Pilot Injection of Reactive Fuels as Ignition Source in Natural Gas Engines With Lean Burn Concept

Recent gas engines developments tend to use more excess air to reduce NO_x emissions. Under these circumstances the ignition in a single cylinder research gas engine with micro pilot injection of highly ignitable fuels has been investigated. Three igniting fuels, Hydrogenated Vegetable Oil (HVO), 2-ethoxyethyl ether (2-EEE) and a Diesel/2-ethylhexyl nitrate blend have been selected by a systematical assessment and their properties have been analyzed. These fuels have been evaluated concerning their aptitude as igniting fuels and compared with diesel as reference fuel. A higher ignitability of igniting fuel reduced the ignition delay of the injected fuel and enabled the diminution of the igniting fuel fraction. A significant share of NO_x emissions have been attributed to the ignition injection, therefore micro pilot injection is necessary to reach emission targets. The micro pilot injection of 2-EEE as a highly ignitable fuel with the highest Cetane Number showed favorably low ignition delay. Depending on the selected fuel and the igniting fuel fraction, the combustion phasing can be controlled directly by the injection timing. In the last section, the results for pilot injection with 2-EEE as an igniting fuel have been compared with the results using a conventional spark plug. Advantages and disadvantages for both ignition systems have been identified at constant Air Fuel Ratio (AFR). A thermodynamical comparison with each ignition system has been performed to explain the different effects on combustion.