1 preface

Particulate matter emitted by diesel engines is harmful and destructive to human beings to varying degrees.

Its ecological environment, therefore, the problem of particulate emission from purifying diesel engines is affected by

People pay more and more attention to it. This paper introduces the diesel engines developed all over the world.

Particulate emission purification technology, and analyze the characteristics and existing problems of various technologies.

Have questions.

2 Diesel particulate emissions

The harmful components in diesel engine emissions mainly include c0, IlC,

No: particles, etc. The composition of particulate matter is mainly soot particulate matter (accounting for)

30%), soluble hydrocarbon organic matter (35%), sulfur and water (35%)

35%). Due to the large specific surface area and strong adsorption (suction) of discharged particles

With polycyclic aromatic hydrocarbons, benzoflowers, etc. ) and small particle size (only

0.0 1-0.05 forest m), light weight, so it can be hung in a large area for a long time.

In qi, it is easily absorbed by human body and deposited in lung cells, which is harmful to human health.

Health is very harmful. Therefore, all countries in the world are making efforts to reduce diesel particulate emissions.

The restrictions on diesel particulate emissions are more and more strict, and Europe restricts diesel particulate emissions.

The following table lists the relevant laws and regulations.

Because the fuel injected into the combustion chamber is not completely mixed with air, the combustion temperature is high.

Warm fuel and gentle gas undergo high-temperature polymerization and decomposition reactions, resulting in high

HC with molecular weight, after further condensation, produces soot.

Particles, although most of them are burned in the subsequent oxygen-enriched zone, are finally mixed.

Incomplete, about 1% of soot particles have not been burned and are relatively heavy.

Particles are discharged immediately. Soluble hydrocarbon organic matter comes from unburned fuel.

Engine oil. Hydrocarbon organics adsorbed on soot particles during condensation.

Surface, become a part of diesel particulate emissions. In order to realize this day.

At present, all countries in the world are actively meeting the requirements of strict emission regulations.

Formulate technical measures to reduce diesel particulate emissions.

3 Main purification measures for diesel particulate emissions

3. 1 in-machine purification measures

In-machine purification focuses on reducing initial carbon particles in the combustion chamber.

Formally, the micro-engine is realized by improving the engine structure or adding additional devices.

The purpose of particle purification.

3. 1. 1 Optimization of combustion system

Combustion process has the greatest influence on particle formation, and it is also the object of study.

Hot topics mainly include the following research directions.

A. Optimization of fuel injection system parameters related to fuel injection

There are mainly fuel injection quantity, nozzle end pressure, nozzle structure and fuel injection quantity.

Forward angle. The performance of fuel injection system directly affects the atomization and mixing of fuel.

The quality of mixed gas will ultimately affect the particulate emission characteristics. For example, it will increase.

The number of nozzle holes, the use of electronic control technology and the increase of injection pressure can make

The fuel is distributed more evenly in the combustion chamber, which reduces the collision between the fuel and the wall and reduces the fuel consumption.

It is beneficial to reduce particulate matter emissions, but it will cause an increase in NO;

B improve the bidding combustion method to reduce particle emission, daily

Both Japan and the United States are studying diesel engines with premixed combustion mode,

Fuel can be fully mixed with air to avoid high temperature and lack of oxygen.

Possibility of fuel cracking into carbon particles;

C. Under the condition of high pressure injection, the optimization of inlet swirl is adopted.

Adopting low swirl ratio is beneficial to reduce particle emission, which is due to the swirl ratio.

Large, improve the intake speed, reduce the inflation efficiency. But at first.

When the machine is actually running, it needs high inlet swirl at low speed; but

Low inlet cyclone flow is required at high speed. Variable vortex air intake is adopted.

The vortex ratio in operation can be changed between 0.2 and 2.5 by wind tunnel technology.

Make the engine performance and particulate emission characteristics within the whole range.

To optimization;

D. Four-valve technology adopts four-valve structure, which makes the piston

The soft burn pit and nozzle on the cylinder head are arranged in the center of the combustion chamber and changed.

