Engine/Transmission (1998.5 - 2002) Will Turbo Bark bite????

[Challenger-II]

A friend of mine who has a '99 recently put on a AFE Mega cannon and has since experienced turbo bark when letting off the pedal quickly. He is running an EZ w/injectors.



What is the exact cause of the bark and will it hurt anything? How do you get rid of it?



Thanks!

 

[jerrethomas]

the turbo bark is caused by a load on the engine and the turbo spools up and builds boost and then u proceed to let off really quick is what will cause this. what actually happens is the comprssor side of the turbo is under pressure and when u lift from the throttle real quick it trys to spin the compressor backwards creating the "turbo bark" and no this maybe sound cool but it is not good for the turbo at all. somebody correct me if i'm wrong. the only to get rid of it is not to leave off really fast from a full throttle accerlation. this is most likey the only time it will does this.

 

[JyRO]

Just an opinion...

I am by no means an expert, I know very little about this. The only time I heard this "bark", was at Thunder in Muncie at the sled pulls. I think they were calling it turbo burp. Anyway, it happened when the sled quit moving, and a driver would quickly let off the go pedal from the floor and stab the clutch at the same time.



From my understanding the turbo is turning such extreme rpm under full boost, and then the engine almost stops (relatively) providing any exhaust heat/pressure. Well the turbo's momentum actually tries to draw exhaust from the engine. And the noise is cause by the quick extreme pressure changes between the exhaust manifold and turbo inlet. When an exhaust valve opens, the pressure goes up a bit in the exhaust manifold, but when all exhaust valves are closed, due to the momentum of the turbo, the pressure goes to a vacuum as the turbo is trying to suck exhaust from the engine.



From the times I've heard this, the length of time the sound happens is only about a second. And that pressure must really be jumping around, during that second. I presume its pretty hard on at least the compressor side of the turbo. I could be wrong on all this, but its my understanding. I've never heard in my truck.



- JyRO

 

[RustyJC]

The actual engineering name is turbocharger compressor surge. The centrifugal compressor impeller on a turbocharger is a dynamic, not a positive displacement compressor. This means that it compresses air by accelerating the incoming air to a high velocity, then converting velocity energy to pressure energy. In simple terms, the only thing keeping the air from flowing backwards from the turbo compressor discharge (a high pressure area) to the turbo compressor inlet (a low pressure area) is that the turbo impeller is "stacking" more air into the discharge at that particular moment.



Ignoring the wastegate for the moment, at full throttle the engine is pumping hot exhaust gas into the exhaust manifold. The turbine section of the turbocharger converts the energy in this high pressure, hot exhaust gas into shaft horsepower which is used to drive the turbocharger's compressor impeller. The turbocharger will theoretically continue to accelerate until the drive horsepower equals the horsepower consumed by the compressor, less frictional losses.



When you suddenly let off the accelerator, the injection pump stops injecting fuel into the engine's cylinders. The exhaust temperature and pressure, relatively speaking, drop like a rock. This deprives the turbocharger's turbine section of drive horsepower. The turbocharger compressor, meanwhile, doesn't know anything has happened and is still compressing air like mad (its horsepower consumption hasn't dropped). Since the drive horsepower has dropped, the turbocharger rotor will decelerate. This deceleration decreases the "head" (discharge pressure minus inlet pressure) produced by the impeller. Since the head produced by the impeller is less than the actual compressor discharge pressure minus the compressor inlet pressure, the high pressure air in the intake system will flow from the discharge to the inlet side of the turbocharger compressor (reverse flow) until the pressure differential across the impeller is less than the impeller's head capacity. At that point, the impeller starts compressing air again.



We still have no drive horsepower with the throttle closed, however. This means the turbocharger rotor is still decelerating and the compressor impeller head continues to drop. Therefore, soon after the impeller begins compressing air again, the head will drop and airflow will reverse once again. This reversal of airflow is the "whoomp" or "bark" that you hear.



Each time the compressor airflow reverses, it unloads the compressor impeller blades and shaft. When the impeller starts compressing again, it loads the compressor impeller blades and shaft. The turbo will continue this surging as it walks its way down the surge line on its performance map as it decelerates.



