The Harmful Effects of Engine Soot

 

Engine soot is a common byproduct in 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 for soot accumulation.

Soot formation is more pronounced in diesel engines than gasoline engines due to the ways 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 have a chance to thoroughly mix, 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, designed to reduce NOx emissions by routing part of the engine’s exhaust stream through an intercooler and back to the intake 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 supply and increasing the fuel-air ratio.

Soot particles are spherical in shape and 98 per­cent carbon by weight. They are 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 of 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 adsorption to 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 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 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 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.