The distribution of intake swirl and oil mist is improved, and the combustion condition is obviously optimized.

At the same time, the cooling conditions of piston and nozzle have also been improved, thus reducing micro-cooling.

Particulate emission:

E. the combustion is made of ceramic materials.

Combustion chamber, piston crown and cylinder liner can improve the heat insulation effect of green combustion chamber.

Moving parts such as valve rocker arm made of ceramic materials can reduce friction resistance.

Force, reduce engine fuel consumption, thereby reducing particle emissions.

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2 air intake pressurization and air cooling technology

Intake pressurization and intercooling can increase the intake volume, which makes her hopeful.

Sufficient oxygen can be obtained under the maximum torque, and critical air can be avoided.

Fuel ratio with variable cross-section supercharger (vCT) and advanced control system.

System, can effectively reduce particle emissions.

3, 1.3 Reduce oil consumption

Unburned engine oil accounts for a large proportion of particulate matter emitted by diesel engines.

Heavy, so we must reduce oil consumption. To this end, the piston,

Piston rings, cylinder liners and other parts are optimally matched.

Gap optimization research, especially under the condition of thermal deformation, to achieve

Reduce oil consumption.

3. 1.4 improvement measures for fuel oil

A. reduce the sulfur content in the combustion process, and the sulfur content in diesel oil is about

98% is converted into S0. The remaining 2% became sulfate particles.

S0 part: further oxidized and combined with HZO produced in the tender firing process.

HZSO。 And sulfate (such as C' 50', etc.). ), which increases the microstructure.

When the S in the bidding materials drops from 0. 12% to 0.05.

%, particulate emissions will be reduced by 8% ~ 65438 00%, but further reduced.

Less sulfur has no effect on particulate emissions. United States 1993 10.

According to the regulation in June, the diesel oil used by cars driving on expressways contains sulfur.

The quantity shall not be higher than 0.05%, and all low-sulfur firewood has been supplied in 1996.

Oil. Japan also supplies all low-sulfur diesel oil in 1997;

B. Reducing the specific gravity of fuel will directly affect indirect injection.

Particulate emission of diesel engine, that is, particulate emission changes with the change of fuel specific gravity.

To increase by increasing:

C the emulsified fuel is water-in-oil emulsified fuel.

Because of the rapid evaporation of water in oil, oil droplets become finer, which is beneficial.

Diffusion combustion can effectively reduce particle emissions.

3.2 purification measures outside the machine

Off-board purification is exhaust aftertreatment, which makes diesel engine exhaust more attractive.

In the post-treatment device of the door, some particles are eliminated before being discharged.

Atmosphere. The main external purification measures are shown in figure 1.

Ultra-thin particle technology is the most widely used technology in off-site purification measures.

Operation, that is, filtering with a specific structure made of high temperature resistant filtering materials.

Body, the particles in the waste gas are trapped in the filter body, so as to achieve net

Purpose. The material and structure of the filter body shall meet the following requirements:

A. The passing characteristics are good and the exhaust resistance is as small as possible;

B. good thermal shock resistance and good mechanical properties.

C. Good thermal stability, able to bear high heat load;

D. high filtration efficiency;

e

.

Adapt to the requirements of regeneration.

At present, the most widely used filter material at home and abroad is wall-flow bee.

Nested ceramics, foamed ceramics, ceramic fibers, etc.

With the continuous accumulation of particles in the filter, the exhaust resistance of diesel engine

As the force increases, the back pressure increases. When the back pressure increases to a certain extent, it will

As a result, the power and filtration efficiency of diesel engine decrease, so it must be cleaned in time.

In addition to filtering particles accumulated in the body.

As we all know, when the diesel engine is at the maximum load and speed,

At this time, the temperature of the cylinder exhaust port can reach 500-600℃

Diesel engine exhaust particles begin to rapidly oxidize and heat up until they catch fire.

Combustion, to reduce particles, so as to achieve the exhaust resistance of the filter body.

