turbo surge

[grumpyvette]

OK I could make this long and complex, but there’s no real need to make things complicated, ideally youll have a long discussion with your turbo manufacturer BEFORE buying a turbo that exactly matches your intended rpm, horsepower and engine displacement, and youll install a blow off valve with an adjustable pressure level and an ignition that has a rev limiter and has the option of adjustable ignition advance as the boost pressure increases.
obviously youll want to maximize your results with parts selected to match your rpm and hp goals.
BTW there’s a good deal more info in this thread
viewtopic.php?f=86&t=1215

Ok first you need to understand how a turbocharger works

http://www.squirrelpf.com/turbocalc/

So read this

http://auto.howstuffworks.com/turbo2.htm

Ill assume you’ve got the basic idea, a turbo uses exhaust flow to spin a turbine on the same axle that spins a matched compressor turbine that sucks in and compresses air to which fuel is added as its stuffed into the engine, the compressed fuel air mix burns and produces an even larger volume of exhaust spinning the exhaust driven turbine even faster so a power loop of rapidly increased pressure on the intake side, or BOOST is created, it will eventually reach the point where no more air can be compressed due to he flow rates and resistance on the intake compressor matching the pressure on the driven turbine or roughly equal pressure on one side being matched with resistance to flow, on both sides of the axle the turbines are spinning on, the turbine housings and impellers size and design on each end of the axle can be changed but they come as a matched set from the factory for a set flow, rpm, and displacement band.

This force generated by the high pressure exhaust on the exhaust turbine shaft generates torque at the compressor end as they are physically connected. Remember that part as its important, because getting things matched incorrectly breaks parts.

the turbocharger is compressing air, forcing it into the engine, the engine is burning it.. And expelling the exhaust and your turbine housing pressure goes up accordingly, the torque applied at the exhaust turbine wheel increases, increasing the spin speed and compressor efficiency at the other end.
End Result? Boost is produced and it climbs rapidly

Ok,
Now imagine this, The turbo begins to spin, as above, but the intake compressor wheel is generating more air than the engine can flow thru its cylinders due to both compressor efficiency and engine component flow capacity and the torque required to spin that intake compressor is not being matched, or exceeded on the exhaust, flow and pressure rates, will equalize or fall.? keeping in mind that unless the air is flowing thru the engine, and exhaust flow into the exhaust turbine housing, our turbine shaft torque will not increase to spin the compressor faster, the compressor is slowing, due too the compressed air resistance eventually equaling the force generated by the exhaust and increase..... youve also got potential detonation damage,to deal with if the pressure spikes to rapidly.

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


So what?
The engine is still rotating and forcing high pressure air, thru the cylinders but the turbo has stopped producing enough exhaust to increase boost, so our engine slows temporarily, once the boost pressure drops a bit, as the engine has now had chance to burn what excess was available and the inlet pressure is now dropped, the exhaust 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 pressurized and as a result, the torque required to spin it has dropped rapidly, a rapid repetitive cycle of speed up, stall, drop off, speed up, stall drop off, etc...

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


http://www.turbobygarrett.com/turbobygarrett/tech_center/turbo_tech103.html



In a quickly closed throttle condition (assume no blowoff valve), your compressor flow

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 intake side of the engine
On hard closed throttle the bypass should open and let that flow off. If You set the blowoff valve spring resistance too high you’ll induce surge , that blowoff valve is there to limit or prevent surge, when you hear that woosh the blowoff valve opened for an instant, doing what it was SUPPOSED TO DO: PREVENT SURGE!

They will keep in this condition for as long as it takes to let flow stabilize, and then close the blowoff valve

 

[grumpyvette]

TONY D POSTED THIS



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 description 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 occurring 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 waste gate 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 don't---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 waste gate 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!