@article{2023_Fuel_LTGC,
  author = {López-Pintor, D. and Dec, J. and Cho, S.},
  title = {"Performance of octane index in LTGC engines from beyond MON to beyond RON"},
  year = {2023},
  journal = {Fuel},
  doi = {10.1016/j.fuel.2023.127625}
}

This work analyzes the inability of the Octane Index (OI) to characterize the autoignition resistance of gasoline-like fuels in a Low-Temperature Gasoline Combustion (LTGC) engine. Experimental studies in the Sandia LTGC engine at naturally aspirated ‘beyond-MON’ and high-boost ‘beyond-RON’ conditions showed a poor correlation between OI and autoignition reactivity. These experiments were successfully replicated in CHEMKIN, allowing detailed studies of the factors affecting the OI correlation. Systematic investigations were conducted to determine why the OI performs less well for LTGC combustion compared to end-gas autoignition (i.e. knock) in spark-ignition (SI) engines. A pseudo-MON condition that follows the pressure–temperature (P-T) trajectory of the real MON test was tested numerically, using 10 different fuels, leading to the conclusion that the OI correlation performs very well for an SI engine running at close-to-MON conditions. The effects of each difference between SI and LTGC combustion were then analyzed for four P-T trajectories: ‘beyond-MON’, ‘MON test’, ‘between RON-MON’, and ‘beyond-RON’ conditions. These differences included changes in: equivalence ratio, engine speed, and residence-time history along the P-T trajectory for piston-only compression, as used for LTGC, compared to compression by the piston plus flame propagation for SI combustion. For LTGC combustion, the OI correlation did not characterize the autoignition quality of gasoline-like fuels well for any P-T trajectory, but the factors causing this poor performance varied with conditions. For example, at naturally aspirated LTGC conditions (beyond-MON), changes in equivalence ratio were the primary cause, followed by differences in the P-T trajectory and changes in residence time. Whereas, for intake-boosted beyond-RON conditions, differences in the P-T trajectory had the largest effect, followed by changes in residence time. Finally, the parameter K that characterizes the OI correlation showed values that contradict the OI theory under realistic LTGC conditions, suggesting that K might be meaningless for LTGC engines.

@article{2023_Fuel_Skeletal,
  author = {Cho, S. and López-Pintor, D. and Sofianopoulus, A.},
  title = {"A Skeletal Mechanism for Gasoline Surrogates: Development, Validation, and CFD application"},
  year = {2023},
  journal = {Fuel},
  doi = {10.1016/j.fuel.2022.126236}
}

In this study, a skeletal chemical kinetic mechanism for gasoline surrogates is developed from a detailed mechanism by applying several reduction techniques. Directed relation graph (DRG) routines and DRG-aided sensitivity analysis methods are applied with worst-case error tolerances equal to 30% and 40%, respectively. Issues with reaction dead-ends in the oxidation paths are resolved by adding adequate intermediate species and reactions. Sensitivity analyses are conducted to identify the most impactful fuel-dependent reactions to ignition delay, and the specific reaction rates of such reactions are optimized to replicate the ignition behavior of detailed mechanism. The final version of the skeletal mechanism consists of 152 species and 563 reactions including NO x chemistry. Experiments are carried out in Sandia’s lowtemperature gasoline combustion research engine with an E10 regular-grade gasoline, termed RD5-87, and the results are replicated in 0D simulations with good accuracies using both detailed mechanism and the skeletal mechanism although a slight overestimation of the low-temperature heat release is observed. Finally, the performance of the skeletal mechanism is also tested in 3D CFD simulation using LES. Simulations are able to predict the pressure evolution and heat release rate, proving that the proposed skeletal mechanism is adequate for simulations.

@article{2023_Fuel_FRED,
  author = {Cho, S. and Wu, A. and Kim, N. and Busch, S. and López-Pintor, D.},
  title = {"Formation of CH2O and UHC emissions during catalyst-heating operation in compression ignition engines: High-speed FID and mid-IR extinction diagnostics"},
  year = {2023},
  journal = {Fuel},
  doi = {10.1016/j.fuel.2023.127832}
}

To comply with increasingly stringent pollutant emissions regulations, catalyst-heating operation in diesel engines is critical to achieving rapid light-off of exhaust aftertreatment catalysts during the first minutes of cold start. Current approaches to catalyst-heating operation typically involve one or more late post injections to retard combustion phasing and increase exhaust gas temperatures. The ability to retard post injection timings while maintaining acceptable pollutant emissions levels is pivotal for improved catalyst-heating operations. This experimental study aims to provide further insight into unburned hydrocarbons (UHC) and formaldehyde (CH2O) formation under catalyst-heating operation. In this study, high-speed laser extinction based measurements of CH2O are demonstrated in an optically accessible exhaust runner, and high-speed UHC measurements are also performed using a fast flame ionization detector analyzer during catalyst-heating operation. Time-resolved CH2O and UHC emission measurements are used to analyze the effect of each injection event in a five-injection strategy (two pilots, one main and two posts) on the pollutant emissions. The first pilot injection generates a significant amount of UHC unevenly distributed in the chamber, with UHC trapped primarily in the upper-cylinder and inside-the-bowl regions. The fuel delivered with the second pilot injection partially burns the unburned fuel from the first pilot injection particularly in inner-bowl region, creating short-chain hydrocarbons corresponding to formation of CH2O. UHC is dramatically reduced in the presence of the main injection, reinforcing the hypothesis by showing large CH2O emission during intermediate and later exhaust process. The first post injection produces UHC and CH2O in the upper-cylinder area due to over-mixing, which is further promoted by the second post injection, while these post-injections do not substantially affect near-bowl emissions.

