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Since the amount of emitted CO2 is directly related to car fuel economy, attention is being drawn to DISI (Direct-Injection Spark-Ignition) engines, which have better fuel economy than conventional gasoline engines. However, it has been a problem that the rich air-fuel mixtures associated with fuel films during cold starts due to spray impingement produce particulate matter (PM). In predicting soot formation, it is important to predict the mixture field precisely. Thus, accurate spray and film models are a prerequisite of the soot model. The previous models were well matched with low-speed collision conditions, such as those of diesel engines, which have a relatively high ambient pressure and long traveling distances. Droplets colliding at low velocities have an order of magnitude of kinetic energy similar to that of the sum of the surface tension energy and the critical energy at which the splash occurs.

The combustion process using two fuels with different reactivity, known as dual-fuel combustion or RCCI is mainly studied to reduce emissions while maintaining thermal efficiency compared to the conventional diesel combustion. Many studies have proven that dual-fuel combustion has a positive prospect in future combustion to achieve ultra-low engine-out emissions with high indicated thermal efficiency. However, a limitation on high-load expansion due to the higher maximum in-cylinder pressure rise rate (mPRR) is a main problem. Thus, it is important to establish the operating strategy and study the effect of in-cylinder flow motion with dual-fuel combustion to achieve a low mPRR and emissions while maintaining high-efficiency. In this research, the characteristics of gasoline-diesel dual-fuel combustion on different hardware were studied to verify the effect of the in-cylinder flow motion on dual-fuel combustion.

Modeling of unburned hydrocarbon oxidation in an SI engine was performed in engine condition using modified one-step oxidation model. The new one-step equation was developed by modifying the Arrhenius reaction rate coefficients of the conventional one-step model. The modified model was well matched with the results of detailed chemical reaction mechanism in terms of 90 % oxidation time of the fuel. In this modification, the effect of pressure and intermediate species in the burnt gas on the oxidation rate investigated and included in developed one-step model. The effect of pressure was also investigated and included as an additional multiplying factor in the reaction equation. To simulate the oxidation process of piston crevice hydrocarbons, a computational mesh was constructed with fine mesh density at the piston crevice region and the number of cell layers in cylinder was controlled according to the motion of piston.

A gasoline homogeneous charged compression ignition (HCCI) called the controlled auto ignition (CAI) engine is an alternative to conventional gasoline engines with higher efficiency and lower emission levels. However, noise and vibration are currently major problems in the CAI engine. The problems result from fast burning speeds during combustion, because in the CAI engine combustion is controlled by auto-ignition rather than the flame. Thus, the ignition delay of the local mixture has to vary according to the location in the combustion chamber to avoid noise and vibration. For making different ignition delays, stratification of temperature or mixing ratio was tested in this study. In charge stratification, which determines the difference between the start of combustion among charges with different properties, two kinds of mixtures with different properties flow into two intake ports.

CAI engine is well known to be advantageous over conventional SI engines because it facilitates higher engine efficiency and lower emission (NOx and smoke). However, its limited operation range, large cyclic variation, and difficulty in heat release control are still unresolved obstacles. Previous studies showed that a high load range of the CAI engine is limited mainly by the combustion noise caused by a stiff pressure rise (knock), and that a low load range is also limited by the combustion instability caused by the high dilution of residual gas. In this study, the characteristics of each cycle were analyzed to find the cause of the cycle variation at the high load limit of CAI operation. Moreover, to improve combustion stability, we tested the in-cylinder fuel stratification by applying nonsymmetrical fuel injection to the intake port. Experiments were performed on a PFI single cylinder research engine equipped with dual CVVT and low lift (2 mm) cam shaft with NVO strategy.

The residual gas in SI engines is one of important factors on emission and performance such as combustion stability. With high residual gas fractions, flame speed and maximum combustion temperature are decreased and there are deeply related with combustion stability, especially at Idle and NOx emission at relatively high engine load. Therefore, there is a need to characterize the residual gas fraction as a function of the engine operating parameters. A model for predicting the residual gas fraction has been formulated in this paper. The model accounts for the contribution due to the back flow of exhaust gas to the cylinder during valve overlap and it includes in-cylinder pressure prediction model during valve overlap. The model is derived from the one dimension flow process during overlap period and a simple ideal cycle model.

