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Turbocharger Lubrication


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Turbocharger Lubrication

The problem of lubricating a shaft spinning inside a sleeve-type journal bearing was solved many years ago. No new science was necessary when the turbocharger came along, even though it presented a couple of new twists. The new twists were the tremendous heat in the turbine side and the cumulative damage to the oil by the migration of this heat into the bearing section. The heat deteriorates the oil and quickly makes it unusable. Solutions to these new twists on the oiling problem have always been readily available but have just recently been implemented. Reasons for delay, one presumes, were economics and fear of sales resistance. The economic aspect of the oil problem was the OEMs1 reluctance to increase prices by the amount necessary to put a water-cooling jacket around the bearing section. The sales problem was the reluc­tance of the sales department to tell the consumer he must change the engine oil with greater frequency—fear, I suppose, that the end user would shy away from what incorrectly appeared to be a high-maintenance product. It's just an­other example of the sad state of affairs when sales and accounting overrule sound engineering. The story ends on an upbeat note, however, as virtually all OEM turbo cars now have water-cooled bearings and the recommendation of frequent oil changes. Had this state of affairs existed from the start of produc­tion of OEM turbos, the English language would be missing the less-than-col­orful phrase 'coked-up turbo bearings.' Pity.

Fig. 4-1, A bearing section and shaft with heavy deposits of charred oil can expect an imminent demise. A coked-up bearing is the result of using oil with insufficient high-temperature stability and/or not changing oil with sufficient frequency.

What Causes a Coked-up Bearing?

Coking its nothing more than charred oil residue accumulating in the turbo bearing section to such an extent that the proper flow of oil to the bearing is eventually blocked. The seriously compromised oil flow will kill the turbo in short order. Four things gang up on the turbo to cause a coking problem:

Oil with inadequate high-temperature capability

Oil with a wide multiviscosity range

Extended oil-change intervals

Excessive heat in the bearing section

Dealing with these problems and the mechanics of a clean, cool oil supply is the focus of this chapter.

** RULE: If your turbo suffers a coked bearing failure, check your own main­tenance diligence before cursing the turbo.

Selecting a Lubricating Oil

The selection of a type, grade, and brand of engine oil should be done with some forethought and perhaps even a little R&D. Please weed out. all old family biases toward Rosie's Red Re-Refined 'cause Dad used it, back on the farm and wouldn't hear of usin' nothin' else. Dad may not have changed much over the years, but engine oil and tractors have made progress measured in orders of magnitude.

Here's what you need to do: First, get a feel for what the lubricant is sup­posed to do for your engine and what special requirements your situation im­poses on the lube. These data will tell you what type of oil will best fit your needs. Second, consider the climate and operating conditions the lube must endure. This info tells you what viscosity and level of severity (grade) lube will do the best job for you. In general, it is best to avoid wide-range, multiviscosity oils, as the materials added that create the multiviscosity capability are the same materials that cause the coking. Thus 20W-50 is clearly better turbo oil than 10W-50. A straight viscosity is best of all, with a ten-point higher viscos­ity in summer. If it is possible to determine the detergent rating and antioxi­dizing rating, good turbo oil will be high in these two categories.

Now you know the type and grade of lube that is your best choice. The one remaining factor is the brand to buy. This boils down to availability, price, and what your R&D efforts tell you is truly the lubricant for your engine. One can be relatively certain that an oil formulated for turbo use, and so advertised, will be an adequate lubricant.

Types of Lubricants

There are two choices here: synthetic-based or mineral-based lube.

Synthetic lubes are manufactured fluids (not necessarily from oil) in which the basic structure of the lube is much more rigidly controlled than in stan­dard hydrocarbon oils. The resultant product is a very consistent, stable fluid with uniform molecular structure, whose properties are highly predictable. Synthetics have clearly demonstrated their capability with respect to friction­al losses, high-temperature stability, and basic toughness of the molecular structure. Mineral-based lubes are less expensive and more likely to coke.

Water-Cooled Bearing Housings

The turbo bearing housing with a water jacket around the bearing chamber has virtually eliminated the problem of oil coking. The cooling capability of the water Is such that the oil seldom reaches the temperature at which it begins to break down. Of course, all oil subject to high-temperature use breaks down slowly over time, so the need for periodic oil changes still exists. The oil-change interval thus becomes only slightly less than with an atmospheric engine.

Oil Flow and Pressure Requirements

The turbo survives with surprisingly low oil pressure and flow. It is virtually certain that all engines in production today have enough excess oil-pumping capacity to adequately take on the additional requirement of lubricating the turbo. If you know a particular engine was shortchanged in the oiling area on original design, it is certainly a good idea to fix it. However, fix it for the en­gine's sake and not for the additional burden of the turbo. Observe the basic lower limits of oil pressure and flow published by the turbo manufacturer and you can't go wrong as far as lubrication is concerned.

