Soot Happens

Excessive soot levels in oil can cause engine issues

All internal combustion engines produce soot as a result of incomplete fuel combustion. However, because of the way that fuel is injected and ignited, soot formation occurs more commonly in diesels than in gasoline engines. With gasoline engines, the fuel/air mixture is injected during the intake stroke and ignited with a spark, according to officials at Growmark’s FS Energy division ( In diesels, the fuel/air mixture is injected during the compression stroke and ignited spontaneously from the high pressure in the combustion chamber.

Growmark is a regional cooperative providing agriculture-related products and services. FS Energy produces agricultural and energy-related products.

The less air that is present, the more favorable the conditions for soot accumulation, they say. Combustion is more efficient in gasoline engines because the air and fuel have a chance to more thoroughly mix than it typically does in diesel engines.


When fuel is combusted in the engine, some of the diesel fuel cannot completely combine with oxygen and that leaves behind small unburned particles of carbon, explains Mark Betner, manager of heavy duty lubricant products for Citgo, a refiner, transporter and marketer of transportation fuels, lubricants, petrochemicals and other industrial products ( These small carbon particles accumulate in the crankcase oil as the engine piston reciprocates during the engine cycle.

Over time, soot will build up in the oil and may lead to problems, he points out. The soot that accumulates in the oil cannot be eliminated by the oil or completely trapped by the oil filter so keeping the soot under control is the challenge.

Engines that have improper fuel combustion or have malfunctioning fuel injectors can cause additional buildup of soot, he adds. Also, operating conditions, such as excessive engine idling or lugging the engine, can increase the level of soot.

The overall result of these factors, says Betner, is reduced combustion efficiency resulting in higher levels of soot particles forming in the engine oil.


In most modern well-maintained diesel engines, the majority of soot will be oxidized within the combustion chamber or later trapped and oxidized downstream in the emissions system, add officials with specialty chemical company Lubrizol Corporation ( However, some soot escapes and gets past the piston rings and ends up in the crankcase oil and soot loading in diesel engine oil can present wear problems.

As the piston goes down for every power stroke, soot can accumulate on the cylinder liners of each bore and can be scraped down by the oil control piston rings, they explain. Soot can be further delivered to the crankcase via blowby of combustion gases past the piston rings, especially if they are worn.

Additionally, the thin motor oil film retained on the bores can partially break down under combustion heat, leaving more soot.


Engine oil is designed with a soot dispersant additive which disperses the soot particles, Citgo’s Betner says. Dispersing the soot particles and preventing the natural tendency for the soot particles to join together prevents, or at least reduces, several lubrication related problems during the engine oil service interval.

There are a number of consequences caused by the buildup of soot in engine oil.

High soot levels in the oil can cause a loss of dispersant additives and ultimately form what is known as sludge, say FE Energy officials. As the dispersants become depleted, the soot particles clump together and attach themselves to engine surfaces. This leads to reduced lubrication due to impeded oil flow through the engine. Particle clumps can also form on oil filters, blocking oil flow and allowing dirty oil into the engine.

This content continues onto the next page...

We Recommend