Research on decoupling of in-cylinder combustion process of diesel/ammonia dual fuel engine

Abstract

Ammonia molecules contain no carbon atoms, which is a kind of clean fuel for diesel engines. The dual-fuel mode, combining ammonia and diesel, is an effective way for ammonia used in diesel engine, which complicates the combustion process. In order to decouple the combustion of diesel and ammonia, a bench test was conducted at 2700 rpm and 75 % load, and the in-cylinder pressure and instantaneous heat release at ammonia substitution rates of 0 %, 10 %, and 20 % were measured and analyzed. In addition, a combustion process model for ammonia and diesel fuel was developed, and a method for decoupling combustion characteristics based on consistent starting time of heat release was proposed. The results indicated that in the dual-fuel combustion mode (diesel/ammonia), the heat release curve was changed from a single peak to a double peak. The addition of ammonia reduced the pre-premixed heat release ratio, leading to a decrease in the peak instantaneous heat release rate and an extension of the combustion duration. As the ammonia substitution rate increased, the rate of ammonia consumption at the same crankshaft angle was significantly reduced. The burned mass center of ammonia (M50-NH₃) was delayed by 4.2°CA, and the amount of unburned ammonia escaping was increased. After ammonia was added, the combustion area in the cylinder was extended toward the combustion chamber wall, and the high-temperature area in the center of the combustion chamber was decreased. With the increase in ammonia substitution rate, NH₂+NO₂=H₂NO + NO became the most sensitive reaction, which was the main reaction that promoted ammonia combustion. The maximum ammonia substitution rate was 40 % at the maximum torque condition.