Soot Formations And Effects On Engine Life
By BRETT WINBERG

Diesel engine exhaust contains partially burned fuel resulting in the formation of (usually) soft, carbonaceous, very fine particles. These particles have the potential to agglomerate and harden under adverse conditions, plugging oil passages or limiting piston ring movement, impairing heat transfer or, in certain cases, neutralizing additive benefits. Soot has a pronounced effect on piston deposits, oil oxidation, and bottom end wear.

Fuel soot has not proved to be practically controllable. While capable of plugging main line filters, it is fine enough to pass through most filter media. Additionally the agglomerate particles in the 3 to 10 micron range tend to do the most damage, a scary number indeed, since most filtering media filters down to around 20 micron.
Soot generation is directly related to exhaust smoke density. This is largely controlled or minimized by keeping engines in tune, especially not over-fueled, as well as maintaining unrestricted air intakes so that maximum oxygen is available to consume fuel as completely as possible during the combustion process. Operating practices also have an effect on soot generation.

The Mack T-5 lubricant test has three operating cycles, two of which are distinctly different in terms of soot generation. An engine at 1400 RPM full load lugging cycle will produce a level of exhaust smoke density five times (42% to 8%) higher than that of the 2100 RPM full load, maximum power cycle. This is due primarily to the richer air-fuel ratio.
The level of soot trapped in piston deposits and in crankcase oils correlate with exhaust smoke density, and the percentage of soot in the piston deposits and the oil insolubles is also five times higher.
 
Accumulations of insolubles in the crankcase oil, and the formation of the piston deposits occur as a result of different, though related mechanisms. Under the worst conditions, a piston’s carbonaceous deposits contain about 20% soot, and quite often as low as 3% soot. The remaining piston deposits are lubricant derived. The “soot effect” does, however dominate the piston deposit formations from other sources such as oil degradation.

The main effect of soot appears to be the increase of carbon deposit formations. Increases in the exhaust smoke density correlates with increases in the carbon and lacquer formations. High-density exhaust smoke can also cause the formation of soot in the combustion chamber and exhaust systems. This can set the stage for an exhaust fire.

Conclusion:
To maintain and increase engine life the soot level must be controlled. High levels of soot in the crank case oil promote piston ring proudness and other carbon formations in the combustion area. Soot coagulation of 2.5 microns and higher causes abrasive wear.

Solution:
Soot levels can be controlled two ways. First, by limiting the cause of soot, which is high exhaust smoke density. This can be greatly aided by assuring more complete combustion through the use of a burn rate modifier or fuel catalyst. Second, by installing a bypass (kidney loop) filtration system, soot is removed from the system before it has a chance to coagulate and carbonize the oil.

The result is less oil degradation and fewer carbon deposits in the combustion area. This will have a dramatic, positive effect on engine life and maintenance budget constraints.

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