@article{2022_IJER_DSL,
  author = {Wu, A. and Cho, S. and López-Pintor, D. and Busch, S. and Perini, F. and Reitz, R.},
  title = {"Effects of a CFD-improved Dimple Stepped-lip Piston on Thermal Efficiency and Emissions in a Medium-duty Diesel Engine"},
  year = {2022},
  journal = {International Journal of Engine Research},
  doi = {10.1177/14680874221117869}
}

Diesel piston-bowl shape is a key design parameter that affects spray-wall interactions and turbulent flow development, and in turn affects the engine’s thermal efficiency and emissions. It is hypothesized that thermal efficiency can be improved by enhancing squish-region vortices as they are hypothesized to promote fuel-air mixing, leading to faster heat-release rates. However, the strength and longevity of these vortices decrease with advanced injection timings for typical stepped-lip (SL) piston geometries. Dimple stepped-lip (DSL) pistons enhance vortex formation at early injection timings. Previous engine experiments with such a bowl show 1.4% thermal efficiency gains over an SL piston. However, soot was increased dramatically [SAE 2022-01-0400]. In a previous study, a new DSL bowl was designed using non-combusting computational fluid dynamic simulations. This improved DSL bowl is predicted to promote stronger, more rotationally energetic vortices than the baseline DSL piston: it employs shallower, narrower, and steeper-curved dimples that are placed further out into the squish region. In the current experimental study, this improved bowl is tested in a medium-duty diesel engine and compared against the SL piston over an injection timing sweep at low-load and part-load operating conditions. No substantial thermal efficiency gains are achieved at the early injection timing with the improved DSL design, but soot emissions are lowered by 45% relative to the production SL piston, likely due to improved air utilization and soot oxidation. However, these benefits are lost at late injection timings, where the DSL piston renders a lower thermal efficiency than that of the SL piston. Energy balance analyses show higher wall heat transfer with the DSL piston than with the SL piston despite a 1.3% reduction in the piston surface area. Vortex enhancement may not necessarily lead to improved efficiency as more energetic squish-region vortices can lead to higher convective heat transfer losses.

@article{2022_Fuel_EHN_RCM,
  author = {Cho, S. and López-Pintor, D. and Goldsborough, S.},
  title = {"Chemical Kinetic Interactions and Sensitivity Analyses for 2-Ethylhexyl Nitrate-doped PRF91 using a Reduced Mechanism"},
  journal = {Fuel},
  year = {2022},
  doi = {10.1016/j.fuel.2022.125503},
  file = {Cho.Fuel.2022_EHN_RCM.pdf}
}

A numerical and experimental investigation about the chemical kinetic interactions between 2ethylhexylnitrate (EHN) and PRF91 was performed in this study. Rapid compression machine experiments were conducted to investigate the effect of EHN onthe autoignition reactivity of the fuel, and a reduced chemical kinetic mechanism was developed including an EHN sub-model. Experiments showed that the ignition delay decreases as the fuel is doped with EHN, but the effect of the doping level of EHN on the ignition is highly non-linear. Moreover, experiments showed that the EHN effectiveness is lowest during the transition between the low-temperature regime and the negative temperature coefficient (NTC) regime, and it rapidly increases as the temperature increases. Both detailed and (developed) reduced mechanisms were validated against the experimental results, allowing a more in-depth EHN-fuel chemistry study. Additionally, ignition delay sensitivity and EHN effectiveness sensitivity analyses were performed with the reduced mechanism to identify the reactions that control the interaction between EHN and the fuel. As the result, EHN thermal decomposition is only relevant for very low temperatures. The chemistry of EHN-doped fuel is more sensitive to intermediate temperature reactions than that of straight fuel, especially at lower temperatures, due to the effect of EHN on the NTC behavior of the fuel. Finally, the chemistry of EHN-doped fuel is very sensitive to NO2-to-NO reactions, especially at high temperatures, because these reactions transform the NO2 generated by EHN into NO, which is a very effective fuel reactivity enhancer.