This unloading and loading can, over time, produce cyclic fatigue failures in either the impeller blades or shaft - whichever is the weak link in the turbocharger's design.



Please note - at no time does the turbocharger rotor reverse its rotation. There's too much rotational inertia in the system for that to happen.



Rusty

 

[John Berger]

I've never experienced the bark and Rusty's explanation gave me a headache. I hear a beer barking around here somewhere. :-laf

 

[JyRO]

Uh huh...

Rusty - I didn't say, and didn't think, that the turbo actually changed direction. I think it would self destruct if that was the case (too much change of momentum to hold together). Your explanation I'm assuming is correct. But I don't understand a few things.



1) Firt of all, clerify what the exhaust side of the turbo is called (impellor, compressor, etc. ). I said I think its hard on the compressor side, but I actually meant the side the exhaust flows on, whatever you call it.



2) You said:

Since the head produced by the impeller is less than the actual compressor inlet pressure, the high pressure air in the intake system will flow from the discharge to the inlet side of the turbocharger compressor (reverse flow)

I'm lost, explain this more clearly, just for my understanding.





If I understand you correctly, you're saying the noise comes from the intake side of the turbo? I wouldn't have thought so. I thought I was hearing the sound more through the exhaust pipe, and was why I thought it was on the exhaust side of the turbo. Interesting stuff. Regardless, it ain't too good for the turbo, I bet.



- JyRO

 

[RustyJC]

Re: Uh huh...

Originally posted by JyRO

Rusty - I didn't say, and didn't think, that the turbo actually changed direction. I think it would self destruct if that was the case (too much change of momentum to hold together). Your explanation I'm assuming is correct. But I don't understand a few things.

Jyro,



My comment wasn't directed to your post. I had just seen a number of "turbo bark" posts that stated the rotor reversed rotation and wanted to clarify that point - it doesn't!

1) First of all, clarify what the exhaust side of the turbo is called (impeller, compressor, etc. ). I said I think its hard on the compressor side, but I actually meant the side the exhaust flows on, whatever you call it.

The "air" side is the compressor side - the "pinwheel", as some call it, is actually called an impeller. The exhaust side is the turbine side - its "pinwheel" is called a turbine or turbine wheel.

2) You said:



Since the head produced by the impeller is less than the actual compressor inlet pressure, the high pressure air in the intake system will flow from the discharge to the inlet side of the turbocharger compressor (reverse flow)



I'm lost, explain this more clearly, just for my understanding.

The faster the impeller spins, the more head (discharge pressure) it can produce. If the intake piping is, say, at 35 psig and the impeller is slowing down, the compressor will reach a point where it can't pump the pressure up to 35 psig - the impeller isn't spinning fast enough. The pressure in the intake piping will then relieve itself by reverse-flowing through the compressor. Once the pressure in the intake piping has dropped below the head capability of the impeller at whatever speed it's spinning, the impeller will start compressing air again. However, since the turbocharger rotor is still slowing down, the head capability will keep dropping lower, and the compressor will "surge" again - and the cycle will keep repeating itself.

If I understand you correctly, you're saying the noise comes from the intake side of the turbo? I wouldn't have thought so. I thought I was hearing the sound more through the exhaust pipe, and was why I thought it was on the exhaust side of the turbo. Interesting stuff. Regardless, it ain't too good for the turbo, I bet.

The flow reversal is taking place on the intake side. If you have a BHAF or K&N, you should hear it clearly. If you have a Psycotty or AFe-type intake system that penetrates the heater plenum, it might blow the cap off your head! Because the intake flow reversal does have an impact on the flow through the exhaust system, you might hear something there as well, but the pressure changes will be more pronounced on the intake side.



Rusty

 

[Challenger-II]

Now that's a scientific answer!! I smell an Engineer here.



OK..... I can follow what creates the bark but, what exactly causes it to start happening? Doesn't happen on a stock truck. Is it the increased air-flow from the AFE and the additional boost from a box/elbow? Anyway of stopping it shy of not letting off the pedal too quick?



Thanks to all who took the lengthy time to explain.

 

[JyRO]

I should be able to hear it?

If you have a BHAF or K&N, you should hear it clearly.

