Getting to the bottom of soot build-up

Oct 21, 2020 | soot build-up, soot damage to engines

Getting to the bottom of soot build-up

Techenomics has the expertise to find the reason and provide treatment for soot build-up
Soot is harmful to engines and if left to accumulate can impact on productivity through poor performing components and can even lead to component breakdown or equipment failure.

At excess levels, soot causes abrasion and high wear in engines and mechanical components, and if it forms clumps, can result in valve train, ring and liner wear. It can also thicken the oil and increase viscosity, which inhibits oil flow.

There are a number of reasons blow-by can form in different scenarios and understanding each of them has significant implication for engine maintenance which makes Techenomics’ oil analysis and fluid management expertise an important tool in overcoming the problem and preventing costly equipment breakdown.

Of soot produced within an engine, just 29 per cent reaches the atmosphere through the exhaust pipe with the remainder deposited on the cylinder walls and piston crown.

With soot retained in the engine most is in the lubricant and 3 per cent of the accumulation is attributable to blow-by gases while the remainder generally results from piston rings scraping away soot deposits in the cylinder, which then end up in the sump. This is then transported around the engine where it can be entrained into component contacts.

Techenomics Australia laboratory manager Sahar Nazari provides details about the reasons for blow-by and soot build-up.

Rings:
Rings are traditionally divided into three types, according to function. Counting from the top of the piston, the first and second rings are compression rings with the task of controlling blow-by.

The middle ring is the scraper, which keeps excess oil from the combustion space, and the last ring is the oil ring, which is serrated to deliver oil to the bore.

Elevated combustion pressure contaminates the oil with blow-by gases and promotes oxidation by raising crankcase oil temperature. That fraction of the oil diverted to the turbocharger can undergo a temperature rise of 80°F in its passage over the bushings.

Compression rings take over the function of sealing and dissipating heat to the cylinder wall and, in addition, are involved in regulating the lubricating oil supply. The sealing function primarily entails preventing combustion gases from passing from the combustion chamber into the crankcase.

Increased blow-by consisting of oil particulates endangers pistons and rings by overheating; disrupts the lubrication on the cylinder wall; and adversely influences the lubricating oil in the crank chamber. If the blow-by gases in an intake system cool down, then the oil particulates usually separate onto the walls.

Oil consumption and blow-by:
blow-by Blow-by, like oil consumption, is a characteristic on which the technical excellence of an engine is based. If oil consumption is satisfactory, blow-by will normally not be a problem although there are cases where excessive blow-by can improve oil consumption by effectively blowing oil down through the ring pack into the crankcase.

This is one area where analytical studies have assisted the ring pack designer to ensure that ring gaps and inter-ring volumes provide a controlled acceptable flow level of combustion gas (blow-by) through the compression rings and preventing reverse flow into the combustion space, which increases oil consumption.

Generally, excessive blow-by seems to be of main concern in high speed engines and even here, the end user takes a widely different view of acceptable levels.

Oil separators:
Combustion products in engine oil seldom only take the form of solid particulates. Rather, blow-by gases consisting of oil particulates, fuel vapour and steam also reach the crankcase chamber through the piston rings. They are drawn off through the separation system into the engine intake system together with oil vapours at high temperatures.

If the blow-by gases in an intake system cool down, then the oil particulates usually separate onto the walls. In this process, the oil vapours initially arrive at a steel wool mesh in which larger oil droplets are separated, while the excess pressure in the intake system feeds drawn off oil vapours to the intake air through a diaphragm. This serves to maintain a constant vacuum in the crankcase, thus reducing oil leaks.

Crankcase pressure:
High crankcase pressure is usually caused by excessive blow-by – gas pressure in the cylinder blowing by the piston rings and into the crankcase. A high-pressure condition indicates the engine is in poor condition.
High crankcase pressure is usually used only as an alarm function.

Severe load:
Under severe load, blow-by gases accumulate faster than they can be vented and escape through the breather filter. These flow reversals, which occur more frequently in turbocharged engines, tend to clog the filter.

Restrictions at the filter allow corrosive gases to linger in the crankcase. A partially functional filter can also pressurise the crankcase under the severe blow-by conditions that accompany heavy loads. This can cause oil seals and gaskets to leak.