The cause of the malfunction of diesel engine black smoke

Abstract: The concept of smoke emission from diesel engine is clarified, and its formation mechanism is further elaborated. The fault cause of smoke exhaust smoke generated during the use of diesel engine is accurately judged and analyzed.

I. Overview:

Since the diesel engine has been designed and put into production, it has a history of nearly one hundred years. The variety, quality and output of diesel engines have been greatly developed and improved. There are hundreds of variants and supporting products of various series, as automobiles and engineering machinery. Ships, power stations, agricultural irrigation and drainage machinery and other industrial supporting power equipment have played an active role in economic development. This paper mainly describes, judges and analyzes the smoke generation mechanism of diesel engine, and carefully observes the exhaust smoke color of diesel engine. The correct judgment and scientific analysis can effectively improve the performance of diesel engine, reduce the failure rate and improve the economic performance of diesel engine. .

When the diesel engine is at normal working temperature, its exhaust smoke color should be colorless or light gray. The so-called colorless is not completely colorless. It can't be colorless like a gasoline engine, but it is accompanied by a light gray in the colorless. This is normal. Exhaust smoke. When the diesel engine is idling, the exhaust smoke color may be heavier, and it may be heavier at high speed and high load. It is necessary to pay attention to observe the normal exhaust smoke color to judge and analyze the abnormal exhaust smoke color. After the diesel fuel is completely burned, the normal color is generally light gray, and the load is slightly gray when it is slightly heavy. Diesel engines often emit smoke during work. There are four types of diesel exhausts: black smoke, blue smoke, white smoke and gray. They are one of the conditions for judging diesel engine failure. The specific analysis is as follows:

Second, the cause of the black smoke of the diesel engine:

(1) The diesel engine exhausts black smoke mainly because the fuel mixture is too rich.

1) Black smoke is also called soot. The black smoke of diesel engine exhaust is mainly caused by excessive fuel mixture, poor formation of combustible mixture or imperfect combustion. Under the high temperature and high pressure combustion conditions, the solid micro-particles with carbon as the main component formed by partial oxygen deficiency, cracking and dehydrogenation are the incomplete performance of fuel combustion in the combustion chamber. Since diesel engines are heterogeneous combustion, the chemical reaction conditions in various regions of the combustion chamber are inconsistent and vary with time, so black smoke is likely to be generated by many different routes. Diesel fuel is a complex hydrocarbon. The unburned diesel fuel injected into the combustion chamber is decomposed by high temperature to form carbon black, which is discharged together with the exhaust gas to form black smoke. Black smoke is an incomplete combustion product, which is produced by the decomposition of hydrocarbons in the pyrolysis process under high temperature and anoxic conditions. In some cases, fuel is injected on the wall of the combustion chamber to form a liquid oil film. The oil film is part of the final evaporation. Its combustion depends on its evaporation rate and the mixing speed of fuel vapor and oxygen. If the concentration of oxygen in the surrounding gas is too low, or the speed of mixing is insufficient, the fuel gas evaporated from the oil film will be decomposed and will produce unburned hydrocarbons, incomplete oxidation products, and black smoke.

2) Through experimental observation, BB Chakraborty and R. Long proposed the mechanism of black smoke formation according to their research and other people's data as shown below:

Open chain hydrocarbon → high temperature cracking dehydrogenation → olefin, mainly ethylene (CH2=CH2)→ further cracking dehydrogenation→alkyne, mainly acetylene (CH≡CH)→polymerization dehydrogenation→vinyl acetylene (CH≡C-CH =CH2) or diacetylene (CH≡CC≡CH)→ further cleavage dehydrogenation, group polymerization → when it is greater than 1000 ° C, the polyacetylene has a plurality of (-C=C-), which is very inactive and has been formed Solid carbon nucleus → polymerized → polymer → ring structure and further dehydrogenation (higher temperature than 1000 ° C) → hydrogen-containing polycyclic carbon complex or insoluble polycyclic hydrocarbon → polycrystalline, expanded → hexagonal crystal The carbon crystal, which is the entity of soot → surface adsorbs foreign matter, itself agglomerates - → soot (black smoke)

According to the above qualitative description, the generation of black smoke during the combustion of the diesel engine can be summarized into three stages - the nucleation stage, the single stage stage, the burning of the single particles disappears or agglomerates into larger flocs.

(2) The main causes of the malfunction of diesel engine exhausting black smoke:

1. Wear of piston rings, cylinder liners, etc.

After the valve, piston ring and cylinder liner wear, the compression pressure is insufficient and the oil is on the combustion chamber, so that the normal ratio of the mixture mixing changes at the end of the compression stroke, so that the fuel is burned under anaerobic conditions, and the combustion process is easy to produce. The carbon deposits and the exhaust gas form a large amount of black smoke.

2. Combustion chamber shape change

The shape of the combustion chamber is degraded due to the quality of manufacture and long-term use. If the compression residual seam is too large, too small, and the position of the piston is wrong, the shape and volume of the combustion chamber will change, which will affect the fuel-air mixing quality and fuel combustion conditions. It is getting worse.

3. Injector malfunction

The exhaust smoke of diesel engine is closely related to the quality of fuel atomization. During the injection process of diesel engine, the injection pressure drops and the atomization quality is poor at the end of each injection, so that the droplet diameter is smaller than that of the main injection stage. 4 to 5 times larger (about 100 times the volume), these oil droplets evaporate and burn for a short time, the concentration of surrounding oxygen is low, and it is easy to produce soot. The injector is not atomized, atomized, or dripped, so that the fuel cannot be sufficiently mixed with the air in the cylinder, nor can it be completely burned. The phenomenon of black smoke caused by poor injector operation is more obvious when the diesel engine runs at low speed, because the intake vortex in the cylinder is weak at low speed, the possibility of oil droplets or oil bundles being dispersed by the airflow is reduced and the waiting time is shorter. Long, it is easier to form carbon black discharge.

4. Excessive oil supply

If the oil supply is too large, the amount of oil entering the cylinder will increase, resulting in less oil and less fuel combustion. In addition, excessive workload, poor fuel quality, and low operating temperatures can cause exhaust fumes. The pyrolysis reaction of fuel in diesel engines is unavoidable, especially for space-mixed combustion diesel engines.

5. Judging the cause of the fault from the exhaust status of the diesel engine, and then taking corresponding solutions.