buletinul institutului politehnic din iaşi - Universitatea Tehnică ...

148 Anca Elena Eliza Sterpu and Anca Iuliana Dumitriu
3.2.5. Distillation Curve Points. Fig. 5 shows the distillation curves for
pure fuels (B0, B100) and for B5, B10, B15 and B20 blends. As can be seen in
this figure, all curves
tend to intercept at a point which defines two zones with
different
behaviour. Before the interception point (10% distilled percentage),
distillation temperature increases with distilled percentage, while after this point
the trend is reversed.
3.2.6. Calculated Cetane Index. Cetane numbers rate the ignition properties
of diesel fuel as a measure of the fuel’s ignition on compression as measured by
ignition
delay. The ignition
delay in a diesel engine is defined as the time
between
the start of fuel injection and the start of combustion. The ignition
quality of a fuel is usually characterized by its cetane number. Higher cetane
number generally means shorter ignition delay (Canakci, 2007). Cetane number
is measured in a single-cylinder engine compared with reference blends of ncetane
and heptamethylnonane according to ASTM D613 or in a constant
volume combustion apparatus following ASTM D6890. However, these tests
are awkward and expensive. For this reason there have been many attempts to
develop methods to estimate the cetane number of a fuel. In order to
differentiate predicted from measured values, the former is called calculated
cetane index (CCI), and the latter cetane number.
Fig.5 – Distillation curves of rapeseed oil biodiesel – diesel fuel blends.
The CCI can be determined using empirical correlations in accordance
with the ASTM D976 and D4737. Using ASTM D976, this parameter
is
btaine d as a function of fuel density at 15 curve
point.
o o
C and the T50 distillation
ASTM D4737 additionally takes into account T10 and T90 values. Figs.
6 and 7 show the CCI values using the two standards. Distillation curve points
were taken according to the measured values shown in Fig. 5 and the values of
density at 15 o C were obtained from Fig. 2. For comparison, the CCI values
were then calculated from values of blend densities at 15 o C and distillation
curve points obtained using Kay’s mixing rule.