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The establishment of highly-diluted combustion strategies is one of the major challenges that the next generation of sustainable internal combustion engines must face. The desirable use of high EGR rates and of lean mixtures clashes with the tolerable combustion stability. To this aim, the development of numerical models able to reproduce the degree of combustion variability is crucial to allow the virtual exploration and optimization of a wide number of innovative combustion strategies. In this study ignition experiments using a conventional coil system are carried out in a closed volume combustion vessel with side-oriented flow generated by a speed-controlled fan. Acquisitions for four combinations of premixed propane/air mixture quality (Φ=0.9,1.2), dilution rate (20%-30%) and lateral flow velocity (1-5 m/s) are used to assess the modelling capabilities of a newly developed spark-ignition model for large-eddy simulation (GLIM, GruMo-UniMORE LES Ignition Model).

Fuel lean combustion and exhaust gas dilution are known to increase the thermal efficiency and reduce NOx emissions. In this study, experiments are performed to understand the effect of equivalence ratio on flame kernel formation and flame propagation around the spark plug for different low turbulent velocities. A series of experiments are carried out for propane-air mixtures to simulate engine-like conditions. For these experiments, equivalence ratios of 0.7 and 0.9 are tested with 20 percent mass-based exhaust gas recirculation (EGR). Turbulence is generated by a shrouded fan design in the vicinity of J-spark plug. A closed loop feedback control system is used for the fan to generate a consistent flow field. The flow profile is characterized by using Particle Image Velocimetry (PIV) technique. High-speed Schlieren visualization is used for the spark formation and flame propagation.

The fifth generation of General Motor's “Small Block” 90-degree V engine family has been developed with a totally new combustion system. This system employs direct fuel injection (DI) and carefully architected in-cylinder flow field development in order to significantly improve all aspects of combustion system performance. Efficiency improvements stem from increased compression ratio, greatly improved dilution tolerance, and excellent knock resistance. The asymmetric, 2-valve (2V) layout of the “Small Block” engine presented unique challenges in developing the combustion system, but also offered unusual opportunities for an elegant solution while retaining the traditional “Small Block” attributes of packaging efficiency and power density.

Optimization of engine startup from crank to catalyst light-off is essential for achieving low emissions. For Spark Ignition Direct Injected (SIDI) engines, this requires optimization of the piston crown features, spray characteristics and control strategy. In this case study, high speed endoscope imaging was used to provide a qualitative confirmation of CFD spray predictions and to provide insight into engine starting in a “real” engine environment. The effect of piston feature was initially evaluated in a single cylinder engine running the dual-injection catalyst heating mode. The piston features were also assessed at part load and wide open throttle. The videos of the spray development were compared to CFD predictions. In the example case reported here, endoscope imaging showed that the baseline piston bowl was not effective in deflecting the spray toward the spark plug. Moving the piston bowl toward the injector gave a visible improvement in the spray deflection.

In internal combustion engines, liquid fuel injection is one of the most prevalent means of fuel delivery and air-fuel mixture preparation. The behavior of the fuel spray and wall film is a key factor in determining air-fuel mixing and hence combustion and emissions. A comprehensive model for the liquid fuel spray has been developed in conjunction with the one-dimensional gas flow code WAVE. The model includes droplet dynamics and evaporation, spray-wall impingement, wall film dynamics and evaporation. The fuel injector can be placed in the manifold, inlet port or cylinder. Liquid fuel droplets are injected with a prescribed size distribution, and their subsequent movement and vaporization are modeled via the discrete particle approach, frequently used in multi-dimensional CFD codes. This approach ensures conservation of mass, momentum and energy between the gas and liquid phases.

In recent years, it has been demonstrated that E-TEC direct injected two-stroke engines are capable of meeting the toughest emissions standards for marine outboard engines. Proper in-cylinder mixture distribution and preparation are essential for achieving low emissions, high performance, and good run-quality. The mixture distribution is driven largely by the momentum exchange between the fuel spray and the scavenging flow. It has been found that different engines can exhibit significantly different behaviors with similar fuel sprays. This difference is attributed to the difference in scavenging flow patterns and its effect on the momentum balance between the fuel spray and the air flow. In order to investigate this phenomenon, a test fixture was designed and built to evaluate fuel sprays into air-counter-flows with velocities of up to 40m/s by recording spray images and measuring spray penetration. Two different sprays were tested in the fixture and in a variety of engines.

