Methods to Increase the Relevancy of the Octane Number Tests

The Octane Number test was unveiled in 1928 with a lukewarm response from the oil and automotive industries. The test represented a noble attempt for capturing the antiknock performance of a fuel given the limited knowledge of knock at the time. The test compares the antiknock performance of a fuel in a test engine to a reference fuel. Though simplistic, the test is ingrained in society and has undergone only minor revision despite dramatic changes in engines and fuels. Many studies have discussed the inadequacies of the test, with recent ones questioning their relevancy. This paper provides an overview of these issues, focusing on how to make the tests relevant to modern engines and fuels. Three techniques are recommended for updating the tests. The first technique adjusts the definition for the antiknock index, which is the “Octane Number” displayed on the fuel pump. The antiknock index is currently the average of the Octane Number measured at two test conditions, but recent studies indicate a more complicated relationship. The second technique changes the test’s references fuels, which are currently paraffins. By replacing iso-octane with toluene in the reference fuel blend, it behaves more similarly to modern fuels. The third technique involves changing the test conditions to better replicate the range of knock-limited conditions in modern engines. In particular, the tests would need to achieve higher in-cylinder pressures and lower in-cylinder temperatures. This paper discusses the merits of each of these three approaches while also looking at the challenges with implementing these changes.