Well, I don't have either one of these. But if I did, I wouldn't hear this unless I did some real abrupt off power change from full throttle, would I? I mean, normal driving I can see there still being a pressure reciprocation (albeit much smaller in this case) in the intake plenum when the compressor is slowing down, but not enough to make this barking or burping noise. Right?



Lastly, I can't "see" an actual reversal of air flow from the intake back into the compressor. I guess I'm going to have to re-read your posts a few times until it can make some sense to me. The airflow into the intake slowing severely, then continuing on seems likely (thus causing pressure reciprocation). But reversing, I'm just having a hard time believing that would actually happen.



- JyRO

 

[RustyJC]

Originally posted by Challenger-II

OK..... I can follow what creates the bark but, what exactly causes it to start happening?

Letting off the throttle suddenly. This cuts the exhaust drive horsepower from a bunch to practically nothing almost instantly.

Doesn't happen on a stock truck. Is it the increased air-flow from the AFE and the additional boost from a box/elbow?

The larger injectors are putting a lot more fuel in the engine and thus creating more drive horsepower on the exhaust side. The turbo is spinning a lot faster than stock to create the extra boost permitted by the box/boost elbow. The BOMBed truck is operating at a much different point on the turbocharger performance map than a stock truck.

Anyway of stopping it shy of not letting off the pedal too quick?

Not practically. Some race cars run a bypass valve that dumps the turbo discharge back to the intake when the throttle is closed to lessen turbo lag and prevent surge, but it's not really practical for our trucks (at least I haven't heard of anyone doing it successfully).



Rusty

 

[RustyJC]

Re: I should be able to hear it?

Originally posted by JyRO

Well, I don't have either one of these. But if I did, I wouldn't hear this unless I did some real abrupt off power change from full throttle, would I? I mean, normal driving I can see there still being a pressure reciprocation (albeit much smaller in this case) in the intake plenum when the compressor is slowing down, but not enough to make this barking or burping noise. Right?

No, normal deceleration doesn't drive the turbo over its surge line. As long as the compressor doesn't surge, there's no pulsation in the intake system as the turbo slows down.

Lastly, I can't "see" an actual reversal of air flow from the intake back into the compressor. I guess I'm going to have to re-read your posts a few times until it can make some sense to me. The airflow into the intake slowing severely, then continuing on seems likely (thus causing pressure reciprocation). But reversing, I'm just having a hard time believing that would actually happen.

JyRO, trust me on this one. I've seen turbo surge blow air filters completely off the engines our company builds. On the old oil-bath air cleaners, it would blow the oil out on the ground. Remember, there's no check valve on a dynamic compressor to keep the air from flowing backwards. If the manifold pressure is higher than the turbocharger's compressor impeller can reach, airflow will reverse!



Rusty

 

[ACoyle]

Just another take.



http://dodgeram.org/tech/dsl/FAQ/turbo_faq.htm

 

[heliopilot]

Rusty, you dead on... .



I was at a plant last week that produces the Exhaust & Inlet wheel for the Holset turbocharger, which is in turn supplied to Cummins for our Dodge diesels. I was told that due to the failure rates of the of the wheels(exhaust & inlet) they had changed the way these are being made, on the inlet side the have gone from a casting to billet aluminum which is then machined and polished to 2 microns and then balanced. On the exhaust side some of the wheels being produced where made from titanium.



I wasnt aware from TDR readers that there was many turbo failures.

 

[RustyJC]

Originally posted by ACoyle

Just another take.



http://dodgeram.org/tech/dsl/FAQ/turbo_faq.htm

The section that deals with turbocharger surge is as follows:

When the engine is loaded, very high turbine speeds are required for the turbo charger to provide intake manifold boost. When the fuel is suddenly cut off, the exhaust volume collapses and the turbocharger rapidly spins down. As the turbine looses speed, it looses it's ability to sustain the manifold pressure and compressed air rushes backward from the manifold and through the turbocharger. The momentary airflow reversal is not harmful and produces the short pSSHTT sound.

In my 29+ years with an industrial engine manufacturer who builds their own turbochargers, I've seen too many surge-initiated turbo failures to buy that as a general across-the-board statement. It really depends on factors such as impeller blade thickness to height ratios, diameter and speed of the impeller, etc. Smaller turbochargers tend to be more forgiving than larger ones, but the cyclic fatigue forces associated with loading and unloading the compressor are still at work. The safe approach is to try to avoid surging the turbocharger whenever possible.



