Compressor surge in pre-compressor injection

[hotrod]

It depends on what your fuel air ratio is. The NACA reports were ONLY interested in the compressor effeciency of the turbine, other effects of the water were irrelevant on a jet engine.

If your fuel air ratio was 12.5:1 , 3% or air would be 37.5% of fuel, if you were running a 10:1 fuel air ratio that same 3% of air mass would be about 30% of fuel. In the case of the war birds they typically ran 50% of fuel for their injection systems.

Larry

[cheekychimp]

John,

I have been thinking about this. I appreciate that in general it makes more sense for a water injection system to be activated by boost pressure because irrespective of where we are injecting we are trying to counteract the effects of heat which increase with boost.

In this particular application (pre turbo injection) however we are looking to extend the compressor map as it reaches it's limit. Now I understand that this is done by trying to get the compression away from isothermic to adiabatic which is temperature related BUT the compressor map usually reaches it's limits at a certain RPM as we reach WOT (wide open throttle).

I'm just wondering if an RPM activated or even possibly a MAF (airflow) activated injection point would keep the turbo in the sweet spot allowing us to extend the compressor range whilst avoiding surge.

Any thoughts!

Paul.

[JohnA]

Obviously rpm on it's own is not a good indicator (you could be at 4Krpm and half throttle, which is not the same as 4Krpm and full throttle!)

Airflow is more promising though, because it is what we are running out of and we'd like to enhance.

Boost (well, absolute pressure) and airflow are the two axis in the compressor maps.
If we were to use one instead of the other, can you think of any pros or cons?

[cheekychimp]

I think there are pros and cons to both and you've brought up an interesting point mentioning why probably RPMs alone is not a good choice to base the activation point on. What I am thinking about here though is very similar, namely that you may reach reach full boost and/or your boost activation point very early on in the rev range maybe as low as 3000-4000 rpms but your compressor might only reach the end of it's efficiency range at 7500 rpms.

I think you'd be hard pushed to get the water injection to start just prior to the compressors efficiency threshold in a boost based system given the above. NOW as you pointed out, you still have the issue that rpms in different gears will also equate most likely to very different boost pressures (think cruising on the freeway and accelerating away in second gear).

Airflow may be the key and maybe a MAF activated activation point based on Hz would be the answer. The compressor reaches the efficiency threshhold when it is unable to flow anymore air. Since that can be affected by boost and rpms perhaps finding a way to meter airflow and activate the injection as the threshhold is reached would allow this injection method to work irrespective of boost and rpms based purely upon load and when the airflow limits of the turbo are reached?

Paul.

[JohnA]

Originally Posted by cheekychimp

I...but your compressor might only reach the end of it's efficiency range at 7500 rpms.

Two points here:

7500rpms are obviously engine rpms. :smile:
These bear little resemblance to compressor rpms, that can peak at ten times or higher.
On top of that, compressor rpms depend on engine LOAD, not just engine rpms, so 7500rmps in 1st gear are a much lower load than the same revs in 6th gear.
That's why some people map their cars 'safely' on the RR using 3rd or 4th gear, then go give it hell on the highway and it blows up. Top gear and aerodynamic resistance create a heavier load for the engine, which in turn forces more exhaust gases than the RR scenario and those gases create more torque on the compressor shaft (leading to more boost at the same revs!)

The end of the efficiency range is not like a brick wall (like the surge line for example!)
The efficiency range more like tapers off, allowing more and more heat to creep in percentage-wise. So there is no sharp limit to avoid, but a general area to shift to.
Also remember that precompressor injection shifts to right not only the efficiency islands, but the surgeline as well, so if it's done too early it can bite back (I've reason to believe it has happened to me, but it's hard to prove)

Cheers,

John

[ride5000]

john, how bad is your surge?

if it's not hideous, i wouldn't worry about it myself. hell, some folks run without a bov/bpv on their car, which i am sure present much higher loads to the wheels when the throttle snaps shut (not me though)!

ken

[raddy]

there is big difference in surge during shifting (caused by closed throttle) and surge during full engine (and turbo) load. First one is almost not damaging turbo, but second one is very destructive to turbo. So, if you experience surge during turbo spool up, it is allways worth to worry about it.

[ride5000]

Originally Posted by raddy

there is big difference in surge during shifting (caused by closed throttle) and surge during full engine (and turbo) load. First one is almost not damaging turbo, but second one is very destructive to turbo. So, if you experience surge during turbo spool up, it is allways worth to worry about it.

how do you figure?

the reason it occurs is exactly the same: too high a pr and too low a flow rate causes the operating point to cross the "surge line" of the compressor characteristic.

the turbo doesn't know the difference. the engine is a "black box" into which it crams air and out of which flows hot exhaust gasses under pressure.

i push the crap out of my vf23 and it surges on cold days in high load/low rpm situations. chuff chuff chuff! that's how i know i'm getting my money's worth. :lol:

ken

[raddy]

I was figured it by exh. gas flow at turbine. During shifting (closed TB) is pressure at turbine much lower (low volume of exh. gase), than kompresor load is lower, as it is spinning mainly by inertia. If surge (opposite airflow) is created at this situation, it try to brake compressor down, but in this situation its spinnig almost free, without power from turbine, so there should be less mechanicall stress to compressor at surge situation.

But opposite situation is during full engine load (acceleraction) when same surge state is created, but compressor have lot of spinning power from turbine, then opposite airflow is come with hard mechanicall stress on it.

Sorry for very simplified description, is mainly becouse of my english, but I hope that is clear what Im try to tell.

I would like to know if my conclusion is correct or not, so please post your comments.

Thank you

Radovan