Abstract
Keywords - dual fuel, combustion, heat release rate, pilot dose, natural gas.
Abstract - Forecasts for the nearest years anticipate significant increase of natural gas consumption in European Union states. Such situation is caused by pro-ecological capabilities of the gas, good deal of resources and common availability. It seams that at the present moment compressed natural gas shall find the main implementation. The gas can be used to feeding of spark ignition or compression ignition engines operating in dual fuel system. The present study shows testing results of a single cylinder; direct injected dual fuel engine fed on compressed natural gas. Combustion parameters with a special consideration of heat release run and impact of gas-air mixture composition on combustion parameters and toxicity of engines exhaust gases were the subject of the analysis. Results of performed experiments have been ascertained by significant differentiation of heat release run in dual fuel engine with respect to the engine fed in traditional way. Heat release run outside stream of liquid fuel as well change of maximal dynamics of gas combustion together with change of gas concentration in the mixture is the reason of such differentiation. Reduction of gas concentration in the mixture results in change of location point of maximal dynamics of gas combustion and its shift in direction of more and more bigger crankshaft rotation angles after TDC. It constitutes one of reasons of reduction of thermal efficiency of dual fuel engine operated at partial load. In this connexion, maximal leaning of gaseous mixture should be limited to excess air ratio of ë CNG<4.0. It enables to restrict a harmful phenomena connected with combustion of lean mixtures at partial load of dual fuel engine. In course of performed experiments it has been found that the engine operating in range of engine revolutions of
2000-2750 rpm at maximal load has reached higher thermal efficiencies comparing with engine fed traditionally. Relative growth of thermal efficiency of dual fuel engine has amounted to 6.5-10.5%. Simultaneously, at assumed limitations of gasair mixture composition, at partial loads, thermal efficiency of the engine was smaller only with relative value of 3.0-5.5% comparing to efficiency of an engine fed traditionally. Usage of natural gas enables to significantly reduce NOx emissions and smokiness of exhaust gases. Reduction of NOx emissions at maximal engine load has amounted to 35-55% with respect to emissions from an engine fed traditionally. Even bigger differences were observed at partial loads where reduction of NOx emissions was nearly threefold. The biggest differences in smokiness of exhaust gases were present at maximal load of dual fuel engine, where more than fourfold reduction of the smokiness has been observed. Dual fuel engines, however, have shown increased emissions of CO and non-burnt THC hydrocarbons. Growth of CO and THC emissions was nearly twice at maximal engine load and had increased at partial load. The reason of increased emissions of CO and THC is worsening of conditions of gas oxidation at partial load and increasing range of flame extinction near cylinder walls.