The filtration efficiency is restored to the original level, that is, the regeneration of the filter body.

At present, the regeneration mode of filter body is mainly "thermal regeneration", that is, benefit.

Full-load regeneration, fuel injection combustion-supporting regeneration, electric heating regeneration and electric spontaneous combustion.

Heating regeneration, throttling regeneration, etc. In addition, reverse injection has been developed.

Non-heating regeneration methods, such as gas regeneration and vibration regeneration, are used.

An external heat source heats and spontaneously ignites particles accumulated in the filter, thereby

Reduce particles in the filter.

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2. 1 diesel engine exhaust aftertreatment system

Diesel particulate aftertreatment system uses foam ceramics to filter particulates.

Particles, and use microwave regeneration filter.

3.2. 1. 1 foam ceramic filter

Ceramic raw materials are made into slurry, and the slurry is mixed with vinegar or polyether foam.

Plastic is formed by impregnation molding and finally fired. This structure is shown in Figure 2.

number

The interior of foam ceramics consists of many small holes (called "air cells")

Each air chamber is connected with a plurality of adjacent air chambers through windows.

This diameter is much smaller than the diameter of the gas chamber, so the particles are captured throughout.

In the air room. Its advantage is that the porous structure makes the flame easy to spread, so

3.2. 1.2 microwave regeneration

Diesel particulate exhaust aftertreatment system was first adopted in China.

Microwave regeneration is a new technology. Compared with other regeneration methods, it has the following advantages.

The following features.

A. Selective heating ceramic materials have very low microwave absorption capacity.

Poor, but the microwave absorption capacity of particles is 100 times that of ceramics.

So in fact, in the process of microwave regeneration, particles are mainly added.

Thermal objects, such selective heating characteristics for improving energy utilization,

It is very beneficial to prolong the life of the filter and improve the regeneration efficiency;

B. Space heating When microwave enters the heated material, it will lead to

Friction caused by high-speed vibration of molecular dipoles of heated substances leads to

Heat. Spatial distribution of microwave energy in filter body. Therefore, micro

Wave regeneration has the characteristics of fast and uniform heating, which makes the regeneration process

The quantity utilization rate is high, and the heating of the filter body caused by uneven heating is reduced.

Possibility of damage.

Diesel particulate emission aftertreatment system is mainly composed of filters and microfilters.

Wave source, vehicle-mounted power supply and automatic control system (Figure 3),

When the diesel engine is working, the exhaust gas passes through the filter body, and some particles in it

The exhaust gas captured and purified by the filter body is discharged into the atmosphere. When the filter body

When the particles accumulate to a certain extent, they begin to regenerate.

The microwave is emitted by the microwave source and located between the filter and the microwave source.

Because the power of microwave source is limited, if there is high-speed gas in the regeneration process,

Flow through the filter body, then regeneration is difficult to achieve. To ensure regeneration

Energy can be generated smoothly, and the bypass valve closes the exhaust pipe leading to the filter during regeneration.

At the same time, the bypass channel is opened, so that the waste gas is directly discharged into the atmosphere.

So as to avoid the impact of airflow on the filter body.

In the regeneration process, the bypass valve, vehicle power supply and microwave source are all controlled by.

This system is automatically controlled.

Through bench test and automobile bench test, its filtration efficiency exceeds

June%. The microwave regeneration efficiency is over 80%, and the whole system works stably.

This method is reliable and has high practical value.

Other filter materials

Wall flow honeycomb ceramics

Wall-flow honeycomb ceramics have many honeycomb holes and adjacent honeycombs.

The two ends of the tunnel are alternately blocked. Engine exhaust gas enters the human body through the population passage.

After that, it must pass through the porous thin wall in the filter body to be discharged. because

The porosity of thin wall is as high as 400 mesh /25.4mm, and the filtration efficiency is high.

High, up to 60%-80%; High structural strength, thermal shock resistance and

Strong mechanical vibration ability. However, honeycomb ceramics are anisotropic, and their radial direction

The expansion coefficient is twice the axial expansion coefficient, and the particles are deposited in the

In the air inlet channel, the heating is uneven during the regeneration process, which is easy to occur.

