Characterizing fuel reactivity in advanced internal combustion engines

Abstract

The urgent need to increase efficiency and reduce exhaust emissions from internal combustion engines has resulted in an increased interest in alternative combustion modes. Premixed or partially premixed compression ignition modes, such as homogeneous-charge compression ignition (HCCI), reactivity-controlled compression ignition (RCCI) and multi-zone stratified compression ignition (MSCI) have been a particular focus because of their potential to deliver enhanced fuel efficiency and meet exhaust emissions mandates without the addition of costly after-treatment technologies. For HCCI and other single fuel, partially premixed compression ignition schemes such as MSCI, many studies have shown that fuels with characteristics intermediate between gasoline and diesel fuel are necessary. Many researchers have shown, however, that existing industry metrics such as Octane Number and Cetane Number are insufficient to represent fuel ignition characteristics for advanced engine combustion modes. In light of the poor performance of traditional metrics, new methods have been proposed to try and better characterize, order, and rank fuels used in HCCI operation. However, studies have since shown that when a broad array of fuels are considered, these recent metrics fail to adequately define a characteristic HCCI fuel index. Described in this work is an analysis of fuel reactivity in traditional and advanced internal combustion engines. Firstly, conventional engine regimes are broken down to their basic components, providing a framework for investigating the context of fuel reactivity. This analysis allows a novel equation to be formulated which links the historic metrics of Octane Number and Cetane Number. As part of this analysis a parameter, the knock length, is developed which explains the underlying principles of the Research and Motor Octane Number scales and further shows why some fuels test differently in these two methods. The knock length is also used to investigate unusual behavior observed in Methane Number reference fuels data - behavior which traditional concepts such as ignition delay and flame speed are unable to explain on their own. Secondly, this work focuses on the application of fuels such as bio-derived alcohols (ethanol and butanol) and fatty acid methyl esters in traditional and advanced combustion applications. Reactivity differences between alcohol and petroleum fuels are described and explained. Lastly, a new metric, the HCCI Number, is developed which allows the prediction of combustion timing in HCCI engines, and is highly amenable toward the development of bench-top laboratory apparatuses to facilitate practical adoption by fuel manufactures. Data from 23 different fuel blends tested in Cooperative Fuel Research (CFR) engines, a Fuel Ignition Tester, and a HCCI engine provide the experimental support for the theory presented herein. Additionally, a new chemical-kinetic mechanism is developed and used to describe combustion of n-butanol/n-heptane fuel mixtures in both conventional and advanced combustion applications (HCCI). Computational modeling is also used to examine the experiments presented herein: single and multi-zone (CHEMKIN) as well as system-level (GT Power) and multi-dimensional (CONVERGE) modeling approaches are developed and discussed. For the HCCI experiments conducted herein, an engine test-bed that allows HCCI examination across a wide array of conditions was also designed and fabricated. In summary, it is hoped that with better understanding of how fuels react in current and future engines, researchers can achieve the control necessary to bring higher performance engines to market and help the world take one step closer to addressing some of the pressing environmental and humanitarian issues at hand.