Causes and issues relating to blow-by and soot build-up

Sep 29, 2020 | soot build-up

Causes and issues relating to blow-by and soot build-up

Soot build-up – Soot is not only detrimental to the atmosphere but can also cause major issues within engines and is not something that can be taken lightly.

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

Of soot retained in the engine, and primarily in the lubricant, 3 per cent is attributable to blow-by gases and the remainder results from piston rings scraping away soot deposits in the cylinder, which end up in the sump. This is then transported around the engine where it can be deposited on various operating components inhibiting performance and even leading to breakdown.

Total fluid management provider Techenomics provides the following information about blow-by and soot build-up prepared by Australian laboratory manager Sahar Nazari .

soot build-up

There are a number of reasons blow-by can form in different scenarios and understanding each of them has implication of your engine maintenance.

Cold starting:
Blow-by has an effect on the amount of trapped air. Owing to low lubricant temperatures during cold starting, expansion and contraction of different engine components takes place which impacts on the clearance between piston rings/liner and the cylinder. When the clearance increases there is more possibility of blow-by losses.

Initial cranking speed:
The lower the cranking speed, the greater time is available for heat and blow-by losses. At extremely low engine speeds the compression temperature is lower than it is at higher speeds and ignition delay is prolonged. Experiments show that a minimum cranking speed exists, below which starting is unviable.

These situations confirm the importance of maintaining high cranking speed under cold starting conditions, even though this results in shorter available time for pre-ignition reactions around TDC, as an effective means of achieving relatively high compression pressure and temperature.

As is the case with compression ratio, a reduction in the minimum starting temperature can be achieved for engines with an increased cranking period and this most probably requires a higher capacity battery. In contrast, whenever conditions of starting are favourable, an increase in the cranking speed does not have much effect.

Thermal stress failures:
Failures due to excessive thermal stress are a common cause of engine breakdown under highly rated conditions. Under extreme conditions – such as burning of exhaust valves following exhaust gas blow-by, or burning of piston crowns or cylinder heads through contact with outflow jets from a pepper-pot pre-chamber type of combustion chamber, or as a result of ‘detonation’ when a diesel engine is operated on gasoline – metals may be removed in appreciable quantities.

Oil deterioration:
Diesel engine lubricants may deteriorate in a variety of ways. They may become contaminated by carbonaceous particles from incomplete combustion of the fuel, unburnt fuel, acidic water from blow-by gases, seawater in marine diesels, oxidation products from the lubricating oil, ash from lubricating oil additives, metal particles from wear of metal parts, or from adventitious matter such as road dust. The viscosity may increase owing to oil oxidation and this will certainly increase as the content of suspended oil-insoluble increases. Any additives will be depleted in the normal course of use.

Engine wear:
Wear can occur on any of the surfaces in relative motion, i.e. piston bearing surfaces, bore, rings, grooves, valve train, main and big-end bearings. However, the important wear issues that control how long an engine can run between overhauls are: the bore at the upper limit of top ring travel; the radial wear of the top ring with resultant increase of ring gap and consequent blow-by; and the increase of side clearance of the top ring due to ring and groove wear.

This wear may be due to abrasion by engine deposits or by foreign matter that enters through the air intake or the crankcase breathers. Abrasion from engine deposits usually increases with an increase in the severity of operating conditions, however, that due to adventitious matter is usually independent of operating conditions.