The overall objectives of this study are to establish the relationship between a spark ignition, or Otto cycle, engine energy efficiency and the octane number (and potentially, the composition) of the fuel through a comprehensive review of recently published literature.
The efficiency of the ideal Otto cycle is a function of the compression ratio (CR), but increasing compression ratio is limited by the onset of knock, which can be prevented by increasing fuel octane number. Hence, in an ideal case, there is a direct connection between engine efficiency and fuel octane number. In the real world, other factors also contribute to the relationship and spark timing is the primary control variable that affects both knock and efficiency. The literature review found that the relationship between octane number and efficiency was influenced by a number of intermediate variables covering engine type, operating condition, and fuel formulation. Hence, it was not possible to simply collate the data from the papers and develop an average relationship; considerable effort was required to organize the reported data into subsets of similar engine types and operating conditions, and to convert the different data metrics into a comparable set of metrics. The analysis was also assisted by inputs from technical experts at three auto-manufacturers obtained from interviews. Experts at all three auto-manufacturers interviewed suggested that this analysis use only the results from the engine studies because they anticipated that in vehicle studies, many variables were vehicle calibration dependent and therefore, largely uncontrolled.
As a result, this review focuses on published data from engine tests where all or most of the data on variables relevant to the study was controlled and/ or documented, but data from vehicle studies were also reviewed for the CRC study to confirm the findings from engine tests. Due to the fact that many other intermediate variables affect the absolute relationship between compression ratio, fuel octane and efficiency, the focus was narrowed to evaluate the marginal effect of increasing CR on octane number requirement and engine efficiency, while holding other parameters not directly linked by geometry to CR nearly constant. This focus does not imply that other parameters are not significant and their influences are mentioned but not explored in detail in this paper. The CRC study[1] completed in 2012 was updated for the Department of Energy in 2016, and the results of both studies are incorporated into this paper.