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Ignition Indices

Cetane Number - The cetane number is an engine-based test that follows ASTM standard D613. It is based on a special engine produced by the Waukesha Engine Company that is similar to the Octane Test Engine used for rating gasolines. The engine is a single cylinder, indirect injection diesel engine. The engine speed is fixed at 900 rpm and while the engine is naturally aspirated, the intake air temperature is held at 150°F. The test is based on a careful adjustment of the fuel/air ratio and the compression ratio to produce a standard ignition delay (the period between the start of fuel injection and the start of combustion).

The test is conductedas follows:
1. The fuel pump rack is adjusted to provide 13 ml/min of fuel flow.

2. The injection timing is adjusted so the start of injection is a 13 degrees before TDC.

3. Now, a handwheel is turned that changes the compression ratio of the engine by increasing or decreasing the prechamber volume. Increasing the compression ratio will shorten the ignition delay and decreasing the compression ratio will lengthen the ignition delay. By turning the handwheel, the ignition delay is adjusted so that it is 13 degrees which means the start of combustion is at TDC.

4. Now the test fuel is removed from the engine and a blend of reference fuels is added. The primary reference fuels are n-cetane (n-hexadecane), which has a cetane number of 100 and heptamethylnonane (HMN) which has a cetane number of 15. Different combinations of these two fuels are added until a blend is found that restores the ignition delay to 13 degrees. When this occurs, the cetane number is computed from the following relationship:

                      Cetane Number = % n-cetane + 0.15 (%HMN)

Since the price of the primary reference fuels is quite high, most commercial cetane testing is done with secondary reference fuels which have been calibrated to known cetane values. Phillips Petroleum supplies these secondary reference fuels.

Cetane Index - The cetane index is a calculated quantity that is intended to approximate the cetane number. It is much cheaper to determine than the engine-based cetane number but its accuracy is limited to the type of fuel on which it is based. It generally does not provide an accurate indication of cetane number if the fuel contains cetane improving additives or for non-petroleum-based alternative fuels. Two methods are available for computing the cetane index.

ASTM standard D976 gives the following empirical equation for the cetane index:
Cetane Index = 454.74 - 1641.416 D + 774.74 D2 - 0.554 T50 + 97.803[log10(T50)]2
where D = fuel density at 15oC in g/ml.
and T50 = the temperature corresponding to the 50% point on the distillation curve in degrees C.
 
ASTM standard D4737 gives the cetane index according to the following four-variable equation:
Cetane Index = 45.2 + 0.0892(T10N) + 0.131(T50N) + 0.0523(T90N)
+ 0.901B(T50N) - 0.420B(T90N) + 4.9x10-4(T10N)2
-4.9x10-4(T90N)2 + 107B + 60 B2
where   T10N = T10 - 215
            T50N = T50 - 260
            T90N = T90 - 310
when T10, T50, and T90 are temperatures at 10%, 50%, and 90% volume distilled in degrees C
and B = [exp(-3.5DN)] – 1 when DN = density at 15oC (kg/liter) - 0.85

The following data were obtained from a commercial fuel testing laboratory for a sample of #2 diesel fuel.

API Gravity 34.1Higher heating Value 19,461 Btu/lbmLower Heating Value 18,309 Btu/lbmT10 413oF (212.0oC)T50 502oF (261.4oC)T90 592oF (311.4oC)

The cetane number was measured by ASTM D613 on two occasions and the following results were obtained: 47.8 and 45.7.

Calculate and compare the calculated cetane indices using the two equations given above with the measured values shown.

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