In this paper, we study the effect of water injection in the fuel supplied to diesel engine, that is, the effect of certain humidity on engine performance, and investigate the effect of water mist in the fuel on combustion. At the same time, through the research of visual combustion, what are the significant effects on the engine performance, such as fuel consumption, smoke concentration and nitrogen oxide concentration in the exhaust, are studied from the following three aspects:
1. Comparison of combustion process between diesel and fuel containing 30% moisture;
2. Influence of fuel temperature on engine performance (use fuel containing 30% moisture);
3. Influence of different water content on engine (use fuel with 20% and 30% water content).
1、 Comparison of visual combustion characteristics
Whether the water contained in the fuel will affect its atomization or re micronization; Whether the chemical reaction between superheated water vapor and oil mist has an impact on the fuel composition; Due to the change of viscosity, the increase of oxygen content required for combustion and higher combustion temperature, the impact on engine performance is briefly introduced in this section.
The following figure shows the photos of in cylinder flame collected by visual engine (optical engine). Under the same experimental conditions, change the water content in the fuel. As shown in the figure, when diesel oil is used, the spark nucleus is formed at the BTDC 10 ° Ca, the center of the combustion chamber. The weak spark nucleus spreads through the diffusion of the surrounding combustible mixture, making the reddish yellow flame fill the combustion chamber. Figure 1 (b) and (c) show the flame photos of fuel oil containing 30% moisture. It is obvious that the formation position of spark nuclei at the initial stage is 5 ° CA later than that of diesel oil. The atomization of spray and the formation of mixture are poor, resulting in wall ignition. The presence of unburned mixture is seen around the flame. The brightness of spark nuclei at the initial stage is dark, the residual light lasts for a long time, and the flame is superimposed, dense cloud, and has no sense of hierarchy, Deterioration of combustion environment.
For aqueous fuel, after the thermal dissociation of water under high temperature and high pressure, the pre flame reaction accelerates, resulting in high initial cylinder pressure and poor fluidity of the mixture, which is easy to densify large particle fuel and cause incomplete combustion. It can be seen from the photos that soot and particulate matter increase; At the same time, the fuel attached to the cylinder wall forms a wall fire, resulting in abnormal cooling water temperature, reduced thermal efficiency, reduced power and torque, and fatal engine failure in serious cases.
2、 Effect of fuel temperature on engine performance
The water in the fuel increases its viscosity due to overheating during combustion, which will have an impact on the atomization and micronization of oil particles and the formation of mixture. In this section, in order to make a sharp contrast, the oil circuit of one engine is heated and insulated, and the external characteristics of the two engines are compared.
Figure 2 shows the effect of different fuel supply temperatures on engine performance when only 30% aqueous fuel is used. With the increase of fuel supply, the performance of both engines shows an upward trend, and there is no change due to the influence of fuel supply temperature, even the exhaust temperature is basically the same. However, the exhaust gas concentration increases sharply with the increase of oil supply temperature.
Due to the rise of fuel supply temperature, the formation of premixed gas volume (combustible mixture) in the initial stage of combustion increases. The rise of fuel temperature increases the viscosity of fuel, reduces the average particles of oil mist injected into the cylinder, and coupled with the thermal decomposition and heat absorption of water, the temperature reflected before the flame of oil mist in the cylinder is low, resulting in lower and relatively concentrated spark nuclear ignition temperature, longer pre flame reaction time, longer ignition delay period and uneven distribution of mixture, With incomplete combustion, the emission smoke concentration increases with the increase of fuel supply, that is, the emission of particulate matter (without fuel gas mixture) is higher.
3、 Influence of different water components on engine performance
Since water has such a great impact on the performance of the engine, how much water is injected or how much water is contained in the fuel? Sell cute here. Smart people have already seen it! We have been using 30% or 20% moisture content for comparative experiments, and then verify whether this content is reasonable.
In order to reduce NOx and particulate matter in the tail gas, it is necessary to increase the water (injection water) in the fuel appropriately. There are two purposes for this: 1 The injected water can reduce the combustion temperature and the content of NOx in the tail gas; 2. In the oxygen enriched combustion environment, make it burn more fully, so that it burns completely. However, considering the fuel economy, we continued to spray water during the experiment until it could not spontaneously ignite, form spark nuclei and burn. After recording and comparing, the fuel economy has reached the optimal state under the condition of 20% - 30% water spray. At this time, the flue gas concentration is low and the engine performance is the best. For example, when the fuel injection is 60mg / St, the explosion pressure of the engine is about PE = 0.6MPa. As shown in the figure above, fuel with 20% moisture content can be adopted at low load; It is more reasonable to use fuel with water content of about 30% at medium and high load.
conclusion
For example, when using the piston with more water, the water consumption in the cylinder can be reduced by about 10% - 30%. At the same time, due to the increase of combustible mixture, the combustion is accelerated and the whole combustion period is greatly shortened; If the injection advance angle is advanced by 5 ° Ca, the noise and vibration caused by the ignition delay period will be significantly reduced and the comfort will be improved;
2. The combustion flame of aqueous fuel is reddish brown, the combustion temperature is low, and the content of nitrogen oxides is significantly reduced. However, due to more moisture and incomplete combustion, the content of particulate matter and soot increases; At the same time, the condensation and thermal corrosion of water vapor also cause great loss to the wear and rust prevention of engine parts.