Rusty

 

[JyRO]

OK, now I can understand...

Rusty - I read the paragraph you quoted above from the Dodgeram website, and it made sense to me. Now that I understand (from the quoted paragraph) what you were trying to explain, I can see that happening. I was on the verge of it but just couldn't quite figure it out. Basically the pressure in the intake manifold will sometimes be much larger than the pressure at the compressor outlet. Therefore the higher pressure air will tend to occupy a lower pressure region. And to do so it would reverse its direction and move back into the compressor. At least, I think that's what the quoted paragraph is saying. Thanks.



- JyRO

 

[Diesel Freak]

Rusty... . very well said!

 

[Hohn]

Physics says that air will ALWAYS flow from an area of high pressure to an area of lower pressure. Thus, if the "bark" is caused by a PRESSURE difference, the flow would HAVE TO reverse.



Yeah, a bigger turbo is a lot more likely to bark because it simply has a lot more rotational inertia (a greater tendency to stay at a certain speed). That means it tends to lag a little more, and tends to "reverse lag" or stay at high speed instead of slowing down. It's this reverse lag that makes barking more likely.



This is one of the reasons that a blow off valve is useful on gassers. Surge on a gasser can cause the throttle plate or carburetor to literallyget launched right off the engine, shredding the bolts that attach it.



On a diesel, we have no throttle valve. The next thing up the tract is- you guessed it- the turbo, so it gets the punishment. Unfortunately, you can't use a blow off valve on diesels though, since all it sees is pressure. That means that it would have to be set so high (to not limit boost) that it wouldn't do jack to prevent barking the turbo.



Maybe you could plumb in a large one-way flapper valve in the intake plumbing so that instead of barking the turbo, you would blow off an intercooler hose or explode the intercooler? Maybe that's not such a bright idea... .



HOHN

 

[Hohn]

OK

Rusty, I just reread my post, and I confused myself on blowoff valves. Where exactly would one mount? And, if they are to limit surging when the throttle is snapped shut (gasser), then do they also limit max boost?



Some guys (bigsaint, jetpilot, HVAC, etc) are using their intake hoses as blow off valves

 

[RustyJC]

Turbo surge on a gasser is a little bit different. You would really need to look at a head/flow curve for the turbocharger to get the picture. What happens on a gasser is that when the throttle valve is snapped shut (assume it's downstream of the turbo), airflow through the compressor immediately goes to zero. This shoves the operating point all the way to the left on the head/flow curve. Two things happen:



1. Since the centrifugal compressor on a turbocharger is a dynamic compressor, it depends on airflow to function. With no flow, the operating point shifts to the left of the surge line - the turbo surges.



2. Generally speaking, with a centrifugal compressor at a given speed, as flow drops, head increases. Therefore, when the throttle is snapped shut, extremely high discharge pressures can be produced between the turbocharger discharge and the throttle plate as the operating point is moving to the left on the head/flow map - at least until the pressure is relieved by flow reversal when the operating point crosses the surge line. That's a big reason some gassers use the "dump" valve - to relieve this momentary pressure spike that can blow the intake system apart.



Edit: As far as mounting the dump valve on a gasser, it would need to be between the turbo discharge and throttle plate. Unless one went to a sophisticated computer-based control system such as we use on the recycle valve of large centrifugal compressors, the simplest approach would be to tie the dump valve to throttle plate position - open the dump valve as the throttle plate closes (to give the turbocharger discharge airflow somewhere to go).



Rusty

 

[rweis]

What a GREAT explanation!



On aircraft engines I think it was compressor stall when the flame came out the front instead of out the back. The engine was trying to stop and turn the other way, but the mass of the axle would not alow that. Would do great damage though. Generally happened when starting the engine with the tail pipe faceing a headwind (wind comming in the rear end during engine start), or quickly retarding from full throttle and not in flight (ie full throttle run on the ground). A definite NO NO.



Basically you are managing the CTD air flow with your foot.



Take proper care of it!



Bob Weis