ASTM D2700 method for motor octane number of spark-ignition engine fuel
12. Test Variable Characteristics
12.1 Cylinder Height Relationship to Octane Number - Cylinder height, an indication of C.R., has a significant effect on fuels and their knocking characteristics. Every fuel has a critical C.R. at which knock begins to occur. As C.R. is increased above this critical threshold, the degree of knock, or severity of knock, increases. The Motor method of test compares sample fuels to PRF blends at a selected knock level termed standard K.I. Guide tables of cylinder height versus O.N. have been empirically determined using PRF blends. They are based on the concept that the K.I. at all O.N. values is constant as detected by the knock measuring instrumentation. Because this test method utilizes three different carburetor venturi sizes, tables specific to each venturi size have been established. Fig. 3 illustrates the slightly nonlinear relationship between Motor O.N. and cylinder height expressed as digital counter reading. Specific guide tables in terms of both digital counter reading and dial indicator reading for each of the three carburetor venturi sizes are in Annex A6 (see Table A6.1 and Table A6.8).
12.2 Barometric Pressure Compensation of Cylinder Height - O.N. values determined by this test method are referenced to standard barometric pressure of 760 mm (29.92 in.) of Hg. Changes in barometric pressure affect the level of knock because the density of the air consumed by the engine is altered. To compensate for a prevailing barometric pressure that is different from standard, the cylinder height is offset so that the K.I. will match that of an engine at standard barometric pressure. For lower than standard barometric pressure conditions, the cylinder height is changed to increase the engine C.R. and thus the knocking level. For higher than standard barometric pressure conditions, the cylinder height is changed to lower C.R. The changes in either digital counter reading or dial indicator reading to compensate for barometric pressure are listed in Annex A6 (see Tables A6.9 and A6.10).
12.2.1 Digital Counter Applications - The digital counter has two indicating counters. The top counter is directly connected to the worm shaft, which rotates the worm wheel that raises or lowers the cylinder in the clamping sleeve. It is the uncompensated digital counter reading. The lower counter can be disengaged from the upper counter for the purpose of off-setting its reading and thus establish the differential or compensation for prevailing barometric pressure. With the differential set, the two counters can be engaged to move together with the lower counter indicating the measure of cylinder height compensated to standard barometric pressure.
12.2.1.1 Digital counter readings decrease as cylinder height is raised and increase as cylinder height is lowered.
12.2.1.2 To index the digital counter unit, position the selector knob to any setting other than 1, change the cylinder height in the proper direction to compensate for the prevailing barometric pressure as given in Annex A6 (see Tables A6.9 and A6.10) so that the lower indicating counter is offset from the upper indicating counter by the amount of the compensation.
12.2.1.3 For barometric pressures lower than 760 mm (29.92 in.) of Hg, the lower indicating counter shall be less than the upper counter. For barometric pressures higher than 760 mm (29.92 in.) of Hg, the lower indicating counter shall be higher than the upper counter.
12.2.1.4 After adjusting to the correct counter readings, reposition the selector knob to 1 so that both indicating counters change when cylinder height changes are made. Check that the proper differential prevails as changes in cylinder height are made.
12.2.1.5 The lower indicating counter represents the measure of cylinder height at standard barometric pressure and is utilized for all comparisons with the values in the guide tables.
12.2.2 Dial Indicator Applications - The dial indicator is installed in a bracket on the side of the cylinder clamping sleeve so that the movable spindle contacts an anvil screw, positioned in a bracket mounted on the cylinder. As the cylinder is raised or lowered, the dial indicator reading measures the cylinder height in thousandths of an inch of travel. When indexed, the dial indicator reading is a measure of cylinder height for engines operating at standard barometric pressure. If the prevailing barometric pressure is other than 760 mm (29.92 in.) of Hg, correct the actual dial indicator reading so that it is compensated to standard barometric pressure. Compensated dial indicator readings apply whenever the reading is pertinent during the rating of sample fuels or when calibrating the engine using PRF blends.
12.2.2.1 Dial indicator readings decrease as cylinder height is lowered and increase as cylinder height is raised.
12.3 Engine Calibration at the Guide Table Cylinder Height - Calibrate the engine to produce standard K.I. at an O.N. level where sample fuels are expected to rate.
12.3.1 Prepare a PRF blend of the selected O.N. and introduce it to the engine.
12.3.2 Set the cylinder height to the appropriate guide table value (compensated for barometric pressure) for the O.N. of the PRF blend.
12.3.3 Determine the fuel level for maximum K.I.
12.3.4 Adjust the meter reading dial of the detonation meter so that the knockmeter reading is 50 +/- 2 divisions.
12.4 Fuel-Air Ratio Characteristic - With the engine operating at a cylinder height that causes knock, variation of the fuel-air mixture has a characteristic effect, typical for all fuels. The peaking or maximizing knock characteristic is illustrated in Fig. 4. This test method specifies that each sample fuel and PRF shall be operated at the mixture condition that produces the maximum K.I. The CFR engine carburetor, utilizing a single vertical jet, provides a simple means to monitor a measure of fuel-air ratio using a sight glass that indicates the fuel level in the vertical jet. Fig. 5 illustrates the relationships of the components. Low fuel levels relate to lean mixtures and higher levels to rich mixtures. Fuel level changes are made to determine the level that produces the maximum knocking condition. To maintain good fuel vaporization, a restrictive orifice or horizontal jet is utilized so that the maximum knock condition occurs for fuel levels between 0.7 and 1.7 in. referenced to the centerline of the carburetor venturi. The mechanics for varying the fuel mixture can be accomplished using various approaches.
12.4.1 Fixed Horizontal Jet–Variable Fuel Level System - Fuel level adjustments are made by raising or lowering the float reservoir in incremental steps. Selection of a horizontal jet having the appropriate hole size establishes the fuel level at which a typical sample fuel achieves maximum knock.
12.4.2 Fixed Fuel Level–Variable Orifice System - A fuel reservoir, in which the fuel can be maintained at a prescribed constant level, supplies an adjustable orifice (special long-tapered needle valve) used in place of the horizontal jet. Fuel mixture is changed by adjustment of the needle valve. Typically, the constant fuel level selected is near the 1.0 level, which satisfies the fuel level specification and also provides good fuel vaporization.
12.4.3 Dynamic or Falling Level System - A fuel reservoir, filled to a higher level than that required for maximum K.I., delivers fuel through either a fixed bore or adjustable horizontal jet. With the engine firing, the fuel level falls as fuel is consumed. Fuel level changes automatically at a specifically selected constant rate that is established by the cross-sectional area of the fuel reservoir and associated sight glass assembly. Maximum K.I. is recorded as the fuel level passes through the critical level.
