Low Temperature Combustion

Hannu Jääskeläinen

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• Introduction
• LTC Approaches
• LTC in φ-T Coordinates
• LTC Fuel Considerations
• Conceptual Combustion Model
• Combustion Noise
• Emissions

Introduction

Since the introduction of diesel engine emission standards that have forced the introduction of aftertreatment systems for NOx and diesel particulate matter, the evolution of the diesel combustion process has been significant. Advanced combustion strategies have attempted to find an in-cylinder approach to either meet these emission standards fully and thus avoiding the need to use aftertreatment or at the very least, to lower the performance demands required from aftertreatment systems and thus reducing their cost and complexity. While the main focus of combustion system development has been to lower emissions of NOx, there is also significant interest to lower PM emissions.

Many of these advanced combustion systems carry numerous handles such as Homogeneous Charge Compression Ignition (HCCI) and Premixed Charge Compression Ignition (PCCI) that may or may not accurately reflect the combustion process.

HCCI was one of the early diesel combustion concepts that differed from the conventional diesel process to attract attention. As the name implies, the goal of early HCCI work was to achieve as homogeneous a mixture of air and fuel as possible before ignition—much the same way as in a conventional spark ignition engine. This can be achieved either by injecting fuel into the intake port or directly into the cylinder and allowing sufficient time between injection and ignition to allow complete mixing of air and fuel. The charge then auto-ignites as it is heated by the compressed gases—no spark or other means of forced ignition is used.

In order to address many of the challenges such as limited load range, controllability and knocking posed by HCCI, a number of other concepts have evolved from this homogeneous charge approach and in many cases, charge stratification was introduced. Since the term HCCI may no longer accurately describe many of these systems, the term Low Temperature Combustion (LTC) can be used as a general term to refer to these and other advanced combustion concepts because the overall goal is to lower combustion temperatures to advantageously alter the chemistry of NOx and/or soot formation.

In the literature, the term HCCI is not used in a consistent manner. In some cases, its use does refer to combustion systems that are indeed based on a relatively homogeneous mixture of air and fuel. In other cases, the term HCCI refers to combustion systems that are not at all homogeneous—they are in fact quite heterogeneous. In this discussion, the term “LTC” will be used when generally referring to these combustion concepts and use of the term “HCCI” will be limited only to those approaches that rely on a relatively homogeneous mixture of air and fuel.

HCCI diesel combustion using diesel fumigation in the intake port was first described in 1958 [1661]. Further work in the late 1970s [1751][1752] reported stable spontaneous auto-ignition in a two stroke gasoline engine with port fuelling that was attributed to the presence of active radicals. While the focus of many of these early publications was on light fuels (gasoline) in two stroke engines, later work described the same type of combustion with diesel fuel in four stroke engines [1717][1737]. These and some of the different approaches that evolved from them are listed in Table 1 [1741].

Early work with HCCI demonstrated that engine-out NOx and PM emissions could be lowered to about 1-10% of the diesel engine technology available at the time. This raised the possibility that the need for aftertreatment devices to meet regulated emission limits could be either eliminated or simplified.

One characteristic of HCCI and many other LTC concepts that have evolved from it share is that either all or a significant amount of fuel is premixed with air before ignition occurs. The combustion rate and ignition timing of such premixed LTC concepts is controlled by the chemical kinetics of the mixture. This greatly complicates the control of the combustion process as well as making it sensitive to fuel properties and in-cylinder conditions. Some premixed LTC concepts benefit from low cetane number fuels with volatility characteristics comparable to gasoline.

It should be noted that premixing of air and fuel can also be an important factor in “conventional” diesel combustion. While the initial stage in conventional diesel combustion is generally premixed, the combustion of a majority of the fuel occurs after this premixed burn at a rate mainly determined by the rate of mixing of air and unburned/partially burned fuel. The conventional diesel combustion process is thus often referred to as mixing-controlled combustion. This mixing control characteristic greatly simplifies the control of the heat release process.

While much of the work with LTC has focused on premixed LTC concepts, it has been demonstrated that mixing-controlled diesel combustion can also be adopted to produce NOx emissions in the 0.2 g/kWh range—comparable to those achievable with some premixed LTC concepts [1676][1675][1738][1637]. Such mixing-controlled approaches could be considered to be the next step in the evolution of conventional diesel combustion beyond the approaches used for example to meet 2004 and 2007 EPA onroad heavy-duty diesel emission standards. They do, however, require advanced “unconventional” hardware to manage PM emissions. These engines require such features as fuel injection systems that provide hight injection pressures (as high as 3000 bar in some prototypes) and air management systems producing levels of boost pressures that require multi-stage turbochargers. Such approaches could be referred to as mixing-controlled LTC concepts. Unlike premixed LTC approaches, it has been shown that mixing-controlled LTC can operate over the entire speed and load range of the engine [1676].

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