Jianwen Yi

Development and Analysis of a Spray-Guided DISI Combustion System Concept

An innovative stratified-charge DISI combustion concept has been developed using a mixture formation method referred to as Vortex Induced Stratification Combustion (VISC). This paper describes the combustion system concept and an initial assessment of it, performed on a single-cylinder test engine and through CFD modeling. This VISC concept utilizes the vortex naturally formed on the outside of a wide spray cone that is enhanced by bulk gas flow control and piston crown design. This vortex transports fuel vapor from the spray cone to the spark gap. This system allows a late injection timing and produces a well-confined mixture, which together provide an improved compromise between combustion phasing and combustion efficiency over typical wall-guided systems. Testing results indicate an 18% fuel consumption reduction, compared with a baseline PFI engine, over a drive cycle (neglecting cold start and transient effects). This represents a 4-7% reduction in fuel consumption compared with previous in-house stratified-charge DISI concepts, while providing similar stability and emissions performance. These benefits are shown to arise from improved combustion phasing, reduced unburned hydrocarbon emissions, and an enlarged window of stratified operation. In addition, the potential use of split injection to expand the window of stratified operation to higher loads is outlined. Potential for improved full-load performance is also shown due to improved air-fuel mixing.

Applications of CFD Modeling in GDI Engine Piston Optimization

This paper describes a CFD modeling based approach to address design challenges in GDI (gasoline direct injection) engine combustion system development. A Ford in-house developed CFD code MESIM (Multidimensional Engine Simulation) was applied to the study. Gasoline fuel is multi-component in nature and behaves very differently from the single component fuel representation under various operating conditions. A multi-component fuel model has been developed and is incorporated in MESIM code. To apply the model in engine simulations, a multi-component fuel recipe that represents the vaporization characteristics of gasoline is also developed using a numerical model that simulates the ASTM D86 fuel distillation experimental procedure. The effect of the multi-component model on the fuel air mixture preparations under different engine conditions is investigated. The modeling approach is applied to guide the GDI engine piston designs. Effects of piston designs on the fuel air mixture preparation are presented. It is found that the multi-component fuel model is critical to the accuracy of the model prediction of the fuel air preparation process, particularly under cold start conditions.

Combustion improvement of a light stratified-charge direct injection engine

In the effort to improve combustion of a Light-load Stratified-Charge Direct-Injection (LSCDI) combustion system, CFD modeling, together with optical engine diagnostics and single cylinder engine testing, was applied to resolve some key technical issues. The issues associated with stratified-charge (SC) operation are combustion stability, smoke emission, and NOx emission. The challenges at homogeneous-charge operation include fuel-air mixing homogeneity at partial load operation, smoke emission and mixing homogeneity at low speed WOT, and engine knock tendency reduction at medium speed WOT operations. In SC operation, the fuel consumption is constrained with the acceptable smoke emission level and stability limit. With the optimization of piston design and injector specification, the smoke emission can be reduced. Concurrently, the combustion stability window and fuel consumption can be also significantly improved. The optimized piston also helps to reduce NOx emission with local mixture enrichment around the spark-plug gap and improved internal residual amount tolerance. The study shows that one of the root causes of smoke emission at low speed WOT is liquid fuel impingement on the valve surface. The smoke emission level can be reduced with injector specification optimization. The mechanism by which split injection improves WOT performance is studied in detail. It is shown that at low speed WOT operation, the split injection improves the liquid spray distribution, thus improves the fuel-air mixing homogeneity and the engine output. At medium speed WOT operation, split injection does not have much effect on the mixing homogeneity, instead it improves the charge temperature distribution, thus reducing the knocking tendency.

Effect of Compression Ratio on Stratified-Charge Direct- Injection Gasoline Combustion

Charge cooling due to fuel evaporation in a direct-injection spark-ignition (DISI) engine typically allows for an increased compression ratio relative to port fuel injection (PFI) engines. It is clear that this results in a thermal efficiency improvement at part load for homogenous-charge DISI engines. However, very little is known regarding the effect of compression ratio on stratified charge operation. In this investigation, DISI combustion data have been collected on a single cylinder engine equipped with a variable compression ratio feature. The results of experiments performed in stratified-charge direct injection (SCDI) mode show that despite its over-advanced phasing, thermal conversion efficiency improves with higher compression ratios. This benefit is quantified and dissected through an efficiency analysis. Furthermore, since the engine was equipped with both wall-guided Dl and PFI systems, direct comparisons are made at part load for fuel consumption and emissions. Interestingly, combustion efficiency deteriorates in SCDI mode as compression ratio increases, albeit not due to crevice loading as is the case in PFI operation. Mechanisms for the observed hydrocarbon emissions behavior are suggested for change in load and compression ratio. The conclusions reached in this investigation provide an experimental basis for adequately selecting a compression ratio in SCDI engines.

Integration of a discrete multi-component fuel evaporation model with a G-equation flame propagation combustion model and its validation

A discrete multi-component fuel evaporation model has been successfully integrated with a G-equation flame propagation combustion model. In the discrete multi-component fuel evaporation model, the individual components of the fuel during the evaporation process are tracked, and the characteristics of the fuel components are determined from fuel libraries. In the G-equation flame propagation combustion model, five improved sub-models developed previously by the authors are used. Two new methods that are necessary for the integration are proposed. To consider the change of local fuel vapor mixture composition, a ‘blending cetane number approach’ is proposed that formulates the relationship between the fuel vapor mixture cetane number and the mole fraction and cetane number of each component. With the mixture cetane number, the research octane number can be calculated locally, and with this research octane number, the flame speed can be calculated. In the combustion stage, a ‘group chemistry method’ is proposed in which one representative species can be selected to represent several fuel components that belong to the same chemical family. This treatment makes it possible to use a smaller size of multi-component fuel chemical kinetic mechanism. Each sub-model included in the integrated model was validated to see if the integration process was correct or not, and finally the integrated model was tested to model complicated gasoline direct injection engine cases. Simulations of single-droplet and spray evaporation with discrete multi-component fuel cases from normal evaporation to flash boiling were performed and compared with available experimental data. The match between simulations and experiments is excellent. The G-equation flame propagation combustion model was also validated with experimental data. Evaporation and the combustion process were simulated for a gasoline direct injection engine. The simulated in-cylinder pressures are in good agreement with the experimental data.