Soot may also result as the indirect by-product of nitrogen oxide and sulphur dioxide reacting in the atmosphere. Soot’s composition often includes hundreds of different chemical elements, including sulphates, ammonium, nitrates, elemental carbon, condensed organic compounds, and even carcinogenic compounds and heavy metals such as arsenic, selenium, cadmium, and zinc.
Diesel engines form soot far more than their gasoline counterparts due the differences in the ways fuel is injected and ignited. In gasoline engines, fuel is injected during the intake stroke and is then ignited with a spark. In diesel engines, the fuel is injected during the compression stroke and is ignited spontaneously from the pressure in the engine. The end result is that gasoline engines are much more efficient from an emissions standpoint since the ignition process is better controlled and the air and fuel are more thoroughly mixed allowing for a better and more efficient burn process and less unburned fuel. In diesel engines, fuel injection takes place later in the cycle, which makes for a less efficient mixture of the fuel and air. As such, diesel exhaust contains far more unburned fuel than gasoline exhaust, which means more soot production.
New EGR (Exhaust Gas Recirculation) diesel engines run hotter and produce more soot and acids than previous diesel engines. Most engine builders have decided that cooled EGR is the most effective way to reduce combustion temperatures. Small amounts of exhaust gas will be piped into the chambers to displace oxygen. Less oxygen means cooler combustion. Before going to the cylinders, some of the exhaust gas heat is absorbed by the engine’s coolant, thus the term “cooled EGR”. Federal regulations now require diesel engines to produce about 50% less nitrous oxide than current models. Despite reducing nitrous oxide, however, these newer EGR motors produce more heat, soot, and acid during combustion.
Excessive soot formation in oil has many possible causes: poor fuel spray patterns, worn out piston rings, clogged or dirty injectors, and idling, as well as incorrect air-to-fuel ratios. 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-to-air ration.
In general, soot particles are small in size and shape. For example, diesel engine soot particles are about 1/30 the diameter of a human hair. The individual soot particles aren’t much cause for concern because the majority leaves through exhaust, but some will end up in the engine oil as it passes the cylinder rings. Once in the engine oil, however, soot can cause engine damage as it accumulates into larger and larger particles which can cause wear on the internal engine parts.
Engine motor oil does contain dispersants designed the reduce the likelihood of this accumulation, but in conditions where soot is much higher (such as EGR engines or engines with poor fuel mixtures), the dispersants just can’t keep up with the soot build-up. High soot levels lead to loss of oil dispersancy as the oil’s dispersant additives are consumed. As dispersancy is lost, soot particles accumulate and form larger particles that build up on engine surfaces. This build-up of soot and sludge will eventually slow and impede oil flow.
Soot and sludge build-up can also form on oil filters, which over time will block oil flow and allow dirty oil back into the engine. Additionally, high soot levels within motor oil increase the oil’s viscosity, further impeding 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.
When soot levels are especially high, carbon particles can form 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 the liner increases. Combustion by-products such as gases and unburned fuels blow into the crankcase, a problem known as “blow-by”, eventually causing expanding gases to lose their ability to drive 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.