The Relevance of Fuel RON and MON to Knock Onset in Modern SI Engines 2008-01-2414
The Octane Index (OI) relates a fuel's knocking characteristics to a Primary Reference Fuel (PRF) that exhibits similar knocking characteristics at the same engine conditions. However, since the OI varies substantially with the engine operating conditions, it is typically measured at two standard 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:
OI = K MON + (1-K) RON
where K is the weighing factor. When the tests were established, K was approximately 0.5. However, recent tests with modern engines have found that K is now negative, indicating that the RON and MON tests no longer bracket the knock-limited operating conditions.
Experiments were performed to measure the OI of different fuels in a modern engine to better understand the role of fuel sensitivity (RON-MON) on knock limits. The experiments were conducted in a single cylinder test engine that had been fitted with a modern pent-roof head. At each test condition, the spark timing was advanced until the engine transitioned into audible knock. Then, at each spark timing, pressure, microphone, and accelerometer data were collected to verify knock onset.
These experimental results show that K is negative for the engine operating conditions tested. Thus, fuels with higher sensitivities (RON-MON), but the same RON, were found to have better anti-knock performance. The results also show that the knock limited spark advance and maximum pressure of the engine increase linearly with increasing fuel sensitivity.
Similar experiments were performed to study the dependency of K on spark location, compression ratio, relative air/fuel ratio, engine speed, intake air temperature, and intake air pressure. The results show that K has a strong dependence on the intake air temperature, engine speed, and intake air pressure.
Recommendations are then made for modifications to the octane number tests to better bracket the knock-limited operating conditions of modern engines.