Too much oil pressure can create problems with turbos. It is possible to force oil past oil seals that are in perfect condition if oil pressure exceeds 65 to 70 psi at the turbo. If a particular engine creates more oil pressure than the seals can handle, it may be necessary to install a restrictor or bypass system to reduce pressure at the turbo.

Problems of oil pressure overpowering the seal are evident in a frequent if not quite constant smoking problem. Anytime oil pressure exceeds the 65-70 psi range and smoking persists, a restrictor or bypass should be installed prior to any other changes.

Fig. 4-2. A water jacket incorporated into the turbo bearing housing and connected to the engine cooling system carries a way most oft he heat that migrates from the exhaust side to the bearing section. Lubri­cating oil is guarded from temperature levels high enough to cause coking.

Fig. 4-3. Temperature comparison of water-cooled and non-water-cooled bearings, illus­trating temperature magnitudes and distributions in the turbo bearing section. The non-water-cooled housing can cause some constant damage to the oil. With 2000-mile oil change intervals, the damaged particles of oil will be removed and not cause coking.

Fig. 4-4. Oil pressure reduction at the turbo by a restrictor. Use of a restrictor requires that pressure available to the turbo bearings be measured and proven

Fig. 4-5. Oil pressure reduction at the turbo by a bypass. This is more reliable than a restrictor in the oil feed line, but the pressure at the bearings must still he known.

These are good guidelines for virtually all turbos:

Min. pressure (psi)

Min. flow (gal/min)

Idle, hot

Maximum load

Oil Coolers

Adding an oil cooler to a high-performance engine is often contemplated in the expectation of improving engine durability. Although it usually does, don't be too hasty to rush out and buy a huge oil cooler without investigating the real requirements of your engine. Oil prefers to operate in a given temperature range that supplies the viscosity needs for protecting the engine, doesn't over­heat the oil on the high end and, when cool, doesn't acid more drag to the sys­tem than necessary. These requirements are all easily met by the right oil type and viscosity operating in the correct temperature range.

Mineral-based oils are not as tolerant of high temperatures as are synthetic oils. For street engines, both synthetic and mineral-based oils have the same lower temperature requirement (l50°F minimum), but synthetics can operate to about 40°F higher (270°F versus 230°F for mineral-based). Therefore, you may need an ail cooler if you use mineral-based oil and perhaps not if you use synthetic.

Fig. 4-6. Oil system fittings and lines must withstand high temper­atures and pressures, the hydrocarbon envi­ronment, and vibration. This oil feed-line instal­lation, with insulation and supporting bracket removed, reveals the sturdiness required, such that oil feed to the turbo is never a problem.

It needs to be understood that oil temperatures below these minimums will degrade durability just as surely as exceeding the maximums. The installation of an oil temperature gauge will tell the whole story. Do that before installing an expensive oil cooler system. There are occasions when both oil and water temperatures are on the high side but neither is out of bounds. This situation is ideal for an oil cooler, which will remove enough extra heat from the entire system to also reduce the water temperature. The presence of a good oil cooler can easily drop the water temperature by 15°. The thermostatically controlled oil system is a good idea: the oil must reach a certain preset temperature before the thermostat diverts it to the oil cooler. Do keep in mind that unlike water cooling systems, the thermostatically controlled oil system will not require the oil to reach the minimum acceptable operating temperature, because the oil thermostat does not block the oil flow but merely diverts it. It has nothing to do with the maximum temperatures either.

Oil Filters

The turbo creates no special filtering requirements. It is certainly within the acceptable range of reason, however, that the real motorhead may want to care for his high-performance engine a bit better than relying on the stock filtra­tion equipment. A wide variety of good components are available.

Oil to and from the Turbo

The plumbing that feeds oil to the turbo and drains it back to the engine is per­haps the weak link in the entire scheme of turbocharging. This is definitely the place for a fifteen-cent part to fail and take oat a seven-hundred-dollar turbo —or, worse yet, an engine bearing. The following should be considered mini­mum requirements. Do a thorough job, and don't hesitate a minute to spend even more $ attempting to establish bragging rights on building the fail-safe turbo lube system.

The oil lines feeding the turbo must meet the requirements of pressure and temperature fuse twice the oil-temperature maximum allowable) and be hydrocarbon-proof. Metal-braid-protected lines are highly desirable from the standpoint of abrasion, chafing, and vibration resistance. Use caution in allow­ing the metal braid line to touch anything, as it will frequently damage the oth­er item If relative motion exists. For example, a stainless steel braid line rubbing on an aluminum valve cover will abrade a slot right into the cover. An­chor the oil line in several places to eliminate relative motion, and support the end fittings. Support of the oil lines near the end fittings will eliminate fatigue-induced failures of these fittings.

