NGINE SOOT IS A COMMON BYPRODUCT of internal combustion engines, formed as the result of incomplete fuel combustion. Most fuels are composed of hydrocarbons, containing both carbon and hydrogen, and when undergoing complete combustion, the only byproducts are carbon dioxide and water. However, no engine is completely efficient and complete combustion does not occur. Complete combustion would require a very lean ratio of fuel to air, whereas real engine conditions exhibit richer fuel mixtures. The less air that is present in the ratio, the more favorable the conditions are for soot accumulation.

Soot formation is more pronounced in diesel engines than in gasoline engines due to the ways fuel is injected and ignited. While the 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 have a chance to thoroughly mix, while the later fuel injection in diesel engines produces fuel-dense pockets in the combustion chamber that produces soot when ignited. Newer exhaust gas recirculation (EGR) diesel engines, designed to reduce NOx emissions by routing part of the engine's exhaust stream through an intercooler and back to the manifold, further compound soot problems in diesel engine oils.

Excessive soot formation in oil can be caused by a number of factors. Worn out rings or injectors, excessive idling, poor fuel spray patterns, and incorrect air- fuel ratios are major 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 air and increasing the fuel-air ratio.

Soot particles are spherical in shape and 98 percent carbon by weight. They are of a very small size of 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 passes through the rings or the combustion chamber and enters the engine oil. As long as these soot particles remain suspended in the oil and are not allowed to 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 become quickly depleted.

High soot load conditions lead to loss of oil dispersancy as an oil's dispersant additives are consumed. As dispersancy is lost, soot particles agglomerate and form larger particles that build up on engine surfaces. This soot and sludge eventually impedes oil flow, and it can also form on oil filters, blocking oil flow and allowing dirty oil into the engine. 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 absorption into soot particles, leading to increased wear and premature engine failure.

Another negative 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 liner, and abrading the ring and liner. As the gap between the ring and liner increases, combustion byproducts such as gases and unburned fuel blow into the crankcase, a problem known as blowby, eventually causing expanding gases to lose ability to push the piston down and generate the power necessary to propel the vehicle. Horsepower is lost and fuel efficiency decreases. Ring sticking and poor heat transfer from the piston to the cylinder wall can also result.

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 17cSt. Excellent wear control was maintained with an average iron content of under 50 ppm. AMSOIL diesel oils provide outstanding protection against viscosity thickening and soot generated wear.