Experimental Optimization of a Small Bore Natural Gas-Diesel Dual Fuel Engine with Direct Fuel Injection

Dual fuel combustion processes, which burn varying ratios of natural gas and diesel, are an attempt to reach high efficiencies similar to diesel engines while exploiting the CO₂ savings potential of natural gas. As shown in earlier studies, the main challenge of this combustion process is the high emission of unburned hydrocarbons during low load operation. Many publications have focused on a layout which utilizes port injection of natural gas and a direct injection of diesel to initiate combustion. However, previous studies indicated that a sequential direct injection of both fuels is more promising. It enables charge stratification of natural gas and air, whereby a remarkable reduction of the unburned hydrocarbon emissions was observed. This work develops this approach further, utilizing a low pressure direct injection of natural gas. The effects of main application parameters such as injection strategy, exhaust gas recirculation, air fuel equivalence ratio and the crank angle of 50 % fuel burned, etc. were investigated systematically on an engine dynamometer. For this purpose, an EU6 certified 4-cylinder diesel engine for passenger cars was modified. The measurements were conducted for two low load and two medium load operating points, which are particularly critical for the hydrocarbon emissions and characteristic for the NEDC test cycle. The aim was to optimize combustion in order to minimize the emission of unburned hydrocarbons. As a result, the total hydrocarbon emissions were reduced by 91 % to 2.2 g/kWh at the operating point n = 1750 rpm / BMEP = 5 bar. In this publication, the influence of the individual application parameters and their combined effect on the combustion is discussed.