Jump to content
HybridZ

What is Compressor Surge? - Explanation Within


ktm

Recommended Posts

I found this when searching for a good explanation of compressore surge. I am familiar with it as I run across it periodically in my line of work, but I could not quite visualize the impacts.

 

This is a simplistic, yet practical, explanation of compressor surge that I thought I would share. We know that turbos flutter under surge, some can point to the surge line on a compressor map, but this gives a basic explanation about what is actually occuring in the engine system.

 

Taken from www.passionford.com:

 

Imagine the way our turbocharger's compressor is driven, right back to basics: The turbine wheel is spun by exhaust gas, utilising also the energy stored in the exhaust gas as latent heat.

 

This force on the turbine shaft generates torque at the compressor end as they are physically connected. Remember that part as its the key element, Torque

 

Ok,

Now lets imagine our turbocharger compressing air, forcing it into the engine, the engine is utilising it.. what happens then? Ok, The engine expells it, and our turbine housing pressure goes up accordingly, the torque applied at the turbine wheel increases, increasing the spin speed and compressor efficiency at the other end. Great chain of events!

 

End Result? boost is climbing nicely.

 

Ok,

Now imagine this scenario:

The turbo begins to spin, as above, but the compressor wheel is generating more air than we can move and the torque required to spin that huge compressor is not being met? Remember that unless the air is processed by the engine, and expelled as hot, rapidly moving air into the turbine housing, our turbine shaft torque will not increase to spin the compressor faster, and it may well diminish.. our compressor is slowing, braked massively by the compressed air its trying to overcome and increase.....

 

Result? Compressor starts to slow a little.. (beginings of a compressor stall)

 

Ok,

So now what?

The engine is still rotating and consuming air, but the turbo has stopped producing an excess, so our engine has now had chance to consume what excess was available and the inlet pressure is now diminishing, the turbine torque is now increasing again...

 

VERY IMPORTANT:

Remember also that at same time, compressor resistance to spin has also dropped due to housing no longer being as pressurised and as a result, the torque required to spin it has dropped massively..

 

Boom.. Suddenly the Turbine wheels torque massively exceeds the compressors resistance and the turbo spins to speed in an instant!! Sounds great, rapid boost climb!! The compressors acceerating at over 2G

 

BUT:

We just hit the same problem again, our huge compressor has made so much boost, so fast, and is trying so hard to push the massive volume or air, that our YB cant use it.... so we start to slow again........

 

So we went: Fast, slow, fast, slow, or in other words: The turbo is Surging.

As you can imagine, it continues this cycle until it hits the revs your engine consumes all the air.. then your away!!

 

 

Please correct where there may be misinformation - TonyD I am looking at you. :) I searched around Hybridz for an explanation, but nothing came up regarding explaining the phenomenom.

Link to comment
Share on other sites

  • Replies 45
  • Created
  • Last Reply

Top Posters In This Topic

Niccceee! I have wondered exactly what compresor surge meant for some time now...just never got around to looking it up lol. If I thought about it, it was while I was away from the comp. In the future I plan on going over to a turbo motor...but have a few more stages to complete before I begin to get into that area! But what you said made a lot of sense, and even if youre off somewhat on the explaination...I, at least learned something!

 

Jeremy

Link to comment
Share on other sites

Surge is the phenomenon whereby laminar flow off the wheel is separated, and reverse flows through the compressor.

 

This can accompany any of the above phenomenon. But surge is when the airflow stops and reverses. It can snap off blades. Blow piping joints. Make for an emergency service call worth $275,000!!!

 

The above discription gets into the forces but is 'muddy' on what it really is.

 

Centrifugal compressors are VERY influenced by speed of the wheel. The curves change dramatically. In industrial compressors as little as a 2% change in input voltage to the driver can slow the compressor enough to cause massive surging.

 

It's easier for me to draw a picture of the compressor curve and show on a Pressure versus Flow graph what happens in each instance. This is how I conduct training.

 

Imagine if you will flow on the horizontal axis, and pressure on the vertical. Zero for each is at the origin, pressure rises as you ascend the vertical axis, flow increases as you traverse the horizontal axis to the right.

