ASTM D613 standard test method for cetane number of diesel fuel oil
11. Calibration and Engine Qualification
11.1 Engine Compliance - It is assumed that the engine has been commissioned and that all settings and operating variables are at equilibrium and in compliance with basic engine and instrument settings and standard operating conditions.
11.1.1 Engine warmup requires typically 1 h to ensure that all critical variables are stable.
11.2 Checking Performance on Check Fuels - This engine test does not have any satisfactory standardization fuel blend or blends to qualify the engine. The Check Fuels are the most helpful means available to permit judgement of good performance.
11.2.1 Test one or more of the Check Fuels.
11.2.2 Engine performance is judged satisfactory if the cetane rating obtained on a Check Fuel is within the Check Fuel tolerance limits calculated as follows:
Tolerance Limits = CN(ARV) more or less 1.5 x S(ARV)
where:
CN(ARV) = the cetane number accepted reference value of the Check Fuel,
1.5 = a selected tolerance limit factor (K) for normal distributions,
S(ARV) = the standard deviation of the Check Fuel data used to determine CN(ARV).
11.2.2.1 In the context of this test method, the statistical tolerance limit factor (K), based on a sample size (n), permits an estimation of the percentage of engines that would be able to rate the Check Fuel within the calculated tolerance limits. Based on a data set of 17 to 20 ratings used to determine the Check Fuel CNARV, and a value of K = 1.5, it is estimated that in the long run, in 19 cases out of 20, at least 70 % of the engines would rate the Check Fuel within the calculated tolerance limits.
11.2.3 If the results are outside this tolerance range, the engine is not acceptable for rating samples and a check of all operating conditions is warranted followed by mechanical maintenance which may require critical parts replacement. The injector nozzle can be a very critical factor and this should be the FIrst item checked or replaced to achieve rating compliance.
