# Numerical Investigation on NOx Emission of a Hydrogen-Fuelled Dual-Cylinder Free-Piston Engine
## 1 Introduction  
### Background and Recent Research  
NOx emissions are known as one of the most important engine emission components that not only affect the atmospheric environment but also do harm to the human body. NOx that is produced from internal combustion engines is comprised of \(NO\), \(NO_{2}\) and \(N_{2}O\), and when the combustion temperature is higher than 1800 K, some of the nitrogen in the air would become liable to be oxidised into NOx. Moreover, under the severe energy shortage, conventional internal combustion engines are on the verge of being eliminated [1, 2], and in the meantime, the free-piston engine has been taken as an excellent substitute for conventional internal combustion engines thanks to its higher efficiency, lower emissions and adaptability to all types of fuel [3]. For the free-piston engine, the elimination of the crankshaft system and the reduction of mechanical components significantly reduce the complexity of the engine [4]. This brings plenty of advantages: high piston speed around top dead centre (TDC) and a fast power stroke expansion enhance fuel-air mixing and reduces the time available for heat transfer losses and the formation of temperature-dependent emissions, such as nitrogen oxides (NOx). The frictional losses should be reduced due to the simplified mechanism and the elimination of the piston side force in conventional reciprocating engines, potentially lower maintenance costs due to a compact design and a multi-fuel/combustion mode operation due to the possibility of a variable compression ratio [5]. Thanks to the multi-fuel/combustion mode operation, hydrogen, which is considered to be one of the most economically viable and high-functioning clean energies of the future, could be successfully applied in the free-piston engine. In recent years, hydrogenapplication on the FPE has attracted considerable research interest, and a great deal of research work has been conducted by scholars and research institutions around the world. [PERSON] et al. investigated the performance of a spark-ignited hydrogen-fuelled FPEG (free-piston engine generator) at Chongqing Jiaotong University [6]. Their results indicated that the hydrogen engine moves with higher velocity and acceleration around the top dead centre (TDC), compared to a corresponding conventional hydrogen engine, which induces lower heat transfer loss and less NO production due to the limited time available. They also found that the combustion rate of hydrogen fuel in the new engine is slower and the combustion duration lasts longer than that in the conventional engine, leading to more intense post-combustion and a lower level of isochoric heat release. They pointed out that the indicated thermal efficiency of the new hydrogen engine is disadvantageous, and it requires proper combustion to accommodate its special piston motion. [PERSON] et al. compared the performance of a spark-ignited gasoline-fuelled FPEG with a conventional gasoline engine at the Beijing Institute of Technology [7]. The results found that the gasoline fuel combusting in the FPEG reacts slower and has a lower peak heat release rate than that in the conventional engine. The thermal efficiency was lower because less heat was released around the TDC. In addition, the diesel fuel combustion in FPEG was experimentally studied by [PERSON] et al. [8; 9]. The features of diesel FPEG are similar to those of hydrogen FPEG. The slow heat release process, intense post-combustion and low thermal efficiency also occur in the diesel FPEG. Globally, several studies have reported the shortcomings of combustion and the efficiency of hydrogen FPEG. Therefore, effectively avoiding the shortcomings is the key issue to advancing the technological progress of the hydrogen FPEG. As a widely used technique in a natural gas engine to improve the combustion of natural gas, diesel pilot-ignition (DPI) technology has the potential to realise the purpose of accelerating combustion through the application of \"multi-point ignition\" [10; 11].