ASTM D2700 method for motor octane number of spark-ignition engine fuel
PROCEDURE A
13. Bracketing - Equilibrium Fuel Level
13.1 Check that all engine operating conditions are in compliance and equilibrated with the engine running on a typical fuel at approximately standard K.I.

13.2 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.

13.3 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.

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

13.3.2 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.

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

13.3.4 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 103 O.N. without resetting. Refer to Annex A4.

13.4 Sample Fuel:
13.4.1 Introduce the sample fuel to the carburetor, purge the fuel system, and if applicable, the sight glass and float 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 float reservoir and the sight glass. (Warning - Sample fuel is extremely flammable and its vapors are harmful if inhaled. Vapors may cause flash fire. See Annex A1.)

13.4.2 Operate the engine on sample fuel.

13.4.3 Adjust the cylinder height to cause a mid-scale knockmeter reading.

13.4.4 Determine the fuel level for maximum K.I. One approach is to first lower the fuel level (float reservoir assembly) and then raise it in small increments (0.1 sight glass divisions or less) until the knockmeter reading peaks and begins to fall off. Reset the float reservoir to the fuel level that produces the maximum knockmeter reading.

13.4.5 Adjust the cylinder height so that the knockmeter reading is 50 +/- 2 divisions.

13.4.6 Record the knockmeter reading.

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

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

13.5.2 Introduce Reference Fuel No. 1 to the engine and, if applicable, purge the fuel lines in the same manner as noted for the sample fuel.

13.5.3 Position the fuel-selector valve to operate the engine on Reference Fuel No. 1 and perform the step-wise adjustments required for determining the fuel level for maximum K.I.

13.5.4 Record the equilibrium knockmeter reading for Reference Fuel No. 1.

13.6 Reference Fuel No. 2:
13.6.1 Select another PRF blend that can be expected to result in a knockmeter reading that causes the readings for the two reference fuels to bracket that of the sample fuel.

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

13.6.3 Prepare a fresh batch of the second PRF blend.

13.6.4 Introduce Reference Fuel No. 2 to the engine and, if applicable, purge the fuel lines in the same manner as noted for the sample fuel.

13.6.5 Position the fuel-selector valve to operate the engine on Reference Fuel No. 2 and perform the required step-wise adjustments for determining the fuel level for maximum K.I.

13.6.6 If the knockmeter reading for the sample fuel is bracketed by those of the two PRF blends, continue the test; otherwise try another PRF blend(s) until the bracketing requirement is satisfied.

13.6.7 Record the equilibrium knockmeter reading for Reference Fuel No. 2.

13.7 Repeat Readings:
13.7.1 Perform the necessary steps to obtain repeat knockmeter readings on the sample fuel, Reference Fuel No. 2, and finally Reference Fuel No. 1. For each fuel, be certain that the fuel level used is that for maximum K.I. and allow operation to reach equilibrium before recording the knockmeter readings. The fuel switching for the complete rating shall be as illustrated in Fig. 6.

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

13.7.3 The two knockmeter 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 (2) the average of the sample fuel knockmeter readings is between 45 and 55.

13.7.4 If the first and second series of knockmeter 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 knockmeter 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 knockmeter readings is between 45 and 55.

13.8 Checking Guide Table Compliance:
13.8.1 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.

13.8.2 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.

13.9 Special Instructions for Sample Fuel Ratings Above 100 O.N.:
13.9.1 Knock characteristics become more erratic and unstable at octane levels above 100 for several reasons. Careful attention to the setting and adjustment of all variables is required to ensure that the rating is representative of the sample fuel quality.

13.9.2 If the sample fuel rating will be above 100 O.N., it is necessary to establish standard K.I. using an isooctane plus TEL PRF blend before sample fuel testing can continue. This may require more than one trial to select the appropriate leaded PRF (one of the two that bracket the sample fuel) and proper cylinder height. It will also necessitate adjustment of the detonation meter METER READING dial to obtain a knockmeter reading of approximately 50 divisions. If the rating is between 100.0 and 100.7 O.N., use the isooctane plus 0.05 mL TEL PRF to establish standard K.I. At the higher O.N. levels, either of the specified leaded PRF blends for the particular O.N. range may be used for this purpose.

13.9.3 Refer to Table 5 when selecting the PRF blends for sample fuels that rate above 100 O.N. Use only the specified PRF pairs for sample fuels that rate in the O.N. ranges 100.0 to 100.7; 100.7 to 101.3; 101.3 to 102.5; and 102.5 to 103.5.

13.9.4 Check that detonation meter spread is maintained as large as possible despite the fact that knockmeter readings will vary considerably and make selection of an average reading tedious.