@article{2022_Fuel_EHN_HCCI,
  author = {Cho, S. and López-Pintor, D.},
  title = {"Understanding the Effects of Doping a Regular E10 Gasoline with EHN in an HCCI Engine: Experimental and Numerical Study"},
  year = {2022},
  journal = {Fuel},
  doi = {10.1016/j.fuel.2022.125456},
  file = {Cho.Fuel.2022_EHN_HCCI.pdf}
}

In this study, the effects of doping a regular E10 gasoline with 2-ethlyhexyl nitrate (EHN) are investigated under homogeneous charge compression ignition conditions. Experiments are performed in a 1-liter single-cylinder engine fueled with both straight and EHN-doped E10 gasoline. Numerical studies are performed with an internal combustion engine single-zone reactor utilizing detailed chemical kinetic mechanism with EHN and NO x chemistry and a surrogate fuel. The kinetic model reproduces the experimental data well for the straight fuel at high initial temperature (TBDC) conditions, whereas the low temperature heat release (LTHR) is under-predicted. Adding EHN reduces the required T BDC , while EHN thermal decomposition rate has to be significantly reduced to accurately reproduce the experimental result, by preventing over-estimation of EHN effect and LTHR. EHN decomposition generates NO2 and 3-heptyl radicals. Using the well-matched mechanism, the numerical results indicate that among the product of EHN decomposition, NO2 decreases the autoignition reactivity whereas the production of 3-heptyl radical is the main source for enhancing the low-to-intermediate chemistry by which OH production is accelerated. The production of 3-heptyl radical is highly sensitive to the EHN decomposition reactions. Despite the reactivity enhancement, increase in NO x emission is observed when the fuel-doping increases.

@article{2021_ECM_OCM,
  author = {Cho, S. and Lee, H. and Lin, Y. and Singh, S. and Northrop, W.},
  title = {"Products of Catalytic Oxidative Coupling of Methane to Improve Thermal Efficiency in Natural Gas Engines"},
  year = {2022},
  journal = {Energy Conversion and Management},
  doi = {10.1016/j.enconman.2022.116030},
  file = {Cho.ECM.2022.pdf}
}

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.

@article{2022_SAE_CatHeating,
  author = {Cho*, S. and Busch, S. and Wu, A. and López-Pintor, D.},
  title = {"Effect of Fuel Cetane Number on the Performance of Catalyst-Heating Operation in a Medium-duty Diesel Engine"},
  year = {2022},
  journal = {SAE International Journal of Advanced & Current Practices in Mobility},
  doi = {10.4271/2022-01-0483},
  file = {Cho.SAE.2022-01-0483.pdf}
}

To comply with increasingly stringent pollutant emissions regulations, diesel engine operation in a catalyst-heating mode is critical to achieve rapid light-off of exhaust aftertreatment catalysts during the first minutes of cold starting. Current approaches to catalyst-heating operation typically involve one or more late post injections to retard combustion phasing and increase exhaust temperatures. The ability to retard post injection timing(s) while maintaining acceptable pollutant emissions levels is pivotal for improved catalyst-heating calibrations. Higher fuel cetane number has been reported to enable later post injections with increased exhaust heat and decreased pollutant emissions, but the mechanism is not well understood. The purpose of this experimental and numerical simulation study is to provide further insight into the ways in which fuel cetane number affects combustion and pollutant formation in a medium-duty diesel engine. Three full boiling-range diesel fuels with cetane numbers of approximately 45, 50, and 55 are employed in this study with a well-controlled set of calibrations employing a five-injection strategy. The two post injections are block-shifted to increasingly retarded timings, and the effects on exhaust heat and pollutant emissions are quantified for each fuel. For a given injection strategy calibration, increasing cetane number enables increased exhaust temperature and decreased hydrocarbon and carbon monoxide emissions for a fixed load. The increase in exhaust temperature is attributed to an increased fueling requirement to compensate for additional wall heat losses caused by earlier, more robust pilot combustion with the more reactive fuels. Formaldehyde is predicted to form in the fuel-lean periphery of the first pilot injection spray and can persist until exhaust valve opening in the absence of direct interactions with subsequent injections. Unreacted fuel-air mixture in the fuel-rich interior of the first-pilot spray is likely too cool for any significant reactions, and can persist until exhaust valve opening in the absence of turbulence/chemistry interactions and/or direct heating through interactions with subsequent injections.

@article{2022_IJER_Knock,
  author = {Cho, S. and Song, C. and Lee, Y. and Kim, N. and Oh, S. and Min, K.},
  title = {"Prediction of Knock Propensity Using Stochastic Modeling in a Spark-Ignition Engine"},
  year = {2022},
  journal = {International Journal of Engine Research},
  doi = {10.1177/14680874221074993},
  file = {Cho.IJER.2022.pdf}
}

To comply with stringent CO2 regulations, enhanced thermal efficiency has been prioritized in internal combustion engine development; however, this has strongly driven the development of engines with operating conditions more prone to knock. Current knock sensors have its limitations to decompose knock signal by degradation so that it required a cross-referencing signal. In addition, knock control intervention is currently preceded by the occurrence of the knock, leading to decrease in thermal efficiency by retarding spark timing. In the present work, a novel prediction model for knock propensity (incidence) is presented, aiming to enable active control of knock or autoignition, and to support conventional knock sensor for cross-referencing by facilitating virtual knock sensor. A zero-dimensional model-based prediction of the in-cylinder pressure is demonstrated to prevent using in-cylinder pressure transducer, along with other incorporated predictive sub-models for the residual gas fraction, heat loss, burn duration, and heat release rate. Ignition delay correlation and Livengood-Wu relation are used to predict the onset of knock, and a burn point-based criterion is newly proposed for application in stochastic modeling for determining the knock propensity. The predicted knock propensity from the combined holistic model shows a remarkable agreement with experimental results.

