Enhanced heat release analysis for advanced multi-mode combustion engine experiments

Advanced combustion strategies, such as Homogeneous-Charge Compression-Ignition (HCCI) and Spark-Assisted HCCI or Spark-Assisted Compression-Ignition (SAC!) hold considerable promise for improving engine efficiencies while maintaining low pollutant emissions, yet few models exist that accurately include the important chemical and physical mechanisms of these advanced combustion strategies. Further, experimental data from advanced combustion engine experiments are not well represented using conventional spark ignited analytical tools. This paper presents new methods for advanced combustion analysis that integrate previous analytical methods with new algorithms to capture the unique features of advanced combustion strategies like SACI. The new analytical capabilities were incorporated into a program which was designated the Advanced Combustion Engine Heat Release Analysis (ACE-HRA) tool. The models developed and applied in ACE-HRA were assessed by comparison with high fidelity engine simulations of HCCI and SACI. The high fidelity simulations provided data sets with detailed predictions of heat release rates, temperatures, auto-ignition timing, flame speeds and other key parameters not resolved or measured in engine experiments. The sensitivity of ACE-HRA estimates to model input data was quantified for important engine performance parameters. The sensitivity analysis showed that estimates for in-cylinder masses have the largest overall impact on the ACE-HRA results (e.g. +/- 10% variation led to changes on the order of +/- 5-10% in peak rate of heat release, burn duration and peak temperature). Noticeable differences in peak heat release rate and ringing intensity were also observed when comparing cycle-by-cycle analysis against ensemble-average analysis, which has implications on how the results are interpreted and applied in modeling work. After validating ACE-HRA with the high-fidelity simulations, ACE-HRA was applied to interpret the data from a recent experimental study of SACI combustion. The ACE-HRA methods were used to infer the effects of flame propagation on in-cylinder gradients and cycle-to-cycle variability, and to provide quantitative estimates for the associated changes in the end-gas burn rate. The trends observed, such as the decrease in burn rate for later auto-ignition and higher burn fraction by flame, provide more in-depth understanding of SACI combustion and demonstrate the insight that can be revealed by the new ACE-HRA tool. (C) 2014 Elsevier Ltd. All rights reserved.