Antiknock efficiency of oxygen-containing compounds in various fuels

Abstract

UDC 621.434.057.3 Extensive research is being conducted today on the possibility of improving the antiknock properties of automative gasolines by the addition of methanol, ethanol, methyl tert-butyl ether (MTBE), and water. Within the framework of this overall research effort, it is particularly important to study the antiknock efficiencies of these additives in fuels with various hydrocarbon compositions, with the aim of optimizing the component composition of gasolines in order to obtain the maximum effect from the use of the additives. Here we are reporting on an investigation of the antiknock efficiencies of methanol, ethanol, MTBE, and water in fuels based on paraffinic or aromatic hydrocarbons, As such fuels we used blends of isooctane with n-heptane (IHB) and toluene with n-heptane (THB), preparing three blends in each series with octane numbers of 50, 64, and 77 (Motor Method). The tests were performed in a UIT-65 unit. The additives were fed to the engine cylinder separately (not blended with the fuel) through a supplementary feed system. The antiknock efficiency of each additive was rated according to the gain in fuel octane number. The results obtained in these tests are presented in Fig. i. A comparison of the data for the IHB and THB with identical octane numbers shows that the susceptibility to methanol is higher for the IHB than for the THB (Fig. la); i.e., methanol, when it is added to the IHB, gives a greater increase in octane number than When it is added to the THE. With increasing octane number of the original fuel (with increasing content of either isooctane or toluene in the blend), this difference in the behavior of methanol is manifested to a greater degree. Analogous results are obtained when ethanol or MTBE was added to the fuels (Figs. ib and c). In contrast, the susceptibility to water is higher for the IHB than for the THB (Fig. id). These results are in good agreement with theoretical concepts of the features of oxidation and flaranability of different classes of hydrocarbons. Most paraffinic hydrocarbons are characterized by two-stage ignition with an intermediate cool-flame stage. In the ignition of aromatic hydrocarbons, the cool-flame stage is either absent or is much less distinct than with paraffinic hydrocarbons. It was shown in [I] that the influence of alcohols and MTBE on the ignition of hydrocarbons consists essentiall Z of a retardation of the appearance of the cool-flame stage and a lowering of the intensity of this stage. In this connection, we should expect less influence of these additives on the ignition of fuels for which the coolflame stage is not distinct. This means that the antiknock efficiency of the additives in such fuels will be lower than in fuels with a well-developed cool-flame stage. The effect of adding water to fuel consists mainly of a lowering of the temperature of the working charge in the engine cylinder. Correspondingly, the effect should depend on the temperature sensitivity of the fuel, i.e., on how much the tendency of the fuel toward detonative ignition changes with changes in the temperature regime of engine operation. The sensitivity of fuels is also determined by thekinetic features of their oxidation. Paraffinic and naphthenic hydrocarbons have low sensitivity to temperature because of their twostage ignition; correspondingly, the ignition lag (and hence the tendency to detonate) will chanse very little over a broad range of temperatures. The ignition lag of aromatic hydrocarbons increases rapidly with decreasing temperature, and hence they have an extremely high temperature sensitivity. For the IHB, the temperature sensitivity, as characterized by the difference between octane numbers determined by the Research and Motor Methods, is equal to 0; for the THB with octane numbers of 50, 64, 77, the respective temperature sensitivities are 6, 8.7, and ii.i. Correspondingly, we should expect that a lowering of the temperature of the working charge due to the addition of water will have a greater effect on the knock resistance of the THB than of the IHB. This difference should be greater in the case of fuels with a higher temperature sensitivity, and this is confirmed by the experimental data.