The Shift in Relevance of Fuel RON and MON to Knock Onset in Modern SI Engines Over the Last 70 Years 2009-01-2622
Since the advent of the spark ignition engine, the maximum engine efficiency has been knock limited. Knock is a phenomena caused by the rapid autoignition of fuel/air mixture (endgas) ahead of the flame front. The propensity of a fuel to autoignite corresponds to its autoignition chemistry at the local endgas temperature and pressure. Since a fuel blend consists of many components, its autoignition chemistry is very complex.
The octane index (OI) simplifies this complex autoignition chemistry by comparing a fuel to a Primary Reference Fuel (PRF), a binary blend of iso-octane and n-heptane. As more iso-octane is added into the blend, the PRF is less likely to autoignite. The OI of a fuel is defined as the volumetric percentage of iso-octane in the PRF blend that exhibits similar knocking characteristics at the same engine conditions.
Since the OI is dependent on the engine operating conditions, it is typically measured at two standard test conditions: the Research and Motor Octane Number (RON and MON) tests. These tests are intended to bracket the knock-limited operating range, and the OI is taken to be a weighted average of RON and MON:
where K is the weighing factor and S is the fuel sensitivity (RON-MON). When the tests were established in 1932, the MON test matched the OI on the road, hence the average value of K was 1; however, recent studies have found that the average value of K is negative.
The parameter K, which is independent of a fuel, can be used to show the relevancy of the RON and MON tests. When K = 0.5, the average of the RON and MON tests is a good indicator of a fuels antiknock performance. However, when K is negative, for a given RON, fuels with a higher sensitivity have better antiknock performance; therefore, an increase in RON or MON does not necessarily correlate to better antiknock performance.
In the decades following the development of the octane number, tests were performed to determine the value of K. However, as K approached a value of 0.5, these tests ceased to be performed.
The Coordinated Research Council (CRC) collected data from 1951 to 1991 to determine the required fuel octane number to avoid fleet vehicles from knocking. The tests typically included both PRFs and reference fuels representative of commercial gasoline, the latter having a non-zero sensitivity. Therefore, a value of K can be determined for each test. Using the data from 1951, 1961, 1971, 1981, and 1991, the average values of K were found for each year.
The study then explores the changes in the engine operating conditions that would cause these changes. To better understand these relationships, the values of K for modern and historic engines were determined by WAVE engine models. The models found that the average value of K is positive for a historic engine, but is negative for modern engines.
This study found that for domestic engines, K is now negative, having decreased from a value of 1 in 1930. This decrease in K is primarily due to better engine cooling, better engine breathing, and the usage of fuel injectors.