Due to the variables in engine design and operating conditions, internal engine oil consumption complaints are often the most difficult to solve. In some cases, oil consumption might be more severe under low-speed operation, in other cases, high-speed operation and, in some cases, intermittent. But, in any case, excessive oil consumption can drastically reduce oxygen sensor and catalytic converter life. In the following text, I’ll deal with some of the more common reasons for internal oil consumption including mechanical and lubrication failures. See Photo 1.
To better understand modern oil consumption issues, let’s begin by taking a look at modern piston ring designs. For example, modern engines use lower-tension piston rings to help reduce rotating friction. And while low-tension piston rings reduce cylinder wall and piston ring wear, varnish and carbon deposits also tend to make them stick in their ring lands, which can cause compression loss and oil consumption.
The design of the top and second compression rings can be better understood if we imagine trimming the outer edge from a saucer, leaving a slightly dish-shaped piston ring that allows the bottom edge of the ring to scrape oil from the cylinder wall during the intake stroke. During the power stroke, the ring flattens against the piston ring land to seal combustion gases inside the cylinder. See Photo 2.
Due to the concentration of heat at the top of the piston, the top piston ring is often filled with molybdenum (moly), which absorbs oil and is scuff resistant at high temperatures. Molybdenum is also very soft, which makes it extremely vulnerable to intake air contaminated with dirt. For this reason, heavy-duty applications often use a chromium-plated top ring for longer life in dusty environments. A steel piston ring can also be used as a top ring in less severe applications.
The third ring is generally a three-piece oil ring consisting of two very narrow “scraper” rings held against the cylinder by a stainless steel expander. In modern applications, the piston ring-to-piston-land tolerances are made very tight to help reduce oil consumption, which again contributes to the possibility of rings sticking in their lands.
To increase piston ring life and reduce oil consumption, modern cylinder wall preparation consists of a honing process that allows the piston ring to quickly seat, but leaves a cross-hatch pattern in the cylinder wall that maintains a film of oil lubrication throughout the cylinder and particularly at the extremely hot upper cylinder area. SeePhoto 3.
Many modern engines use hypereutectic aluminum alloy pistons that have about three times the silicon content of conventional cast aluminum pistons. In brief, this high silicon content tends to reduce piston-to-cylinder wall expansion by containing combustion heat at the top or “dome” of the piston. In effect, the piston dome runs hotter, which increases the engine’s thermal efficiency. And, since the piston skirts run cooler, the piston-to-cylinder wall clearances are reduced to hold the piston rings square against the cylinder wall. While conventional cast pistons are similarly designed to maintain piston ring contact by reducing piston “rock” as it reverses travel in the cylinder, they don’t have the same thermal characteristics as hypereutectic.
Oil consumption past the piston rings is generally typified by the edge of the piston dome being washed clean of carbon by the incoming oil. This wash pattern can be caused by worn out piston rings or by the oil scraper ring sticking in a compressed position. Worn compression rings normally cause excessive combustion blow-by around the piston into the crankcase.
In many instances, this excessive blow-by will overload the positive crankcase ventilation (PCV) valve, which allows the oil-saturated blow-by gases to exit through the PCV inlet into the intake air stream and be consumed by the engine.
In cases of extreme overheating or under-lubrication, the rings can be held in the compressed position by scuffed
aluminum around the piston ring lands. In most cases, worn or stuck piston rings will tend to consume more oil at high engine speeds when oil temperature is at its highest and oil viscosity is at its lowest value. In addition, an engine running at higher speeds tends to flood the piston rings with oil, which makes oil control more difficult.
CYLINDER HEAD DESIGN
Modern OHC, aluminum cylinder heads generally use an alloy valve stem guide insert and alloy valve stems to reduce valve guide wear. In addition, modern aluminum cylinder heads incorporate very efficient valve stem seals to prevent oil from leaking through the valve guide into the cylinder. Although a cylinder head can be flooded with oil due to clogged oil return passages, the condition is relatively rare because most modern designs incorporate very large oil drain passages. Oil consumption through the cylinder head is generally through worn or cracked valve stem seals. This high oil consumption is then produced when engines are operating at high intake manifold vacuum values created under light load, low-speed conditions. Keep in mind that using an incorrect or poorly designed PCV valve can also mimic worn valve stem seals by allowing oil to enter the intake air stream.
While some engines are designed to use “generic” 5W-30 motor oils, others require synthetic-based oils with additive packages that are compounded to deal various engine design issues. As piston and piston ring design changed during the 1990s, many auto manufacturers began requiring lower-viscosity, synthetic-based oils that would lubricate efficiently at either engine oil temperature extreme.
In cold weather, the oil should quickly circulate and efficiently lubricate the upper cylinder area. In hot weather, the oil should protect the pistons against scuffing, the piston rings against sticking in the piston lands and the crankshaft bearings against oil shear wear.
In addition, modern extended oil change intervals require base oils that will not oxidize or deplete their additive packages within the 7,500 or more miles before the oil change reminder light illuminates. Unfortunately, most substitute oils don’t have those base oil properties.
In some applications, the use of an incorrect oil will cause the very narrow piston rings in use today to micro-weld against the cylinder walls at high engine speeds. In other cases, excessive varnish will cause the low-tension oil scraper ring to stick in the compressed position.
Lastly, the low flash points of some substitute oils will cause oil consumption by vaporizing in the extremely hot upper cylinder area. All of these failures will cause internal engine oil consumption when a generic substitute is used in place of the recommended, application-specific motor oil.
Clearly, there’s a lot to be said about using the correct engine oil and changing it at the recommended intervals. Because modern piston rings are especially sensitive to dirt and other abrasive particles, the oil filter is equally important because it must efficiently remove these particles throughout the extended oil change interval.
When dealing with an oil consumption complaint, begin by inspecting the engine for external oil leaks. If no serious leaks are apparent, remove the oil filter cap or possibly the camshaft cover to inspect the engine for internal varnish and sludge. If the engine is heavily sludged, it might be impossible to reverse the damage caused by neglected oil change intervals. In many cases, using an oil flush additive will simply dislodge the sludge, clogging the oil drain passages and the oil pump inlet screen.
Mild varnish conditions can often be remedied by simply changing to the recommended engine oil and changing it at reduced intervals. In addition, inspect the PCV system for clogging and for the correct PCV valve part number. On some applications, it’s possible to remove the spark plug to inspect the internal condition of the cylinder with a borescope or stem light. Here again, the piston dome should exhibit an even coat of carbon across its surface. Oil wash at the center often indicates leaking valve stems, while oil wash at the edges of the piston indicates sticking or worn piston rings.
Whatever the case, always try using the correct oil and recommending adequate engine warm-up and drive times to reduce potential gasoline contamination in the engine oil.
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