ASTM D2700 method for motor octane number of spark-ignition engine fuel
PROCEDURE B
14. Bracketing - Dynamic Fuel Level:
14.1 Applicable O.N. Rating Range - This procedure shall only apply for ratings within the range from 80 to 100 O.N.

14.2 Check that all engine operating conditions are in compliance and equilibrated with the engine running on a typical fuel at approximately standard K.I.

14.3 Perform engine fit-for-use testing utilizing a TSF blend applicable for the O.N. range in which sample fuels are expected to rate. If TSF blend temperature tuning is to be used, determine the proper intake MIXT required. Perform this rating in the same manner described below for a sample fuel except that the TSF blend shall be rated without carburetor cooling.

14.4 Establish standard K.I. by engine calibration using a PRF blend having an O.N. close to that of the sample fuels to be rated.

14.4.1 Set the cylinder height to the barometric pressure compensated value for the O.N. of the selected PRF.

14.4.2 Determine the fuel level for maximum K.I. and then adjust the detonation meter METER READING dial to produce a knockmeter reading of 50 +/- 2 divisions.

14.4.3 Check that detonation meter SPREAD is maximized commensurate with satisfactory knockmeter stability.

14.4.4 Detonation meter spread set at 12 to 14 K.I. divisions per O.N. at the 90 O.N. level will typically provide suitably optimized spread settings for the range 80 to 100 O.N. without resetting. Refer to Annex A4.

14.5 Sample Fuel:
14.5.1 Introduce the sample fuel to an empty fuel reservoir. Purge the fuel line, sight glass, and fuel reservoir by opening and then closing the sight glass drain valve several times and observing that there are no bubbles in the clear plastic tubing between the fuel reservoir and the sight glass. Top off the level so that the fuel level is at approximately 0.4 in the sight glass. Where experience demonstrates the critical maximum K.I. occurs near a specific fuel level, filling to a level 0.3 above the typical level is acceptable. (Warning - Sample fuel is extremely flammable and its vapors are harmful if inhaled. Vapors may cause flash fire. See Annex A1.)

14.5.2 Position the fuel-selector valve to operate the engine on the sample fuel and observe that the fuel level begins to fall in the sight glass.

14.5.3 When applying this falling level technique, stop the sequence by switching to another fuel when the K.I. reading passes its maximum value and decreases approximately ten divisions. Closely monitor each falling fuel level sequence to ensure the engine is always supplied with fuel and that knocking conditions prevail for a high proportion of rating time to maintain operating temperature conditions.

14.5.4 If the K.I. reading changes significantly from midscale, adjust the cylinder height to bring the engine close to the standard K.I. condition.

NOTE 2 - Proficiency in making this initial adjustment of cylinder height is achieved with experience.

14.5.5 Refill the fuel reservoir to the appropriate rich mixture sight glass level for each successive repetition of the trial-and-error process.

14.5.6 After the cylinder height is approximately determined, it may be necessary to make a final adjustment to ensure that (1) the fuel level for maximum K.I. occurs at a sight glass level within the critical range from 0.7 to 1.7 in. and (2) the maximum K.I. reading is between 45 and 55 divisions.

14.5.7 Record the maximum K.I. reading, or if a K.I. recorder is being used, mark the trace to indicate the sample identification and highlight the maximum reading.

14.5.8 Observe the cylinder height reading, compensated to standard barometric pressure, and using the appropriate guide table, determine the estimated O.N. of the sample fuel.

14.6 Reference Fuel No. 1:
14.6.1 Prepare a fresh batch of a PRF blend that has an O.N. estimated to be close to that of the sample fuel.

14.6.2 Introduce Reference Fuel No. 1 to one of the unused fuel reservoirs, taking care to purge the fuel line, sight glass, and fuel reservoir in the same manner as noted for the sample fuel.

14.6.3 Position the fuel-selector valve to operate the engine on Reference Fuel No. 1 and record, or mark the recorder tracing, to indicate the maximum K.I. reading that occurs as the fuel level falls. Care shall be taken to observe that the maximum K.I. condition occurs at a fuel level within the specified 0.7 to 1.7 in. range.

14.7 Reference Fuel No. 2:
14.7.1 Select another PRF blend that can be expected to result in a maximum K.I. reading that causes the readings for the two reference fuels to bracket that of the sample fuel.

14.7.2 The maximum permissible difference between the two reference fuels is dependent on the O.N. of the sample fuel. Refer to Table 5.

14.7.3 Prepare a fresh batch of the selected PRF blend.

14.7.4 Introduce Reference Fuel No. 2 to one of the unused fuel reservoirs, taking care to purge the fuel line, sight glass, and fuel reservoir in the same manner as noted for the sample fuel.

14.7.5 Position the fuel-selector valve to operate the engine on Reference Fuel No. 2 and record, or mark the recorder tracing, to indicate the maximum K.I. reading which occurs as the fuel level falls. Care shall be taken to observe that the maximum K.I. condition occurs at a fuel level within the specified 0.7 to 1.7 in. range.

14.7.6 If the maximum K.I. reading for the sample fuel is bracketed by those of the two reference fuel blends, continue the rating; otherwise try another reference fuel blend(s) until the bracketing requirement is satisfied.

14.8 Repeat Readings:
14.8.1 Perform the necessary steps to obtain repeat K.I. readings on the sample fuel, Reference Fuel No. 2, and finally Reference Fuel No. 1. The fuel switching for the complete rating shall be as illustrated in Fig. 6.

14.8.2 Refer to Section 16 for detailed interpolation and calculation procedure.

14.8.3 The two maximum K.I. readings for the sample fuel and two for each of the PRF blends constitute a rating provided (1) the difference between the rating calculated from the first and second series of readings is no greater than 0.3 O.N., and the (2) average of the sample fuel K.I. readings is between 45 and 55.

14.8.4 If the first and second series of K.I. readings do not meet the criteria, a third series of readings may be obtained. The fuel switching order for this set shall be sample fuel, Reference Fuel No. 1, and finally Reference Fuel No. 2. The second and third series of maximum K.I. readings shall then constitute a rating provided the difference between the rating calculated from the second and third series of readings is no greater than 0.3 O.N., and the average of the last two sample fuel K.I. readings is between 45 and 55.

14.9 Checking Guide Table Compliance:
14.9.1 Check that the cylinder height, compensated for barometric pressure, used for the rating is within the prescribed limits of the applicable guide table value of cylinder height for the sample fuel O.N. At all O.N. levels, the digital counter reading shall be within +/- 20 of the guide table value. The dial indicator reading shall be within +/- 0.014 in. of the guide table value.

14.9.2 If the cylinder height of the sample fuel rating is outside the guide table limit, repeat the rating after readjustment of the detonation meter to obtain standard K.I. using a PRF blend whose O.N. is close to that of the sample fuel.