FI Engine Surge at Idle

[lr172]

Originally Posted by Taildrgr

Carl,
I have done 3 GAMI tests while flying at 75% and 65% power. All had a GAMI range of 0.4 gpm.

Would this test show the spider and injectors are ok?

Also, all my EGT's at cruise are within 50 degr F when leaned to 100 ROP = 1300 degr to 1350 Degr. Probes are 2 inches from exhaust flange.

Max EGT spread at idle is 100 degr. Lowest is 900 degr and highest is 1000 degr.

With that spread of EGT's at idle, I wouldn't think you have a plugged bypass port. I believe (i.e. my opinion) that surging comes from systemic issues, not an individual cylinder issue, which usually presents as roughness.

It is difficult to compare absolute EGT numbers across different platforms. However, My peak EGTs are in 1400-1500 range, similar to yours. I took a look at a recent datalog and I showed a range of 900-940 at a 750 RPM idle (the range goes up and down with RPM). I was 830-880 at 650 RPM. You should compare these with yours. Given that our peaks are similar, you can use that as a guideline. By comparing mine to yours, I could possibly infer that you are running leaner at idle. My setup is on the lean side with barely a bump in RPM at ICO.

Again, difficult to compare across planes, but given this and your symptoms, I would try richening up the idle mixture a bit and see if your surging goes away. Surging is a classic symptom of lean idle. Do you ever get a hesitation when increasing the throttle quickly from idle early in the acceleration? That would be another symptom of a lean idle.

You can also do some research about lean surging on the web. 4 stroke engines all behave the same way. Our systems, like FI are different, but the fundamentals of fuel mixture are the same and much can be learned. It will also help to tighten up your definition of surging to be sure we're on the same page. Surging is not that common and can only come from a few things. It usually requires dymanic changing of the air volume, air fuel ratio or ignition timing (we can pretty much rule this out with mags) or a very lean condition that develops it's own dynamics (think of a falling leaf series of stalls).

Good luck,

Larry

[Carl Froehlich]

Originally Posted by Taildrgr

Carl,
I have done 3 GAMI tests while flying at 75% and 65% power. All had a GAMI range of 0.4 gpm.

Would this test show the spider and injectors are ok?

Also, all my EGT's at cruise are within 50 degr F when leaned to 100 ROP = 1300 degr to 1350 Degr. Probes are 2 inches from exhaust flange.

Max EGT spread at idle is 100 degr. Lowest is 900 degr and highest is 1000 degr.

This thread has nice photos and description of how the spider works:http://www.vansairforce.com/communit...ghlight=spider

In short, the spider functions differently at idle compared to at power. Recommend you:
- With the engine at 1000 RPM or so, lean while watching EGTs. Is one cylinder EGT rising faster than the others? If one cylinder goes down while leaning it is already LOP - and is a problem.
- If the above test shows a problem, do the shot glass flow test (throttle set at fast idle, move the mixture to full rich, engine off, boost pump on). You are looking for close to the same fuel out of each spider tube.

On EGTs, absolute value is nice to know but is only a rough indication. Fuel flow when each cylinder peaks is the data you want.

0.4 gph GAMI spread is what one would expect with stock injector nozzles (0.028"). Find out which cylinder is peaking last (more fuel than desired) and install a 0.027" or 0.0275 injector. Don at Airflow Performance sells these in various sizes for ~$21 each.

I assume you have already eliminate intake leaks or ignition/spark plug issues, and have verified mag timing.

Carl

[Taildrgr]

Maybe these pictures will help.

Here's one of my GAMI tests that showed 0.1 spread. This was back in January on a cold day. About 38F OAT.






Here is the EGT right after landing on my last flight that had the surging. Note that the engine is NOT surging here. Taxi was normal. Only started surging after fueling up at the pumps.
Only showing this to see about the EGT's at taxi leaning. I did not download the file for the taxi from the pumps to the hangar.
In the middle of the graph at 1:55 was when I increased RPM to 1100 to turn off the runway and then I leaned the mixture for taxi.
You can see that EGT 2 was low by 110 degr before leaning and 80 degr after leaning. Does not look too significant.






