the first phase, all the fuel was delivered through the RePOx prechamber, while in the second phase, part of the fuel was premixed in the main chamber prior to compression and the balance was delivered through the prechamber. In both phases, the effects of extraction mass, extraction timing, injection timing, pre-chamber volume, and overall equivalence ratio were examined. Varying pre-chamber volume did not show any effect on the performance or combustion phasing under the conditions and assumptions of this study. In the first phase, advancing injection timing by 5o and 10o crank angle (CA) has advanced the combustion phasing by 1.8o and 3.3o CA respectively. With the premixed charge, the combustion in the main engine chamber exhibited low temperature heat release (LTHR) after 30o crank angle (CA) before top dead center (BTDC) compression. This LTHR varied this trend. When the injection was before LTHR (before -30o CA) the trend is similar to first phase. When the injection is after LTHR (-20o CA), the rise in temperature during LTHR advanced the combustion by 7o CA when compared to -30o CA. In both phases when extraction mass is 5% or above, the combustion is advanced with increased extraction amount. When the extraction mass is below 3%, the incomplete alkane oxidation in pre-chamber caused LTHR in the main chamber after injection causing advanced combustion. Late extraction has delayed the combustion in both phases when there is no LTHR. When there is LTHR, the effect of temperature rise due to LTHR dominated the effect of late extraction and there is no variation in combustion phasing. Increasing overall equivalence ratio without premixing from 04 to 0.5 and 0.6 advanced the combustion phasing by 2o and 3o respectively. Under the conditions of the investigation, the RePOx system without premixing was able to operate at lower overall equivalence ratio than pure HCCI. The (NVO) strategy was incorporated into a 'conventional' HCCI engine and was also modeled and evaluated using a complex reactor network in CHEMKIN-PRO. In this case, however, actual experimental data was available from the literature to validate the system as modeled. The data showed that start of injection timing during NVO (NVO_SOI) effected the fuel reformation and varied the main combustion phasing. The main combustion phasing is delayed as the NVO_SOI is intitally retarded since the later injection caused less heat release during NVO, which reduced the temperatures after closing the intake valve (IVC). However, once a particular threshold was reached, additional delay in NVO_SOI resulted in advanced main combustion phasing. The model showed that this was because the reduced time for reformation during NVO caused more alkanes from the reformed fuel to be present during compression of the main combustion event. This triggered low temperature heat release (LTHR) during compression, from which the associated temperature rise caused advanced main combustion. While the model showed the same heat release timing trend as the experimental work, the point of reversing the trend due to LTHR occurred with NVO_SOI 10o crank angle earlier than as it occurred in the experimental results. When both RePOx and NVO systems are compared using the same engine displacement, the RePOx system has more than twice the power output than NVO because the full displacement can be used for fresh charge, whereas the volumetric efficiency is significantly impacted by the NVO valve timing. The RePOx system has more controlling parameters than the NVO system to control the combustion phasing and optimizing performance and emissions. The current research work demonstrates that presence of LTHR effectively minimizes the effect of othe parameters on combustion phasing in both RePOx and NVO systems. LTHR can be minimized by reforming the fuel and controlling the concentrations of species such as HO2, alkenes and alkanes. This work shows that both fuel reforming strategies investigated can be effectively used to control the combustion phasing in LTC systems.
