**2. What are autoignition combustion and HCCI?**

The phenomenon of autoignition combustion is still under investigation, even though HCCI combustion has been applied in a two-stroke engine in a commercial motorcycle [9]. Does HCCI combustion and "hot spots" in the burned area in SI engines propagate in the same way? Is there a flame front propagation present in an HCCI engine? How does autoignition combustion propagate in an HCCI engine? Does turbulent mixing affect HCCI combustion? What fuel properties drive cool flame combustion and what the main combustion? What engine parameters affect HCCI combustion? And most importantly of all, how can HCCI combustion be controlled in the most effective way? This section presents an overview of the nature of the autoignition combustion and what is believed to define HCCI combustion, regardless of the fuel used or the engine parameters.

Autoignition combustion can be described by the oxidation of the fuel driven solely by chemical reactions governed by chain-branching mechanisms [1],[2]. Furthermore, two temperature regimes exists – one below 850K (low temperature oxidation or cool flame combustion) and one around 1050K (high temperature oxidation or main combustion) – that can define the autoignition process [10]-[13]. At high temperatures, the chain branching reactions primarily responsible for the autoignition, are given by

*H O*2• + *RH* →*H*2*O*<sup>2</sup> + *R* • *H*2*O*<sup>2</sup> + *M* →*OH* • + *OH* • + *M*

where R• is any hydrocarbon radical and M are other molecules in the system. At low temperatures, the decomposition of H2O2 is quite slow and the reaction mechanisms respon‐ sible for the low-temperature combustion are:

*H O*2*R* '• + *O*2↔*H O*2*R* '*O*2• *H O*2*R* '*O*2• + *RH* →*H O*2*R* '*O*2*H* + *R* • *H O*2*R* '*O*2*H* →*H O*2*R* '*O* • + *HO* • *H O*2*R* '*O* • →*OR* '*O* + *HO* • *H O*2*R* '*O*2• →*H O*2*R* • ' '*O*2*H H O*2*R* • ' '*O*2*H* →*H O*2*R* ' '*O* + *HO* • *H O*2*R* ' '*O* →*OR* '*O* • + *HO* •

Depending on the structure of the fuel, under engine conditions some fuels would exhibit cool flame combustion and some others will not. In general, long straight chain alkanes, normal paraffins, and low Research Octane Number (RON) fuels would exhibit cool flame combustion while branched-chain alkanes, aromatics and high RON fuels would not [14], [15]. However it was also shown [16] that *iso*-octane may also exhibit cool flame combus‐ tion under certain conditions. Furthermore, Kalghatgi [17],[18] has also shown that the temperature is not the only parameter that affects the aforementioned mechanisms and that depending on the fuel composition and the engine conditions, the autoignition process varies significantly. It was therefore suggested that other parameters affect the autoigni‐ tion process and that a better understanding on the fuel property is needed. Neither the Motor Octane Number (MON) nor RON of different fuels alone can be used to describe the fuel characteristics and it was proposed that the Octane Index (OI) of a fuel should be used as defined by:

a historical review on the early research on autoignition is presented. In section 4, HCCI combustion is presented in more detail, including aspects such as the effect of fuels, and fuel additives, engine design, etc, as well as the HCCI engines in production. In Section 5, a theory on controlling HCCI is presented, with emphasis on fuel injection strategies, Exhaust Gas Recirculation (EGR) and temperature inhomogeneities. In the final Section, the conclusions of

The phenomenon of autoignition combustion is still under investigation, even though HCCI combustion has been applied in a two-stroke engine in a commercial motorcycle [9]. Does HCCI combustion and "hot spots" in the burned area in SI engines propagate in the same way? Is there a flame front propagation present in an HCCI engine? How does autoignition combustion propagate in an HCCI engine? Does turbulent mixing affect HCCI combustion? What fuel properties drive cool flame combustion and what the main combustion? What engine parameters affect HCCI combustion? And most importantly of all, how can HCCI combustion be controlled in the most effective way? This section presents an overview of the nature of the autoignition combustion and what is believed to define HCCI combustion,

Autoignition combustion can be described by the oxidation of the fuel driven solely by chemical reactions governed by chain-branching mechanisms [1],[2]. Furthermore, two temperature regimes exists – one below 850K (low temperature oxidation or cool flame combustion) and one around 1050K (high temperature oxidation or main combustion) – that can define the autoignition process [10]-[13]. At high temperatures, the chain branching

where R• is any hydrocarbon radical and M are other molecules in the system. At low temperatures, the decomposition of H2O2 is quite slow and the reaction mechanisms respon‐

the chapter are presented.

120 Advances in Internal Combustion Engines and Fuel Technologies

*H O*2• + *RH* →*H*2*O*<sup>2</sup> + *R* • *H*2*O*<sup>2</sup> + *M* →*OH* • + *OH* • + *M*

*H O*2*R* '• + *O*2↔*H O*2*R* '*O*2•

*H O*2*R* '*O* • →*OR* '*O* + *HO* •

*H O*2*R* ' '*O* →*OR* '*O* • + *HO* •

*H O*2*R* '*O*2• →*H O*2*R*

*H O*2*R* • '

*H O*2*R* '*O*2• + *RH* →*H O*2*R* '*O*2*H* + *R* • *H O*2*R* '*O*2*H* →*H O*2*R* '*O* • + *HO* •

> • ''*O*2*H*

'*O*2*H* →*H O*2*R* ' '*O* + *HO* •

**2. What are autoignition combustion and HCCI?**

regardless of the fuel used or the engine parameters.

sible for the low-temperature combustion are:

reactions primarily responsible for the autoignition, are given by

$$\text{OI} = \text{RON} - \text{KS} \tag{1}$$

where S=RON-MON and K is a variable that is determined by the engine parameters and operating conditions.