An LPG engine for heavy duty vehicles has been developed using liquid phase LPG injection (hereafter LPLI) system, which has regarded as as one of next generation LPG fuel supply systems. In this work the optimized piston cavities were investigated and chosen for an LPLI engine system. While the mass production of piston cavities is considered, three piston cavities were tested: Dog-dish type, bathtub type and top-land-cut bathtub type. From the experiments the bathtub type showed the extension of lean limit while achieving the stable combustion, compared to the dog-dish type at the same injection timing. Throughout CFD analysis, it was revealed that the extension of lean limit was due to an increase of turbulence intensity by the enlarged crevice area, and the enlargement of flame front surface owing to the shape of the bathtub piston cavity compared to that of the dog-dish type.

The role of 1D simulation tool is growing as the engine system is becoming more complex with the adoption of a variety of new technologies. For the reliability of the 1D simulation results, it is necessary to improve the accuracy and applicability of the combustion model implemented in the 1D simulation tool. Since the combustion process in SI engine is mainly determined by the turbulence, many models have been concentrating on the prediction of the evolution of in-cylinder turbulence intensity. In this study, two turbulence models which can resemble the turbulence intensity close to that of 3D CFD tool were utilized. The first model is dedicated to predicting the evolution of turbulence intensity during intake and compression strokes so that the turbulence intensity at the spark timing can be estimated properly. The second model is responsible for predicting the turbulence intensity of burned and unburned zone during the combustion process.

In recent years, Large-Eddy Simulation (LES) is spotlighted as an engineering tool and severe research efforts are carried out on its applicability to Internal Combustion Engines (ICEs). However, there is a general lack of definitive conclusions on LES quality criteria for ICE. This paper focuses on the application of LES quality criteria to ICE and to their correlation, in order to draw a solid background on future LES quality assessments for ICE. In this paper, TCC-III single-cylinder optical engine from University of Michigan is investigated and the analysis is conducted under motored condition. LES quality is mainly affected by grid size and type, sub-grid scale (SGS) model, numeric schemes. In this study, the same grid size and type are used in order to focus on the effect on LES quality of SGS models and blending factors of numeric scheme only.

Due to the direct injection of fuel into a combustion chamber, particulate emission is a challenge in DISI engines. Specifically, a significant amount of particulate emission is produced under the cold start condition. In this research, the main interest was to investigate particulate emission characteristics under the catalyst heating condition because it is one of the significant particulate-emissionproducing stages under the cold start condition. A single-cylinder optically accessible engine was used to investigate the effect of injection strategies on particulate emission characteristics under the catalyst heating condition. The split injection strategy was applied during intake stroke with various injection pressures and injection timings. Using luminosity analysis of the soot radiation during combustion, the particulate formation characteristics of each injection strategy were studied. Moreover, the factors that affect PM formation were analyzed via fuel injection visualization.

Much research has been devoted to reducing NOx and soot emissions simultaneously in diesel engines. The low temperature combustion (LTC) concept has the potential to reduce these emissions at the same time, but it has limitations to its commercialization. In-cylinder EGR stratification is another combustion concept meant to reduce both types of emissions simultaneously using non-uniform in-cylinder EGR gas distribution. The EGR stratification concept uses a locally high EGR region of the in-cylinder so that the emissions can be reduced without increasing the overall EGR rate. In this study, the EGR stratification concept was improved with a CFD-based analysis. First, a two-step piston was developed to maximize the stratified EGR effect. Then, the feasibility of combustion and emission control by stratified EGR was evaluated under cases of artificially distributed EGR stratification and conventional diesel engine conditions.

Large-eddy simulation (LES) applications for internal combustion engine (ICE) flows are constantly growing due to the increase of computing resources and the availability of suitable CFD codes, methods and practices. The LES superior capability for modeling spatial and temporal evolution of turbulent flow structures with reference to RANS makes it a promising tool for describing, and possibly motivating, ICE cycle-to-cycle variability (CCV) and cycle-resolved events such as knock and misfire. Despite the growing interest towards LES in the academic community, applications to ICE flows are still limited. One of the reasons for such discrepancy is the uncertainty in the estimation of the LES computational cost. This in turn is mainly dependent on grid density, the CFD domain extent, the time step size and the overall number of cycles to be run. Grid density is directly linked to the possibility of reducing modeling assumptions for sub-grid scales.

The More precise control of the multiple-injection is required in common-rail injection system of direct injection diesel engine to meet the low NOx emission and optimal PM filter system. The main parameter for obtaining the multiple-injections is the mechanism controlling the injector needle energizing and movement. In this study, a piezo-driven diesel injector, as a new method driven by piezoelectric energy, has been applied with a purpose to develop the analysis model of the piezo actuator to predict the dynamics characteristics of the hydraulic component (injector) by using the AMESim code and to evaluate the effect of this control capability on spray formation processes. Aimed at simulating the hydraulic behavior of the piezo-driven injector, the circuit model has been developed and verified by comparison with the experimental results.