The simplest surge line is a slope starting at origin and raising (for argument) at a 30 degree angle to the right. The line represents where the compressor will surge if crossing that line. It is a line denoting the minimum stable flow for the pressure and flow given.

 

Now, somewhere to the right of that surge curve there will be an 'eyebrow' showing the design curve of the wheel being used. Design point is on this curve. Below that point, you will get more flow for imparted horsepower to a point. Until you reach stonewall, a phenomenon where the wheel, due to the physical size, can not move any more air and flow will stagnate. It is still flowing, but regardless of horsepower applied, the pressure won't rise, and you can't get any more flow. This is in the nebulous region somewhere in or off the lower right of this graph. It is also a curve...but lets skip that!

 

In a drop throttle condition (assume no blowoff valve), your flow moves to the LEFT rapidly(Instantly) along the horizontal axis while simultaneously RISING along the vertical axis.... This is because the flow requirement from the engine is STOPPED, and the pressure rises simultaneously due to this. Bad Surge that continues to decrease in severity as air is discharged back through the compressor to the intake ductwork and the pressure reaches a point where the flow that exists lets the air reattach itself to the wheel and flow smoothly again through the compressor.

 

In a lift throttle conditon-say to half throttle, the movement to the left does not move all the way to the vertical origin line, there is still a flow demand, but the compressor will continue making air at high speed till the torque slip mentioned above equalizes. This condition will cause the same pressure rise and it may be at a higher pressure---so the surge is louder due to higher pressure, but because flow is still occuring it usually is only a couple of times and quieter---'honking' is usually what people refer to it as...

 

On WOT, and hammering max boost you may reach a natural surge point---that is sufficient flow to stay stable but due to increasing turbine speed the pressure keeps climbing higher and higher. You are a boostaholic so you have wired your wastegate closed. The pressure reaches a point where somewhere almost vertically from design point you will cross the surge line. WHAM! This one is LOUD. It can be repetitive and in quick succession. You lift and then it starts 'honking'...damned if you do damned if you dont---take a look at what you did when you drop throttled afte a surge: You has a pressure situation where you were ABOVE the surge line to begin with, and then moved HARD LEFT along the horizontal axis while ALREADY in surge. Compound this, the wastegate pops open and SLOWS the compressor wheel---MOVING THE SURGE LINE AS WELL!

 

The whole thing to understand is the way to stop surge is to throw FLOW to the compressor. FLOW may result in stonewall, but that won't break things.

 

This is why bypass valves should open on partial throttle IMMEDIATELY! Even LIGHT lifts of the throttle should result in a 'sigh' from the turbo piping. This drops pressure on the graph towards the horizontal, and moves the flow along the vertical to the RIGHT.

 

When in doubt: induce FLOW! On hard drop throttle the bypass should open and let that flow off. You set the spring 'hard' and what you see when you graph it is a slight spike towards that surge line, in addition to the same quick move to the left, before the valve opens to vent pressure and move the flow-pressure point down towards the horizontal axis and to the right towards stonewall. With a 'hard set ricer woosh spring' it is VERY possible that the toy you are using for sound effects is INDUCING a surge before doing what it was SUPPOSED TO DO: PREVENT SURGE!

 

Most industrial compressors will sense the pressure differentials, or the amperage change of a surge, and IMMEDIATELY pop open an unloading valve to get flow stable. They will keep in this condition for a few seconds to let flow stabilize, and then close the blowoff (unloading) valve slowly to bring the pressure in the system back up. There is more to it than this with PID loops controlling the inlet and unloading valves independently, but generally it opens WIDE to induce stable flow. Some units using older control systems may sense a surge and then just unload until there is an operator input.

 

If you wonder why Indy Cars never seemed to surge...they controlled boost through blowing off excess pressure to atmosphere off the top of the plenum. The big BOOM you would hear when they manually shifted (I hate the sequential boxes and paddle shifters!) was that valve opening after they dropped throttle and it had to open and vent the plenum (throttle rotors in the head...) SSSS BOOM SSSSSSSSS BOOM SSSSSSSS BOOM. It's almost like a wired close wastegate on a stock Z and running off the emergency relief valve. But this has the thing spooled and running at maximum flow almost all the time! Now, the F1 Engines of the early 80's... oooooh... gaaaaarrrrgh! F1 Turbo Cars.......gaaagggaaaargggrrrrgggghhhh!

