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CliffBrown CliffBrown is offline
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Join Date: Feb 2004
Posts: 1,317
What others say about turbocharger failures
caused by hot shutdowns & oil lag.

Operators and owners can help themselves to get maximum service life from their turbochargers if a few good practices are followed:-

Start Up
When starting the engine use minimum throttle and run in idle mode for approximately one minute. Full working oil pressure builds up within seconds but it is useful to allow the turbocharger moving parts to warm up under good lubricating conditions. Revving the engine within the first few seconds of start up causes the turbocharger to rotate at high speeds with marginal lubrication which can lead to early failure of the turbocharger.

Shut Down
Before shutting your engine down, let the turbocharger cool down. When an engine runs at maximum power/high torque, the turbocharger is operating at very high temperatures and speeds. Hot shut down can cause reduced service life which is avoidable by a minute or two of idling. Most mobile equipment applications include an adequate cooling period during parking or mooring procedures.
Courtesy Holset Turbochargers

A failure can occur if the lubricating oil being supplied to the turbocharger is not sufficient to lubricate the thrust and journal bearings, stabilize the journal bearings and shaft, and cool the bearing and journal surfaces, even for periods as short as 5 seconds
Courtesy Mining News

Many turbocharger failures are due to oil supply problems. Heat soak after hot shutdown can cause the engine oil in the turbocharger and oil lines to "coke.''
Courtesy Auto Zone

Preventing Turbo Failures
Warm up the engine for two to five minutes prior to throttling up the engine. This procedure assures proper oil pressure to the turbocharger prior to operation under load conditions. Let the engine idle for approximately two minutes prior to engine shut down. This cool-down period prevents oil coking and oil varnishing on the turbine wheel and shaft. Varnishing is a build up of oil on the shaft which increases clearances and decreases the flow of oil to cool and lubricate the shaft.
Courtesy Rick Hehman and David Keiner Schwitzer U.S. Inc.

The most common failure of automotive turbos is due to hot shutdown. This occurs when the vehicle has been running at a constant speed for a period of time and the vehicle is shut off before the turbo has had time to slow down. A turbo can spin at speeds exceeding 100,000 rpm, the faster the vehicle goes or the harder it works, the faster the turbo will spin. If a vehicle is shutoff suddenly the turbo will continue to spin without oil. Each time this occurs, the life of the turbo is shortened because of wear occurring from no lubrication. Eventually there will be enough wear to allow one of the wheels on the turbo to contact its housing. This causes the wheel to be out of balance. This causes even more contact and the turbo is usually destroyed. Allowing the vehicle to idle for a few minutes after its been running hard or allowing the exhaust temperature to cool to below 500 degrees will greatly reduce the risk of premature turbo failure.
Courtesy 01 Motors

1.Operate the engine above idle only after normal engine oil pressure has been established. Revving the engine immediately after start-up can force the turbocharger to operate at maximum speed before the bearings are adequately lubricated. A turbocharger running with insufficient lubrication can suffer some amount of bearing damage. Repeated occurrence may lead to premature turbocharger failure.

2.Take steps to reduce temperatures and speeds from their maximums before shutting the engine down. turbochargers operate at speeds and continuous temperatures that are higher than most other machines. When an engine is run at maximum power/torque output, turbocharger speeds and temperatures are also at maximum. Problems can be caused for the engine and more so for the turbocharger when the engine is shut off at this point. This may require operation at medium idle or at "light load" conditions while still maintaining full engine oil pressure and airflow over the cooling system. Following these guidelines will prevent prolonged turbocharger rotation without lubrication and the formation of carbon deposits that can form as residual oil is decomposed in the bearings and center housing (heat soak-back).
Courtesy Joss Elliot "All about turbochargers"

Wyant has a final thought on engines: avoid hot shutdowns. "Don't turn the machine off without letting the engine idle down," he says. "If you're an owner, and your excavators are equipped with ECM engine monitoring systems, you need to check the data they output and remind your operators not to do this. A hot shutdown is a sure-fire way to burn up the engine's turbocharger. Remind your operators that when they shut the engine off, the oil pump shuts down as well. The engine's turbo is still spinning at full speed, but it's not getting any lubrication. It's an expensive item to repair, but a repair job that can be easily avoided with a little diligence on your part."
Courtesy Equipment World

Causes of failure by type and corrective measures:
A. Lack of lubrication and/or oil lag.
1. This type of failure occurs when the oil pressure and flow is insufficient to:
a. Lubricate the journal and thrust bearings.
b. Stabilize the shaft and journal bearings.
c. Reach bearings before unit is accelerated to high speeds.
2. The turbocharger bearing's need for oil increases as the turbocharger speed and engine
load increases. Insufficient oil to the turbocharger bearings for period as short as a few
seconds during a heavy load cycle when shaft speed is high will cause bearing failure.
Courtesy Betan Turbochargers Specialists

The oil supplied to the turbo provides not only the bearing wedge, it also acts as a heat sink, assisting in maintaining workable temperatures within the bearing housing. One of the most common causes of turbo bearing failure is a result of the lube oil's exposure to extremely high heat. When a turbocharged diesel is run under heavy load for extended periods, it must be allowed to cool off before being shut down. If this turbo cool-off procedure is not observed, a process known as carbonizing occurs. In this process, the lube oil left in the turbo bearing journals literally cooks, leaving behind an abrasive carbon deposit. The next time the engine is started, this gritty substance scores the bearings and clogs oil-supply ports and drains, dramatically shortening the life of the turbocharger and perhaps the engine. When running under heavy loads, these unfortunate circumstances can be forestalled by idling a turbocharged diesel for five minutes before shutting down.
Courtesy Ocean Navigation

In the case of turbocharging, the problem is more the effect of the turbocharger turbine rotating at high speed for up to two minutes after the engine has been shut off. The problem here is twofold. First, at shutdown, the turbine spins in an unrenewed oil supply. Hot, and no longer under pressure, the oil is forced off the turbine bearings, leaving these closely machined tolerances to run in a diminishing oil film. In short order, the turbo bearings wear themselves out, the turbine no longer fits its housing properly, and things begin to deteriorate rapidly.
Herein lies the reason most automotive manufacturers request that the engine be run at idle for two to three minutes before being shut down in an effort to help slow and cool the turbine and its bearings. The hidden enemy here is heat soaked oil, which develops a condition referred to as "coking." Basically, coking is the effect of the oil burning, not unlike burnt pudding; it forms a sticky film that is scorched to the surface. And, though small in total volume, this oil's lubricating qualities are destroyed; then it mixes with the engine's principal oil supply, slowly contaminating it, resulting in the destruction of the engine's lubrication protection.
Courtesy Lubrication Research

Following a hot shutdown of a turbocharger, heat soak begins. This means that the heat in the head, exhaust manifold, and turbine housing finds it way to the turbo's center housing, raising its temperature. These extreme temperatures in the center housing can result in oil coking.
Courtesy Garrett Turbochargers

If the turbo isn't given that cooling-off period, oil around the bearing stops flowing immediately after the engine is stopped. It vaporizes and forms abrasive deposits -- called "coking" -- that can result in failure of the close-tolerance turbo wheel. That exhaust-driven wheel driving a compressor, has to spin at 100,000-250,000 rpm to achieve the performance boost.
Courtesy Wards Auto World

Turbocharged engines should be idled for several minutes after starting and before shutdown to prevent "oil lag" failures to turbocharger bearings. This is particularly important during cold weather or when the equipment has not been in use for extended periods.
Courtesy Arrow Engine Company
__________________
Clifton Brown
https://www.mancalamarketing.com
Old 10-07-2007, 09:53 AM
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