Why more lube oil consumed by an engine?

Oil consumption in an engine can occur in two ways – either by the oil being “burnt” within the cylinders, or by external leakage. The first condition is usually indicated by a “smoky” exhaust, especially when accelerating after a period of engine over-run, or under heavy load. The second condition is usually more obvious, as oil leaks appear around engine seals (front crankshaft and rear main bearing seals) and gaskets, such as timing cover, sump, rocker/cam covers, etc. Listed below are some of the conditions that can develop during an engine’s life span, along with possible effects on engine oil consumption.

Piston rings: Piston rings have two main functions: to maintain compression within the cylinder and to “scrape” excess oil from the cylinder walls. The compression rings, usually two per piston maintain the cylinder compression by acting as a seal between the piston and cylinder wall. The compression rings not only rely upon residual “wall tension”, they also utilise gas pressure, acting behind the rings (between the ring and its groove), to provide the necessary pressure to maintain compression. The second ring also provides some scraping action to assist the oil control rings. The oil control rings remove excess oil from the cylinder walls to prevent it from being “burnt” during the combustion process. In order for the oil scraper rings to function, their ring grooves have a series of oil drain holes to return the excess oil “scraped” from the cylinder walls back to the engine sump.

Because of these drain holes, oil rings are not assisted by gas pressure to provide the necessary scraping action. They have to rely on their in-built wall tension, plus additional tension provided by the piston ring “expander” for the control of excess oil. When ring wear occurs, the compression rings are no longer able to completely retain the combustion gases and a percentage, known as “blow-by gases”, pass down the side of the piston and cause excess “crankcase pressure”. This crankcase pressure tends to “blow” the oil mist within the engine past both internal and external oil seals, causing oil consumption problems.

Internal leakage occurs when oil leaks past valve stem oil seals. This allows oil to enter directly into the combustion chamber where it is burnt. External leaks will be evident around engine seals and gaskets, especially where these may be worn or deteriorated through age. The crankcase pressure  caused by “blow-by” also tends to push the oil mist in the crankcase up into the cylinders where the already worn rings have difficulty in preventing it from entering the combustion chamber and burning. Similar results occur in the case of broken piston rings, however the problems are usually more severe with broken rings. The amount of “blow-by” is increased considerably as the broken ring/s cannot maintain any pressure on the cylinder wall to control the excess oil. When the oil control rings wear, they have difficulty controlling any excess oil on the cylinder walls. This lack of control allows the excess oil to pass the compression rings and to be burnt in the combustion chamber.

Pistons:Worn pistons can also contribute to high oil consumption. Any wear on the piston skirt results in the piston not accurately following the cylinder bore contours. This “irregular” motion of the piston as it moves up and down the cylinder bore results in “flutter” of the rings and loss of oil control, allowing “blow-by” as described above. Worn piston ring grooves also allow “ring flutter”, along with loss of gas pressure behind the rings.

This contributes to “blow-by” and its inherent problems. Cracked and/or broken ring lands result in complete loss of ring pressurisation by the gases and virtually complete loss of ring control within the piston itself, resulting in a high degree of “blow-by” in the cylinder concerned. Piston rings that have become “stuck” in their grooves also contribute to “blow-by” as they do not allow freedom of the rings to maintain cylinder wall tension. Problems of ring sticking are usually associated with over-extended oil drain intervals and/or dirty oil and filters.

Cylinders: Any problems that results in any distortion of the engine cylinders can have an effect on engine oil consumption. Long-term wear of cylinders results in uneven, tapered or out of round bores and, coupled with the piston problems already described, contributes to loss of oil control and “blow-by”. Cylinder distortion can also be caused in extreme cases by cooling system problems, where “hot-spots” caused by poor circulation, or abnormal wear due to over-cooling, result in poor piston-to-cylinder relative movement and loss of oil control.

Engine bearings

Worn or damaged main and, more predominantly, big-end bearings result in an increased amount of oil being thrown onto the cylinder walls. This in turn increases the job that the oil rings have to do to control this excess oil and prevent it from passing the rings and entering the combustion chamber and burning. The amount of oil thrown onto cylinder walls and piston under-crowns is critical as it is the only method of lubricating piston skirts, rings, gudgeon pins as well as providing piston under-crown cooling. Problems with either too little or too much lubrication often due to excess bearing clearances, means that the piston rings are unable to control it and increased oil consumption results.

Valve mechanisms:

Any wear in engine valve stems, valve guides or valve stem oil seals, especially the inlet valves, results in oil being drawn past these components directly into the combustion chamber where it is burnt, resulting in increased oil consumption. A secondary problem of inlet carbon buildup can occur when oil is drawn past the inlet valve stems/guides/seals. This carbon build-up in the inlet port and the back of the inlet valve results in loss of engine performance and increased fuel

consumption.

Increase in crankcase pressure  :any increase in crankcase pressure(For clogged PCV Valves) results in increased oil consumption due to its effect of forcing the fine droplets comprising the oil mist past piston rings and valve stem oil seals. This increase in crankcase pressure also contributes to external oil leaks as the fine droplets are forced past crankshaft seals and engine gaskets. PCV valves “clogged” with dirty engine oil and sludge cause problems with engine breathing and build up of crankcase “blow-by” pressure and thus contribute to oil consumption problems as well as leaks.

Dirty Engine Oil – Dirty engine oil, associated with over-extended drain intervals coupled with poor oil filter maintenance (not being changed on a regular basis), can have quite serious short-term and long-term effects on an engine and its oil consumption. In the shorter term, dirty oil can clog PCV valves and cause the

problems already outlined when these components do not function correctly. In the long-term, dirty oil can contribute to piston ring sticking and can even contribute to serious engine wear as it thickens and its ability to flow is reduced. Both of these situations are conducive to the generation of “blow-by” and the end result is again increased oil consumption.

Fuel Dilution :When oil is diluted with fuel its viscosity is initially substantially reduced, and this “thinning” of the oil, with its lower surface tension, allows it to flow much more freely. This means that it has the ability to more easily leak past valve stems and valve guides and stem seals, and also to leak past external engine seals, contributing to higher oil consumption.

Low Oil Viscosity :Similarly, the use of an engine oil with a lower viscosity than that recommended by the engine manufacturer can also contribute to increased oil consumption, as it is able to more easily leak past internal and external seals. If wear occurs in these areas from the use of oil with a lower viscosity (either too low initially, or from fuel dilution) then the problems of “blow-by”, described above, can take place.

High Oil Viscosity :The use of an oil with a significantly higher viscosity than recommended by the manufacturer can have long-term effects on the engine, especially at start-up when, due to the thicker oil’s resistance to flow, initial oil starvation to critical engine components may occur. This can cause wear over the long-term and this wear can result in the same problems of “blow-by” and result in increased oil consumption as the engine ages.

Engine Sump Overfull : Running an engine with more oil in the sump than it is designed to utilise results in more oil being splashed up onto the cylinder walls by the crankshaft and big-end bearings. This then means additional work for the oil control rings to cope with this excess oil, contributing to increased oil consumption.