2.1 Fundamentals of compression- compression-ignition combustion

were made by several investigators (Chiu et al. 1976; Faeth 1977) to develop models for diesel spray mixing and to describe the nature of combustion processes in diesel engines. Chiu et al. (1976) investigated transient fuel sprays in a high pressure, high temperature chamber and proposed a diesel spray model consisting of the fuel rich diesel core surrounded by diesel fuel-air mixtures of progressively leaner equivalence ratios. The non-uniformity in stoichiometry in the outer spray region exists due to the different degrees of entrainment of air into the spray. Faeth (1977) looked into the steady state aspect of compression-ignition combustion and described the spray structure as a diesel fuel rich central core, surrounded by a layer of diesel fuel and entrained air mixture, where combustion occurred upon the mixture reaching an appropriate stoichiometry. The lack of advancement in computational capabilities and optical diagnostic technologies prevented a detailed correlation of the spray and flame structure with combustion stoichiometry, flame temperatures and cylinder wall interactions. In recent years, the understanding of in-cylinder processes associated with diesel spray combustion has been greatly improved due to conceptual models of diesel combustion, such as the one proposed by Dec (1997), who utilised laser-imaging diagnostics to provide a temporal description of diesel fuel consumption and define zones of varying fuel-air mixture stoichiometry in the flame structure, as well as detail the location of various burning processes that occur within these zones (Flynn et al. 1999). The events occurring

Chapter 2 2.1 Fundamentals of compression-ignition combustion

during diesel combustion, as proposed by this conceptual model (Dec 1997), have been summarised in Figure 2.1 and are discussed in detail in the following paragraphs.

Diesel combustion commences with the injection of diesel fuel at a high pressure (500 -1000 bar), through a narrow injector orifice into the cylinder of an engine which contains compressed intake air at a temperature higher than the ignition temperature of the fuel. As diesel fuel is injected into the cylinder, it entrains the surrounding hot air and undergoes atomisation and droplet formation. The fuel droplets encounter aerodynamic resistance, slow down, gain heat from the entrained air and evaporate in the process. High-speed imaging of the fuel injection process showed that the fuel jet length is limited due to this vaporisation process. The fuel vapour gets pushed to the periphery of the spray by the later-injected fuel which forms a narrow core densely filled with liquid fuel droplets (Heywood 1988).

Rayleigh scatter images showed (Figure 2.1, number 2) that the layer downstream of this core has a well-defined boundary and consists of a uniform fuel rich diesel fuel-air mixture with an equivalence ratio of between two to four, and that there is only a very narrow fringe of stoichiometric mixture at this edge.

Chapter 2 2.1 Fundamentals of compression-ignition combustion

The fuel vapour gets blown around by the swirling motion of the air, and the turbulent environment inside the chamber promotes the mixing of fuel vapour with the intake charge mainly in the regions between the individual sprays. The mixing process of fuel vapour with air and the energy transferred to the fuel from the hot air initiates the chemical breakdown of the fuel molecules; the kinetic reaction mechanisms between fuel and air molecules in the entrained air govern the duration of ignition delay. Ignition delay is defined here as the duration in CAD between the start of diesel fuel injection (SOI) and the start of combustion (SOC). SOI is taken to be the time when the actuation signal is sent to the injector, whereas the SOC is defined as the first incidence of detectable heat release following autoignition of the diesel fuel (Figure 2.2). Autoignition first occurs around fuel vapour-air pockets

Figure 2.1: Diesel conceptual model describing fuel mixing and combustion, up to the end of injection (Dec 1997; Musculus et al. 2012)

Chapter 2 2.1 Fundamentals of compression-ignition combustion

of the appropriate stoichiometry and temperature, which have had time to mix during the ignition delay period; therefore, this phase is generally referred to as the premixed combustion phase (Heywood 1988). The rapid chemical energy release, following ignition, is accompanied by a significant rise in the combustion chamber temperature, resulting in high in-cylinder gas pressures, characteristic of the high peak heat release rates of the premixed combustion phase (Figure 2.2). Dec (1997) employed the use of natural flame chemiluminescence to detect the beginning of heat release, and reported that chemiluminescence is first observed in the fuel vapour regions near the injector tip, and it progressively increases in intensity downstream of the injector tip, with the highest levels of chemiluminescence detected in the solely vapour zone in the leading portion of the jet (Figure 2.1-number 3). Furthermore, chemiluminescence was detected at multiple points throughout the leading edge of the fuel-air mixture, providing evidence that diesel

Figure 2.2: Typical heat release rate (J/deg) curve of a DI compression-ignition engine showing the different phases of combustion (SOI-start of fuel injection, EOI-end of fuel injection) (Heywood 1988)

Chapter 2 2.1 Fundamentals of compression-ignition combustion

fuel ignition occurs at numerous multiple locations spontaneously where fuel and air have mixed to within flammability limits.

During the initial stages of premixed combustion, as the heat release rate rises steeply, fuel molecules break down into smaller chain hydrocarbons and intermediate radicals, some of which subsequently polymerise to form polyaromatic hydrocarbons (PAH). These PAHs are precursors to soot, and were detected by Dec (1997) using planar laser induced fluorescence (PLIF) (Figure 2.1-number 4).

The conceptual model by Dec (1997) of diesel combustion describes the premixed fuel vapour-air pockets having an equivalence ratio between one to one-and-a-half.

In-cylinder gas sampling studies indicating high levels of CO within the flame enveloped spray (Heywood 1988) provide further evidence that the premixed burn phase comprises mainly of the combustion of a fuel rich mixture. As premixed burning proceeds, small soot particles begin to appear in the fuel rich regions, as evidenced by laser induced incandescence images obtained by Dec (1997) (Figure 2.1-number 5). Subsequent detection of larger soot particles, using Rayleigh light scattering (Dec 1997) at the periphery of the premixed combustion zone, suggests that soot particles undergo growth as they move towards the edge of the rich premixed flame.

As combustion proceeds during the premixed burn phase, a thin diffusion flame develops along the periphery of the premixed burn region now. As the premixed

Chapter 2 2.1 Fundamentals of compression-ignition combustion

phase is completed, a fully developed diffusion flame encompasses the entire downstream region of the spray cone and extends upstream along the jet as far as the flame lift off point. This is referred to as the diffusion burn, or mixing rate controlled phase (Figure 2.2), and the heat release during this phase is limited by the rate at which the fuel and air sufficiently mix to combustible levels. Therefore, this phase of combustion is characterised by lower heat release rates and occurs over a longer period of time as compared to the premixed combustion phase. The existence of a diffusion flame was indicated by the presence of OH radicals observed by Dec (1997) using planar laser induced fluorescence (PLIF) (Figure 2.1-number 6). OH radicals occur in regions where the fuel-air mixture is close to stoichiometric (Turns 1996). No OH radicals were detected during the initial premixed burn phase, when the mixture stoichiometry was between two and four. Following peak heat release, OH radicals were first observed in small pockets along the contours of the premixed burn region. These radicals were subsequently seen encompassing the flame front and extending up to just downstream of the injector tip.

PLIF measurements done for the detection of nitrogen oxide (NO) by Dec &

Canaan (1998) provided further evidence of the presence of a diffusion flame (Figure 2.1-number 7). NO formation is normally favoured in a high temperature environment, which can be found in a diffusion flame region due to the fuel and air mixture equivalence ratio being near stoichiometric (Heywood 1988). Work done

Chapter 2 2.2 Emissions formation in