 

Did I make that clear as mud, or what? It's better with graphic aids...

 

My photo is decidely non-corporate! WONK WONK, GEEK GEEK!

http://www.fs-elliott.com/template_cat.aspx?page=locations&grpid=211&catid=0&subid=0

Link to comment
Share on other sites

Tony

I've got a question, I recently aquired a vortech supercharger dirt cheap (s-trim) and wanted to try some fiddling with it. my goal was to attach it to a vg30e and run ~10 lbs of boost thru the thing. the problem is, it's waaaaaay too big for the motor. before someone goes spouting off about how I just need to put on a bigger pully to slow the speed let me explain. I want to pully the supercharger up so it's operating at it's max rpm when the engine is operating at it's max rpm. now this poses a big problem since even under normal operating conditions the engine can't consume the required amount of air to prevent a surging problem given the setup I want. now since the supercharger speed is tied directly to the engine rpm and not the exhaust volume, how would you prevent surge from occuring during a normal run up in rpm. the goal is to have 10lbs of boost from 3-4000 rpm to red line like a turbo but without the surge. I've got an idea of how to do this but want to get your input first.

 

jesse

Link to comment
Share on other sites

Put a blow off valve and blow off the excess air you don't need.

 

Your blower will consume more shaft horsepower than it 'normally' would were the pressure lower. But the effeciency you loose playing with the tip speed of the impeller is worse.

 

A big plate on the plenum that 'lifts' at the given PSI would work, and the more area you have to dissipate the flowing air, the quieter it will be.

 

The centrifugal compressor has a minimum flow where it will remain stable. You just need to know where that is and make the excess flow blow off to keep it stable. Normally what would happen under overfeed is you boost higher than normal, and as rpms rise and it gets 'matched' you actually see less 'boost'. The variable speed portion of it skews the surge line all over the place, so it has a 'variable' surge point depending on the tip speed of the impeller, clearances, pressure of operation.

 

Slowing it down would be an experiment worth trying just to see how the boost curve is affected IMO.

 

Time for me to hit the sack, I gotta be up in 7 hours for work!

Link to comment
Share on other sites

Tony

I was kinda thinking that route but let me show you what I'm talking about. Below is the compressor map for the s-trim with a line drawn assuming that the supercharger is setup for max rpm and an engine rpm of 6000rpm. my initial idea was to have a wastegate actuator connected to a throttle body of sorts after the compressor and before the intercooler to open when the desired boost level is reached and just vent the rest (hence the knee). I have now come to the realization that it will become unstable before reaching the desired boost level.

 

vortech s-trim on vg30e, drawn line for 3000-6000 engine speed/air consumption

vortech_kits_s_trim_map-vg30e.GIF

 

now if I'm hearing your right, there is really no way of producing stable flow without increasing the flow thru the supercharger. well,.......what if there was a one way valve after the compressor that started to vent at say 5psi and would be large enough in size to allow just enough "extra" flow to prevent the unstable condition? that would effectively change the angle of the line to put it just inside the stable region of the compressor. does this sound correct? what happens if I do the wastegate thing at the higher supercharger rpm? how might the surge line change with a higher impleller speed at a governed boost level?

Link to comment
Share on other sites

now if I'm hearing your right, there is really no way of producing stable flow without increasing the flow thru the supercharger. well,.......what if there was a one way valve after the compressor that started to vent at say 5psi and would be large enough in size to allow just enough "extra" flow to prevent the unstable condition? that would effectively change the angle of the line to put it just inside the stable region of the compressor. does this sound correct? what happens if I do the wastegate thing at the higher supercharger rpm? how might the surge line change with a higher impleller speed at a governed boost level?

 

I thought that's what I said?:mrgreen:

 

Yes, simply bleed off the excess imparting an excess flow, and as your rpms rise the engine will suck more, causing that venting valve to close accordingly. You change the slope of the line and that is exactly what you have to do. I'm glad you realized the slope crossed and went into surge!

