As experienced technicians, we have all encountered, diagnosed, and repaired mundane causes of turbocharger failures like excessive free play in shafts, bearings, and thrust washers, which are in turn, usually caused by poor lubrication. However, when we encounter exotic, explosive failures like shattered compressor and/or turbine wheels, the causes are not always immediately clear or apparent. Thus, in this article, we will discuss the most likely causes of explosive turbocharger wheel failures, as opposed to failures that do not always involve the disintegration of rotating parts, starting with this question-
While we are not discussing explosive turbocharger failures on highly modified competition-grade engines, it must be stated that in competitive environments, these types of failures are extremely common.
However, in the real world where most of our customers live, the chances of a branded turbocharger suffering an explosive failure, aka “wheel burst” are vanishing small. Of course, this assumes that the engine is well maintained, the fuel and engine management systems are in standard trim, the boost control system is fully functional, and that the engine is operated only within its recommended operating range.
Nonetheless, modern passenger vehicle engines are developing at a rapid pace, and exhaust gas temperatures, engine speeds, and compression ratios are all higher than what they were ten years ago. If we add increasingly severe turbocharger duty cycles to this mix, even branded turbochargers are fast approaching the point where their reliability and structural integrity are being severely tested.
Turbocharger manufacturers are well aware of this fact, and all reputable manufacturers are continually developing new rotating-component designs, as well as new materials from which to fabricate these new designs. Manufacturers also spend vast amounts of money on developing stronger, more ductile, and more impact-resistant turbocharger casings that are able to safely withstand and contain wheel bursts, given the fact that fragments of shattered turbine and compressor wheels can, and do, reach ballistic speeds and energy levels.
Sadly though, there are many manufacturers that turn out unbranded turbochargers by the thousands, and while these may not affect the OE market, the aftermarket is flooded with turbochargers that offer users almost no guarantee against explosive failures, which begs this question-
In simple terms, an explosive failure of one or more turbocharger wheels happens when a rapidly spinning compressor or turbine wheel cannot hold itself together against the combined effects of very high temperatures and enormous centrifugal forces.
For instance, the rotating parts of even a small turbocharger, such as might be found on most mid-range passenger vehicles, can reach rotational speeds in excess of 200 000 RPM in the presence of exhaust gasses that can be as hot as 9000C, and sometimes higher. Larger units, such as might be found on large truck engines can safely spin at 90 000 RPM, and under these conditions, the structural integrity of the rotating parts is the only thing that keeps the spinning wheels intact.
As a practical matter, the centrifugal forces any given turbine wheel must withstand are directly proportional to the wheels’ rotational speed squared, but even on branded turbochargers, the strength of the wheel decreases sharply above a maximum allowable threshold. Therefore, in order to resist explosive failures, the materials that turbocharger compressor and turbine wheels are made from must be able to withstand both the centrifugal forces that result from their high rotational speeds, as well as the effects that very high temperatures have on those materials. Note though temperature is the most important factor that determines whether or not a turbocharger wheel will fail explosively, which can happen in one of two ways, these being-
Blade failures
Blade failures on both turbine and compressor wheels happen when centrifugal forces overcome the structural strength of the material the wheel is made from. These types of failures occur when a blade is literally flung off the hub, with the fracture usually occurring at the blade root. If the failure occurs on the compressor wheel, the severed blade impacts the turbocharger casing at energy levels that are high enough to shatter the blade, and on engines that do not have intercoolers that can trap the fragments, the fragments can cause severe damage when they are blown into the engine.
Hub failures
These are extreme examples of explosive failures that occur when the spinning hub breaks up explosively into two or more large pieces through the wheel’s centreline. Although the hub is more robust than even the strongest blade it is also vastly more massive than a single blade, and since the rotational centreline of the hub coincides with its geometrical centre, the stresses that act on the wheel are greatest at, or close to the hub’s centre.
In some cases, a hub failure follows immediately after a blade failure as the result of a fatal imbalance caused by the loss of a blade. Regardless of the actual cause of a hub failure though, the most massive spinning part of the hub has the greatest potential to cause extensive damage to the turbocharger casing, since it releases the most energy when it impacts the casing wall, which brings us to the-
For the most part, explosive turbocharger wheel failures have only three major causes, these being-
Over speeding
Most turbocharger compressor wheels are made of aluminium, which has a significantly lower structural strength that the various Inconel steels turbine wheels are made of. Therefore, the limits at which compressor wheels fly apart are normally a bit higher than those of turbine wheels.
Since the structural strength of aluminium is significantly lower than that of Inconel steels, the prevalence of compressor wheel failures is proportionally higher than turbine wheel failures. However, apart from compressing the intake air, a compressor wheel also transfers some of its inertia to the turbine wheel via the shaft between them, with this inertia making a significant contribution to the turbine wheel’s rotational speed.
In practice then, should a compressor wheel fail or become detached from the shaft, the braking action that compressing the intake air causes is lost, and the turbine wheel is therefore free to instantly speed up to way past its maximum allowable speed. If, moreover, the engine is running at high speed when the compressor wheel failure occurs, the high velocity exhaust gas speeds up the turbine wheel even more, with the explosive failure of the turbine wheel being almost inevitable.
Fatigue failures
All metals and metal alloys have a specified fatigue life, which can be defined as the total number of cyclical loads a metal can endure before failing, even though the forces that act on the metal are not strong enough to cause a failure in the absence of cyclical loads.
In practice, the structural strength of aluminium is so low that even reputable turbocharger manufacturers constantly consider this weakness when they develop new wheel designs. At issue is the fact that constantly accelerating an aluminium compressor wheel, and then slowing it down again over many cycles can induce low-cycle fatigue failures in compressor wheels even at rotational speed that are considerably lower than their maximum allowable speeds.
While manufacturers of branded turbochargers usually manage to strike a happy balance between durability and reduced turbo lag because of the lower specific weight of aluminium, many cheap aftermarket turbochargers contain compressor wheels that are poorly made. In fact, many, if not most explosive failures of compressor wheels on unbranded turbochargers can be traced back to structural defects in the aluminium the wheels were made from.
Ingestion of foreign objects
The type of damage a foreign object causes depends on several things: the type, mass, and size of the object, as well as on where the object enters the turbocharger. In many cases, a small object will only rip the blades off the compressor wheel, leaving the hub largely intact. For the most part, a foreign object has to be big enough to jam the compressor wheel before the hub will fail, which may or may not cause the turbine wheel to fail as a result.
However, exhaust valve failures are relatively common on high-revving, modified engines, and should a piece of a broken valve be blown into the turbocharger and come into contact with the spinning turbine wheel, the wheel usually explodes violently. This often breaks the shaft, which then causes the rapidly spinning compressor wheel to attempt to “bore” itself out of the casing via the air intake.
It should be noted that while compressor wheels usually do not exit the casing in one piece, the fragments possess several times the energy levels of large-calibre rifle bullets, and on applications where the turbocharger inlet is open, these fragments can kill bystanders or cause extensive damage to engine parts and components if they ricochet off a closed bonnet.
While explosive failures of branded turbochargers are extremely rare, the same cannot be said for cheap unbranded units, whose origin and/or provenance is often untraceable. Moreover, if one adds the fact that most reputable turbocharger rebuilders will often refuse to work on an unbranded turbocharger, the message should be clear- avoid aftermarket turbochargers that are not branded, or backed-up by a written guarantee that the unit conforms to all generally accepted safety and production standards.
