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Turbo Failure!
Major Causes of turbo damage
Major Causes of Damage:
Most turbocharger damage can be traced to one of a few basic causes:
- Lubrication Related Problems
- Foreign Object Damage
- Operational Temperature Extremes
Careful examination of the parts from a damaged turbocharger and proper interpretation of their condition will usually pinpoint the cause of a unit´s problem. If a turbocharger is ruined by an external fault and that fault is not found and corrected, the replacement unit will soon fail in the same way as the original.
Lubrication Related Problems:
Insufficient Lubrication
Lack of proper lubrication can ruin a turbocharger within mere seconds of operation. With shaft rotation speeds approaching 200,000 rpm in some models, all bearings must receive a supply of oil that can stabilize, lubricate and cool. When lubrication is slowed, stopped or interrupted for any reason, metal to metal contact first occurs at the journal bearings/shaft journals/bearing bores of the center housing. Without the oil, friction will generate enough heat to turn the shaft and/or thrust collar blue (This bluing is often not as evident in units with liquid-cooled center housings.) The thrust bearing, thrust collar and/or backplate mating surface will also show signs of damage. Once the bearings are damaged, "shaft motion" increases. Instead of rotating in a perfect circular motion, the shaft begins to orbit. As bearing clearances are enlarged, shaft motion becomes more intense, and damage accumulates. The center housing seal bores, shaft hubs, and thrust components (thrust collar, thrust spacer, and backplate assembly/thrust bearing, as applicable) are damaged next. This additional damage, in turn, allows for more violent shaft motion. After shaft motion reaches a certain point, the turbine and compressor wheels will contact their respective housings damaging the blades and possibly backing off the shaft nut. When blades are damaged by housing rub, the precise balance of the wheel/rotating assembly is destroyed, leading to even more shaft motion. The shaft may eventually break from a combination of motion stress, fatigue and destabilization (due to the original problem, insufficient lubrication).
Foreign Material In Oil:
Foreign material in the oil includes various sizes of abrasive particles, corrosive chemical compounds, and dilution by coolant or fuel. Foreign material in the engine lubrication system first damages the journal and thrust bearings. When foreign material in the oil is found to be the primary cause of damage, steps should be taken to identify the foreign material or eliminate its entry into the system.
Abrasive Particles:
Fine abrasive contaminants will score and wear virtually every bearing surface . If material particles are large enough, the contaminate related damage is more confined to journal bearing outside diameters and center housing bearing bores because centrifugal force keeps the contaminants away from the spinning shaft.
Corrosive Chemicals;
Corrosive chemical compounds alter surface finishes and clearances. When surface finishes are too rough, oil films are penetrated allowing metal to metal contact.
Diluted Oil;
Diluted oil has its film strengths reduced and does not easily adhere to surfaces. The lack of surface adherence does not allow the shaft to be dampened and supported by its encompassing oil film.
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How do I know if my turbocharger is in need of service?
If you have noticed a drop off in the performance or economy of your turbocharged car, and suspect the turbo might be the problem, there are several ways to inspect your system for proper operation. Assuming that the air filter, exhaust system and fuel system are in good operating condition, you can focus directly on the turbocharger itself. You will need to remove the air filter system to gain access to the front of the turbocharger, making notes on any amounts of oil or loose connections you might find.
Once you can visually see the compressor wheel, look for the following signs: You should find a clean, oil-free wheel, without any signs of nicks, dings, or evidence of rubbing between the wheel and the compressor housing. Making sure the turbo is cool enough to touch, check to see if the turbocharger rotates freely, with minimal amounts of up and down movement. The wheel will have some movement in the bearings in a radial motion, but there should never be enough play to allow the wheel to contact the housing. Next, you will want to push and pull on the compressor shaft feeling for axial movement. There should never be any movement that you can feel in this direction; the nominal clearance is usually less than .003”. If it passes inspection on the compressor side, it is time to move over to the exhaust side. You will have to remove the exhaust pipe all the way at the turbocharger, and be able to see the turbine wheel.
The wheel should be fairly clean, without any signs of wetness from oil, large amounts of carbon build up, etc. The wheel should be very dry to the touch, without any scale or other contaminants built up on its surface. There should be no signs of rubbing or damage to the blades. The turbine wheel shaft directly connects it to the compressor wheel, so spinning one should also spin the other. While you are looking at the exhaust side of the turbo, inspect the housing for any signs of cracks or leaks, any of which can cause a lack in performance.
The usual signs of a turbo that is failing will be the accumulation of oil on one or the other housings, excessive bearing movement or wheel to housing contact.
How does the Turbo work?
TURBINE
A turbine wheel is driven by the otherwise 'waste' exhaust gases. The turbine is connected to a compressor wheel and the whole assembly rotates at speeds up to 230,000 rpm.
COMPRESSOR
Ambient air is drawn into the turbo via an air filter, then, as it passes over the compressor wheel it accelerates up to very high speed.
Once past the compressor wheel it passes through a 'diffuser' which is formed between the face of the backplate (or centre housing) and the machined face of the compressor housing.
As it passes through this area the air moving at high speed slows down and this continues as it passes though the 'volute' of the compressor housing (shaped like a snail's shell!). This turns the Kinetic energy of the moving air into high pressure air.
Turbocharger Gas & Air Flow
Turbocharger Gas & Air Flow - On Engine
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