## 1 Introduction  
At present, the problem of environmental pollution has attracted more and more attention, especially the problem of nitrogen oxide pollution, which has perplexed us for a long time. The generation of NO\({}_{x}\) is divided into thermal NO\({}_{x}\), fast NO\({}_{x}\) and fuel NO\({}_{x}\). Thermal NO\({}_{x}\) refers to nitrogen oxides produced by oxidation of N\({}_{2}\) in the air at high temperature. The generation mechanism of the fast NO\({}_{x}\) is that a large amount of NO\({}_{x}\) is produced rapidly in the surface of the flame when the excess air coefficient is less than 1. The generation of fuel NO\({}_{x}\) depends on the nitrogen compounds in the fuel. Previous studies have demonstrated that the components of nitrogen oxides emission produced during the combustion process are mainly NO, and a small amount of NO\({}_{2}\), N\({}_{2}\)O and so on. But NO will be reoxidized to NO\({}_{2}\) in the air. In the control technologies of reducing nitrogen oxides emission, combustion with moderate water addition is an effective way [1,2]. Oil water emulsion and water injection are two common measures to introduce water into the combustion reaction, which is also regarded as the fuel improvement techniques. [PERSON] et al. [3]performed a set of engine experiments at the condition of three engine speeds (1000, 2100 and 3000 rpm), three engine loads (20%, 50% and 80%), and three different levels of water concentration (5%, 10% and 15%). They concluded that the nitrogen oxides (NO\({}_{x}\)) were significantly reduced with the increase of water content in the emulsion compared to their bases. One advantage of water injection when compared with EGR is the possible reduction of NO\({}_{x}\) emissions either at low engine loads and high engine loads without a substantial increase in PM emissions [4]. [PERSON] et al. [5] investigated the performance and efficiency of a diesel engine with a steam injection system by means of experiment and simulation. They come to conclusion that the steam injection had a positive effect on performance and NO emissions at all speed and didn't affect CO, CO\({}_{2}\), HC emissions considerably. [PERSON] et al. [6] conducted an engine experiment operated with biodiesel directly injected in the cylinder and water injected in the manifold. Some conclusion was got that when the water injection rate was 3 kg/h, NO\({}_{x}\) was reduced by about 50% and this measure didn't result in significant changes in fuel consumption. In addition, the water injection in the intake manifold had little effect on the cylinder pressure and the heat release rate [7]. [PERSON] et al. [8] performed an experimental study with gasoline-ethanol-water ternary mixtures. They pointed out that the Otto engine requires no modifications to burn E40 and E40h (10% hydrous) efficiently with advantage of NO\({}_{x}\) emissions reduction. [PERSON] et al. [9,10] proposed that the combustion temperature of fuel is the most important factor that directly affecting the NO\({}_{x}\) emissions from the engine. For biodiesel, the NO\({}_{x}\) emission increases a little with increasing of biodiesel blend ratio in diesel engines fueled with biodiesel blend fuel. This could attribute to that the combustion chamber temperature of biodiesel is higher than that of diesel fuel. Because biodiesel is an oxygenated fuel. All the references mentioned above are focused on the generation quantity of NO\({}_{x}\), which remained at the macro and phenomenal levels. Researchers are more inclined to explain the phenomenon that water addition into the combustion chamber can inhibit the generation of nitrogen oxides based on the physical property of water. Apparently, the latent heat of vaporization of water is high (40.8 kJ/mole at 100 \({}^{\circ}\)C, 1 atm), after introducing water into the combustion chamber, the adiabatic flame temperature reduces due to water vaporization. And it is generally believed that the generation conditions of NO\({}_{x}\) are high temperature and oxygen enrichment.