Diesel Combustion and Emissions
Author: Society of Automotive EngineersPublisher:
ISBN:
Category : Air
Languages : en
Pages : 154
Book Description
Fundamental Interactions in Gasoline Compression Ignition Engines with Fuel Stratification
Author: Benjamin Matthew WolkPublisher:
ISBN:
Category :
Languages : en
Pages : 115
Book Description
Transportation accounted for 28% of the total U.S. energy demand in 2011, with 93% of U.S. transportation energy coming from petroleum. The large impact of the transportation sector on global climate change necessitates more-efficient, cleaner-burning internal combustion engine operating strategies. One such strategy that has received substantial research attention in the last decade is Homogeneous Charge Compression Ignition (HCCI). Although the efficiency and emissions benefits of HCCI are well established, practical limits on the operating range of HCCI engines have inhibited their application in consumer vehicles. One such limit is at high load, where the pressure rise rate in the combustion chamber becomes excessively large. Fuel stratification is a potential strategy for reducing the maximum pressure rise rate in HCCI engines. The aim is to introduce reactivity gradients through fuel stratification to promote sequential auto-ignition rather than a bulk-ignition, as in the homogeneous case. A gasoline-fueled compression ignition engine with fuel stratification is termed a Gasoline Compression Ignition (GCI) engine. Although a reasonable amount of experimental research has been performed for fuel stratification in GCI engines, a clear understanding of how the fundamental in-cylinder processes of fuel spray evaporation, mixing, and heat release contribute to the observed phenomena is lacking. Of particular interest is gasoline's pressure sensitive low-temperature chemistry and how it impacts the sequential auto-ignition of the stratified charge. In order to computationally study GCI with fuel stratification using three-dimensional computational fluid dynamics (CFD) and chemical kinetics, two reduced mechanisms have been developed. The reduced mechanisms were developed from a large, detailed mechanism with about 1400 species for a 4-component gasoline surrogate. The two versions of the reduced mechanism developed in this work are: (1) a 96-species version and (2) a 98-species version including nitric oxide formation reactions. Development of reduced mechanisms is necessary because the detailed mechanism is computationally prohibitive in three-dimensional CFD and chemical kinetics simulations. Simulations of Partial Fuel Stratification (PFS), a GCI strategy, have been performed using CONVERGE with the 96-species reduced mechanism developed in this work for a 4-component gasoline surrogate. Comparison is made to experimental data from the Sandia HCCI/GCI engine at a compression ratio 14:1 at intake pressures of 1 bar and 2 bar. Analysis of the heat release and temperature in the different equivalence ratio regions reveals that sequential auto-ignition of the stratified charge occurs in order of increasing equivalence ratio for 1 bar intake pressure and in order of decreasing equivalence ratio for 2 bar intake pressure. Increased low- and intermediate-temperature heat release with increasing equivalence ratio at 2 bar intake pressure compensates for decreased temperatures in higher-equivalence ratio regions due to evaporative cooling from the liquid fuel spray and decreased compression heating from lower values of the ratio of specific heats. The presence of low- and intermediate-temperature heat release at 2 bar intake pressure alters the temperature distribution of the mixture stratification before hot-ignition, promoting the desired sequential auto-ignition. At 1 bar intake pressure, the sequential auto-ignition occurs in the reverse order compared to 2 bar intake pressure and too fast for useful reduction of the maximum pressure rise rate compared to HCCI. Additionally, the premixed portion of the charge auto-ignites before the highest-equivalence ratio regions. Conversely, at 2 bar intake pressure, the premixed portion of the charge auto-ignites last, after the higher-equivalence ratio regions. More importantly, the sequential auto-ignition occurs over a longer time period for 2 bar intake pressure than at 1 bar intake pressure such that a sizable reduction in the maximum pressure rise rate compared to HCCI can be achieved.
Using Large Eddy Simulations to Study Diesel DI-HCCI Engine Flow Structure, Mixing and Combustion
Author: Rahul JhavarHCCI and CAI Engines for the Automotive Industry
Author: Hua ZhaoPublisher: CRC Press
ISBN:
Category : Technology & Engineering
Languages : en
Pages : 562
Book Description
Homogeneous charge compression ignition (HCCI)/controlled auto-ignition (CAI) has emerged as one of the most promising engine technologies with the potential to combine fuel efficiency and improved emissions performance, offering reduced nitrous oxides and particulate matter alongside efficiency comparable with modern diesel engines. Despite the considerable advantages, its operational range is rather limited and controlling the combustion (timing of ignition and rate of energy release) is still an area of on-going research. Commercial applications are, however, close to reality. HCCI a.
Author: Jacob Richard Zuehl