One-dimensional unsteady gas dynamics dominate the prediction and optimization of two-stroke engine performance. Its application in engines with complicated geometry is, however, limited because the flow through the engine is three dimensional in nature. Multidimensional CFD has the capacity to capture the effect of complicated flow fields. However, most existing CFD studies include either only one cylinder with a partial exhaust system or just a separate exhaust manifold, and boundary conditions need to be fed from experimental data. It is found in this study that such simplifications may yield misleading results. In a previous study, the authors extended a multidimensional CFD code, KIVA to simulate a multi-cylinder engine together with a full exhaust manifold. The need for exhaust pressure boundary conditions was thus eliminated. In continuation of this study, a crankcase model was first developed to dynamically predict the crankcase pressure.

This paper presents a numerical study on air-fuel mixing in a two-stroke direct injection spark ignition engine under homogeneous operation. The simulated engine is loop scavenged and uses an outwardly opening swirl injector. A generic mesh-snapping algorithm is developed to enable the moving piston to snap through transfer ports with complicated geometry. A spray model based on Linear Instability Sheet Atomization is used to describe the primary breakup of fuel sprays, and the initial rotational velocity of the conical sheet is determined from a CFD simulation of the nozzle internal flow. A wall film model accounting for the effect of contacting area is also developed to avoid the severe grid-dependence of the original film model in KIVA. Comparisons between simulations and experiments were made for sprays in quiescent ambient conditions, and a good agreement of the spray characteristics was obtained. The simulations were performed for four different injection timings.

This paper presents a study on multi-dimensional CFD modeling of scavenging and plugging in a twin-cylinder two-stroke engine. A general-purpose CFD code, KIVA, was extended to track an arbitrary number of moving pistons. The code was also modified to allow piston snapping through complicated transfer ports. Thus, a multi-cylinder simulation together with a full exhaust manifold to fully account for the interaction between scavenging and plugging becomes possible. The developed code is intended to be a numerical tool for exhaust-manifold design and optimization. The studied engine is a five-port loop scavenged twin-cylinder engine with a cylinder displacement of 432 cc. The computed exhaust pressure was compared with measured data, and reasonably good agreement was obtained. The results were also compared with those from a one-dimensional gas dynamics model, which over-predicts the plugging intensity while under-predicting the pressure loss in the exhaust manifold.

This paper describes important aspects of the development process for meeting CARB's “Ultra-Low” 3-Star emissions with engines using the new E-TEC™ direct injection system. In-house research and analysis of data from other state-of the-art engines were used to determine achievable emission levels and to set the development targets. A detailed mode-point-specific analysis of the emissions potential of the FICHT® direct injection system revealed excellent system capability in homogeneous operation and limited potential for stratified operation. Based on these results, the development work was focused on the reduction of stratified hydrocarbon emissions. Wall impingement of the fuel spray onto the piston surface was identified as a major source of hydrocarbon emissions during stratified operation. A zero-dimensional simulation of various parameters affecting wall impingement indicates that droplet size, in-cylinder temperature, and penetration velocity are the three major factors.

This paper describes a numerical study on air/fuel preparation process in a direct-injected spark-ignition engine under partial load stratified conditions. The fuel is represented as a mixture of four components with a distillation curve similar to that of actual gasoline, and its vaporization processes are simulated by two recently formulated multicomponent vaporization models for droplet and film, respectively. The models include major mechanisms such as non-ideal behavior in high-pressure environments, preferential vaporization, internal circulation, surface regression, and finite diffusion in the liquid phase. A spray/wall impingement model with the effect of surface roughness is used to represent the interaction between the fuel spray and the solid wall. Computations of single droplet and film on a flat plate were first performed to study the impact of fuel representation and vaporization model on the droplet and film vaporization processes.

A numerical investigation of air-fuel mixing in gasoline direct-injection (GDI) engines is presented in this paper. The primary goal of this study is to demonstrate the importance of fuel representation. In the past studies, fuel has been usually modeled as a single component substance. However, most fuels are mixtures of hydrocarbons with diverse boiling points, resulting in mixture vaporization behavior substantially different from single-component behavior. This study presents a newly developed multicomponent vaporization model, which takes into account important mechanisms such as preferential vaporization, internal circulation, surface regression, and non-ideal behavior in high-pressure environments. A sheet spray atomization model was also used to calculate the disintegration of the liquid sheet and the breakup of the subsequent droplets. The results of a single-component fuel representation and a multicomponent fuel representation were compared.