This paper presents recently derived relationships between gasoline octane quality and vehicle fuel economy measured on a chassis dynamometer. Data are reported from a number of vehicle types, which include both port injection and direct injection technologies. Fuel economy was measured over a fixed test cycle on a matrix of ten fuels. Hence we established an engine/vehicle response to changes in gasoline octane number, in terms of fuel economy. This is comparable with previously reported relationships between gasoline octane quality and engine power output. Finally, fuel economy was measured over a number of industry-standard test cycles, when retail fuels of different octane grade were tested in vehicles. Statistically significant changes in vehicle fuel economy were measured for vehicles that ran on different fuel grades.
For over 40 years the octane number of gasoline has been defined by measurements of RON (Research Octane Number) and MON (Motor Octane Number), made in a CFR (Cooperative Fuel Research) engine. Conventional wisdom acknowledges a direct and positive relationship between octane quality and potential engine performance; for example, in modern knock sensor-equipped vehicles, increasing octane number can result in higher engine power output. However, it has been shown that increasing the MON of gasoline can have unexpected impacts on engine power output, with some vehicles showing almost no response to gasoline MON, or even a negative response, Kalghatgi [1]. In this paper, the same is shown to be true for vehicle fuel economy.