Environmental Engineering Reference
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than that of diesel. This is because the constituents with higher oxygen content are adequate to
ensure complete combustion of the fuel that is left over during the main combustion phase, and they
continue to burn in the later combustion phase.
The maximal rate of rise in pressure and heat release rate were found to be lower for JB with 15%
EGR in comparison to JB without EGR (Pradeep and Sharma 2007).
14.5.1.3 Ignition delay
The ignition delay has been compared among diesel, JB, and JBB at various loads in Table 14.10.
A shorter ignition delay has been found in the case of JB and JBB. The delay is consistently shorter
for JB, i.e., 4.2° CA lower than diesel at peak load, and the difference increases with an increase in
engine load for a 6-kW diesel engine (Sahoo and Das 2009b). Biodiesel usually includes a small per-
centage of diglycerides having higher boiling points than diesel. However, the chemical reactions
during the injection of biodiesel at high temperature results in the breakdown of the high-molecular-
weight esters. These complex chemical reactions lead to the formation of gases of low molecular
weight. Rapid gasification of this lighter oil in the fringe of the spray spreads out the jet, and thus
volatile combustion compounds ignite earlier and reduce the delay period.
However, while studying the combustion characteristics of JOME, SOME, and HOME in a four-
stroke, 5.7-kW, single-cylinder diesel engine, Banapurmath et al. (2008) observed that the injection
delay of biodiesels was longer than that of diesel, and it decreased with an increase in the brake
power. The values of the ignition delay for JOME, SOME, and HOME were found to be 11.5, 10.5
and 11° CA, respectively, as compared with 9.91° CA with diesel operating at 80% load. Among the
methyl esters tested, SOME had a shorter ignition delay when compared with HOME and JOME.
The combustion duration for all esters as compared with diesel was found to increase because of
a longer diffusion combustion phase. SOME and HOME resulted in an improved heat release rate
as compared with JOME, which resulted in better brake thermal efficiency. On the whole, it was
observed that operation of the engine was smooth on SOME, HOME, and JOME. While analyzing
the combustion parameters of a 3.7-kW diesel engine using JB with and without EGR, the combus-
tion duration for JBB with an optimized value of 15% EGR was found to be increased by 1° than
with the no-EGR condition. An overall analysis of combustion parameters indicates comparable
heat release rates, cylinder pressures, cumulative heat release, combustion duration, and noise-free
operation with and without EGR (Pradeep and Sharma 2007).
Kumar et al. (2003a) used JB, JO, and blends of JO with methanol to study the combustion char-
acteristics of a 3.7-kW single-cylinder diesel engine. The ignition delay was found to be 11° CA with
JO. The same trend was observed with the 30% blend of methanol with JO and in dual-fuel opera-
tion, i.e., ignition delays of 12° and 13° CA were observed for the JO methanol blend and in dual-fuel
operation, respectively. In the dual-fuel mode, ignition delay was increased because of a cooling effect
produced by the methanol as it vaporized. Combustion duration increases with a rise in power output
due to an increase in the quantity of fuel injected. Higher combustion duration was observed with JO
than diesel. The increase in combustion duration is mainly due to the slow combustion of the injected
fuel. However, it is reduced with JB and the blend as compared with JO. In the dual-fuel operation due
to burning of the inducted methanol by flame propagation, the combustion duration is increased. The
premixed burning is more with diesel. Once the autoignition of the fuel commences, the pressure rises,
entering into the rapid or premixed combustion phase. The diffusion-burning phase indicated under
the second stage is greater for JO. At the time of ignition, less fuel-air mixture is prepared for combus-
tion with JO; therefore, more burning occurs in the diffusion phase rather than the premixed phase.
14.5.2 p ErformancE c haractEriSticS
The performance parameters such as brake specific fuel consumption (BSFC), brake thermal effi-
ciency (BTE), and exhaust gas temperature (EGT) of CI engines when operated with JB, JBB, and
JO are summarized in Table 14.11.
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