Here's the cruise part of the flight running 100 degr ROP just for reference.



The engine runs smooth so I have not checked the resistance on the plugs, but they are all gapped correctly.
Even when it is surging, it is not like a stumble. It is more like a rolling surge. I will get a file downloaded if it does it on my next startup.
Thanks for all the help on this puzzle guys.
I will plug the sniffle valve on the next flight.

[AX-O]

like this video?
https://www.youtube.com/watch?v=K7TXL_p23bw

related thread.
http://www.vansairforce.com/communit...d.php?t=134609

[Taildrgr]

Yes the video is exactly like mine. A rolling rpm change of about 100 to 150 rpm.

Did you pull the injector and find any trash?

The fact that mine went away and returned many hours later may support the trash theory.

[AX-O]

I have done everything basically except to pull the motor. No change. very frustrating.

[RV10inOz]

Originally Posted by Kevin Horton

Many Lycoming engines with mechanical fuel injection (i.e. Bendix, Airflow Performance) do this when the engine is hot, as the fuel boils in the fuel injection lines above the engine when the fuel flow is low. This causes uneven fuel flow and rpm variations. The surging stops when you increase power, because the higher fuel flow requires higher pressure in the lines, and the boiling point of the fuel is now higher than the actual temperature.

Switching to slightly smaller injection nozzles (sold by Airflow Performance) increases the pressure in the fuel injection lines, which increases the boiling point of the fuel. But, the smaller nozzles pass less fuel at full rich, so there is a risk of being too lean at high power. I've chosen to live with the issue, as I don't want to risk being too lean at full power.

Yep, this is normal, and Kevin nailed it?..as he often does

[mike newall]

Seconded here - same when hot, not when cold.

[Taildrgr]

Originally Posted by RV10inOz

Yep, this is normal, and Kevin nailed it?..as he often does

Why would it continue to surge on startup after the engine is cold?

Would there be residual bubbles in the lines left from the prior shutdown? Even with the boost pump on?

I haven't been to the hangar due to the holiday last weekend but once I check for leaks and remove the sniffle valve I'll report back with any new data.

Thanks to all.

[Don at Airflow]

From the aircraft that customers bring to our facility for rough idle performance, 90% of the time we find the installation is the culprit. That?s not to say that there is an issue with part of the fuel injection system, ignition, or engine itself. We typically find excess hose routing, flow meters mounted on the firewall, gascolators on the firewall, loops in the hoses that can trap air bubbles etc. Sometimes we find issues in the airframe, poor flairs on tubing, flow meters on the suction side of the pumps, cut O-rings and bad pipe thread connections. All these things contribute to poor hot idle operation and hot restart problems.
Even if the installation is correct, poor idle operation can be an issue in tightly cowled or pressure cowled installations. There?s no magic here, we all know avgas boils at around 130 degrees F at sea level. So if the nozzle line temperature gets to 130, the fuel boils in the nozzle lines at idle where the pressure at that point at idle is around 0.04 PSI for a 4-cylinder engine. Raising the idle nozzle line fuel pressure to 5 PSI would be great but then you would need 900 PSI to make the system work at 16 GPH. Certainly you can install smaller nozzles up to a point and that would help. And NO, it will not reduce the take-off fuel flow if the size is calculated to work with the existing inlet fuel pressure and system pressure drops.
One thing to check is to run the engine on the ground with the cowl off. If proper running is achieved, then you know you have a heat issue due to the cowling, so then concentrate in trying to reduce under cowl temperature, reduce the heat transfer to the fuel or increase the nozzle line pressure. If poor operation at idle remains with the cowl off, then look for air entering the fuel supply (typically this is on the suction side of the pumps).
Running a re-circulation system may improve hot idle operation, but then there are fuel management and plumbing issues to deal with.