Turbochargers and reliability: we find out what kills a turbo...

Nearly every new car launched boasts a turbocharger but buyers’ reliability concerns remain. We investigate what could cause one to fail...

"Turbochargers are inherently extremely reliable.” So says Chris Kambouris, managing director of TurboDirect SA, when I pay him a visit to understand why turbochargers fail. According to Chris, in the 20 years he has been involved in the turbo trade and doing forensic analysis, there have been only a few cases leading to warranty claims; the majority of turbo problems are caused by external factors. A turbocharger is a sensitive component and, if mistreated, can fail, much like any other part.

Background

Engines are turbocharged to allow downsizing of their capacity without affecting outputs, with the added benefit of lower fuel consumption plus reduced CO2 and other harmful emissions.

Engine power is limited by the amount of air (oxygen) that is available for combustion; any additional fuel injected goes to waste (this is especially true in a petrol engine with a fixed air-fuel ratio). A turbo can be seen as an air pump utilising normally wasted exhaust energy to compress the intake air above atmospheric pressure and force it into the combustion chambers, which in turn allows more fuel to be burnt.

How a turbocharger works

• The exhaust valve opens and the remaining combustion energy (heat and pressure) is released down the exhaust ports.
• The expelled exhaust gas is forced through the turbine side of the turbocharger harnessing some of the energy by spinning the turbo shaft connected to the compressor side.
• The compressor receives fresh atmospheric air from the air-intake system and raises the pressure to above atmospheric in the intake manifold.
• The intercooler stage increases the efficiency by cooling the fresh-air charge after the compressor stage (resulting in denser air) before it reaches the intake manifold.
• The engine breathes air at the intake valve at a higher pressure than atmospheric, which results in more oxygen filling the combustion chambers than in a naturally aspirated engine of similar capacity.

Turbo components

A turbo consists of many parts manufactured from special materials that have to endure temperatures approaching 1 000 degrees Celsius while the rotating parts may exceed 200 000 r/min. Therefore, all the rotating components are individually balanced by a machine cutting away small amounts of excess material.

The final centre rotating-hub assembly is then balanced to a fine degree on a machine similar in operation to a tyre-balancing unit but which is able to test at maximum speeds employing compressed air. The final adjustment is done by removing minimal material from the assembly nut on the shaft.

The following main components are important when it comes to reliability.

Housing

The compressor (and turbine) housing’s shape is defined by the area and radius dimensions, and forms the outer shell of the turbo.

Wheels

The compressor and turbine wheels consist of radial blades designed to compress the intake air and extract energy from the exhaust gas. The design, size and shape determine the air-flow, pressure ratio and efficiency on the compressor side and power capability (and efficiency) on the turbine side. These two wheels need to be perfectly matched to each other and the engine application.

Shaft

The shaft connects the com-pressor wheel to the turbine wheel and runs through the journal (or ball) bearings.

Bearings

Journal bearings lubricated with engine oil are most common, although high-end turbochargers now employ ball bearings.

Thrust bearing

This bearing plate deals with the axial loads on the shaft of the turbocharger.

Turbo “seal”

A seal is an improper term to describe the component because it’s actually closer to a piston ring. This metal ring prevents oil leaking to the compressor or turbine side through the sealing action, but also by allowing pressurised air (or exhaust gas) through the ring gap to keep the oil back into the centre housing.

How to take care of your turbo

• Always service your engine according to the service schedule, paying special attention to the specification of oil.
• Fix engine problems immediately, including intake and boost leaks.
• As good practice (although not strictly needed with modern watercooled turbos supplied by electric water pumps), let the engine idle for a minute after hard driving.
• When a turbo fails, find the root cause before replacing the unit.
• Request the turbo oil-feed pipes to be replaced when a turbo change is required (or after 100 000 km), as a coked supply line has the same effect as cholesterol to arteries.
• Never dry-start a new turbo. Ensure the proper lubrication procedures are followed when a new turbo is installed.

Turbo killers

It is a common mistake to view a turbocharger as an add-on component to an engine, like you would an alternator. Therefore, the assumption is, when it fails, it is the turbo’s fault and a new unit is needed. As the turbo is such an integral part of an engine, external problems often cause failure and will do so again to the new unit if the root cause isn’t addressed.

Lack of lubrication

The number one killer is a lack of lubrication. Owing to the fact a turbo can spin quicker than 200 000 r/min means the boundary-layer lubrication at the bearings is critical. If for some reason the correct-specification oil does not reach these areas at the correct pressure, turbo failure is imminent. Lubrication problems may include incorrect oil, faulty oil pumps and a blocked oil feed (or drain) pipe. When a turbo is replaced, a dry start-up – if the correct lubrication procedures were not followed – can kill a turbo in a matter of seconds. Even at idle, the turbo spins at about 10 000 r/min.

Oil contamination

It is not just sufficient for the turbo to receive oil; the liquid also needs to be clean. As the turboshaft spins at such high rotational speeds in the bearings, any particles or debris in the oil act as grinding paste and wear away the shaft and brass bearings in the case of the journal type. Forensic analysis of this failure mode reveals scoring on the shaft and bearing surfaces. The wear can result in bearing failure and seizure but will initially lead to other problems.

Exceptional operating conditions

If a turbo runs outside its design envelope, failure can be catastrophic. This includes over-speeding of the turbo leading to burst compressor and turbine wheels if, for example, there is a problem with the wastegate control (or variable-nozzle turbo actuator). The turbo materials are designed for a specific temperature range; exceeding this is terminal. Hot shutdown is a known killer because oil starts coking to bearing surfaces. Although modern watercooled turbos with electric water pumps are designed to cope with engine stop/start systems, Chris Kambouris feels there’s merit in letting a turbo cool down by allowing the engine to idle after sustained hard driving.

Foreign object damage

Imagine throwing a rock into the blades of a room fan. The result is similar to a foreign object entering a turbocharger but the damage is amplified because of the elevated speeds. Run the vehicle without an air filter and the dust particles will sand-blast the compressor blades. On the turbine side, any debris the engine coughs out can be lethal. Even the sealant wrongly applied by some turbo installers on the turbine flange can harden and break off in tiny pieces, ripping the blades to shards.

Why do turbos leak oil?

The misconception is the metal rings (informally referred to as “turbo seals”) are worn, which is true only in rare cases. The more likely cause is an imbalance in the compressor-turbine, leading to gas pressure forcing oil to the low-pressure side. Intake, boost or even exhaust leaks are common causes.

The other main culprit is the oil-return line running back to the engine crank case under gravity. If this line is blocked, oil has nowhere to go and is forced past the metal ring seals. Causes can be a kink in the line; too high oil level in the engine; or excessive blow-by gases in the crank case (or blocked positive-pressure crankcase ventilation valve).

How much does a turbo cost?

Turbos are expensive, especially when purchased at the dealer. We compared four popular units’ price with similar units (same brand and part number) from importer and distributor TurboDirect SA.

Thank you to TurboDirect SA for supplying the information and allowing us access to its premises for photography. More info at turbodirect.co.za...

Author: Nicol Louw

Original article from Car