@article{2022_Fuel_ShortCom,
  author = {López-Pintor, D. and Cho, S.},
  title = {"Effects of the stability of 2-methyl furan and 2, 5 dimethyl furan on the autoignition and combustion characteristics of a gasoline-like fuel"},
  year = {2022},
  journal = {Fuel},
  doi = {10.1016/j.fuel.2021.122990}
}

Renewable liquid fuels have potential to greatly reduce the carbon footprint of the transportation sector while leveraging existing powertrain technologies and infrastructure. Prior studies identified a mixture of 2-methyl furan and 2, 5 dimethyl furan as one of the most promising components for formulating renewable gasoline fuel blends. Within the Co-Optima initiative, this furan mixture was used in the formulation of a custom renewable gasoline-like fuel, termed CB#2, that contains 40% of the furans in volume. Although CB#2 has demonstrated better performance than regular E10 gasoline for advanced compression ignition and boosted spark ignition engines, several concerns arose regarding stability of furans. Stability can be improved by adding antioxidant additives, but the effect of furans’ stability and the antioxidants on the ignition and combustion characteristics of gasoline fuels is unknown. In the present work, the effects of the stability of 2-methyl furan and 2, 5 dimethyl furan and the addition of butylhydroxytoluene on the autoignition and combustion characteristics of CB#2 were studied. Experimental measurements were performed using unstabilized CB#2 and CB#2 stabilized using butylhydroxytoluene (added in a concentration of 250 ppm to the furan species). From the test result, both fuel batches showed the same octane rating. Moreover, homogeneous charge compression ignition engine experiments at naturally aspirated and lean conditions showed that the two batches have the same autoignition reactivity and combustion characteristics. Therefore, fuel performance was not affected by the stability of the furans and the addition of butylhydroxytoluene at the concentration explored in this paper.

@article{2022_ECM_GasCatHeat,
  author = {Kim, N. and Chung, J. and Kim, J. and Cho, S. and Min, K.},
  title = {"Effect of Injection Parameters on Combustion and Emission Characteristics under Catalyst Heating Operation in a Direct-Injection Spark-Ignition Engine"},
  year = {2022},
  journal = {Energy Conversion and Management},
  doi = {10.1016/j.enconman.2021.115059}
}

Clean and stable operation during the catalyst heating mode in a direct-injection spark-ignition engine is crucial to meet increasingly stringent emission regulations. Although the injection strategy has a profound effect on both emission and combustion characteristics, the relative importance of various injection parameters is not well understood under catalyst heating operation. Consequently, a parametric sweep of injection timing, injection pressure, and a number of split injections was conducted to reveal the relative impact of each parameter on particulate emissions, unburned hydrocarbon emissions, and combustion characteristics. A single-cylinder research engine with a side-mounted multi-hole injector was used in the experiments. The fuel was injected during the early intake stroke with 2–4 split injections using 10 and 20 MPa of injection pressure. Retarding the injection timing had the most significant influence on the accumulation-mode particulate emission. However, combustion stability can deteriorate if the injection timing is retarded beyond a certain point, possibly because of unfavorable fuel distribution near the spark plug. The higher number of split injections and/or injection pressure also contributed to lower accumulation-mode particulates. However, the combination of higher injection pressure and split numbers led to increased combustion variability due to the injector operating in a ballistic regime. The unburned hydrocarbon emissions were found to be insensitive to the injection parameters. Utilizing the parametric sweep results, it was hypothesized that the curtailment of the amount of fuel injected while the piston is located closer to the injector tip contributes to a lower fuel film on the piston surface and hence reduces engine-out particulate emissions. The estimation of fuel films using three-dimensional computational fluid dynamics confirmed that the hypothesis is valid under the catalyst heating operation used in this study. In the last step, the split ratio was varied such that the injection duration per injection increased gradually. Such a split ratio enabled further reduction of particulate emissions without incurring a penalty in unburned hydrocarbon emissions and combustion stability.