Thermal shock damage.

3.2.2.2 Woven Ceramic Fiber

Woven ceramic fibers have high surface area and good high resistance.

The temperature capacity not limited by fixed size determines the shape of the inner hole of the filter.

And the distribution of holes provides a wide range of choices, and by changing various

The design parameters optimize the application. Due to the fiber surface in the filter body,

They are all effective filtration areas, so the filtration efficiency is as high as 95%. however

However, ceramic fiber is a brittle material, although it can adapt to catalysts.

The disadvantage of vegetables is that the production process is complex and easy to be damaged.

Due to the fiber surface in the filter body, the parameters are designed to optimize the application.

They are all effective filtration areas, so the filtration efficiency is as high as 95%. however

However, ceramic fiber is a brittle material, although it can adapt to catalysts.

The disadvantage of vegetables is that the production process is complex and easy to be damaged.

3.2.3 No filtering technology-70%-80% of diesel exhaust particles are charged.

State, each charged particle has about 1-5 basic positive or negative charges.

The charge is generally electrically neutral. At home and abroad, attempts have been made to use adhesion to enhance the electric field.

The electrostatic adsorption experiment of charged smoke particles was carried out and the experimental results were obtained.

Some test results. However, due to the large size and complicated structure of the additional equipment,

The supply of miscellaneous and high-voltage power supplies brings some difficulties to practicality.

Difficult.

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2.4 Non-filtering technology-voltage capture technology

The upstream and downstream of the diesel engine exhaust pipe are respectively provided with a metal mesh and a mesh.

Add a DC voltage of about 50 v. Generally, the upstream metal mesh is large and

When a negative voltage is applied, the downstream metal mesh is denser and a positive voltage is applied. while

Particles are negatively charged when passing through the upstream metal mesh, and positively charged when passing through the downstream metal mesh.

The metal mesh is adsorbed, thus achieving the purpose of purifying particles. this

The method has simple equipment and high filtration efficiency.

3.2.5 Catalytic technology for particulate aftertreatment

Commonly used catalysts are precious metals (platinum, handle, etc. ) and base metals.

(oxides of manganese, diamond, vanadium and chromium) and rare earth, and the catalyst may

It can be added to the bidding materials or soaked on the filter body for the following purposes:

. Promote the oxidation of carbon particles in the filter element and effectively reduce carbon particles.

Ignition temperature makes the filter easier to "regenerate";

b

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Effectively reduce HC and C0 content in waste gas. But because

The oxidation of catalyst, sulfur in waste gas is easy to form sulfide, not

It will only lead to the poisoning of catalyst'', and it is deposited by sulfide in

Filter the car body, reduce the filtration efficiency and increase the exhaust resistance.

Sample, when the combustion products of metal additives flow through the filter, there will be a

Part of the sediment will also affect the filtration efficiency and increase the exhaust resistance.

The noble metal catalyst will not cause secondary pollution and has high catalytic efficiency.

Expensive base metal and rare earth catalysts are cheap, but

The catalytic efficiency is not as good as that of noble metal catalyst. China is a rare earth in the world.

China has the largest reserves, so rare earth catalysts have great prospects in China.

4 conclusion

Purification technology of diesel particulate emissions, whether in-machine purification

Or off-site purification, although all have a certain purification effect, but for no

The same diesel engine of the same model or the same diesel engine under different working conditions, each

All technologies have certain limitations, and some technologies are still immature.

The formation mechanism of diesel engine exhaust particles is different from N0, so it is necessary to adopt it.

Measures to reduce particulate emissions usually lead to an increase in No emissions.

In addition, the focus of future research is mainly:

A. Further explore the mechanism of diesel particulate emission;

B. Continuously develop new internal and external purification measures and seek

Internal and external purification measures are organically combined to achieve a more ideal purification effect.

Effect;

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Develop technologies that can simultaneously reduce diesel particulate and No emissions.

Surgical measures.