Octane Number is one of the most important quality of gasoline. The influence of chemical structure of gasoline components such as hydrocarbons and oxygenates on octane number is reviewed. In addition, how the chemical structure of hydrocarbons changes under petroleum refinery process is described.
It is well known that octane number systematically changes depending on the chemical structure. According to this empirical rule, octane number increases with the number of branches, and decreases with the separation between branches. This paper aims to provide some insight into the chemical interpretation of octane number. At first, the low temperature oxidation and H2O2 loop mechanism are introduced in order to understand the auto-ignition process of hydrocarbon compounds, and then, the effects of branched structure on the low temperature oxidation mechanism have been discussed by using a steady state analysis. This analysis shows that both tertiary and quaternary carbon decrease the effective rate constants of chain branching process for alkylperoxy radical (ROO•) reactions. A tertiary carbon inhibits intramolecular hydrogen shift reaction of hydroperoxyalkylperoxy radical (•OOQOOH). On the other hand, a quaternary carbon inhibits mainly intramolecular hydrogen shift reactions of ROO• and promotes cyclic-ether formation reactions of hydroperoxy radical (•QOOH). The correlations between chemical structure and octane number can be partly explained by these kinetic effects.
For the isomers of hexane, heptane, and octane as typical candidates for gasoline of motor vehicles and propeller airplanes correlation between the research octane number (RON) and the molecular structure of alkane was studied. Three topological indices, Z (Hosoya index), p (3-step number of Wiener), and B (Balaban's centric index) were selected for this QSAR (QSPR) study not only to attain good correlation but also to clarify the mechanism of gasoline combustion in the engine. As generally accepted RON increases with the degree of branching, correlated well with B and -Z. Another known feature of this problem that RON decreases with the number of carbon atoms can be taken into account by -Z. On the other hand, p, which shows the highest correlation with the density of liquid, has almost nothing to do with RON by itself. Although B has the highest correlation with RON among various single indices, it cannot explain the difference among the mode of branching, such as 2,2-dimethyl and 2,3-dimethy substitution. This weak point of B can be overcome by the combination with p and Z as B+4p-2Z, leading to the conclusion that the more spherical the alkane molecule becomes the lower the boiling point and the more dense the unburned liquid gasoline can be pressed for effective burning without knocking. Finally, it should be pointed out that the strange behavior of 2,2,3,3-tetramethylbutane, with extraordinary high melting point but predicted to have high RON, cannot be explained by this simple QSAR study.
Spark ignition engines are widely used for a small generator as well as a passenger vehicle. The exhaust gas emissions no longer have pollution problem by adopting a combination of three-way catalyst and stoichiometric mixture control, but an improvement of thermal efficiency is an urgent task. To achieve this, an increase of compression ratio and down-sizing with a turbo-charger are solutions. However, employing these methods causes abnormal combustion of such as knocking and pre-ignition that will bring about damages to engine components. In this article, why these phenomena occur and how to solve the issues are briefly mentioned with some author's ideas: since autoignition itself does not lead to a heavy knocking, a quick expansion at early expansion stroke and a spatial temperature distribution of mixture inside the cylinder can control the knocking intensity. As pre-ignition at low engine speed is not well analyzed, this phenomenon must be quantitatively examined.
Homogeneous charge compression ignition (HCCI) combustion can be achieved further improvement in thermal efficiency by increasing both compression and specific heat ratios. Toward the commercial production of gasoline HCCI engines, it is important to understand the effect of fuel characteristics on HCCI and SI combustion. The model fuels used in this study have the two parameter, octane number and fuel components. Using these model fuels, the effect of fuel components against two Auto-Ignition phenomena, which are 1) HCCI combustion and 2) knocking in SI combustion, was analyzed with experimental approaches.
