10 Procedure
10.1 General
Check that all engine operating conditions are in compliance and equilibrated with the engine running on a typical diesel fuel oil.
NOTE 26 ASTM D613, Appendix X2 (Operating Techniques - Adjustment of Variables) provides additional useful details of engine operation and the adjustment of each of the individual operating variables for application of this International Standard.
Always position the mode selector switch on the ignition delay meter (Mark II and earlier models) to "calibrate" before proceeding with fuel switching, so that violent meter-needle full-scale pegging does not occur. Calibration adjustment should be checked before each rating but never changed during a rating.
10.2 Sample introduction
Pour the sample into an empty fuel tank, rinse the fuel burette, purge any air from the fuel line to the pump and position the fuel-selector-valve to operate the engine on this fuel.
10.3 Fuel flow rate
Check the fuel flow rate and adjust the flow-rate-micrometer of the fuel pump to obtain 13 ml/min consumption. The final flow-rate measurement shall be made over a full 60 s +/- 1 s period. Note the flow-rate-micrometer reading for reference.
10.4 Fuel injection timing
After establishing the fuel flow-rate, adjust the injection-timing-micrometer of the fuel pump assembly to obtain a 13.0° +/- 0.2° injection advance reading. Note the injection-timing-micrometer reading for reference.
10.5 Ignition delay
Adjust the handwheel to change the compression ratio and obtain a 13.0° +/- 0.2° ignition delay reading. Make the final handwheel adjustment in the clockwise direction (viewed from the front of the engine) to eliminate backlash in the handwheel mechanism and a potential error.
10.6 Equilibration
Achieve stable injection advance and ignition delay readings.
NOTE 27 Stable readings should typically occur within 5 min to 10 min.
The time used for the sample and each of the reference fuels should be consistent and shall not be less than 3 min.
10.7 Handwheel reading
Observe and record the handwheel reading as the representative indication of the combustion characteristic for this fuel sample.
10.8 Reference fuel No.1
Select a secondary reference fuel ("T fuel" and "U fuel") blend close to the CN of the sample. Prepare a fresh 400 ml or 500 ml batch of the selected reference fuel blend. Introduce reference fuel No.1 to one of the unused fuel tanks, taking care to flush the fuel lines in the same manner as noted for the sample. Perform the same adjustment and measurement steps used for the sample and record the resulting handwheel reading.
NOTE 28 The handwheel reading versus cetane number relationship based on this procedure is engine- and overhaul-dependent but it can be established for each engine as testing experience is gained after each overhaul. A plot or table of handwheel readings provides a simple guide to selection of the reference fuel.
10.9 Reference fuel No.2
Select another secondary reference fuel blend which can be expected to result in a handwheel reading that causes the two reference-fuel handwheel readings to bracket that for the sample. The difference between the two reference-fuel handwheel blends shall not exceed 5.5 CN. Typically, blends differing by 5 % ( V/V ) "T fuel" will span approximately 2.7 CN and those differing by 10 % (V/V) "T fuel" will span approximately 5.3 CN. Prepare a fresh 400 ml or 500 ml batch of the selected reference fuel blend. Introduce reference fuel No.2 to the third fuel tank, taking care to flush the fuel lines in the same manner as noted for the sample. Perform the same adjustment and measurement steps used for the sample and first reference fuel and record the resulting handwheel reading.
NOTE 29 Typically, the fuel-flow-rate should be the same for both reference fuels because they are sufficiently similar in composition.
10.10 Number of blends of reference fuels
If the handwheel reading for the sample is bracketed by those of the reference fuel blends, continue the test: otherwise try an additional reference fuel blend(s) until this requirement is satisfied.
10.11 Repeat readings
After operation on a satisfactory second reference fuel blend, perform the necessary steps to rerun reference fuel No.1, then the sample, and finally reference fuel No.2. For each fuel, be certain to check all parameters carefully and allow operation to reach equilibrium before recording the handwheel readings. The fuel switching shall be as shown in figure 3 - Sample and reference fuel reading sequence A.
If a sample is tested immediately following one for which the reference fuel No.2 will be applicable, that reference fuel handwheel reading can be utilized for the new sample. The fuel switching shall thus be as shown in figure 3 - Sample and reference fuel reading sequence B.
11 Calculation
11.1 Calculate the average handwheel readings for the sample and each of the reference fuel blends.
11.2 Calculate the CN by interpolation of these average handwheel readings proportioned to the cetane numbers of the bracketing reference fuel blends in accordance with the appropriate formula. Do not interpolate using reference fuel blend volume of "T fuel" values and convert that equivalent to CN.
11.3 For this handwheel bracketing procedure the equation is as follows:
where
CNS is the CN of the sample;
CNLRF is the CN of the low reference fuel;
CNHRF is the CN of the high reference fuel;
HWS is the handwheel reading of the sample;
HWLRF is the handwheel reading of the low reference fuel;
HWHRF is the handwheel reading of the high reference fuel.
EXAMPLE
