A truism in the world of automotive design says that although just about anybody is free to think of clever ways to improve existing technologies or systems, not everybody should be allowed to design or implement their clever innovations. For various legal reasons, we will refrain from naming names and listing instances of poorly implemented design innovations here but this writer can state without fear of contradiction that Volvo's 2.0L T6 engine embodies several hugely successful technical innovations.
However, even though the Volvo T6 engine produces more power per litre of displacement than some late-model Ferrari and McLaren supercars, the T6 engine is not perfect and supercharger failures on older T6 engines are now becoming prevalent. In fact, this writer recently encountered supercharger failures on four different T6 engines in less than a month, but this was almost certainly just a series of coincidences. However, the fact is that the superchargers on T6 engines are arguably now the most common failure points on T6 engines dating from the early to mid-2010s.
In practice, though, this writer is aware that higher-end Volvos are not exactly a common sight in many smaller independent workshops, so if you have not had much exposure to Volvo engines in general and to T6 engines in particular, you are not alone. Therefore, in this two-part article, we will take a closer look at Volvo's T6 engines and explain the most common causes of supercharger failures on these engines. Before we get to specifics, though, let us look at-
Image source:https://www.swedespeed.com/threads/how-volvos-2-0l-t6-makes-more-power-per-liter-than-a-ferrari.636722/
This image shows a general view of a 2.0L Volvo T6 engine, a power plant that produces 224 kW at 5600 RPM and a maximum torque of 440.0 Nm of torque at only 2100 RPM, although it develops significantly more power in Polestar trim. These figures mean that the T6 engine in standard trim develops more power per litre of displacement than the engine in the McLaren 720S or Ferrari 488 GTB and very nearly as much power per litre of displacement as the 12-cylinder engine in the Bugatti Chiron. This is very impressive for a 2.0L four-cylinder engine since even the low-power version of the T6 engine's power-to-displacement ratio ranks among the top 10 power-to-displacement ratios of all current mass-produced engines, which, incidentally, places it well ahead of the Nissan GT-R Nismo.
So why do we mention any of the above? You may well ask because we need to understand how and why Volvo’s T6 engines work as well as they do before we can understand why their forced induction systems fail, so let us look at-
T6 engines use both superchargers and turbochargers to force air into the cylinders, albeit not always, at the same time. In practice, only the engine-driven supercharger supplies compressed air to the engine from idling speed to 1600 RPM. During this range, the turbocharger’s inlet remains closed, but once the engine’s speed reaches around 3500 RPM, the turbocharger’s inlet opens and exhaust gas pressure begins to spool up the turbocharger.
This is where the magic begins to happen: when the boost pressure from the turbocharger reaches a pre-defined threshold, which, depending on the model, happens between about 3500 RPM and about 3800 RPM, the engine management system disengages the clutch that drives the supercharger. From this moment, only the turbocharger supplies boost pressure through an intercooler, but since the supercharger supplied boost pressure while the turbocharger was spooling up, there is no turbo lag.
Moreover, since the supercharger disengages at engine speeds above about 3500 RPM, Volvo had optimized its drive system to produce high boost pressures at low engine speeds, meaning that throttle responses are immediate and predictable throughout the engine’s entire torque band, which peaks at around 5600 RPM for standard versions and at about 6000 RPM for Polestar variants.
There is one more refinement, or from our perspective as mechanics and technicians, one more layer of complexity to consider, this refinement being the fact that T6 engines can also operate as naturally aspirated engines during some operating conditions, such as slow city driving. Here is how this works-
Under light-load conditions, it does not matter if the turbocharger slows down to the point where it no longer generates boost pressure because the supercharger is switchable, meaning that under some driving conditions, the supercharger can be re-engaged to supply boost pressure. However, to save fuel, the supercharger will only re-engage if the throttle pedal's range and rate of movement exceeds pre-programmed limits/parameters, which means that for as long as the throttle pedals' movements remain within strictly defined limits, neither the supercharger nor the turbocharger will supply boost pressure.
However, large throttle inputs will re-engage the supercharger immediately to supply the engine with compressed air until the engine speed reaches the point where the turbocharger can supply a sufficient boost pressure, and the supercharger disengages again. As a practical matter, switching between supercharger and turbocharger-derived boost pressure happens seamlessly, and average drivers are generally unable to detect the switchover points.
Of course, it will be readily understood that this dual forced induction system only works seamlessly if all its components, including its control system and its dedicated input devices, work as designed and expected under all operating conditions. It is probably worth mentioning at this point that the T6 engine featuring the dual forced induction system is now in use in or available as, an option in theXC40, XC60, XC90, S60, S90, and V90 model ranges, which brings us to-
The principal defining characteristic of the superchargers on T6 engines is that they are incredibly tough, durable, and reliable, and provided a T6 engine is serviced regularly, a supercharger on a Volvo T6 engine will endure many years of exposure to vibration and thousands of thermal cycles before failing. So, what’s the problem, you may ask?
You might very well ask because it has been said by some technicians (including this writer), that Volvo has designed and built these superchargers to be indestructible. Well, that statement is perhaps somewhat hyperbolic because superchargers on T6 engines are not exactly indestructible; they do fail eventually, but not in the way you might think or expect.
For example, while many superchargers fail explosively or suffer severe to fatal mechanical damage when the ceramic coating on the rotors/screws flakes off, this writer has never seen or heard of an explosive failure of a supercharger on a Volvo T6 engine. Of course, this is not the same as saying that a supercharger on a Volvo T6 engine cannot fail explosively, this writer is unaware of such failures.
Nonetheless, since the turbochargers on T6 engines are switchable, they contain a fairly large number of silicone or synthetic rubber seals, plastic or aluminium valves, resonators, and others that control the boost pressure and a clutch mechanism that engages and disengages the rotating parts from the engine.
Limited space precludes even a cursory discussion of the role and function of each of these parts but it is important to note that the failure of any one (or more) of these parts can affect the overall operation and functioning of the supercharger. Note that such failures typically cause one or more control modules to set fault codes such as P0171 – [Fuel) System Too Lean and other air/fuel metering codes that may or may not include fuel trim codes. Moreover, depending on the nature of the failure, one or more manufacturer-specific trouble codes may also be present.
As a practical matter, typical parts failures occur as the result of seals hardening and cracking or the sealing surfaces valves wearing to the point where unmetered air enters the engine via one or more possible paths, which leaves us with-
In Part 2 of this article, we will discuss the mechanics of supercharger failure on Volvo T6 engines and present a case study of such a failure that has set code P0171 on a 2017 Volvo XC90.
