Review
Fundamental phenomena affecting low temperature combustion and HCCI engines, high load limits and strategies for extending these limits

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Abstract

Low temperature combustion (LTC) engines are an emerging engine technology that offers an alternative to spark-ignited and diesel engines. One type of LTC engine, the homogeneous charge compression ignition (HCCI) engine, uses a well-mixed fuel–air charge like spark-ignited engines and relies on compression ignition like diesel engines. Similar to diesel engines, the use of high compression ratios and removal of the throttling valve in HCCI allow for high efficiency operation, thereby allowing lower CO2 emissions per unit of work delivered by the engine. The use of a highly diluted well-mixed fuel–air charge allows for low emissions of nitrogen oxides, soot and particulate matters, and the use of oxidation catalysts can allow low emissions of unburned hydrocarbons and carbon monoxide. As a result, HCCI offers the ability to achieve high efficiencies comparable with diesel while also allowing clean emissions while using relatively inexpensive aftertreatment technologies.

HCCI is not, however, without its challenges. Traditionally, two important problems prohibiting market penetration of HCCI are 1) inability to achieve high load, and 2) difficulty in controlling combustion timing. Recent research has significantly mitigated these challenges, and thus HCCI has a promising future for automotive and power generation applications.

This article begins by providing a comprehensive review of the physical phenomena governing HCCI operation, with particular emphasis on high load conditions. Emissions characteristics are then discussed, with suggestions on how to inexpensively enable low emissions of all regulated emissions. The operating limits that govern the high load conditions are discussed in detail, and finally a review of recent research which expands the high load limits of HCCI is discussed. Although this article focuses on the fundamental phenomena governing HCCI operation, it is also useful for understanding the fundamental phenomena in reactivity controlled compression ignition (RCCI), partial fuel stratification (PFS), partially premixed compression ignition, spark-assisted HCCI, and all forms of low temperature combustion (LTC).

Section snippets

Characteristics of HCCI, spark-ignited, and diesel engines

For health and environmental reasons, modern engines must meet increasingly tight emissions regulations for urban pollutants such as soot, particulate matter, nitrogen oxides, unburned hydrocarbons and carbon monoxide. Simultaneously, as society increasingly realizes the impact of CO2 on global warming, engines must achieve higher efficiency levels to minimize emissions of the global pollutant CO2.

Traditionally, spark-ignited (SI) engines with 3-way catalysts have been effective at minimizing

Fundamental phenomena affecting high load HCCI

In this section, the important physical phenomena governing HCCI operation are reviewed. An emphasis is placed on physics and chemistry occurring at the high load limits, however these phenomena influence the entire HCCI operating range. Chemical kinetics and important reactions governing HCCI operation and fuel characteristics are first discussed. The effects of variations in intake charge are then reviewed, including intake pressure, temperature, equivalence ratio, and overall charge

Emissions

HCCI typically has low emissions of nitrogen oxides, soot, and particulate matter, and higher emissions of unburned hydrocarbons and carbon monoxide. Two particular characteristics of HCCI govern the emissions: in-cylinder temperatures and charge composition (discussed in Sections 2.2.1 Intake charge conditions, 2.2.2 Exhaust residuals) [113], [153], [154], [155], [156], [157].

Typical load levels achieved in HCCI experiments

In internal combustion engines power output is determined by load and engine speed. For a fixed engine speed, different load levels are achievable in spark ignited, diesel and HCCI engines and each engine type is controlled using different methods. Engine load is usually reported using the IMEP or BMEP because these variables allow comparison between different engine sizes and technologies. In automotive applications, the maximum BMEP is around 12 bar for light duty SI engines and 18 bar for

Strategies for avoiding & expanding the operating limits for HCCI and LTC

Section 4 discussed the different limits that confine the maximum load in HCCI engines. One of the primary limiting phenomena constraining maximum power output is the need to prevent excessive ringing [70], [90], [91], [151], [203], [205], [228], [229], which is discussed in Section 4.2. Avoiding excessive rates of heat release and excessive pressure rise rates is the primary means for avoiding excessive ringing [5], thus the strategies for increasing maximum power output discussed in Sections

Summary of important concepts

A detailed review of the fundamental phenomena in HCCI engines and their interactions was presented in this article, particularly in relation to the high load operating limits. First, a review of hydrocarbon fuel breakdown was presented, including the chemical pathways for low and intermediate temperature chemistry and hot ignition. The characteristics of different fuels were discussed, with a focus on single- and two-stage ignition, the influence of molecular structure on fuel vaporization,

Acknowledgments

The authors wish to acknowledge many colleagues in the engine research community for their thoughts and insights regarding the fundamental phenomena discussed in this article. The participants at the bi-annual U.S. DOE Advanced Engine Consortium meetings have provided valuable understanding about low temperature combustion engine technologies. For the fruitful discussions that helped improve the authors' understanding on the topics covered in this article, the authors particularly acknowledge

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