Experimental investigations on high octane number gasoline formulations for internal combustion engines

The attempt to achieve higher and higher levels of specific power output and efficiency has increased the complexity of the design process of both the engine and its management system. Among the different issues, it is also necessary to take into account the chemical and physical characteristics of the available fuels. In order to study the behavior of some gasoline formulations characterized by a high octane number, the authors have carried out an experimental activity to understand how each fuel sample could improve the performances of a modern naturally aspirated SI (spark ignition) engine for passenger cars. The new fuel formulations were characterized by different contents of olefins and oxygen, the latter through the presence of oxygenated compounds like ethyl tert-butyl ether (ETBE) or methyl tert-butyl ether (MTBE). The experimental campaign consisted in measurements of the maximum brake torque (MBT) curve up to knock onset and the corresponding knock intensity, at wide open throttle (WOT) and partial load operating conditions, for each tested gasoline sample. The results of the data analysis show that the evaluation of the enhanced characteristics of a gasoline cannot be done by considering only the increase of the knock limit. Although a gasoline is generally labeled only by its RON (Research Octane Number), it can extend the benefits due to particular chemical formulation from full load up to part load conditions and, may be, in transient situations, as pointed out in this work. A naturally aspirated SI engine, under steady operation and fueled by high octane number gasolines, cannot provide a higher power output at WOT condition only by modifying the spark timing, if this engine has already been correctly optimized by the manufacturer before introducing it on the market. As a result, to achieve higher levels of power output it is necessary to modify the compression ratio, the Variable Intake System (VIS) and Valve Timing (VVT) strategies and so on, in order to exploit the high octane number offered by the new gasolines tested. Moreover, the analysis of the experimental data has also confirmed that the Research Octane Number (RON) index is less important than the Motor Octane Number (MON) at high engine speeds and loads, useful to characterize the octane requirement for modern engines. Furthermore, a standard deviation analysis has been conducted on the BMF (Burned Mass Fraction) parameter to understand if the different characteristics of gasolines could give advantages in terms of reduction of the CCV (Cyclic Combustion Variability). The results of the data analysis showed that the fuel formulations with higher content of oxygenated compounds exhibit a better behavior, highlighting a smaller CV (Coefficient of Variation) especially when reducing the load. This aspect could be considered one of the possible reasons of an improvement of the vehicle drivability.