Pre-compressor injection: could it induce surge?

[JohnA]

I find pre-compressor injection to be an interesting idea.
The possibility of making the compressor (under boost) act as if it is a bigger one, is just something that captures my imagination.
No need for a hybrid, just stick a nozzle.
The fact that reliability may also be enhanced, makes it even more compelling.

I'm already running a turbo with a big compressor (so it can sustain high boost without the turbo shaft overspinning)
With the pre-compressor injection the turbo feels different. Weird, as if it is close to the surge line. Well, maybe it is - after all, wouldn't this be expected from an abnormally large compressor? (compared to the turbine of course)

Any ideas? :?:

[Richard L]

I need some time to think about this.

[PuntoRex]

Surge is big boost with small flow, as my rough understanding.

How is the "real" situation?

My turbo has been repaired twice. Each time they told me the compressor wheel should be enlarged. Which I don't know why. The boost was coming on 2600~2700rpm & touched the top on 3000 in stock form. After enlarged twice, now it won't go to the max until 3600~3700. I feel the power band is shifted upward by 800~1000rpm overall.

Since then, I noticed when in 5th gear light boost mild acceleration on motorway, the small opening throttle & boost are fighting with each other. Even the dump valve air flow can be heard. It's boosting & dumping at the same time. What a waste! To avoid stumbling, I have to use the softest spring in my Forge dump valve.

Is this some kind of "surge"? Boost on small flow?

Or, in another situation, when dialing up boost severely, it would be skyroketing in the midrange without related performance output. Turbo is screaming but the engine seems not able to swallow the air untill the rpm comes high enough.

If these are "surge", then adding WI prior to the compressor will make it worse? By this, maybe the trigger point should be higher.

Any thoughts? Or I misunderstand all these?

[hotrod]

My experience indicates it can in some conditions. I have an upgraded turbo that I just put on a while ago, and it is on the ragged edge of surge except in hot weather. I noticed the symptoms of surge were less when I shut off the pre-compressor injection.

I believe if your right on the ragged edge like me it may be an issue. I am considering some solutions to it.

One would be to put an rpm switch in line with the pre-comressor solenoid so that it can't spray until the engine reaches a certain rpm ( ie air flow).

I am first going to work with other things to slow the turbos spool a bit, as I did too good of a job with my porting and extrude hone work to maximize its effeciency. I am also looking around to see whats available in boost controllers that allow you to map boost limits to rpm.

Here at high altitude turbo's run at substantially higher pressure ratios than the do at sea level and that effect moves the entire pressure flow plot up on the compressor map from where it would normally be. In my case that pushes the knee portion of the plot when the turbo first reaches max boost around 3000 rpm up far enough that it bumps the surge line.

In hot weather its a non-issue as the lower air density, slows boost increase enough to avoid the problem.

Larry

[PuntoRex]

Larry,

What exactly is the symptom of surge?
Boost bounces up & down in a small range like shaking?
Or others?
Is this going to hurt engine or turbo itself only?

Your experience sharing is highly appreciated :smile:

Rex

[hotrod]

It varies depending on severity and engine load. At low rpm and moderate load, mild surge is sort of a fluttering sound, and a gentle fluctuation in engine power. As engine load goes up it gets more agressive.

Severe surge under high load can be an eye opener, it resemble repeated hard fuel cuts, or complete ignition cut outs. It can be very violent and at this level is not good for the turbo or the engine/transmission due to the violent power fluctuations.

Its not a fun experience when it is bad. One tip off that it is surge is that it gets worse in colder weather as the turbo gets to full boost sooner.

At least that's my experience.

Larry

[PuntoRex]

Not sure where to post this, quite a few pre-compressor injection topics lately...


Well, I was happy last night on the first few runs with the pre-compressor injection.

....

Anyway, it works!

Lukily I didn't experience the WOT bog down as Gelf. Yet not so lucky as Hodrod to get a rise in boost. Car just pulls harder on boost, especially in the midrange. The performance gain is easily sensible with a quiter turbo hiss.
....




Today on my way to work & the short business trip, I noticed some fluctuation on WOT & the process from WOT to gradually lift-off. And the car felt softer, not so powerful & energetic as last night. ( the sparkle is fading so soon? No! )

The fluctuation on WOT was slight, feel like ripples. When lifting-off, some time it became waves. Funny that it's not every time. Maybe 50/50 in my impressions. And sometimes the engine felt harsh along with the fluctuation.

When it happened on WOT, the boost came up normally.

I'm running 0.5mm pre-compressor & 0.6 pre-throttle without check valves currently. When I pulled over & checked the engine bay, I noticed the water lines were almost empty between HSV & jets. Only some fine droplets on the inner wall randomly, water here was siphoned out almost completely.

This might cause some delay on supply, but I didn't notice any det on the sharp throttle action & fast rising boost, so the cooling effect should be sufficient & in time, I guess. (injection starting @10psi with IDC of about 40% and up, max boost 20 with 70~80% IDC in high rev range ).

The fluctuation was in the sustained high boost situation. On the wave-like fluctuation when slowly lifting off, the boost was still higher then 10psi. Under that, everything returned normal & smooth.


Some different conditions between last night & today are:

1. Ambient temp. High teens to low 20-ish 'C last night. High 20-ish to even 30 'C today (higher around noon)

2. Loading. Me only last night vs me plus a passenger today. ( my colleague is not very big though, maybe 70-ish kg I guess ) Please note that this is a tiny car. I can always feel the performance impact caused by passenger(s). And A/C off last night vs A/C on today.

3. Driving style. Hard-on-the-throttle/rev-to-the-red style last night vs much more calm & gentle style today. I just couldn't enjoy myself when there's any passenger. My colleague was amazed when I pushed it on the highway, while I was somewhat disappointed & frustrated.

....

Any thoughts?

[PuntoRex]

Well, I tried dialing down the boost a little, about 1~2psi. And it seemed fine this morning, the previous fluctuation was barely sensible.

The boost taper on high rev range almost inexists by the helps of pre-compressor injection. It was not very noticeable before either, say, maybe 2psi drop near the red. And now I get an almost constant boost from 4000~6000rpm @18-ish ~ 19psi.

I'm not sure it's that the extra 1~2psi of boost made it flucturated. (maybe sort of miss fire) Which I can be sure is now it's running better at this slightly lower boost, equally responsive, barely any slower. It's just my greedy hunger for boost is crying for more :twisted: I guess the poor little engine has already been pushed near the limit at its stock form.

Anyway, I'm happy again :smile: