The depletion of fossil fuels and its emissions promoted the researchers to search for substitute fuels and their controlled combustion. Hydrogen is considered as one of the best fuels for internal combustion engines because of its unique combustion properties. Currently, there are very few commercial devices that utilize hydrogen combustion for the production of heat, which is mainly due to the limited availability of hydrogen fuel. As the accompanying environmental legislation will clearly favour clean technologies, the emergence of hydrogen as an energy carrier will modify this situation. To achieve controlled combustion, an attempt was made at investigating the effect of change of piston geometry on the emission characteristics of diesel engine enriched with hydrogen at optimum flow rate. Experiments were conducted to study the effect of varied piston bowl geometry on the emission characteristics of diesel engine enriched with hydrogen at a flow rate of 6 lpm on four stroke single cylinder diesel engine at constant speed of 1500 rpm for different loads. For flow rates above 6 lpm knocking tendency was observed due to raise in temperature and peak pressures with addition of hydrogen. The experiments were conducted with standard hemispherical, toroidal and re-entrant toroidal piston bowl geometry at 6 lpm flow rate of hydrogen duly ensuring the same compression ratio in all three cases. The emissions for diesel engine enriched with hydrogen in hemispherical combustion chamber at 6 lpm flow rate were reduced by 27.1%, 37.5% and 10.8% of unburnt hydrocarbons (UHC), Carbon monoxide (CO) and smoke density respectively when compared to diesel fuel alone operation at rated load. This is mainly due to high combustion temperatures which lead to complete burning of fuel and reduction in carbon content with addition of hydrogen. However, there was a 14% increase in oxides of Nitrogen (NOx) emission due to high combustion temperatures by hydrogen induction. With toroidal and reentrant geometry of the combustion chambers at 6 lpm flow rate of hydrogen, the emission parameters were further reduced notably. Further there is an increase in NOx emission was observed in dual fuel mode compared to standard piston due to high cylinder temperatures and pressures. The obtained results show that at part load conditions with enriched hydrogen, the percentage reduction of NOx emission was engine load dependent, being least increase at low loads and high increase at high loads. The reduction in emission particulates with varied combustion chamber bowl geometry was due to improved swirl motion of high turbulence of air in the combustion.

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