Why Would Diesel Engine Have Black Smoke?

At normal operating temperature, the fuel of the water cooled diesel engine generator can be relatively completely burned in the cylinder. The exhaust gas should be colorless and transparent gas or a light blue or light gray gas. The so-called colorless is not completely without color, but accompanied by a light gray in colorless, this is the normal exhaust smoke color.

The exhaust smoke of a diesel engine may be heavier at idle speed and heavier at high speed and high load. It is necessary to observe the normal exhaust smoke color in order to judge and analyze the abnormal exhaust smoke color.

After the diesel fuel is completely burned, the normal color is generally light gray, and dark gray when the load is slightly heavy. During the operation of diesel engines, there will often be smoke. There are four types of diesel exhaust: black smoke, blue smoke, white smoke and yellow-black smoke. They are one of the conditions for judging the failure of diesel engines. The following mainly describes the reasons and main features of diesel engines emitting black smoke. Whether you have a high speed 6 cylinder diesel engine or a high speed 12 cylinder diesel engine, 6 cylinder water cooled diesel engine generator set or 12 cylinder water cooled new diesel engine generator set, the basic reason for the black smoke color of the diesel engine is similar.

Main features of diesel engines emitting black smoke

The main characteristics of black smoke (carbon smoke) in the exhaust are difficult to start and exhaust black smoke; the exhaust is serious at low speed, the exhaust is reduced or disappeared at high speed; the exhaust is normal at low and medium speed, and the exhaust is serious when accelerated ; under various speed conditions, continuous or intermittent smoke exhaust, accompanied by abnormal metal knocking sound.

Black smoke is also called soot. The black smoke from the exhaust of diesel engines is mainly caused by excessive fuel mixture, poor formation of combustible mixture or imperfect combustion. Under the high-temperature and high-pressure combustion conditions of diesel engines, the solid fine particles with carbon as the main component formed by partial hypoxia, cracking and dehydrogenation are manifestations of incomplete combustion of fuel in the combustion chamber. Because diesel engines are inhomogeneously combusted, the chemical reaction conditions in each area of the combustion chamber are inconsistent and change with time, so black smoke is likely to be generated by many different ways. Diesel is a complex hydrocarbon. Unburned diesel injected into the combustion chamber is decomposed by high temperature to form carbon black. When exhausted, it is discharged together with the exhaust to form black smoke.

Black smoke is a product of incomplete combustion, which is released and polymerized by the cracking process of hydrocarbon combustion 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 the last part of evaporation. Its combustion depends on its evaporation rate and the mixing speed of fuel vapor and oxygen.

Five Causes of black smoke from diesel engines

The generation of black smoke during the combustion process of diesel engines can be summarized into three stages-nucleation stage, single particle stage, single particle combustion disappears or agglomerates into larger flocs. The main reasons affecting its generation are:

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

After the valve, piston ring and cylinder liner are worn out, the compression pressure is insufficient and the oil flows into the combustion chamber. At the end of the compression stroke, the normal ratio of the mixture of the mixed gas changes, so that the fuel is burned under oxygen-free conditions, and the combustion process is easy to produce. Carbon deposits and exhaust fumes form a large amount of black smoke.

2. The shape of the combustion chamber changes

The shape of the combustion chamber is reduced due to manufacturing quality and long-term use. The compression clearance is too large, too small, and the piston position is installed incorrectly. These will change the shape and volume of the combustion chamber, which will affect the mixing quality of fuel and air.

3.  Bad injector work

The exhaust smoke of diesel engine is closely related to the quality of fuel atomization. During the fuel injection process of the diesel engine, at the end of each injection, the injection pressure drops and the atomization quality is poor. The evaporation and burning time of these oil droplets is short, and the surrounding oxygen concentration is low, so it is easy to produce soot. No atomization, poor atomization or dripping of the fuel injector will make the size distribution of diesel particles injected into the cylinder uneven, so that the fuel cannot be fully mixed with the air in the cylinder, nor can it be completely burned.

4.  Fuel supply is too large

Too much oil supply will increase the amount of oil entering the cylinder, resulting in more oil and less gas, and incomplete fuel combustion. In addition, heavy work load, poor fuel quality, and low working temperature can also cause black smoke in the exhaust.

The high-temperature cracking reaction of fuel in diesel engines is inevitable, especially in space-mixed combustion diesel engines, because the high-temperature gas surrounds the liquid oil droplets, which creates conditions conducive to the cracking reaction, so a large amount of carbon is generated in the early stage of combustion This has been confirmed by high-speed photography of the combustion process. During normal combustion of a diesel engine, before the exhaust valve opens, a large amount of carbon particles formed in the initial stage of combustion can be basically burned out, and the exhaust gas is basically smokeless. Only under certain adverse working conditions, the carbon particles cannot be burned in time, but instead agglomerate and adsorb, forming larger soot particles or flocs in the cylinder and during the exhaust process, making the exhaust emit black smoke. You can choose a high quality turbocharger diesel engine. To a certain extent, it can help extend the life of diesel engines.

5.  Improper adjustment of fuel supply advance angle

The fuel supply advance angle is too large, the fuel is injected into the combustion chamber prematurely, because the pressure and temperature in the cylinder are low at this time, the fuel cannot be ignited and burned. When the piston goes up and the cylinder reaches a certain pressure and temperature, the combustible gas mixture burns. In direct injection diesel engines, when other parameters remain unchanged, increasing the injection advance angle can reduce exhaust smoke. However, premature fuel injection increases the amount of pre-mixed fuel, makes the diesel engine work rudely, increases the combustion noise, and causes a larger mechanical load and more black smoke (COX) emissions.

The fuel supply advance angle is too small, the fuel injected into the cylinder is too late, and the piston has descended at the end of the final injection. The volume in the cylinder increases, the pressure decreases, and the temperature decreases. At this time, part of the fuel is separated or discharged before it can form a combustible mixture, so that part of the fuel material discharged with the exhaust gas in the exhaust pipe is decomposed and burned at high temperature, forming black smoke and discharged with the exhaust gas.