Improving fuel efficiency and emission characteristics are significant issues in engine research. Because the engine has complex systems and various operating parameters, the experimental research is limited by cost and time. One-dimensional (1D) simulation has attracted the attention of researchers because of its effectiveness and relatively high accuracy. In a 1D simulation, the applied model must be accurate for the reliability of the simulation results. Because in-cylinder turbulence mainly determines the combustion characteristics, and mean flow velocity affects the in-cylinder heat transfer and efficiency in a spark-ignited (SI) engine, a number of sophisticated models have been developed to predict in-cylinder turbulence and mean flow velocity. In particular, tumble is a significant factor of in-cylinder turbulence in SI engine.

In recent years, emission regulations have been getting increasingly strict. In the development of engines that comply with these regulations, in-cylinder pressure plays a fundamental role, as it is necessary to analyze combustion characteristics and control combustion-related parameters. The analysis of in-cylinder pressure data enables the modelling of exhaust emissions in which characteristic temperature can be derived from the in-cylinder pressure, and the pressure can be used for other investigations, such as optimizing efficiency and emissions through controlling combustion. Therefore, a piezoelectric pressure sensor to measure in-cylinder pressure is an essential element in the engine research field. However, it is difficult to practice the installation of this pressure sensor on all engines and on-road vehicles owing to cost issues.

As emissions regulations become stricter, a variety of advanced combustion concepts that can reduce emissions with a higher thermal efficiency have been suggested. Dual-fuel combustion is one of the alternatives that has both premixed and non-premixed combustion characteristics. Knowing the effects of the mixture formation in dual-fuel combustion is important because it determines the ignition location and the following combustion phase. Hence, a thorough investigation on the related factors, such as the engine hardware or fuel spray, is required. Meanwhile, Computational Fluid Dynamics (CFD) is a good technique to visualize the in-cylinder phenomena and enables quantitative investigations into the detailed combustion characteristics. In this paper, a 3-dimensional CFD simulation was used to investigate the effects of the mixture formation in dual-fuel combustion. The combustion model consists of two parts.

As global emission standards are becoming more stringent, it is necessary to increase thermal efficiency through the high compression ratio in spark-ignited engines. Various studies are being conducted to mitigate knocking caused by an increased compression ratio, which requires an understanding of the combustion phenomena inside the combustion chamber. In particular, the in-cylinder flow is a major factor affecting the entire combustion process from the generation to the propagation of flames. In the field of spark-ignited engine research, where interest in the concept of lean combustion and the expansion of the EGR supply is increasing, flow analysis is essential to ensure a rapid flame propagation speed and stable combustion process. In this study, the flow around the spark plug was measured by the Laser Doppler Velocimetry system, and the correlation with combustion in spark-ignited engines was analyzed.

Regulations regarding vehicles’ CO2 emissions are continuing to become stricter due to global warming. The CO2 regulations urge automobile manufacturers to develop gasoline engines with improved efficiency; however, the main obstacle to the improvement is the knock phenomenon in spark-ignition engines. If knock is predicted, the efficiency potential can be maximized in an engine by applying modest spark timing. Several research regarding knock prediction modeling have been conducted, and typically Livengood-Wu integral model is used to predict the knock occurrence. For the prediction, knock onset should be determined on a given pressure signal of given knock cycles for establishing the 0D ignition delay model. Several methodologies for knock onset determination have been developed because checking all the knock onset position by hand is impossible considering the breadth of data sets.

As a result of stringent global regulations on fuel economy and CO2 emissions, the development of high-efficiency SI engines is more urgent now than ever before. Along with advanced techniques in friction reduction, many researchers endeavor to decrease the B/S (bore-to-stroke) ratio from 1.0 (square) to a certain value, which is expected to reduce the heat loss and enhance the burning rate of SI engines. In this study, the effects of B/S ratios were investigated in aspects of efficiency and knock characteristics using a single-cylinder LIVC (late intake valve closing) GDI (gasoline direct injection) engine. Three B/S ratios (0.68, 0.83 and 1.00) were tested under the same mechanical compression ratio of 12:1 and the same displacement volume of 0.5 L. The head tumble ratio was maintained at the same level to solely investigate the effects of geometrical changes caused by variations in the B/S ratio.

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.