Abstract:
The use fossil fuels in internal combustion engines has contributed highly to the emission
of greenhouse gases. For this reason, stringent regulations are being set to minimise
their emission to the environment. Researchers have conceived a concept of dual
fuel engines that use alternative fuels to replace a percentage of fossil fuels in internal
combustion engines. Biogas as one of the renewable source of energy has advantages
in dual fuel application because of its ability to mix with air, high flammability and the
ability to reduce NOX emission and particulate matter to the environment.
In this study, a variable compression ratio CI engine was modified to operate as a dual
fuel engine using biogas. The modification included designing of an air-gas mixer
to introduce biogas into the combustion chamber as air-biogas mixture charge and an
exhaust gas recirculation (EGR) system to recirculate exhaust gas back into the combustion
chamber. Operating the engine at a constant speed of 1500 rpm and varying
engine load, combustion, performance and emission characteristics of the dual fuel engine
were studied under different engine parameters.
Experiments were carried out to study the effects of compression ratio, injection timing
and emission characteristics of a dual fuel engine. The effect of EGR on the NOX
emission of a dual fuel engine fueled with biogas and diesel was also conducted. Investigation
of compression ratio was done with the engine running with the injection
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timing of 23 before top dead center (bTDC) while varying the compression ratios.
The study for injection timing was done with the engine compression set to 18:1 and
the experiments to study the effect of biogas flow rate were done with the engine compression
ratio set at 18:1 and injection timing of 29.5 bTDC.
Increasing the compression ratio and advancing fuel injection timing increased the
thermal efficiency, in-cylinder pressure and net heat release rate. The highest brake
thermal efficiency, in-cylinder pressure, net heat release and mechanical efficiency of
25.7%, 49 bar, 60 J/deg and 49.9% respectively were found to be at compression ratio
of 18:1 and injection timing of 29.5 bTDC. Increasing the compression ratio from 14
to 18 improved the brake specific fuel consumption of the engine by 26%. It was found
that NOX and CO2 emission increased with the increase in compression ratio and was
highest at compression ratio of 18:1. HC and CO emissions reduced with increase in
compression ratio and were highest at the compression ratio of 14:1. This study found
out that at compression ratio of 18:1, injection timing of 29.5 bTDC, EGR ratio of
20%, 40% biogas and 60% diesel, load of 12kg and engine speed of 1500 rpm, the
dual fuel engine had superior performance and emission characteristics.
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