4. Summary of Test Method
4.1 he Research O.N. of a spark-ignition engine fuel is determined using a standard test engine and operating condi- tions to compare its knock characteristic with those of PRF blends of known O.N. Compression ratio and fuel-air ratio are adjusted to produce standard K.I. for the sample fuel, as measured by a specific electronic detonation meter instrument system. A K.I. guide table relates engine C. R. to O.N level for this method. The fuel-air ratio for the sample fuel and each of the primary reference fuel blends is adjusted to maximize K.I. for each fuel
4.1.1 The fuel-air ratio for maximum K.I. may be obtained (1) by making incremental step changes in mixture strength, observing the equilibrium K.I. value for each step, and then selecting the condition that maximizes the reading or (2) by picking the maximum K.I. as the mixture strength is changed from either rich-to-lean or lean-to-rich at a constant rate.
4.2 Bracketing Procedures The engine is calibrated to operate at standard K.I. in accordance with the guide table. The fuel-air ratio of the sample fuel is adjusted to maximize the K.I., and then the cylinder height is adjusted so that standard K.I. is achieved. Without changing cylinder height, two PRF blends are selected such that, at their fuel-air ratio for maxi- mum K.I., one knocks harder (higher K.I.) and the other softer (lower K.I.) than the sample fuei. A second set of K.I measurements for sample fuel and PRF blends is required, and the sample fuel octane number is calculated by interpolation in proportion to the differences in average K.I. readings. A final condition requires that the cylinder height used shall be within prescribed limits around the guide table value for the calculated O.N. Bracketing procedure ratings may be determined using either the equilibrium fuel level or dynam fuel level fuel-air ratio approach
4.3 C. R. Procedure A calibration is performed to establish standard K.I. using the cylinder height specified by the guide table for the O N. of the selected PRE he fuel-air ratio of the sample fuel is adjusted to max the K I. under equilibrium imize conditions; the cylinder height js adjusted so that standard K.I. is achieved. The calibration is reconfirmed and the sample fuel rating is repeated to establish the proper eonditions a second time. The average cylinder height reading for the sample fuel, compensated for barometric pressure is converted directly to O.N., using the guide table. A final condition for the rating requires that the sample fuel G.N. be thin prescribed limits wi around that of the O N. of the single PRF blend used to calibrate the engine to the gu ide table standard K.I. condition