Junseok Chang

New Heat Transfer Correlation for an HCCI Engine Derived from Measurements of Instantaneous Surface Heat Flux

An experimental study has been carried out to provide qualitative and quantitative insight into gas to wall heat transfer in a gasoline fueled Homogeneous Charge Compression Ignition (HCCI) engine. Fast response thermocouples are embedded in the piston top and cylinder head surface to measure instantaneous wall temperature and heat flux. Heat flux measurements obtained at multiple locations show small spatial variations, thus confirming relative uniformity of incylinder conditions in a HCCI engine operating with premixed charge. Consequently, the spatially-averaged heat flux represents well the global heat transfer from the gas to the combustion chamber walls in the premixed HCCI engine, as confirmed through the gross heat release analysis. Heat flux measurements were used for assessing several existing heat transfer correlations. One of the most popular models, the Woschni expression, was shown to be inadequate for the HCCI engine. The problem is traced back to the flame propagation term which is not appropriate for the HCCI combustion. Subsequently, a modified model is proposed which significantly improves the prediction of heat transfer in a gasoline HCCI engine and shows very good agreement over a range of conditions.

Investigation of Mixture Preparation Effects on Gasoline HCCI Combustion Aided by Measurements of Wall Heat Flux

Expanding the range of HCCI operation will be critical for maximizing the fuel economy benefits in future vehicle applications. The mixture stratification, both thermal and compositional, can have very tangible impact on HCCI combustion, and gaining a deeper insight into these effects is critical for expanding the HCCI range of operation. This paper presents results of the comprehensive experimental investigation of the mixture preparation effects on a single-cylinder gasoline HCCI engine with exhaust reinduction. The effects include the type of mixture preparation (external mixing versus direct injection), charge motion, and injection timing. A combination of pressure-based combustion diagnostics, emission analysis, and heat flux measurements on the combustion chamber wall quantifies the effects on combustion and provides insight into reasons for observed engine behavior. As an example, the instantaneous temperature and heat flux measurements show the fuel impingement locations and allow assessing the fuel film dynamics and their effect on mixture stratification. The effects of direct injection and partial closing of the swirl control valve are relatively small compared with extending the injection timing late into the intake process or completely closing the swirl control valve and allowing charge storage in the intake port.

The Effect of Intake Valve Deactivation on Lean Stratified Charge Combustion at an Idling Condition of a Spark Ignition Direct Injection Engine

A combined experimental and analytical study was carried out to understand the improvement in combustion performance of a 4-valve SIDI wall-guided engine operating at lean, stratified idle with enhanced in-cylinder charge motion by deactivating one of the two intake valves. A fully warmed-up engine was operated at low speed, light load by injecting the fuel from a pressure-swirl injector during the compression stroke to produce a stratified fuel cloud surrounding the spark plug at the time of ignition. Steady state flow-bench measurements and CFD calculations showed that valve deactivation primarily increased the in-cylinder swirl intensity as compared with opening both intake valves. Engine dynamometer measurements showed an increase in charge motion led to improved combustion stability, increased combustion efficiency, lower fuel consumption, and higher dilution tolerance. A CFD study was conducted using in-house models of spray and combustion to simulate the engine operating with and without valve deactivation. The computations demonstrated that the improved combustion was primarily driven by higher laminar flame speeds through enhanced mixing of internal residual gases, better containment of the fuel cloud within the piston bowl, and higher post-flame diffusion burn rates during the initial, main, and late stages of the combustion process, respectively.Copyright