Experimental Investigations on Natural Gas-Diesel Dual Fuel Combustion
Abstract: Natural gas has been traditionally applied in spark-ignited combustion engines due to similar combustion characteristics for methane gas and gasoline. However, spark ignition requires a low compression ratio to avoid knock problems and therefore, gas engines have lower efficiency than diesel engines. A combustion concept that has been successfully applied on large stationary engines and to some extent on heavy duty engines is dual fuel combustion, where a compression ignited diesel pilot injection is used to ignite a homogeneous charge of natural gas and air. This dual fuel combustion concept is well established for large stationary engines and exists as an after-market solution for heavy duty engines but does not exist at all for light duty engines. This concept offers a high degree of flexibility for the engine operation because dual fuel combustion does not require heavy modifications of the original diesel engine architecture, so diesel operation could remain unaltered. The scope of this thesis is to explore the implementation of this dual fuel concept on different types of applications using the diesel injection system as an alternative ignition system for lean natural gas mixtures.The main objectives are the identification of pathways to increase combustion efficiency at low loads, improve the understanding of how pilot injection should be controlled over the entire operating range and finally, analyze the interaction between the pilot and main air-gas charge. Experiments resulted in robust and efficient high gas fraction dual fuel operation from 5 bar IMEPg. Below this load, insufficient exhaust energy limits the applicability of this concept. The pathways explored to increase combustion efficiency at low load resulted in a minimum gross indicated efficiency of 40% from 3 bar IMEPg. Moreover, the use of alternative pilot injection strategies allows a simultaneous reduction of TUHC and NOx emissions and knockfree operation. Under certain conditions, operation without NOx after-treatment system is possible. This requires operation near the lean limit, which has been experimentally evaluated and a gas equivalence ratio of 0.44 is proposed, considering methane emissions and exhaust temperature levels.
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