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The Harmful Effects of Engine Soot And How To Prevent

The Harmful Effects of Engine Soot And How To Prevent

Learn more about engine soot, why it is more prevalent in diesel engines, and how to prevent it from accumulating in your engine.
Dave Mann
March 18, 2024
5 min read
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The Harmful Effects of Engine Soot And How To Prevent

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Meet The Author

“Hi, I’m Dave - a former Ford Truck/Automotive Engineer and the face behind Performance Oil Technology. My trucks, heavy equipment, farm and powersports vehicles experience extreme use, and I use AMSOIL products in every single one; learn more about me here. Have a question? Contact me here. Enjoy the site!”

What is engine soot?

Engine soot is a black, tar-like substance that is a common byproduct in internal combustion engines, formed due to incomplete fuel combustion. Most fuels are composed of hydrocarbons containing carbon and hydrogen, and when undergoing complete combustion, the only byproducts are carbon dioxide and water. However, no engine is 100% efficient, and complete combustion does not occur. Complete combustion would require a very lean ratio of fuel to air, whereas actual engine conditions exhibit richer fuel mixtures. The less air that is present in the ratio, the more favorable the conditions for soot accumulation.

Engine soot that has collected in a tailpipe

Why is engine soot more common in diesels?

Soot formation is more pronounced in diesel engines than in gasoline engines due to how fuel is injected and ignited. While fuel is injected during the intake stroke and ignited with a spark in gasoline engines, it is injected during the compression stroke and ignited spontaneously from the pressure in diesel engines. Combustion is more efficient in gasoline engines because the air and fuel can mix thoroughly, while the late fuel injection in diesel engines produces fuel-dense pockets in the combustion chamber that produce soot when ignited. Newer exhaust gas recirculation (EGR) diesel engines are designed to reduce NOx emissions by routing part of the exhaust stream through an intercooler and back to the intake manifold, further compounding soot problems in diesel engine oils.

What causes soot to form in oil?

Several factors can cause excessive soot formation in oil. Worn-out rings or injectors, excessive idling, poor fuel spray patterns, and incorrect air-fuel ratios are significant causes of soot formation. A faulty fuel nozzle may spray more fuel than desired, increasing the fuel-to-air ratio and causing incomplete combustion and soot accumulation, or the air filter may become clogged, decreasing the air supply and increasing the fuel-air ratio.

Soot particles are spherical in shape and 98 percent carbon by weight. They are tiny, around 0.03 microns, but they often agglomerate to form larger particles. Although the majority of soot produced during combustion exits through the exhaust, some pass through the rings of the combustion chamber and enter the engine oil. As long as these soot particles remain suspended in the oil and cannot agglomerate, they pose little risk to engine parts. It is up to the motor oil's dispersants to keep soot particles dispersed. However, in high soot conditions, dispersants can quickly deplete.

Loss of Oil Dispersancy

High soot load conditions lead to the loss of oil dispersancy as an oil's dispersant additives are consumed. Oil dispersancy is the oil's ability to suspend and carry away pollutants like engine soot and metallic particles from wear. 

As dispersancy is lost, soot particles agglomerate, forming larger particles that build up on engine surfaces. This soot and sludge eventually impede oil flow, and it can also form on oil filters, blocking oil flow and allowing dirty oil into the engine. 

More Soot Means Higher Oil Viscosity

In addition, high soot levels within a motor oil increase its viscosity, further impeding oil flow and increasing engine wear. Anti-wear additive performance is also affected in high soot conditions as additives are gradually removed from the oil by adsorption to soot particles, leading to increased wear and premature engine failure.

Degradation of Oil Seal in the Combustion Chamber

Another adverse effect of high soot conditions is the formation of carbon particles on the piston ring grooves, causing degradation of the oil seal between the ring and cylinder line and abrading the ring and liner. As the gap between the ring and liner increases, combustion byproducts such as gases and unburned fuels blow into the crankcase, a problem known as blowby, eventually causing expanding gases to lose the ability to push the piston down and generate the power necessary to propel the vehicle.

Engine soot (carbon) buildup on a piston inside an internal combustion engine

Horsepower is lost, and fuel efficiency decreases. Ring sticking and poor heat transfer from the piston to the cylinder wall can also result.

How can engine soot be prevented in diesel engines?

AMSOIL synthetic diesel oils are formulated with robust additive packages that effectively disperse soot particles so they do not agglomerate and cause engine damage. In actual on-highway heavy-duty truck field trials, at soot levels as high as 10 percent volume and higher, AMSOIL diesel oils maintained an extremely low viscosity of 17 cSt. Excellent wear control was maintained with an average iron content under 50 ppm. AMSOIL diesel oils provide outstanding protection against viscosity thickening and soot-generated wear.

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