Tang-Wei Kuo

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.

A Mean-Value Model for Control of Homogeneous Charge Compression Ignition (HCCI) Engines

A Mean Value Model (MVM) for a Homogeneous Charge Compression Ignition (HCCI) engine is presented. Using a phenomenological zero-dimensional approach with five continuous and three discrete states we first model the effects of the Exhaust Gas Recirculation (EGR) valve, the exhaust Rebreathing Lift (RBL), and the fueling rate on the state of charge in the cylinder at intake valve closing. An Arrhenius integral is then used to model the start of combustion, θ soc . A series of simple algebraic relations that captures the combustion duration and heat release is finally used to model the state of charge after the HCCI combustion and the Location of Peak Pressure (LPP). The model is parametrized and validated using steady-state test data from an experimental gasoline engine at the General Motors Corporation. The simple model captures the temperature, pressure, air-to-fuel ratio, and inert gas fraction of the exhausted mass flow. This characterization is important for the overall HCCI dynamics because the thermodynamic state (pressure, temperature) and concentration (oxygen and inert gas) of the exhausted mass flow affect the next combustion event. The high dilution level in HCCI engines increases the significance of this internal feedback that generally exists to a smaller extent in conventional spark-ignition and compression-ignition internal combustion engines.

Characterizing the Effect of Combustion Chamber Deposits on a Gasoline HCCI Engine

Homogenous Charge Compression Ignition (HCCI) engines offer a good potential for achieving high fuel efficiency while virtually eliminating NOx and soot emissions from the exhaust. However, realizing the full fuel economy potential at the vehicle level depends on the size of the HCCI operating range. The usable HCCI range is determined by the knock limit on the upper end and the misfire limit at the lower end. Previously proven high sensitivity of the HCCI process to thermal conditions leads to a hypothesis that combustion chamber deposits (CCD) could directly affect HCCI combustion, and that insight about this effect can be helpful in expanding the low-load limit. A combustion chamber conditioning process was carried out in a single-cylinder gasoline-fueled engine with exhaust rebreathing to study CCD formation rates and their effect on combustion. Burn rates accelerated significantly over the forty hours of running under typical HCCI operating conditions. Variations of burn rates diminished after approximately 36 hours, thus indicating equilibrium conditions. Observed trends suggest that deposits change dynamic thermal boundary conditions at the wall and this in turn strongly affects chemical kinetics and bulk burning. In addition, this work presents a methodology for investigating the thermal diffusivity of deposits without their removal. The experimental technique relies on a combination of instantaneous surface temperature and CCD thickness measurements. Results demonstrate a strong correlation between deposit thickness and the diffusivity of the CCD layer.

Computation of Premixed-Charge Combustion in Pancake and Pent-Roof Engines

Multidimensional computations were made of spark-ignited premixed-charge combustion in a pancake-combustion-chamber engine with a centrally located spark plug and in two pent-roof-chamber engines, one with a central spark plug and the other with dual lateral spark plugs. A global combustion submodel was used that accounts for laminar kinetics and turbulent mixing effects. The predictions are compared with available measurements in the pancake-chamber engine over a range of loads, speeds, and equivalence ratios. A sensitivity study was performed to understand the sources of discrepancies between the predictions and measurements. It was found that the discrepancies could be explained through uncertainties in the gas temperature, turbulence intensity and length scale existing in the chamber prior to combustion.

Evaluation of four mixing correlations for performance and soot-emission characteristics for a small open-chamber diesel engine

A quasi-steady gas-jet model was applied to examine the spray penetration and deflection in swirling flow during the ignition-delay period in an open-chamber diesel engine timed to start combustion at top dead center. The input to the gas-jet model included measured values of ignition delay and mean fuel-injection velocity. Attempts were made to correlate measured fuel-consumption and soot-emissions data with mixing parameters based on calculated spray penetration and deflection. The engine parameters examined were piston-bowl geometry, compression ratio, speed, and overall air-fuel ratio

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

The gasoline HCCI engine holds a promise of achieving very high part-load efficiency combined with extremely low NOx and soot emissions. However, the load range of HCCI operation is limited by the misfire limit at the low end, and knock limit at the high end. Therefore, the future practical implementation will likely be a dual-mode engine, operating in the HCCI mode at part load and switching to SI at higher loads. Expanding the limits will be critical for maximizing the fuel economy benefits in the vehicle. 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 engine with exhaust re-induction. The effects include type of mixture preparation (external mixing vs. direct injection), charge motion, and injection timing. A combination of pressure-based combustion diagnostics, emissions 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 were relatively small compared to extending the injection timing late into the intake process or completely closing the swirl control valve and allowing charge storage in the port.Copyright