Establish standard K.I. by engine calibration, using a PRF blend O.N. close to the sample fuel to be rated.
Set the cylinder height to the barometric pressure compensated value for the O.N. of the selected PRF.
Determine the fuel level for maximum K.I. Adjust the detonation meter to produce a knockmeter reading of 50 ± 2 divisions, and record this value.
Check that detonation meter spread is maximized commensurate with satisfactory knockmeter stability.
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.
Introduce the sample fuel to an empty fuel bowl. Purge the fuel line and observing that there are no bubbles in tubing.
Operate the engine on sample fuel. Adjust the cylinder height to cause a mid-scale knockmeter reading for the sample fuel.
Determine the fuel level for maximum K.I.
Adjust the cylinder height. So that the knockmeter reading is within ±2 divisions of the standard K.I. reading recorded for the applicable PRF blend.
Allow equilibrium to occur. If necessary, make any slight adjustment in cylinder height to obtain a valid standard K.I. reading.
Upset engine equilibrium by opening the drain valve 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 ±1 division, readjust the cylinder height to obtain the standard K.I. value. And when equilibrium is achieved, repeat the fuel level upset check for repeatability of readings.
Read and record the compensated digital counter reading.
Convert the compensated digital counter reading to O.N. using the appropriate guide table in ASTM D2699.
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 ±3 divisions of the original reading, record the value and switch back to the sample fuel. If outside the ±3 division limit, standard K.I. must be reset before again rating the sample fuel.
Check the sample fuel by adjusting the cylinder height, so that the knockmeter reading is within ±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.
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.
Checking PRF Limit Compliance:
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 5 ASTM D2699.
When the O.N. difference between the sample fuel and the PRF exceeds the limits in Table 5 ASTM D2699. 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 ± 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.
Octane Rating Sample Fuels of Similar O.N.:
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. ate each of the sample fuels and then check that the standard K.I. for the PRF is within ±1 division of the initial value.
A check of standard K.I. shall, in any event, be made after every fourth sample fuel measurement.