Fuel Pressure fluctuations

[Flowczar]

Have turbocharged fuel injected engine on SF901 with very erratic pressures during run up, particularly worse within rpm ranges. Engine is fired sequentially and alternates rails. Have checked supply and system all the way to engine, great volume, even pressure is stable up to our decided limit of 5 bar. Once engine is added pressure swings of 1.5 bar are registered on guage and in expansion box to test data. All air is bled from system. Engine power does vary with fuel pressure and is very unrepeatable. Fuel is fed from engine stand with high pressure pump into tee, then to each rail assembly, thru rails to regulator, return to surge tank on dyno stand. Adding damper resulted in condition becoming WORSE! All lines are -6 AN, all 90 degree fittings are bent tube. Also present if manifold disconnected from fuel pressure regulator, or routing is changed to series loop. Alternate fuel pump and regulator were also tried in vain. Any suggestions welcome.

 

[jeremy4v]

I would say you must have a restriction some where or your fuel rail volume is not large enought. Try a gauge before the T to the fuel rails and see if it is eratic there, if it is less eratic or constant I'd guess your rails are not flowing enough. just a guess though. anyone think I'm nuts?

cheers

 

[patprimmer]

I know it sounds ridiculouse, but I once had a variable fuel pressure problem. It was a leak in the tank, wedged between the pick up and a baffel. under certain conditions it bent enough to partly cover the pick up tube.

Also, is the split to the two rails, a "T" or a "Y".

You might be generating some cavitation if it is a "T"

Are you sure the lines, pump and filter are big enough for your power output.

I have also seen rubber delaminate from thew inside of a line and restrict flow.

Also a bend in a rubber line can collaps a bit. This might vary from time to time depending on temp and any other loads like the engine moving a bit on soft mounts or whatever

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[andyv8]

I've also had a similar problem in the past going from a large volume single rail on a V6 to a pair of smaller volume rails. The system was returnless & running at 4 bar, but here is what I found:

Damper should be close to the fuel rails on the pump side of the rails. It should be tuned to damp out pressure pulses from the pump as well as resonance effects from the fuel line to the pump (calculate the resonant frequency as if the fuel line was a helmholz resonator).

Twin rails should be linked at the far end (opposite the damper) with rubber or plastic pipe. This prevents resonance in the rails and damps out the pressure spikes caused by the injectors closing.

I didn't find any differences changing pipework connections from T's to Y's as all of the effects could be explained by resonance lengths excited by the injector closing frequencies.

All of the above was carried out on an injector flow rig at Bosch in Germany, which showed individual flows through injectors and showed deviations in flow rate as an offset from nominal. The worst system I tried showed 20% fuelling offsets at some engine speeds!! This could also be picked up on a dyno using thermocouples in the exhaust ports.

If you get really stuck, the Bosch charges about £600 (UK) to run a days worth of tests. (I can give you a contact if required).

Cheers,

Andy

 

[Flowczar]

We have made some headway. Pressure has been measured in 2 locations; before "y" fitting (replacing "t" of earlier testing), and one end of rail near connecting "loop" line. Dampers have been added to each rail, low volume type by Mototron. Pulses have decreased to a just tolerable 5 psi oscillation at 60 psi. My only other idea is to replace the connecting stainless steel braid hoses with the Nomex jacketed rubber lines.

We attempted to model using Helmholtz equation but gave up after extensive discussion about the compressibility. We are certain this has become a mechanical vibration.

An observation includes the idea that the on the left rail, in sequential mode, the injectors actually fire right down the rail. We have even changed the fittings feeding the ends of the rails to a style which do not have a flare to the O.D. of the fitting. Increased input line from pump to "y" fititng to -8, no difference.

I think we can rig our injection test bench to work the injectors sequentially, but had really hoped to work this out on the engine.

 

[enginesrus]

Any small air leak on the suction side could do it too.
Like a pump seal, or in the pick-up thus cavitation.
What is the ID of the rail?

 

[Flowczar]

I likely should have been more clear in the initial mail. The fuel system works well when the engine is not running. Fuel comes from outside room via high volume, low pressure pump, through two filter assemblies, to measurement turbine, then to 1 gallon surge tank which feeds high pressure pump. Another filter and into the engine plumbing. Return line from injection returns to the top of surge tank so all fuel used comes through 1 turbine. It would be different if I hadn't just run an engine similar, but more powerful, to this two weeks before...and had no problem like this.

When checking the system without the engine we have regulated "use" of fuel to container and modified pressure using regulator at engine and run system to 5 bar, no fluctuations, and no air bubbles to line. Pump capacity at 5 bar is 0.55 gallons per minute. The I.D. of rail is 0.562". I have checked all hoses for delamination and all connections to fittings for assembly errors, nothing.

 

[Andy330hp]

Where is the fuel pressure regulator referenced to?
Are the fuel lines routed near any substantial heat sources?
Does the problem begin immediately or gradually escalate?

 

[Flowczar]

Eureka!! The installation of several (3) dampers on the system did the trick, in reviewing the data taken during a test we found the fluctuations reduced to under 4 psi, and then only during certain rpm ranges. Those ranges were compensated with extra fuel. The system was tested both with the regulator referenced to manifold and atmosphere, the manifold referenced tests were slightly more erratic, but I suspect it was really due to the elevated pressure. Fuel temp very stable throughout system.

Thanks for the help and interest.