Turbo compressor surging explained...

[RedRobin]

I'm not sure where best to post this because although it was written specifically relating to GTI discussions, this useful information applies to turbo'd cars in general. But it's not a Tuning subject. Anyway, Mike from Forge (UK) based in Florida wrote the following:

"Compressor surging is an often discussed occurrence in the world of turbochargers.

The problem, however, is that the explanations and/or definitions typically presented as “fact” are generally one paragraph long, and are so brief, generic and rudimentary in their descriptions that other common aspects and operational parameters of turbocharged applications are easily confused to be related to or completely misinterpreted to be compressor surging.

With all of these ill-written definitions being used so haphazardly providing more misinformation than factual, I would like to take the time to more thoroughly and rationally explain the concepts behind surging in a way that will shed some better light on the phenomenon and give everyone a better understanding of it and provide an increased level of confidence about undertaking modifications to and tuning of their turbocharged vehicles. I only hope that this text pervasively replaces all of the inappropriate and ill-written explanations cited by your average weekend warrior.

Why are there no photos, diagrams, graphs, charts or anything else aside from 8 pages of text you may ask? Because none of it is needed nor will any of it better explain anything to you. This is all based on fundamental properties of physics that anyone with even the most basic understanding of engines will comprehend,

The simplest definition that I or anyone else can give you is that compressor surging is exactly what its name implies. It is a surging or rapid change or oscillation to the speed or possibly even the rotational direction of the compressor wheel of the turbocharger when under load.

This is caused by one thing and one thing only, of which the origins of will be explain more thoroughly later. Compressor surging is caused by two opposing pressures acting against one another on the impeller and compressor wheels of the turbocharger which share a common drive shaft.

These two opposing pressures are exhaust gas pressure which spins the impeller wheel and then a reversion of boost pressure acting against the normal rotational direction of the compressor wheel which is driven by the impeller wheel.

The opposing pressures act on their respective wheels in pulses which are caused by intake and exhaust valve events (based on cam phasing, timing, and engine RPM). The opening of the exhaust valves within the head of the motor leads to exhaust gas pulses that spin the impeller wheel of the turbo which then drives the compressor wheel which generates boost. The pulses of pressure waves that revert back into the compressor wheel are similarly caused by intake valve events (opening and closing) however the buildup and subsequent reversion of pressure itself is caused by something entirely different. (See the next paragraph)

Though the term surging can seemingly imply many things in the world of turbochargers today, the term was originally derived from a condition in which the turbo (compressor wheel) is actually spooling and generating boost pressure faster than the motor can “ingest” it. In such a scenario, where airflow volume exceeds the rate of consumption at which the engine itself is capable of operating, as the compressor wheel is spooling and boost pressure is building, when it reaches a point at which the motor can no longer accept any more airflow, a buildup of pressure in the intake manifold and charge piping will occur that will cause the reversion of boost pressure (pulse timed by intake valve events) as the turbo attempts to overcome any and all flow restrictions.

The reason that the pulsating reversion of pressure into the compressor wheel is dictated by intake valve events and not by throttle position is that the forces (pressures) that cause surging are only present to act against one another at a high RPM wide open throttle condition in which the throttle plate does not act as a restrictor as it is fully open.

The surging caused as a result of the two opposing pressures acting against one another on the impeller and compressor wheels is manifested by an actual surging or rapid oscillation of the speed of the wheels until such a time that the throttle plate closes or a fail safe devise like a wastegate or bypass valve is actuated. The pulse timing of the exhaust gas pressure acting on the impeller wheel and the pulse timing of the pressure reversion into the compressor wheel and their unique and combined harmonics will determine the frequency of the surging.

(Here, the term frequency implies the frequency of the wheel speed and pressure oscillations. It does not mean the frequency of the occurrence.)

Compressor surging was more of a common phenomenon prior to the introduction of the above mentioned fail safe devices, wastegates and bypass valves, that are now commonly both mechanically and electronically manipulated and controlled by OEM manufacturers and individual tuners/users alike in an attempt to minimize such occurrences and even secondarily to control the boost curve and turbo performance.

Compressor surging is incredibly difficult to empirically measure, however, through using a turbocharger turbine speed sensor (yes, they exist), it becomes possible to directly measure the speed of the compressor wheel under all kinds of different load and boost conditions, but because of the incredibly high speeds at which turbocharger impeller and compressor wheels spin and even gain and lose speed (accelerate and decelerate respectively), the highest sampling rate possible must be used as consistently as possible otherwise inconsistent wheel speed readings will result. Using a turbine wheel speed sensor to measure surging, however is not its primary function and not necessarily the best method which to do so, but it would certainly be the most accurate of any currently available option to measure such a phenomenon.