@article{2021_SAE_CatHeating,
  author = {Busch, S. and Wu, A. and Cho, S.},
  title = {"Catalyst heating operation in a medium-duty diesel engine: operating strategy calibration, fuel reactivity, and fuel oxygen effects"},
  year = {2022},
  journal = {SAE International Journal of Advanced & Current Practices in Mobility},
  doi = {10.4271/2021-01-1182}
}

Compliance with future ultra-low nitrogen oxide regulations with diesel engines requires the fastest possible heating of the exhaust aftertreatment system to its proper operating temperature upon cold starting. Late post injections are commonly integrated into catalyst-heating operating strategies. This experimental study provides insight into the complex interactions between the injection-strategy calibration and the tradeoffs between exhaust heat and pollutant emissions. Experiments are performed with certification diesel fuel and blends of diesel fuel with butylal and hexyl hexanoate. Further analyses of experimental data provide insight into fuel reactivity and oxygen content as potential enablers for improved catalyst-heating operation. A statistical design-of-experiments approach is developed to investigate a wide range of injection strategy calibrations at three different intake dilution levels. Thermodynamic and exhaust emissions measurements are taken using a new medium-duty, single-cylinder research engine. Analysis of the results provides insight into the effects of exhaust gas recirculation, oxygenated fuel blends, and fuel reactivity on exhaust heat and pollutant emissions. Late-cycle heat release is an important factor in determining exhaust temperatures. Intake dilution and fuel properties certainly affect late-cycle heat release, but the methods applied in this work are not sufficient to reproduce or explain the mechanisms by which improved fuel cetane rating promotes operation with hotter exhaust and lower pollutant emissions.

@article{2021_ATE,
  author = {Cho, S. and Song, C. and Kim, N. and Oh, S. and Han, D. and Min, K.},
  title = {"Influence of the Wall Temperatures of the Combustion Chamber and Intake Ports on the Charge Temperature and Knock Characteristics in a Spark-ignited Engine"},
  year = {2021},
  journal = {Applied Thermal Engineering},
  doi = {10.1016/j.applthermaleng.2020.116000},
  file = {Cho.ATE.2021.pdf}
}

Reducing wall temperatures is a promising method to suppress knocking behavior in spark-ignited engine. However, this may increase undesirable heat loss which acts as countereffect, so a strategic cooling approach is required. In this study, a multidisciplinary investigation of the wall temperature effect was demonstrated using experiments and simulations. By experiments under full load and part load conditions, improvements in the indicated thermal efficiency achieved by knock-limited spark advancement were obtained, and detailed analyses were incorporated. Under cooled conditions, it was found that an improved thermal efficiency was achieved by not only the advanced combustion phasing but also the reduced compression work obtained from increased gas density, particularly under part load conditions. By categorizing and evaluating the heat transfer phases using simulations, it was found that the cooled wall temperature did not provide a significant gas temperature drop via compression and combustion processes. Unexpectedly, a notable contribution to gas heat transfer reduction arose during the early gas induction stage because of not only the extended period of heat transfer but also the large surface area and initial low temperature before compression. An enhanced cooling on cylinder head resulted in a larger effect on knock mitigation than enhanced liner cooling under normal conditions, attributed to the large heat transfer at the intake port wall. From the assessment, as the liner coolant dominated the piston surface, it was found that the contribution of the liner wall temperature to the gas temperature reduction was significantly influential, even showing a higher knock mitigation effect after intake port insulation was applied. Intensified tumble flow showed a high potential of gas temperature decrease by increasing the heat transfer from gas to wall during the compression stroke, and the effect of the enhanced cooling on the liner was more significant than that of the normal intake port due to the high velocity and turbulence of the air. The simulation results revealed that enhanced liner cooling could decrease the in-cylinder temperature by more than 18 K when insulation and intensification were both applied to the intake port design.

@article{2021_IJER_Soot,
  author = {Kim, J. and Chung, J. and Kim, N. and Cho, S. and Lee, J. and Oh, S. and Song, C. and Min, K.},
  title = {"Numerical Investigation of Soot Emission Sources in a Direct-Injection Spark-Ignition Engine Based on Comprehensive Breakup Model Validation"},
  year = {2021},
  journal = {International Journal of Engine Research},
  doi = {10.1177/14680874211047524}
}

Direct injection system is widely adopted in spark-ignition engines to achieve higher thermal efficiency, but it accompanies a penalty in particulate emission, especially when engine is not fully warmed-up. Split injection strategy is known to be an effective measure to reduce engine-out particulate emissions. To better understand the role of split injections, this study aims to analyze the effect of split injection strategy on the sources of soot formation using computational fluid dynamics simulation. To accurately predict changes in particulate mass and number associated with split injection strategy, it is vital that spray models be carefully validated against the experimental data since spray dynamics govern the formation of soot emission sources, such as local fuel-rich mixtures and wall-deposited fuel-films. To this end, a set of spray experiments for free sprays is performed to measure liquid penetration length and droplet size distribution, and hence a comprehensive validation is conducted for spray breakup models. Then, engine simulations are carried out to predict the change in soot sources according to split injection, and the trend of simulation results is compared against the measured engine-out particulate mass and number. Simulation results indicate that breakup model validation using both penetration length and droplet size data is critical for predicting fuel spray dynamics and formation of sources of soot emission. It is also revealed that the piston wetting decreases as the number of injections increases because less amount of fuel is injected when piston is closer to the injector. Lastly, the late evaporation of heavy gasoline components from fuel-film appears to be a significant contributor to soot precursors formation.

