ASTM D6890 Standard Test Method for Determination of Ignition Delay and Derived Cetane Number (DCN) of Diesel Fuel Oils by Combustion in a Constant Volume Chamber
4. Summary of Test Method
4.1 A small specimen of diesel fuel oil is injected into a heated, temperature-controlled constant volume chamber, which has previously been charged with compressed air. Each injection produces a single-shot, compression ignition combustion cycle. ID is measured using sensors that detect the start of fuel injection and the start of significant combustion for each cycle. A complete sequence comprises 15 preliminary cycles and 32 further cycles. The ID measurements for the last 32 cycles are averaged to produce the ID result. An equation converts the ID result to DCN (derived cetane number), which is correlated to cetane number by Test Method D613.
5. Significance and Use
5.1 The ID and DCN values determined by this test method can provide a measure of the ignition characteristics of diesel fuel oil in compression ignition engines.
5.2 This test can be used in commerce as a specification aid to relate or match fuels and engines. It can also be useful in research or when there is interest in the ignition delay of a diesel fuel under the conditions of this test method.
5.3 The relationship of diesel fuel oil DCN determinations to the performance of full-scale, variable-speed, variable-load diesel engines is not completely understood.
5.4 This test may be applied to non-conventional fuels. It is recognized that the performance of non-conventional fuels in full-scale engines is not completely understood. The user is therefore cautioned to investigate the suitability of ignition characteristic measurements for predicting performance in full-scale engines for these types of fuels.
5.5 This test determines ignition characteristics and requires a sample of approximately 100 mL and a test time of approximately 20 min on a fit-for-use instrument.
6. Interferences
6.1 Minimize exposure of sample fuels, calibration reference materials, QC samples, and check standard to sunlight or fluorescent lamp UV emissions to minimize induced chemical reactions that can affect ignition delay measurements.
6.1.1 Exposure of these fuels and materials to UV wavelengths shorter than 550 nanometers for a short period of time may significantly affect ignition delay measurements.
NOTE 1 - The formation of peroxide and radicals can effect ignition delay measurement. These formations are minimized when the sample or material is stored in the dark in a cold room at a temperature of less than 10°C, and covered by a blanket of nitrogen.
6.2 Statistical analysis of data from a sequential testing study (Note 2) revealed a possible carryover effect in succeeding tests on samples containing 2-ethylhexylnitrate cetane improver at concentrations above 2000 ppm.
NOTE 2 - In the sequential testing study, a fuel without cetane improver was tested three times back-to-back. Then a fuel with 2-ethylhexylnitrate cetane improver at concentrations above 2000 ppm was tested. Subsequently, the same fuel without cetane improver was tested three times. Statistical analyses of repeat data on two units were examined for evidence of hysteresis.
