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Effect of humidity on engine combustion performance

In this paper, we studied the effect of certain humidity on engine performance when spraying water into the fuel supplied to the diesel engine, and examined how the water mist existing in the fuel has an effect on combustion. At the same time, through the study of visual combustion, the engine's fuel consumption, smoke concentration, nitrogen oxide concentration in the exhaust gas and other engine performance have a significant impact, from the following three aspects:
 
1. Comparison of the combustion process between diesel and fuel containing 30% moisture;
 
2. The effect of fuel temperature on engine performance (using fuel with 30% moisture);
 
3. The effect of different water content on the engine (using fuel with 20% and 30% moisture).
 
First, visual combustion characteristics comparison
 
The moisture contained in the fuel will have an effect on its atomization or re-micronization; whether the chemical reaction between the superheated steam and the oil mist has an effect on the fuel composition; due to the change in viscosity, it is required for combustion. The effect of increasing the amount of oxygen, the higher combustion temperature, etc. on the performance of the engine is briefly described in the combustion characteristics of this section.
 
The picture below shows a photograph of the in-cylinder flame collected by the visual engine (optical engine). Under the same experimental conditions, the moisture content in the fuel was varied. As shown, in the case of diesel fuel, the spark nucleus is formed at a position of BTDC 10 ° CA, the center of the combustion chamber, and the weak spark core propagates through the diffusion of the surrounding combustible mixture to fill the combustion chamber with a red-yellow flame. (b) and (c) of Fig. 1 are photographs of flames using a fuel containing 30% of water. It is obvious that the initial spark nucleus is formed at a position delayed by 5 ° CA compared with the use of diesel, and the atomization and mixture of the spray are The formation is poor, and the wall surface is ignited. The presence of unburned mixture is seen around the flame. The initial spark nucleus has a darker glow, the residual light lasts longer, and the flame is superimposed, densely clouded, and layerless. The combustion environment has deteriorated.
 
Aqueous fuel, after the heat dissociation of water under high temperature and high pressure, the pre-flame reaction accelerates and the initial cylinder pressure is high. The poor fluidity of the mixture is easy to make the large particle fuel dense, resulting in incomplete combustion. The soot and particulate matter increase; at the same time, the fuel adhered to the cylinder wall forms a fire on the wall surface, causing abnormal temperature of the cooling water, lowering the thermal efficiency, reducing the power and torque, and causing a fatal engine failure in severe cases.
 
Second, the impact of fuel temperature on engine performance
 
The moisture in the fuel is increased due to overheating during combustion, and it affects the atomization and atomization of the oil particles and the formation of the mixture. In this section, for the sake of sharp contrast, the oil circuit of one of the engines is heated and insulated to compare the external characteristics of the two engines.
 
Figure 2 shows the effect of different oil supply temperatures on engine performance when only 30% aqueous fuel is used. With the increase of oil supply, the performance of the two engines is on the rise, and there is no change due to the influence of the oil supply temperature, and even the exhaust gas temperature is basically the same. However, the smoke exhaustion concentration increases drastically with the increase of the oil supply temperature.
 
Due to the increase in the oil supply temperature, the amount of premixed gas (combustible mixture) increases at the initial stage of combustion, and the increase in fuel temperature increases the viscosity of the fuel, and the average particle size of the oil mist injected into the cylinder decreases, together with the heat of the water. Decomposition and endothermic, the temperature reflected by the flame in the cylinder is lower, resulting in a lower spark ignition temperature and relatively concentrated, the reaction time before the flame becomes longer, the retardation period becomes longer, the mixture gas is unevenly formed, and the combustion is not Completely, the concentration of emitted smoke increases with the increase of oil supply, that is, the concentration of particulate matter (not fuel gas mixture) is higher.
 
Third, the impact of different water components on engine performance
 
Since moisture has such a large influence on the performance of the engine, how much water is sprayed or how much water is contained in the fuel? Selling a cute, smart person has already seen it! We have been using a 30%, or 20% moisture content for comparison experiments, and then verify that this content is reasonable.
 
In order to reduce NOx and particulate matter in the exhaust gas, it is necessary to increase the amount of moisture (spray moisture) in the fuel. The purpose of this is twofold: 1. The injected water can lower the combustion temperature and reduce the content of NOx in the exhaust gas; 2. Oxygen-rich In a burning environment, it burns more fully, so that it burns completely. However, considering the fuel economy, we continued to spray water until it could not spontaneously ignite, and it could not form a spark core that could not be burned. After recording and comparison, the fuel economy has reached the optimal state under the condition of 20%----30% of water spray. At this time, the smoke concentration is low and the engine performance is optimal. For example, when the fuel is sprayed at 60 mg/st, the burst pressure of the engine is about Pe=0.6 MPa. As shown in the above figure, at low load, fuel with 20% moisture content can be used; for medium and high load, it is reasonable to use about 30% water content.
 
in conclusion
 
1. When using 20%-30% moisture content of fuel, or when spraying water into the cylinder, plus good air flow, such as using a piston that produces eddy currents and more squirting, the fuel consumption rate can be reduced. About 10%. At the same time, due to the increase of the amount of combustible mixture, the combustion is accelerated, and the entire combustion period is greatly shortened; if the injection advance angle is advanced by 5°CA, the noise and vibration caused by the retardation period will be significantly reduced, and the comfort will be improved;
 
2. The combustion flame of water-containing fuel is reddish brown, the combustion temperature is low, and the content of nitrogen oxides is significantly reduced. However, due to the large amount of water, the incomplete combustion increases the particulate matter and soot content. At the same time, the condensation and hot corrosion of water vapor cause a great loss of wear and rust on the engine parts.

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