Fig. 4-7. Tap: The oil-line brace at the frame forces the oil line and fittings to carry loads induced by engine rack­ing. The motion must be absorbed by the short distance 'A'; therefore, the loads are potentially large and damaging. Bottom: With the brace attached to the engine, the fittings will not experience any bending load. Alt flexing of the oil line occurs over the long, flexible portion 'B', inducing only low stresses and helping to eliminate failures.

Oil Drain System

Oil return line design is even more stringent than oil feed line design. Even the position of the turbo relative to the engine should take into account the require­ments of positioning the oil drain line. The turbo must be positioned high enough to allow a downhill drain to the oil sump. The focus of the problem is that the oil seals in the turbo do not operate well if they are completely bathed in oil. Oil that has passed through the turbo bearings must be free to drain out quickly and without any serious restriction, Gravity is the only force available to rid the bearing section of oil, and gravity is, by all relative standards, a wimp.

The layout of the oil drain system has a few fundamentals that should be ob­served:

Exit angle from the turbo. Virtually all turbos allow a 360° rotation of the bearing section relative to the exhaust and intake housings. This is to permit a near-vertical downward alignment of the oil drain hole. Vertical is the ideal alignment, but where necessary, the deviation may be as great as 30°.

Size of drain hose. Where possible, a minimum inside diameter of 1/2 inch should be observed. It is frequently necessary to compromise the 1/2-inch ID, and this is permissible when other factors are favorable. For example, a 1/4-inch ID restriction at the fitting where the oil passes back into the engine may work just fine, but it is unlikely to work at all at the turbo end of the line. Keep in mind that no oil pressure exists after the bearing, and low-pressure Flow re­quires much greater flow area for equal flow rates.

Routing of the drain hose. Ideally, the drain hose should swoop smoothly downward and arc gently over into the oil pan with no kinks, sharp bends, or rises. Equipment hanging off the side of the engine rarely permits the ideal to be achieved.

Fig. 4-8 Oil inlet and outlet drain positions. These must always be within 30° of vertical to assure a gravity drain from the bearing section back to the oil samp.

Fig. 4-9. A variety of methods exist to attach the oil drain piling to the sump. Sturdiness and the least number of parts determine the joint's merit.

Effort and forethought are necessary here. Keep the hose clear of heat radiated from the exhaust housing and manifolding. Insure that it is not subject to damage from road debris or is suitably protected.

Special Requirements for Low-Mounted Turbos

Situations frequently dictate mounting the turbo so low in the chassis that gravity drain back to the oil pan is out of the question. While gravity is still the prime mover to get the oil out of the bearing cavity, a sump or small reservoir immediately below the turbo will be necessary to collect the oil which can then be returned to the engine oil sump via a pump system. Perhaps the cleverest de­vice in this circumstance is the oil-pressure- powered scavenge pump. The oil flow to the turbo is used to power a pump that in turn scavenges the oil sump.

Oil System Aids

A wide variety of devices on the market endeavor to provide oil flow to the tur­bo bearings when the engine is not running.

Fig. 4-10. An oil sump is inquired for a low-mounted turbo when a gravity drain is not possible, Electrical and mechanical pumps can do the job well. High flow capability should be avoided, as the risk of cavitation will be greater.

These mechanisms are attempt­ing to solve three basic problems, as perceived by their designers:

supplying lubricant to the turbo prior to start-up, to replace oil that drains away while the turbo is stationary

supplying lubricant to the turbo after engine shut-off stops the oil pump

pumping a given amount of oil through the turbo after shut-off, to help remove heat from the bearing cavity, reducing the oil's tendency to coke

White all these intentions are honorable enough, there are a few flaws in the scheme;

All of the oil does not flow out of the turbo bearing. Further, the turbo does not leap into action on start-up. Rather, it achieves a rotational speed at idle similar to that of your ceiling fan.

When an engine is turned off, the instant the spark is discontinued, heat available to the turbo for its driving power is removed, and the turbo Stops. Generally, the turbo will stop before the engine's rotation ceases, A nonrotating turbo needs no lubrication.

Removing heat from the turbo is always a good idea. However, a turbo that is already air cooled, oil cooled, and probably water cooled is going to enjoy little extra benefit from one more quart or so of oil pumped through it to cool it. Not cost-effective.

Determine precisely what an oil system aid will do for you and for the man­ner in which you operate your automobile. If the aid suits your needs, buy it, and good luck to you.

And Furthermore . . .

What is all that jazz about coking your turbo bearings?

Although I tend to think journalists are responsible for coked turbo bear­ings, it might be that never changing the oil is a more likely culprit. In actual practice, if one lets the engine idle for 30 seconds before shutdown, changes the oil every 2000 miles, and uses high-quality oil, turbo lubrication failures aren't going to happen. Water-cooled bearings assure that bearing housing

temperatures never reach the oil breakdown temperature. Please resist the idea of 'oilers and lubers' ( oil system aids) as the savior of turbo bearings. The advertised merit of these devises is. based on fallacious information. In my opinion, they are worthless.

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