In order for such opposing pressures to even exist in order to be able to cause surging, the engine will need to be operating at a very high RPM, and the turbo too must be operating at a high RPM to be generating more pressure than the motor can ingest. A 200 RPM change or fluctuation in compressor wheel speed is not indicative of surging, particularly on turbochargers that can sometimes see wheel speeds in excess of 120,000 + RPM. Such a small change could easily be attributed to a wheel speed sensor anomaly based on sampling rate or even fluctuations in the volumetric density and speed of the exhaust gas pulses which are driving the compressor wheel by way of the impeller wheel. Even a 2000 RPM fluctuation in wheel speed is not indicative of surging to any degree. It would take a larger change in wheel speed, and more appropriately, a rapid oscillation of wheel speed more proportional to the maximum compressor wheel speed of the application to result in surging that would be noticeable or even detrimental to the turbo in any way.

Let’s also clarify, too, the distinction between, the “acceleration” of the compressor wheel, it’s “deceleration”, and “surging”. This difference ties into the previously mentioned idea that the term itself will most accurately describe the condition. The acceleration of the compressor wheel, which is obviously a function of an increasing rate of exhaust gas pulses driving the impeller wheel that then drives the compressor wheel, is not surging. Logic dictates that a constant rising rate of wheel speed is not equivalent to rapid oscillations. The deceleration of the wheel too, which is a function of lower or no throttle input that won’t result in combustion that won’t result in exhaust gas pulses that won’t drive the impeller wheel that won’t then drive the compressor wheel, is also not surging. Deceleration does not equal rapid oscillation.

Compressor surging can theoretically occur on ANY turbocharged application, however, in most cases, detrimental and noticeable surging will occur in such a case where the turbocharger is abnormally large for the application, which is rare in and of itself when a turbo is appropriately match to the size of the engine, but it does happen.

In addition to a “flow exceeding consumption” scenario, on an abnormally large turbo, the exhaust gas of the motor can sometimes be insufficient to spool the turbocharger to within its optimal operating speed range, thusly it will already be spinning markedly slower than a more appropriately sized turbo would be on the same application. When a reversion of pressure into the compressor wheel occurs and is accompanied by continuous exhaust gas pressure acting on the impeller wheel, it will be acting against the lower levels of pressure generated by a much slower spinning compressor wheel, thusly more easily causing surging, but at a much lower level than the term normally implies and not dangerously. A more appropriately sized turbo should always be spinning at a higher rate of optimal speed, within its efficiency range of course, which will then make it less prone to surging.

The “surge line”, or point at which any turbo may be prone to surging on any particular application or at a particular boost level, is usually clearly noted on the compressor map of any given turbocharger.

To go off on a small but relevant tangent, even at idle, on virtually all turbocharged applications, a turbocharger is generating positive boost pressure between the discharge end of the compressor cover and the throttle body. Between these two components lies the charge piping often plumbed with an intercooler and a bypass (blow-off/diverter) valve.

The most common incorrect usage for the term “surging” nowadays implies that during an open throttle condition, boost pressure is exiting the compressor cover discharge traveling through the charge piping and the throttle body into the intake manifold and the engine. At such a time that the throttle plate inside the throttle body closes, however, the compressor wheel is still spinning and generating positive boost pressure, yet it cannot enter the engine causing a reversion of boost pressure into the compressor wheel.

This scenario too, can be considered similar to the previously mentioned occurrence of flow exceeding consumption. In this case, however, the amount of boost pressure is not restricted by the airflow capacity of the engine itself, rather it is being manually limited by the flow rate of the throttle body at any given throttle position input level (controlled by your right foot).

This is not an accurate description of compressor surging at all, however, as one of the pressures/forces required for surging to occur is no longer present. As the throttle body has been closed limiting the rate and volume of airflow required for high RPM combustion, the exhaust gas flow is no longer nearly as high as it was when the throttle plate was open which would otherwise accelerate or maintain the speed of the impeller wheel to drive the compressor wheel. As the larger volume and velocity of exhaust gas is no longer present to act against the possible reversion of pressure into the compressor wheel, the entire rotating assembly will just decelerate, not surge. (Remember the distinction?)

Even if the throttle is quickly reapplied, certain fail safe devices will have actuated and there is still a period of time during which the pressures will not be acting against one another thusly allowing the turbo to decelerate not causing surging. Even on some applications in which the throttle is electronically operated and not necessarily closed immediately as your foot lifts off the pedal, the electronic actuation of a fail safe devise or a brief pause in fuel injection and spark is all that is required to slow combustion and to prevent surging.

Be sure to consider, however, that even when a throttle body closes, it never closes completely and it will still offer a given amount of airflow into the engine to maintain idle. This is often accomplished either by actually keeping the throttle plate open to a certain percentage or by actually closing the throttle plate completely but allowing airflow past it through some type of perforation (hole) that is sized accordingly to maintain a given idle RPM for the application.

