21-12-2012, 03:18 PM
Biodiesel engine performance and emissions in low
temperature combustion
Biodiesel engine performance and emissions.pdf (Size: 540.18 KB / Downloads: 75)
Abstract
Engine performance and emission comparisons were made between the use of soy, Canola and yellow grease derived B100 biodiesel
fuels and an ultra-low sulphur diesel fuel in the high load engine operating conditions. Compared to the diesel fuel engine-out emissions
of nitrogen oxides (NOx), a high-cetane number (CN) biodiesel fuel produced comparable NOx while the biodiesel with a CN similar to
the diesel fuel produced relatively higher NOx at a fixed start of injection. The soot, carbon monoxide and un-burnt hydrocarbon emissions
were generally lower for the biodiesel-fuelled engine. Exhaust gas recirculation (EGR) was then extensively applied to initiate low
temperature combustion (LTC) mode at medium and low load conditions. An intake throttling valve was implemented to increase the
differential pressure between the intake and exhaust in order to increase and enhance the EGR. Simultaneous reduction of NOx and soot
was achieved when the ignition delay was prolonged by more than 50% from the case with 0% EGR at low load conditions. Furthermore,
a preliminary ignition delay correlation under the influence of EGR at steady-state conditions was developed. The correlation considered
the fuel CN and oxygen concentrations in the intake air and fuel. The research intends to achieve simultaneous reductions of NOx and
soot emissions in modern production diesel engines when biodiesel is applied.
2007 Elsevier Ltd. All rights reserved.
Introduction
Alkyl mono-esters of vegetable oil, waste animal fats
and waste restaurant greases (yellow grease), commonly
known as biodiesel fuel, have been identified as one of
the future contenders to fulfil the demand gap produced
by the depletion of fossil diesel fuels [1–14]. In comparison
with conventional diesel fuels, biodiesel fuel is renewable
and the fuel-borne oxygen in biodiesels, which could be
over 10% by mass, may promote a more complete combustion
and thus effectively reduce engine-out emissions of
particulate matter (PM), carbon monoxide (CO), and unburnt
hydrocarbons (UHC) in modern four-stroke compression-
ignition engines. However, a slight increase in
emissions of nitrogen oxides (NOx), which could be partially
caused by the fuel property incurred combustion-timing
variations, has been observed in the use of oxygenated
fuels in general [2,4,5,11,13,15,16]. Another possible explanation
for the increased NOx formation in biodiesel-fuelled
engines has been attributed to the lowered in-cylinder soot
levels, thus lower radiation heat transfer resulting in higher
in-cylinder temperatures [17].
Experimental details
The tests were conducted on a naturally-aspirated, fourstroke,
single cylinder DI diesel engine that was coupled to
a DC motoring dynamometer. Fig. 1 is a schematic representation
of the experimental setup while Table 1 presents
the original manufacturer geometrical and injection characteristics
for the test engine. The LabVIEW programming
environment was utilised for equipment control and data
acquisition and on-line monitoring of the pressures, temperatures
and emissions, among others. The original engine
configuration was modified to include independent control
of EGR, sequential port-injection, intake air pre-heating
and throttling, which has been reported in the authors’ previous
work [14]. In the present study, the extended use of
EGR and intake throttling were implemented and the
effects analysed in detail.
Conclusion
Neat biodiesel engine tests were conducted and the
engine performance and emission comparisons made with
an ultra-low sulphur diesel fuel. A naturally-aspirated
four-stroke single-cylinder DI diesel engine was instrumented
for the tests. For the investigated conditions, the
test results could be summarised as follows:
In the high load operating condition, the engine-out
NOx emissions were dependent on the fuel CN, for the
same SOI. The biodiesel fuel with a CN similar to the diesel
fuel produced higher NOx emissions than the diesel fuel.
The biodiesel fuels with a higher CN, however, had comparable
NOx emissions with the diesel fuel. A higher CN
would result in a shortened ignition delay period thereby
allowing less time for the air/fuel mixing before the premixed
burning phase. Consequently, a weaker mixture
would be generated and burnt during the premixed phase
resulting in relatively reduced NOx formation. Generally
the emissions of soot, CO and THC were lower for the
engine fuelled with biodiesel.