In this article, we studied the effect of a certain humidity on the performance of the engine when injecting water into the fuel supplied to the diesel engine, and investigated how the water mist in the fuel affects the combustion. At the same time, through the study of visual combustion, what are the major impacts on engine performance such as fuel consumption, exhaust smoke concentration, and nitrogen oxide concentration in exhaust gas?
1. Comparison of the combustion process of diesel and fuel containing 30% moisture;
2. The effect of fuel temperature on engine performance (use fuel with 30% moisture);
3. The effect of different water content on the engine (using fuel with 20% and 30% moisture).
1. Visual comparison of combustion characteristics
Does the moisture contained in the fuel have any effect on its atomization or reparticulation; whether the chemical reaction between superheated steam and oil mist has an effect on the fuel composition; due to the change in viscosity, it is required for combustion The effect of increasing oxygen content and higher combustion temperature on the performance of the engine is briefly introduced in the combustion characteristics of this section.
The picture below is a picture of the flame in the cylinder collected by the visualization engine (optical engine). Under the same experimental conditions, change the moisture content in the fuel. As shown in the figure, in the case of using diesel fuel, the formation of spark nuclei is at BTDC 10 ° CA, the center of the combustion chamber, and the weak spark nuclei spread through the surrounding combustible mixture to make the red flame fill the combustion chamber. Figures 1 (b) and (c) are photos of flames using a fuel containing 30% moisture. It is obvious that the initial spark nucleus formation position is delayed by 5 ° CA than when diesel is used. The formation is poor, and the phenomenon of wall ignition is formed. The presence of unburned gas mixture is seen around the flame. The initial brightness of the spark core is dark, the residual light lasts for a long time, and the flame is superimposed, dense cloud-shaped, and has no sense of hierarchy. The combustion environment deteriorates.
For water-containing fuels, after the water is thermally dissociated under high temperature and high pressure, the pre-flame reaction accelerates and the initial pressure in the cylinder is high. The poor fluidity of the mixed gas is easy to make large particles of fuel dense and cause incomplete combustion, as seen in the photo At the same time, the soot and particulate matter increase; meanwhile, the fuel attached to the cylinder wall ignites to cause the temperature of the cooling water to be abnormal, the thermal efficiency is reduced, and the power and torque are reduced. In severe cases, it will cause a fatal engine failure.
2. The influence of fuel temperature on engine performance
Moisture in the fuel increases its viscosity due to overheating during the combustion process, how it affects the atomization and atomization of the oil particles, and the formation of the mixed gas. In this section, for a clear comparison, 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 fuel supply temperatures on engine performance when only 30% water-containing fuel is used. With the increase in fuel supply, the performance of both engines is on the rise, and there is no change due to the effect of fuel supply temperature, even the exhaust temperature is basically the same. However, the exhaust gas concentration increased dramatically with the increase of the oil supply temperature.
Due to the increase in fuel supply temperature, an increase in the amount of premix (combustible mixture) is formed at the initial stage of combustion. The increase in fuel temperature increases the viscosity of the fuel, the average particle size of the oil mist injected into the cylinder decreases, and the heat of moisture Decomposition and heat absorption, the temperature reflected by the flame before the occurrence of oil mist in the cylinder is low, which leads to the low and relatively concentrated ignition temperature of the spark core, the reaction time before the flame becomes longer, the lag period is longer, the mixture is unevenly distributed, and the combustion is not Completely, the emission smoke concentration increases with the increase in fuel supply, that is, the (unfueled gas mixture) particulate matter concentration is higher.
3. The influence of different water content on engine performance
Since water has such a great influence and effect on the performance of the engine, how much water should be injected or how much water is contained in the fuel? Sell a cute here, smart people have long 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 exhaust gas, it is necessary to increase the amount of water (injected water) in the fuel in an appropriate amount. There are two purposes for this: 1. The injected water can reduce the combustion temperature and reduce the content of exhaust gas NOX; Under the combustion environment, make it burn more fully, so that it is completely burned. However, considering the fuel economy, we continued to spray water until we were unable to spontaneously ignite and could not form a spark core and could not burn. After recording and comparison, the fuel economy has reached the optimal state under the condition of 20%-30% water injection. At this time, the flue gas concentration is low and the engine performance is optimal. For example, when fuel injection is 60mg / st, the burst pressure of the engine is around Pe = 0.6MPa. As shown in the figure above, at low load, 20% moisture can be taken
Fuel content; it is more reasonable to use fuel with a moisture content of about 30% at medium and high loads.
in conclusion
1. When using fuel with 20% -30% moisture content, or when spraying water into the cylinder, plus good air flow, such as the use of pistons that generate vortex and squeeze flow, can reduce the fuel consumption About 10%. At the same time, due to the increase in the amount of combustible gas, 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 lag period will be significantly reduced, and the comfort will be improved;
2. The burning 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 more moisture, incomplete combustion increases the particulate matter and soot content; at the same time, the condensation and thermal corrosion of water vapor also cause a large degree of wear and tear on engine parts and rust.