 

When you have higher rpms, you get more efficient. You could simply alter the blowoff like you mention, OR...

 

Divide the flow, using your wastegate to control some set ammount of flow, the the bleeder to control the rest. Say 50% each of them contributing to the flow situation. After a given point, with something like a fuzzy logic boost controller, you could signal the wastegatge to close and provide more boost after a given rpm. This would let the curve go vertically from that point, while the other bleeder kept you to the right of surge.

 

Basically, your orifice being simply a hole in the piping would move that line to the right quite simply. Maybe put an Allied Wittan Muffler on it so it's not happily whistling hot compressed air all the time... but with so much excess capacity that may be the easiest way to go about figuring out how to make it do what you want. Just dump the excess through an orifice that you progressively bore larger between passes until you find a point where you are blowing too much and can't maintain your boost. From there, you have a practical limit of orifice, and given you know the pressure you can back calculate the flow you're dumping.

 

Then back to the graph to figure out where you want to make the line 'go up straight' for the boost spike towards higher rpms.

 

Split the difference and then start tuning the characteristics of the valves.

 

Oh, if my job were that easy. Today, I'm beat up and 'have no face'---the Japanese are brutal. And I got suck with responsibility for the whole country. When you know the answer and depend on others to come through...and then they don't, ARRGH!

 

But I digress... At least this one isn't surging!

Link to comment
Share on other sites

I've got a book on the shelf that goes through the development of the Convair B-36. It ran six P&W Wasp major R4360 engines that used a supercharger and a tuborcharger (in the -41 version). The turbocharger was only "turned on" at higher altitudes to maintain rated power (3,600 shp and 7,986 ft. lbs).

 

In the book they have a checklist the flight engineer had to go through to engage the turbochargers on all six engines to prevent "sympathetic intake surging" between the supercharger and turbocharger.

Link to comment
Share on other sites

I spent many an hour secreted away in the B36 on static display at Chanute AFB while attending technical training there. That plane was just TOO COOL not to break into and check out.

 

Alas, it's no longer there (had an Alley-Oop for nose art) when I last returned. I wonder where it went. It would be a shame if it got wasted... I hope some municipality of museum took it in, it was just neat overall. Suprisingly BIG...

 

All I have is photos of the nose from outside. Having flashes go off after dark was a way to get the SP's on your tail and a 'special gift from the government'....

Link to comment
Share on other sites

  • 2 weeks later...

Looks like a visit to Castle AFB to visit my old friend will be in order, thanks a lot for that!

 

I have officially been outdone as well... That guy in Ohio who has 'pieces of the B36 being used as storage sheds' on his farm is definately got it worse than I do...

 

I mean, I would love to have the opportunity to bid and buy something like that, just to stick in the back yard and PO the neighbors when I'm 80 (er... or 50...) but MAN! To actually live the dream! TOO cool. That is a definate 'one up': "Yeah, well I got a disassembled Strategic Bomber in my back yard!" LOL

Link to comment
Share on other sites

  • 2 months later...
  • 2 weeks later...

OK, I completely understand everything above. But I have one question.

 

I actually own a DSM. Recently I put a 60-1 on the car. Well I was getting surge when I let off the throttle. Fixed most of that problem by changing my BOV. Well now from 2psi to 10 psi I get surge, from 10psi to 15psi I'm not getting any surge. My BOV is recirculated and my MAF is before my turbo, so I can tell when I get the slightest bit of surge from a log through DSMLink. Is there anyway to get rid of this? Would relieveing my wastegate help any? Before I put it on I made sure that it closed completely and it had a snug fit. Also, how would I go about porting the compressor housing like in the diagram on the turbobygarrett.com page? Would that help in the lower boost areas like how I am getting it or would that only help in the higher boost levels? I want the car to be tuned on 25psi but I need to eliminate the surge before I can touch the tune because it's causing inaccurate MAF readings so I can't adjust fuel. ANY information is extremely appreciated and should be very helpful! Thank you.

Link to comment
Share on other sites


×
×
  • Create New...