@article{2019_Energies,
  author = {Cho, S. and Park, J. and Song, C. and Oh, S. and Lee, S. and Kim, M. and Min, K.},
  title = {"Prediction Modeling and Analysis of Knocking Combustion with an Improved 0D RGF Model and Supervised Deep Learning"},
  year = {2019},
  journal = {Energies},
  doi = {10.3390/en12050844},
  file = {Cho.Energies.2019.pdf}
}

The knock phenomenon is one of the major hindrances for enhancing the thermal efficiency in spark-ignited engines. Due to the stochastic behavior of knocking combustion, analytical cycle studies are required. However, there are many problems to be addressed with regard to the individual cycle analysis of in-cylinder pressure data. This study thus proposes novel, comprehensive and efficient methodologies for evaluating the knocking combustion in the internal combustion engine. The proposed methodologies include a filtering method for the in-cylinder pressure, the determination of the knock onset, and the calculation of the residual gas fraction. Consequently, a smart knock onset model with high accuracy could be developed using a supervised deep learning that was not available in the past. Moreover, an improved zero-dimensional (0D) estimation model for the residual gas fraction was developed to obtain better accuracy for closed system analysis. Finally, based on a cyclic analysis, a knock prediction model is suggested; the model uses 0D ignition delay correlation under various experimental conditions including aggressive cam phase shifting by a dual variable valve timing (VVT) system. Using the proposed analysis method, insight into stochastic knocking combustion can be obtained, and a faster combustion speed can lead to a higher knock intensity in a steady-state operation.

@article{2019_Energies_0D,
  author = {Kim, Y. and Kim, M. and Oh, S. and Shin, W. and Cho, S. and Song, HH},
  title = {"A New Physics-based Modeling Approach for a 0D Turbulence Model to Reflect the Intake Port and Chamber Geometries and the Corresponding Flow Structures in High-Tumble Spark-Ignition Engines"},
  year = {2019},
  journal = {Energies},
  doi = {10.3390/en12101898}
}

Turbulence is one of the most important aspects in spark-ignition engines as it can significantly affect burn rates, heat transfer rates, and combustion stability, and thus the performance. Turbulence originates from a large-scale mean motion that occurs during the induction process, which mainly consists of tumble motion in modern spark-ignition engines with a pentroof cylinder head. Despite its significance, most 0D turbulence models rely on calibration factors when calculating the evolution of tumble motion and its conversion into turbulence. In this study, the 0D tumble model has been improved based on the physical phenomena, as an attempt to develop a comprehensive model that predicts flow dynamics inside the cylinder. The generation and decay rates of tumble motion are expressed with regards of the flow structure in a realistic combustion chamber geometry, while the effects of port geometry on both charging efficiency and tumble generation rate are reflected by supplementary steady CFD. The developed tumble model was integrated with the standard k-ε model, and the new turbulence model has been validated with engine experimental data for various changes in operating conditions including engine speed, load, valve timing, and engine geometry. The calculated results showed a reasonable correlation with the measured combustion duration, verifying this physics-based model can properly predict turbulence characteristics without any additional calibration process. This model can suggest greater insights on engine operation and is expected to assist the optimization process of engine design and operating strategies.

@article{2019_SAE_SB,
  author = {Oh, S. and Cho, S. and Seol, E. and Song, C. and Shin, W. and Min, K. and Song, HH},
  title = {"An Experimental Study on the Effect of Stroke-to-Bore Ratio of Atkinson DISI Engines with Variable Valve Timing"},
  year = {2018},
  journal = {SAE Int. J. Engines},
  doi = {10.4271/2018-01-1419}
}

In this study, fundamental questions in improving thermal efficiency of spark-ignition engine were revisited, regarding two principal factors, that is, stroke-to-bore (S/B) ratio and valve timings. In our experiment, late intake valve closing (LIVC) camshaft and variable valve timing (VVT) module for valve timing control were equipped in the single-cylinder, direct-injection spark-ignition (DISI) engine with three different S/B ratios (1.00, 1.20, and 1.47). In these three setups, displacement volume and compression ratio (CR) were fixed. In addition, the tumble ratio for cylinder head was also kept the same to minimize the flow effect on the flame propagation caused by cylinder head while focusing on the sole effect of changing the S/B ratio. The experiments were performed in two steps: Firstly, univariate analysis based on the basic input variables-intake camshaft timing, exhaust camshaft timing, and start of injection (SOI)-was conducted to understand the effect of each variable in various load conditions of each S/B ratio. Secondly, design of experiment (DoE) was conducted to find the point of the optimum indicated thermal efficiency of each engine, considering the mutual effect among these input variables. The optimum results showed that at low-load operation (net indicated mean effective pressure (IMEP) 4.5 bar), the values of indicated efficiency are in the order of S/B ratio 1.20 > 1.00 > 1.47, mainly attributed by increased cooling and exhaust loss at higher S/B ratio (i.e., 1.47). However, in case of IMEP 6.5 bar, knock occurrence at lower S/B ratio (i.e., 1.00) led to retarded ignition timing, incurring higher exhaust loss and slower burning rate. In consequence, the best values of the net indicated specific fuel consumption (nisfc) at IMEP 6.5 bar are in the order of S/B ratio 1.20 > 1.47 > 1.00; changing S/B ratio from 1.0 to 1.2 improved nisfc by 1.36%, while changing S/B ratio from 1.2 to 1.47 degraded nisfc by 1.11%.

