Analysis of Poor Engine Response Caused by MTBE-Blended Gasoline from the Standpoint of Fuel Evaporation

Fifty percent distillation temperature (T₅₀) can be used as a warm-up driveability indicator for a hydrocarbon-type gasoline. MTBE-blended gasoline, however, provides poorer driveability than a hydrocarbon-type gasoline with the same T₅₀. The purposes of this paper are to examine the reason for poor engine driveability caused by MTBE-blended gasolines, and to propose a new driveability indicator for gasolines including MTBE-blended gasolines. The static and dynamic evaporation characteristics of MTBE-blended gasolines such as the evaporation rate and the behavior of each component during evaporation were analyzed mainly by using Gas Chromatography/Mass Spectrometry. The results of the analysis show that the MTBE concentration in the vapor, evaporated at ambient temperature (e.g. 24°C), is higher than that in the original gasoline. Accordingly, the fuel vapor with enriched MTBE flows into the combustion chamber of an engine just after the throttle valve is opened. This MTBE-rich mixture sometimes causes misfire and lower engine torque because of more excessive air-fuel ratio and lower combustion energy, respectively. These phenomena degrade engine driveability. The combustion energy, obtained as a function of the evaporation rate and the calorific value, correlates with the engine response time. In this paper, the percentages of the fractions recovered at some specific temperature (e.g. 70°C) under distillation were used as the above evaporation rate. Also, the calorific values measured for the above fractions were used as the above calorific values. Therefore, the combustion energy mentioned above proved to be a useful indicator of driveability during engine warm-up.