(542j) Realization of Compression Ignition Engine Using Ammonia As a Sole Fuel with New Combustion Strategy

As regulations on fossil fuels and their emissions have become more stringent, various efforts have been made to improve emissions management and fuel combustion efficiency to meet regulatory standards. In recent years, many efforts have been made to use renewable energy to fundamentally reduce emissions. Among them, research on hydrogen has been actively conducted. However, hydrogen is difficult transport and store, and studies on developing various hydrogen carriers have been carried out. Ammonia is considered a promising hydrogen carrier with good storability and transportability. Ammonia can also be used as a carbon-free fuel as needed. In order to use ammonia as a hydrogen carrier, it is essential to develop an energy conversion device that uses the chemical energy stored in ammonia as another type of energy. There are several candidates for energy conversion systems using ammonia, but we want to focus on the internal combustion engine used in automobiles and power plants. There has been much research on using ammonia as fuel for engines. However, studies on engines using ammonia as a sole fuel have not made progress over the last 40 years because of the very high compression ratio due to ammonia combustion problems. Recent studies on engine operation using ammonia have involved mixing ammonia with other commercial fuels (e.g., gasoline, diesel, etc.) at a certain rate. In 2017, a new combustion strategy for an internal combustion engine fueled solely by ammonia was proposed to resolve the limitations of previous studies, where additional fuel was used as a combustion promoter.

D. Lee presented the following new combustion strategy in his simulation study. First, a very small amount of ammonia was injected into the cylinder during the intake process or the initial compression process. The injected ammonia homogenously mixed with the air in the cylinder through the compression process and self-ignited at the end of the compression process as pressure and temperature rose. This self-ignition caused the temperature and pressure inside the cylinder to rise further. Then, the main fuel was injected and combustion occurs as in the diesel engine. After the main-combustion ends, ammonia is post-injected to remove NOx generated during combustion. This new engine combustion strategy can make experimental conditions more feasible.

In this study, we examined the realization of an engine fueled by ammonia using the combustion process described above. First, we used MATLAB to determine the expected operation range of the engine used in the experiment. It is difficult to set the compression ratio of the engine used in the experiment to 35:1, as in the previous study, because of the geometry factor. Therefore, we created an operating map that allowed the engine to run within the realistic compression ratio range of the engine used in the experiment(16~20:1). Although there are many factors that affect engine performance, it was important to realize the normal operation of the engine. Thus, an operation map for the intake temperature and pressure according to the compression ratio of a general compression ignition(CI) engine was created. Since ammonia is highly corrosive, in order to prevent this, ammonia-only fuel engine experimental equipment setup was carried out with attention to wetting point by ammonia. The CI engine, which runs on ammonia, was operated in the operable region of the operating map using a mass flow controller and heater.

We demonstrated the possibility of operating a CI engine with ammonia as fuel using the initial pressurization and heating of air at the compression ratios commonly used by commercial diesel engines. In addition, we examined the trends of various variables (e.g., in-cylinder pressure profile, IMEP, etc.) that can be obtained by varying the ratio between the main and pilot injections, as shown in the results of previous studies. Depending on the timing of the main injection, start of injection (SOI) timing that is too advanced could lead to charge-cooling, which prevents the premixed ammonia and air mixtures from burning, resulting in poor efficiency. On the other hand, an SOI timing that is too late causes combustion during the expansion process and incomplete combustion by cooling, which also results in low efficiency. In this way, the new ammonia engine combustion model from the previous study can be validated, and the possibility of conducting future experiments under different conditions can be verified using the simulation model.