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The Influence Of Humidity On Engine Combustion Performance

- Sep 21, 2018 -

In this paper, we study the influence of certain humidity on the performance of diesel engine when water is injected into fuel for diesel engine, and investigate the influence of water mist existing in fuel on combustion. At the same time, through the study of visual combustion, which has a significant impact on engine performance such as fuel consumption, smoke emission concentration, nitrogen oxide concentration in exhaust, etc., the following three aspects are studied:

1. Comparison of combustion process between diesel oil and fuel containing 30% water;

2. Influence of fuel temperature on engine performance (use fuel with 30% moisture);

3. Influence of different water contents on the engine (use fuel containing 20% and 30% water).

1. Visual combustion characteristics comparison

Does the wter contained in the fuel have any effect on its atomization or reatomization? Whether the chemical reaction between superheated water vapor and oil mist has any influence on the fuel composition; Due to the change of viscosity and the increase of oxygen content required for combustion, the effect of higher combustion temperature on engine performance is briefly introduced in this section of combustion characteristics.

The following figure is a picture of the cylinder flame collected by the visual engine (optical engine). Under the same experimental conditions, change the moisture content of the fuel. As is shown above, under the condition of using diesel spark nucleation position in BTDC 10 ° CA, the center of the combustion chamber, small spark of nuclear spread through the mixture of around make red yellow flame combustion chamber. Figure 1 (b) and (c) is the use of the fuel contained 30% moisture of flame images, obviously the formation of initial spark nuclear position than using diesel delay 5 ° CA, spray atomization and mixture formation is poor, the formation of the phenomenon of fire wall and saw not the existence of the burning mixture around the flame, the initial spark nuclear luminance darker, death, a longer duration of the flame is superposition, miyun, without administrative levels, the combustion environment deterioration.

As for water-containing fuel, after thermal dissociation of water under high temperature and high pressure, the pre-flame reaction is accelerated, resulting in high pressure in the cylinder at the initial stage. The poor fluidity of the mixture can easily densify large-particle fuel, resulting in incomplete combustion. As can be seen from the photo, carbon smoke and particulate matter increase. At the same time, the fuel attached to the cylinder wall forms wall fire, resulting in abnormal cooling water temperature, low thermal efficiency, reduced power and torque, and in serious cases, fatal engine failure.

The influence of fuel temperature on engine performance

Water in the fuel increases in viscosity due to overheating during combustion, which has an impact on the atomization and micronization of oil particles and the formation of mixed gases. In this section, for the sake of sharp contrast, the oil path of one of the engines is heated and insulated to compare the external characteristics of two engines.

Figure 2 shows the influence of different oil supply temperatures on engine performance when only 30% water-containing fuel is used. With the increase of oil supply, the performance of both engines showed an upward trend, and there was no change due to the influence of oil supply temperature, and even the exhaust temperature was basically the same. However, the smoke emission concentration increases sharply with the increase of oil supply temperature.

Due to the increasing oil temperature, combustion formed early in premixed gas (syngas) increases, the rising temperature of the fuel oil viscosity increases, spray into the average oil mist particles in the air cylinder to reduce, coupled with the thermal decomposition of water absorption of heat, the temperature of the oil mist occurred before the flame in cylinder reflects low led to spark ignition temperature is lower, and relatively concentrated, flame before the longer reaction time, ignition delay period longer, uneven mixture formation, incomplete combustion, emissions along with the increase of oil smoke concentration increased, the particle concentration (without fuel gas mixture) emissions is higher.

The influence of different water components on engine performance

Given that water has such a big impact on engine performance, how much water is right to spray or how much water is in the fuel? Here to sell a cute, smart people have long seen it! We have been comparing the water content of 30%, or 20%, to see if this content is reasonable.

In order to reduce NOx and particulate matter in the exhaust, a proper amount of water (water injection) in the fuel should be increased. The purpose of this is twofold. 2. The oxygen-rich combustion environment makes it burn more fully and completely. However, considering the fuel economy, we adopted continuous spraying water until it could not spontaneously ignite, and the spark-core could not be burned. After recording and comparing, under the condition of spraying water 20% to 30%, the fuel economy has reached the optimal state. At this time, the flue gas concentration is low and the engine performance is optimal. For example, when fuel injection is 60mg/st, the explosion pressure of the engine is around 0.6mpa. As shown in the figure above, 20% water content of fuel can be taken at low loads; It is reasonable to use 30% water content fuel at high load.


1. When using fuel with 20%-30% water content or spray combustion in the cylinder, plus good air flow, such as using pistons that generate eddy current and are more crowded, the fuel consumption rate can be reduced by about 10%. At the same time, due to the increase of combustible mixture, combustion is accelerated and the whole combustion period is shortened. If the injection advance Angle in advance 5 ° CA, noise and vibration caused by delay period will be significantly lower, improve comfort;

2. The flame of water-containing fuel is reddish-brown, with low combustion temperature and significantly reduced content of nitrogen and oxygen compounds. However, the content of particulate matter and carbon smoke increased due to more water content and incomplete combustion. At the same time, the condensation of water vapor and the heat corrosion of the engine parts wear, rust prevention also cause a great degree of loss.

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