@article{2015_Fuel,
  author = {Kim, N. and Cho, S. and Min, K.},
  title = {"A study on the combustion and emission characteristics of an SI engine under full load conditions with ethanol port injection and gasoline direct injection"},
  year = {2015},
  journal = {Fuel},
  doi = {10.1016/j.fuel.2015.06.025}
}

Ethanol has the potential to improve engine efficiency and reduce harmful emissions when used as fuel in a spark-ignited engine. Ethanol is mostly supplied in a splash-blended form with gasoline or in a pure form; however, this is not an optimal way of using ethanol because the use of ethanol leads to increased brake specific fuel consumption. To fully utilize the merits of ethanol, on-demand control of ethanol and gasoline is required so that the fuel-blending ratio can be altered according to the engine operating conditions. This study investigated the effect of ethanol port fuel injection and gasoline direct injection systems on engine combustion and emission characteristics under full load conditions. The experiment was conducted using two different compression ratios and various ethanol injection timings. Knock occurrence decreased as the ethanol injection timing was held while intake valves were open. Minor reductions in carbon monoxide, total hydrocarbon, and particulate emissions were observed under a compression ratio of 9.5, while the reduction in emissions became significant under a compression ratio of 13.3 as the amount of ethanol injection increased.

@article{2015_Energy,
  author = {Lee, K. and Cho, S. and Kim, N. and Min, K.},
  title = {"A study on combustion control and operating range expansion of gasoline HCCI"},
  year = {2015},
  journal = {Energy},
  doi = {10.1016/j.energy.2015.08.031},
  file = {Lee.Energy.2015.pdf}
}

Because of the combustion principle of a gasoline HCCI (homogeneous charge compression ignition) engine, the operating range is limited within a narrow area. The objectives of this study were to extend the operating range of gasoline HCCI combustion and to develop control logic. To extend the high load operating range, several strategies including external EGR (exhaust gas recirculation), EGR stratification, fuel stratification and valve timing swing were adopted. Among these strategies, EGR stratification, asymmetric injection and open valve injection are novel techniques. The high load boundary of the low speed region was improved more than that of the high speed region. The improvement in the low load boundary was due to the direct injection during negative valve overlap. In terms of stabilizing the HCCI combustion phase, the peak pressure value and pressure rising rate of a cycle were important factors when considering the ringing intensity equation. Coefficient of variation of combustion was also used to judge the stabilization of the combustion. In this study, using these variables, the engine was controlled within the maps which were determined from the experiment. The indicated mean effective pressure calculated from real-time data followed the target load successfully without severe problems.

@conferences{SAE Technical Paper 2023-01-0262,
  doi = {10.4271/2023-01-0262},
  author = {Cho*, S. and Wu, A. and López-Pintor, D.},
  year = {2023},
  title = {"Understanding Hydrocarbon Emissions to Improve the Performance of Catalyst-Heating Operation in a Medium-Duty Diesel Engine"},
  booktitle = {SAE Technical Paper 2023-01-0262},
  address = {Detroit, Michigan, USA}
}

@conferences{SAE Technical Paper 2022-01-0400,
  doi = {10.4271/2022-01-0400},
  author = {Wu, A. and Perini, F. and Cho, S. and Busch, S. and López-Pintor, D. and Reitz, R.},
  year = {2022},
  title = {"Numerical Studies of a Novel Dimpled Stepped-Lip Piston Design on Turbulent Flow Development in a Medium-Duty Diesel Engine"},
  booktitle = {SAE Technical Paper 2022-01-0400},
  address = {Detroit, Michigan, USA}
}

@conferences{SAE Technical paper 2021-01-0383,
  doi = {10.4271/2021-01-0383},
  author = {Park, J. and Lee, S. and Cho, S. and Shin, S. and Kim, M. and Song, C. and Min, K.},
  year = {2021},
  title = {"Improvement of Knock Onset Determination Based on Supervised Deep Learning Using Data Filtering"},
  booktitle = {SAE Technical Paper 2021-01-0383},
  address = {Detroit, Michigan, USA}
}

@conferences{SIA 2019,
  author = {Cho, S. and Song, C. and Park, J. and Song, HH and Min, K.},
  year = {2019},
  title = {"Development of Knock Prediction Model for On-board Control in a Spark-Ignited Engine"},
  booktitle = {SIA 2019 Paris Powertrain & Electronics},
  address = {Port-Marly, France},
  file = {Cho.SIA.2019.pdf}
}

