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Experiments were performed to identify the knock trends of lean hydrocarbon-air mixtures, and such mixtures enhanced with hydrogen (H2) and carbon monoxide (CO). These enhanced mixtures simulated 15% and 30% of the engine's gasoline being reformed in a plasmatron fuel reformer [1]. Knock trends were determined by measuring the octane number (ON) of the primary reference fuel (mixture of isooctane and n-heptane) supplied to the engine that just produced audible knock. Experimental results show that leaner operation does not decrease the knock tendency of an engine under conditions where a fixed output torque is maintained; rather it slightly increases the octane requirement. The knock tendency does decrease with lean operation when the intake pressure is held constant, but engine torque is then reduced.

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.