To briefly touch on some of the fail safe devices used in turbocharged applications, the wastegate, either built “internally” into the exhaust housing of the turbo, or incorporated “externally” typically on the exhaust manifold prior to the turbocharger, exists in the system to bypass exhaust gas away from the impeller wheel of the turbocharger (which drives the compressor wheel) in order to prevent the turbo from continuing to generate boot pressure beyond the pressure level it has reach at the time the wastegate opens and begins bypassing exhaust gas. Additionally, if a bypass valve is present in the charge piping system to any degree, whether it is setup to recirculate into the intake side of the system or to vent to the atmosphere, and it is both sized and tuned appropriately to the application, it is still relieving this residual amount of pressure that needs to be vented at throttle lift, thusly it is preventing a buildup and reversion of boost pressure altogether.

As the wastegate is used to bypass exhaust gas to limit the increase of boost pressure and indirectly prevent surging, and a bypass valve is used to prevent pressure reversion to prevent surging, it becomes even clearer that the two forces together are required for surging to occur. Exhaust gas pressure alone will not create surging and a reversion of boost pressure alone will not do so either. They must occur in unison for surging to result. If the above mentioned, incredibly effective fail safes are used to independently limit and even prevent both contributing factors to surging, it is altogether infinitely more unlikely it will happen than if neither fail safe is incorporated at all.

Back to bypass valves, though. As mechanical style valves operate based on a pressure differential, they will open until such a time that the pressure differential (between the intake manifold and charge piping) is equalized. Electronically controlled valves are manipulated in accordance to prerecorded and predetermined pressure differential control variables that try to mimic mechanical valve operation as closely as possible under every possible condition. These control variables are stipulated in code in the ECU that can then be tied to other variables such as throttle position which will actuate the valve when certain predetermined conditions are met.

Under certain circumstances, even when a bypass valve is utilized, a reversion of boost pressure into the compressor wheel MAY still result, however, it is often as a result of some other underlying cause, such as improper tuning of the valve or a failure of the valve to operate altogether, but as mentioned above, this reversion of pressure alone will NOT result in surging.

If a valve does not offer enough flow volume to discharge the entire amount of residual boost pressure that needs to be discharged, either because it is too small for the application or it is adjusted too stiffly to allow enough flow volume, whatever volume of air that cannot be vented will remain in the system and MAY cause a pressure reversion if and only if that remaining volume of pressure is such that it is not forced through the partially open or perforated throttle plate by the constant pressure being generated by the compressor wheel. A very small pressure spike in the charge piping may occur, but for it to cause a pressure reversion and then surging, as mentioned before, it will need to measurably slow down and, together with the exhaust gas pulses, create an oscillation to the incredibly high rate of speed at which the compressor wheel is already spinning.

If a valve is somehow not operating properly or it is not venting at all, or similarly if a valve is not present in the system at all, clearly the residual amount of pressure that needs to be vented will remain in the charge piping either partially or entirely and a pressure reversion may result but if and only if the failure of an incorporated valve is such that it is prevented from venting any amount of pressure at all and such an occurrence measurably slows the speed of the compressor wheel.

Consider that even if a vacuum reference to a mechanical style valve is leaking or blocked in some way preventing the valve from operating properly, certain valves may still open and discharge pressure when a spike occurs in the charge piping, thusly preventing pressure reversion. In this regard, improper valve operation will not always and in fact rarely result in pressure reversion.

This is where a distinction between “pull type” and “push type” mechanical valves can come into play. “Pull type” (ie; HKS SSQ) valves are solely operated by vacuum, in that the plunger of the valve which seals the charge piping is opened solely in response to vacuum acting on a sealing surface that is completely enclosed in a separate sealed chamber. This means that the pressure in the charge piping acting on the plunger will not open the valve until the vacuum reference of the intake manifold is such that it can overcome the pressure in the charge piping needing to vent that is acting on the plunger holding it closed.

“Push type” valves do not have independent chambers in which the sealing surface and the plunger are located, thusly allowing both the intake manifold vacuum reference AND pressure in the charge piping to open the valve either together OR independently of one another. At throttle lift, both the residual pressure within the charge piping, AND the return of the intake manifold to vacuum will both "push" and "pull" the sealing surface of the valve open thus releasing the residual pressure from the charge piping.

On the subject of bypass valves, there is a common attribute to their individual operation that is often misinterpreted or confused to be a problem not only to the valve’s operation, but it is also often confused to be related to or even to actually be compressor surging.

This occurrence is “valve fluttering”, and it should be fully understood that it is NOT an indication of compressor surging occurring, nor does it “cause” compressor surging in any way.

Bypass valve fluttering will occur under various circumstances, so please consider under what situations you are experiencing valve fluttering before you presume that a pressure reversion or even compressor surging is taking place at all, or more importantly, before it is assumed that a problem with the valve even exists at all."

This info has a bearing on discussion about the new Forge DV for the 2.0T FSI engine so I'll post a link in that existing thread.

I certainly learnt something from this well written piece and I hope others will benefit.

[SamD]

Already here Robin

[RedRobin]

....Duh, Robin! I didn't spot that one. Nevertheless it kinda goes beyond the GTI subject imo. Perhaps the Mods will close or link this thread.

Sorry, guys!