Development of a Postprocessing Methodology for Studying Thermal Stratification in an HCCI Engine

Abstract

Development of a Postprocessing Methodology for Studying Thermal Stratification in an HCCI EngineNaturally occurring thermal stratification signiflcantly impacts the characteristics of homogeneous charge compression ignition (HCCI) combustion. The in-cylinder gas temperature distributions prior to combustion dictate the ignition phasing, burn rates, combustion efficiency, and unburned hydrocarbon and CO emissions associated with HCCI operation. Characterizing the gas temperature fields in an HCCI engine and correlating them to HCCI burn rates is a prerequisite for developing strategies to expand the HCCI operating range. To study the development of thermal stratification in more detail, a new analysis methodology for postprocessing experimental HCCI engine data is proposed. This analysis tool uses the autoignition integral in the context of the mass fraction burned cwve to infer information about the distribution of temperature that exists in the cylinder prior to combustion. An assumption is made about the shape of the charge temperature profiles of tlie unburned gas during compression and after combustion starts elsewhere in the cylinder. Second, it is assumed that chemical reaction rates proceed very rapidly in comparison to tire staggering of ignition phasing from thermal stratification. The autoignition integral is then coupled to the mass fraction burned curve to produce temperature-mass distributions that are representative of a particular combustion event. Due to the computational efficiency associated with this zero-dimensional calculation, a large number of zones can be simulated at very little computational expense. The temperature-mass distributions are then studied over a coolant temperature sweep. The results show that veiy small changes to compression heat transfer can shift the distribution of mass and temperature in the cylinder enough to significantly affect HCCI burn rates and emissions.The proposed methodology breaks the cylinder into a large number of different "regions" or "zones," each with a different temperature. These "zones" are different from the "zones" traditionally described in multizonal HCCI combustion modeling [21][22][23][24] due to the fact that the "zones" in this analysis are not assigned a spatial location and, at least initially, there is no specific amount of mass attached to each "zone." For the lack of a Journal of Engineering for Gas Turbines and Power

Computational fluid dynamics investigations of the effect of water injection timing on thermal stratification and heat release in thermally stratified compression ignition combustion

“…The mass PDFs are calculated from the experimental heat release characteristics using a unique post-processing technique called thermal stratification analysis (TSA). For more information about the development and validation of the TSA technique, refer to Lawler and colleagues, 57,58 and for other examples of the application of the TSA, refer to Lawler and colleagues. 59,60 Generally speaking, the experimental mass PDFs show the same trend as the simulation results which provide confidence in the trends and conclusions.…”

Computational Fluid Dynamics Simulations of the Effect of Water Injection Characteristics on TSCI: A New, Load-Flexible, Advanced Combustion Concept

“…There is an analytical post-processing technique called the Thermal Stratification Analysis (TSA) which can determine a mass-temperature distribution and thermal width from the experimental data and heat release characteristics (see [21,[53][54][55][56] for more details on the analytical post-processing technique). The 25%-75% thermal width for the pure HCCI case and the 19.3 injection pressure from the experimental and the TSA technique were 56 K and 67 K [21] which agree very well with the values at -10 CAD from the CFD of 53 K and 69 K. The good agreement between the CFD and the experimental thermal widths provides confidence in the accuracy of the results and the analysis.…”

Numerical Investigation on Effects of Assigned EGR Stratification on a Heavy Duty Diesel Engine with Two-Stage Fuel Injection

“…Therefore, in recent years, high exhaust gas recirculation (EGR) and flexible two-stage fuel injection [4][5][6][7][8][9] are becoming increasingly popular due to their capability to control combustion temperature and fuel/oxygen ratio. Homogeneous charge compression ignition (HCCI) combustion [10,11], low temperature combustion [12] and high dilution combustion [13,14] are successful new combustion concepts for reducing engine-out toxic emissions by reducing oxygen and combustion temperature using high EGR, however, at the expense of high fuel consumption [15] and a narrow operation range in HCCI combustion [16]. Combustion efficiency also deteriorates [17] due to the high dilution effects of EGR.…”