ASTM D2699 test method for Research Octane Number of Spark-Ignition Engine Fuel
PROCEDURE C
15. Compression Ratio
15.1 Cylinder Height Measurement - This procedure shall only be used if the CFR engine is equipped with a digital counter for measurement of cylinder height in order to maximize the resolution of the measurement of this primary variable.
15.2 Applicable O.N. Rating Range - This procedure shall only apply for ratings within the range from 80 to 100 O.N.
15.3 Check that all engine operating conditions are in compliance and equilibrated with the engine running on a typical fuel at approximately standard K.I.
15.4 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 IAT required. This rating shall be performed in the same manner described below for a sample fuel except that the TSF blend shall be rated without carburetor cooling.
15.5 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.
15.5.1 Set the cylinder height to the barometric pressure compensated value for the O.N. of the selected PRF.
15.5.2 Determine the fuel level for maximum K.I., adjust the detonation meter, METER READING dial to produce a knockmeter reading of 50 more or less 2 divisions, and record this value.
15.5.3 Check that detonation meter SPREAD is maximized commensurate with satisfactory knockmeter stability.
15.5.4 Detonation meter spread set to 12 to 15 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.
15.6 Sample Fuel:
15.6.1 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.)
15.6.2 Operate the engine on sample fuel. If the engine knock changes drastically and results in either a very low or very high knockmeter reading, adjust cylinder height in the proper direction to reestablish a mid-scale knockmeter reading. This shift in O.N. level may require establishing standard K.I. with a different PRF blend whose O.N. can be estimated from the guide table for the cylinder height reading that has just been determined.
15.6.3 Adjust the cylinder height to cause a mid-scale knockmeter reading for the sample fuel.
15.6.4 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.
15.6.5 Adjust the cylinder height so that the knockmeter reading is within more or less 2 divisions of the standard K.I. reading recorded for the applicable PRF blend.
15.6.6 Allow equilibrium to occur, and if necessary, make any slight adjustment in cylinder height to obtain a valid standard K.I. reading. Do not extend the operating time beyond approximately 5 min as measured from the time at which the fuel level setting is finalized.
15.6.7 Upset engine equilibrium by opening the sight glass drain valve momentarily to cause the fuel level to fall and any trapped vapor bubbles to be removed. After closing the drain valve, observe that the knockmeter reading returns to the previous value. If the knockmeter reading does not repeat within more or less 1 division, readjust the cylinder height to obtain the standard K.I. value for the applicable PRF blend and when equilibrium is achieved, repeat the fuel level upset check for repeatability of readings.
15.6.8 Read and record the compensated digital counter reading.
15.6.9 Convert the compensated digital counter reading to O.N. using the appropriate guide table.
15.7 Repeat Reading:
15.7.1 Check standard K.I. by operation on the PRF blend at the compensated digital counter reading for the O.N. of this blend. If the knockmeter reading is within more or less 3 divisions of the original reading, record the value and switch back to the sample fuel. If the knockmeter reading is outside the more or less 3 division limit, standard K.I. must be reset before again rating the sample fuel.
15.7.2 Check the sample fuel by adjusting the cylinder height so that the knockmeter reading is within more or less 2 divisions of the standard K.I. reading recorded for the PRF blend and convert the compensated digital counter reading to O.N. using the appropriate guide table.
15.7.3 The average of the two sample fuel O.N. results constitute a rating provided the difference between them is no greater than 0.3 O.N.
15.8 Checking PRF Limit Compliance:
15.8.1 The average O.N. of the sample fuel is acceptable if it does not differ from the O.N. of the PRF used to establish standard K.I., by more than the value in Table 4.
15.8.2 When the O.N. difference between the sample fuel and the PRF exceeds the limits in Table 4, check standard K.I. using a new PRF whose O.N. is within the indicated limits. If the new PRF knockmeter reading at the cylinder height for its O.N. is within 50 more or less 1 divisions, the previously determined rating may be accepted. If not, perform a new engine calibration using the selected PRF and repeat the sample fuel rating.
15.9 Testing Sample Fuels of Similar O.N.:
15.9.1 If the O.N. values of several sample fuels are known to be similar, it is permissible to determine standard K.I. using an appropriate PRF, rate each of the sample fuels and then check that the standard K.I. for the PRF is within more or less 1 division of the initial value.
15.9.2 A check of standard K.I. shall, in any event, be made after every fourth sample fuel measurement.