Regardless of the chemical reactions associated with autoignition, the spatial initiation and the development or "propagation" of the autoignition sites is another point of interest. Chemiluminescence and Planar Laser Induced Fluorescence (PLIF) imaging of the autoigni‐ tion phenomenon has shown that autoignition would start at various locations through‐ out the combustion chamber [3],[4],[6] probably due to local inhomogeneities. Due to the heat released from the burn regions, the temperature and pressure in the cylinder increase and therefore more autoignition sites appear, until the whole fuel-air mixture is ignited. It was also shown [19] using both chemiluminescence and formaldehyde PLIF imaging in a highly stratified engine (hot EGR gases and cold fresh fuel/air mixture) that these autoig‐ nition sites initiated at neither the location of maximum temperature nor the location of maximum fuel concentration, but at the boundary of these two regions. Once the first autoignition sites appeared, double-exposure PLIF or chemiluminescence imaging showed that these sites grow in size at different speeds – more or less they can appear to be "flame fronts" in the absence of any other information (i.e. A/F ratio, in-cylinder temperature, "flame front" speed, double-exposure timings).

This autoignition phenomenon has been applied in IC engines as an alternative to SI and CI engines, and is generally referred to as HCCI combustion. Since under HCCI combus‐ tion the fuel/air mixture does not rely on the use of a spark plug or direct injection near Top Dead Centre (TDC) to be ignited, overall lean mixtures can be used resulting to high fuel economy. Thus, the combustion temperature remains low and therefore NOx emis‐ sions decrease significantly [20],[21] compared to SI and CI operation. An illustration of the combustion differences between the three modes of IC operation is shown in Figure 1.

(RPM) and the Compression Ratio (CR) was varied by altering the cylinder position. The twostage combustion of *n*-heptane was observed by recording the pressure traces. It was also observed that the ignition temperature (above which an explosion took place), depended both on the properties of the combustible substances (i.e. octane number), on the conditions of the experiments (i.e. CR, initial temperature and pressure) and on the rate of heat losses from the gas.Furthermore,anequationwasderivedforthetimeforcompletecombustionoftheexplosive mixtures of gases when suddenly compressed to a temperature above its ignition temperature. A rapid-compression machine capable of producing CRs up to 15:1 was used in the 1950s [3], [29],[30] to investigate the effect of fuel composition, compression ratio and fuel-air ratio on the autoignition characteristics and especially the ignition delay (i.e. the time from when the mixture was suddenly compressed until autoignition) of several fuels that included heptane, *iso-*octane, benzene, butane and triptane. An air-fuel mixing tank was used to ensure the correct ratio, the pressure records were taken with a catenary-type strain-gage indicator and a Fastax camera (operated at a rate of 10,000 frames per second) was used in taking flame and Schlieren photographs. It was concluded that all fuels had a minimum value of ignition delay at their chemically correct air-fuel ratio that increased with decreasing compression ratio. Further‐ more, the detonating or knocking properties of the fuels depended both on the ignition delay and on the rate of combustion after autoignition. The flame photographs recorded [3] revealed that autoignition in the rapid-compression machine fell in three loose classifications that is also

Homogenous Charge Compression Ignition (HCCI) Engines

http://dx.doi.org/10.5772/55807

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**•** Isolated points of autoignition that developed sporadically in all parts of the combustion

**•** The inflammation began in small regions and proceeded across the chamber in the form of

The possibility of fuel droplets, non-homogeneity in the air-fuel mixture, dust particles, piston contact with cylinder head and temperature gradients causing the non-uniformity in the autoignition process was also investigate by using flame and Schlieren images [30]. They have concluded – in the absence of any data to provide a different reason – that temperature gradi‐ ents are the primary reason for the inhomogeneities in the autoignition process. It was ob‐ served that before the main ignition of the mixture, a first-stage smaller-scale reaction, called "cool flame" was also present for some hydrocarbons. It was found that the pressure required toinitiate the first-stage reactionwasalinearfunctionofthe compressionpressureatTDC,while dependingonthefuel,therequiredcompressionpressuretoinitiateautoignitiondecreasedwith

Onishi *et al*. in 1979 [27] were amongst the first researchers to investigate the possibility of using autoignition combustion as a combustion mode in an engine. They have applied autoignition combustion using gasoline in a two-stroke gasoline engine and named this process ATAC. They showed that there was very small Cycle-By-Cycle Variations (CBCV) in the peak combustion pressure, the reaction occurred spontaneously at many points and combustion proceeded slowly. They investigated the significance of the hydroxyl, OH,

increasing fuel concentration. However, no analysis of the result was presented.

evident in modern IC engines:

chamber.

a "flame front".

**•** Uniform combustion throughout the combustion chamber.

**Figure 1.** Combustion differences between the three modes of IC operation.

Under optimum operating conditions, HCCI combustion can offer comparable Carbon Monoxide, CO, and HydroCarbon, HC, emissions with SI and CI combustion, but under very lean conditions – and thus low combustion temperatures (approximately below 1500K) – incomplete combustion can occur in the bulk regions leading to partial oxidation of the fuel, decrease in combustion efficiency and increase in CO and HC emissions [12],[22]-[24]. Furthermore, since a homogeneous fuel/air mixture can be prepared in the manifold, low soot can be achieved [20]. However, when HCCI combustion is operated at richer fuel/air mixtures, knocking can occur. In conclusion, HCCI combustion in a production engine is therefore limited by two main regimes [25],[26]:

