# Numerical Investigation on NOx Emission of a Hydrogen-Fuelled Dual-Cylinder Free-Piston Engine
## 1 Introduction
### Background and Recent Research
The engine in which natural gas acts as the main fuel and diesel acts as the small pilot-ignition fuel is called the diesel pilot-ignited natural gas engine. In the intake stroke, the homogeneous mixture of natural gas and air enters the cylinder, and in the compression stroke, when the piston approaches the TDC, the injector sprays a small amount of ignition diesel. Moreover, in 2018, [PERSON] et al. tried adding different proportions of hydrogen to the FPE (free-piston engine) through intake ports; the different piston displacements and boundary conditions would be obtained by multiple iterations [12]. Hydrogen can accelerate the combustion speed of the mixture in the cylinder; on the other hand, the compression ratio, movement frequency and residual exhaust gas coefficient that vary with hydrogen content also make the mixing and combustion processes change. These factors have different effects on the combustion and emissions of the FPE. Adding some hydrogen to the diesel FPE has an impact on the evaporation, breakup, collision and wall interaction processes of diesel fuel, which change the combustion process in the cylinder. Moreover, the pressure, temperature and oxygen concentrations change accordingly due to the combustion of hydrogen, which eventually leads to different emission performances under the conditions of variable RH2 compared with the condition of pure diesel. In diesel engines, the emissions of CO and HC are very limited. In addition, because of the high diffusivity of hydrogen fuel, the mixture of diesel and air would be more uniform, and the combustion condition in the cylinder would be further improved. As a result, the emissions of CO and HC are further reduced, and the main emissions would be NO and soot production only. [PERSON], [PERSON] and their research team have conducted a comprehensive experiment globally for the first time: a first-of-its-kind hydrogen-fuelled dual-piston FPEG is experimentally investigated in both two-stroke and four-stroke thermodynamic cycles [13]. They found that the four-stroke cycle mode exhibits slightly higher peak in-cylinder pressure and higher overall in-cylinder pressure from the peak pressure position to the BDC (bottom dead centre) when compared to the two-stroke cycle mode. It can be observed that combustion is longer in four-stroke cycle compared to the two-strokecycle, and the reason for higher in-cylinder combustion pressure and longer combustion time observed in the four-stroke cycle is because of the superior charge mixture capability and higher scavenging efficiency inherited in the four-stroke cycle process. In four-stroke mode, the FPEG produced 13% more indicated efficiency as compared to the two-stroke mode of operation. For the emission section, the nitrogen oxide emission analysis from the FPEG at an operating frequency of 5 Hz and an equivalence ratio of 0.4365. The two-stroke mode of FPEG produces NOx emissions of 44 ppm, which are less than the four-stroke mode NOx emission of 84 ppm. This is mainly due to the in-cylinder temperature, which is higher in the four-stroke cycle than in the two-stroke cycle.