Zhiyu Han

Spray/wall interaction models for multidimensional engine simulation

Abstract Models were developed to describe the spray wall impingement processes that take place in internal combustion engines. In this report focus is placed on the model formulation and experiment assessment of the spray/wall interaction submodels. It is identified that the Leidenfrost phenomenon is very unlikely to occur in a spark ignition (SI) engine including stratified-charge operation in a direct injection spark ignition (DISI) engine. A more comprehensive splashing/deposition threshold function is proposed to include the effects of surface roughness and pre-existing liquid film. Based on the wave phenomena observed on the surface of the liquid crown formed during drop impingement, a new splash breakup model is developed using linear instability analysis. The predicted drop size agrees well with available single-drop impingement experimental data. A new formulation for the post-impingement droplet velocity is also given which uses statistical sampling and jet impingement theory. The proposed models were assessed by comparing computations with two sets of experimental sprays impinging on a flat plate with the use of a pintle nozzle injector for port fuel injection (PFI) engines. The computed spray shape, normal and tangential penetration and droplet size show good agreement with experimental data.

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.

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.