@conferences{SAE Technical paper 2019-01-1138,
  doi = {10.4271/2019-01-1138},
  author = {Cho, S. and Oh, S. and Song, C. and Shin, W. and Song, HH and Lee, B. and Jung, D. and Woo, SH and Min, K.},
  year = {2019},
  title = {"Effects of Bore-to-Stroke Ratio on Efficiency and Knock Characteristics in a Single-cylinder GDI Engine"},
  booktitle = {SAE Technical Paper 2019-01-1138},
  address = {Detroit, Michigan, USA},
  file = {Cho.SAE.2019-01-1138.pdf}
}

@conferences{SAE Technical paper 2018-01-0213,
  doi = {10.4271/2018-01-0213},
  author = {Cho, S. and Song, C. and Oh, S. and Min, K.},
  year = {2019},
  title = {"An Experimental Study on the Knock Mitigation Effect of Coolant and Thermal Boundary Temperatures in Spark Ignited Engines"},
  booktitle = {SAE Technical paper 2018-01-0213},
  address = {Detroit, Michigan, USA},
  file = {Cho.SAE.2018-01-0213.pdf}
}

@conferences{SIA 2017,
  author = {Cho, S. and Song, C. and Kim, M. and Ha, K. and Kim, B. and Suh, I. and Min, K.},
  year = {2019},
  title = {"The Effect of Thermal Boundary Conditions on Knock Characteristics in a Single Cylinder Spark Ignited Engine"},
  booktitle = {SIA Powertrain Conference 2017},
  address = {Versailles, France},
  file = {Cho.SIA.2017.pdf}
}

@conferences{FISITA 2018,
  author = {Min, K. and Song, C. and Cho, S.},
  year = {2018},
  title = {"A Study on the Effect of Wall Temperature on Knock Phenomena using a Single Cylinder Spark-Ignited Engine"},
  booktitle = {FISITA 2018},
  address = {Chennai, India},
  file = {Min.FISITA.2018.pdf}
}

@conferences{SAE Technical paper 2015-01-0764,
  doi = {10.4271/2015-01-0764},
  author = {Cho, S. and Kim, N. and Chung, J. and Min, K.},
  year = {2015},
  title = {"The Effect of Ethanol Injection Strategy on Knock Suppression of the Gasoline/Ethanol Dual Fuel Combustion in a Spark-Ignited Engine"},
  booktitle = {SAE Technical paper 2015-01-0764},
  address = {Detroit, Michigan, USA},
  file = {Cho.SAE.2015-01-0764.pdf}
}

@conferences{SAE Technical paper 2014-01-1208,
  doi = {10.4271/2014-01-1208},
  author = {Kim, N. and Cho, S. and H. Choi, HH Song and Min, K.},
  year = {2014},
  title = {"The Efficiency and Emission Characteristics of Dual Fuel Combustion Using Gasoline Direct Injection and Ethanol Port Injection in an SI Engine"},
  booktitle = {SAE Technical paper 2014-01-1208},
  address = {Detroit, Michigan, USA}
}

@unpublished{2023_OCM2,
  author = {Lee, H. and Cho, S. and Northrop, W.},
  title = {"Understanding Oxidative Coupling of Methane of Engine-Scale Reactor in HCCI Combustion"},
  journal = {Undefined journal},
  year = {Undefined year}
}

@unpublished{2023_HFS,
  author = {Cho, S. and Busch, S. and Wu, A. and López-Pintor, D. and Hong, K.},
  title = {"Impact and Sensitivity Analysis of Design Parameters in MEMS-based Fast-Response Heat Flux Sensor usuing 1D Numerical Simulation"},
  journal = {Undefined journal},
  year = {Undefined year}
}

@unpublished{2023_Fuel_Volatility,
  author = {Cho, S. and López-Pintor, D. and Busch, S.},
  title = {"Effect of Fuel Volatility on Catalyst-Heating Operating in a Compression Ignition Engine"},
  journal = {Undefined journal},
  year = {Undefined year}
}

@unpublished{2022_Fuel_OctaneIndex,
  author = {López-Pintor, D. and Mehl, M. and Cho, S. and Dec, J.},
  title = {"A Methodology to Replicate LTGC Engine Conditions in a Single-zone Model and Validation of a Surrogate Fuel for an AKI 88.5­ – E10 Research-grade Gasoline Versus Experimental Measurements"},
  journal = {Undefined journal},
  year = {Undefined year}
}

@unpublished{2023_PEL,
  author = {Cho, S. and López-Pintor, D. and Nguyen, T. and Wu, A. and Hwang, J.},
  title = {"Large Eddy Simulation of Multi-injection Strategy during Catalyst-Heating Operation in a Compression Ignition Engine"},
  journal = {Undefined journal},
  year = {Undefined year}
}