During large scale CAMUI-type motor development, the authors frequently encountered anomalous combustion, a sudden pressure increase leading to destroy of the motor. Repeated static firing tests finally revealed that the cause of the anomalous combustion is the low initial fuel temperature. However, the mechanism responsible for the anomalous combustion is still unclear. Although a series of firing tests with a small combustor could not reproduce the anomalous combustion successfully, results showed a clear correlation between the initial fuel temperature and chamber pressure overshoot; chamber pressure overshoot does not occur when the fuel temperature is above the Leidenfrost point. From this result, the authors offer a hypothesis that the low fuel temperature below the Leidenfrost point enhanced heat transfer from the fuel to liquid oxygen and caused local blowoff. Accumulation of combustible mixture follows the blowoff and may cause the anomalous combustion. A possible reason why the firing tests could not reproduce the anomalous combustion is that Damkohler number in the combustion chamber was larger than those in the large-scale CAMUI-type motors. A preliminary experiment showed that the small combustor could reproduce the anomalous combustion by decreasing the Damkohler number in the combustion chamber. Detailed experimental study will follow the preliminary experiment to clarify the mechanism of the anomalous combustion.
To clarify the stabilization mechanism of coaxial oxygen-jet diffusion flames at high pressure, experiments on flame lifting and blowout for pure-oxygen jet diffusion flame for two types of double tube burners were performed under the conditions of various jet and co-flow velocities and various methane mole fractions in co-flow diluted with nitrogen. At high pressure of 0.5 MPa, the flame stability was significantly enhanced in comparison with that at atmospheric pressure. Flame was formed in co-flow side at atmospheric pressure, while at high pressure the flame position moved to the jet-flow side. At atmospheric pressure, the flame base located close to the lip and then moves downstream of the lip with increase in methane mole fraction in co-flow and increase in jet velocity. In contrast, at high pressure, the flame base located close to the lip regardless of methane mole fraction in co-flow and approached the lip with increase in jet velocity. According to the observation results of stream line near the lip, at atmospheric pressure, the characteristic length scale of the recirculation zone was small and the flame base was located downstream of the recirculation zones, indicating that the oxygen-jet diffusion flame was stabilized by the balance between the local gas velocity and burning velocity of the premixed gas formed by diffusion. In contrast, at high pressure, the recirculation zone was formed in almost all experimental conditions, its characteristic length scale being larger than that at atmospheric pressure, and flame base located in the recirculation zones in some cases. This means that the stabilization of the flame base at high pressure is dominated by the existence of the recirculation zone. Therefore, the stabilization mechanism of the coaxial oxygen-jet diffusion flame changes depending of the ambient pressure and stability of the flame base is enhanced at high pressure.
This paper presents a simple theory that addresses the interaction between two identical jet diffusion microflames. Point-source diffusion flames under a uniform flow are considered. Since the dimensionless burner-burner distance, defined as the distance between burner axes divided by the burner diameter, is the only parameter in the dimensionless system adopted, its influence on the predicted flame shape is analytically studied. It is found, similarly to the interaction between two burning droplets, that (1) when the burner-burner distance is sufficiently large, each microflame behaves as an isolated flame, (2) two flames approach each other with a decrease in the burner-burner distance, (3) two flames merge when the burner-burner distance is less than a critical value, and finally (4) two flames unite and behave like a single flame when the burner-burner distance is close to the burner diameter. The critical burner-burner distance in which two flames touch each other is derived.
In this study, we simulated the combustion field in a triple port burner. There are four flame configurations, consisting of attached flames, inner attached/outer lifted flames, inner lifted/outer attached flames, and twin lifted flames. Focusing on the transition process of these flames, the flame behavior and flow field were investigated when the external air flow velocity was increased at constant internal air flow and fuel flow velocities. Except for the attached flame, when the flame is lifted, the axial velocity toward the leading-edge flame gradually decreases downstream, takes its minimum, and then increases very rapidly. This minimum velocity corresponds approximately to the burning velocity of the leading-edge flame. Since two flames are formed in the triple port burner, the inner or outer flame is affected by the other flame located more upstream. Due to the thermal expansion of the upstream outer flame, the downstream inner flame is pushed inward. Since the flame is stabilized at the position where the burning velocity of leading-edge flame and the incoming flow velocity are balanced, the axial velocity toward the leading-edge flame is important to discuss the transition of flames. The inner flame re-attachment is caused by the velocity change toward the leading-edge flame. As a result, the unique flame behavior of flip-flop between inner and outer flames occurs.