Technical: Why increase fuel rail pressure?

[rbattelle]

Why are companies coming out with performance boxes that increase fuel rail pressure? The fuel injector is an orifice, and we know that mass flow rate through an orifice is independent of drive pressure (as long as drive pressure is greater than about twice the cylinder static pressue). So increasing fuel rail pressure will NOT increase the quantity of fuel injected.



So, is the injector opening-time modulated by rail pressure by the ECM?

 

[rbattelle]

I think I just answered my own question. Increasing rail pressure probably gives you smaller fuel droplets, which will reduce ignition lag (delay angle) and rate of pressure rise, which in turn decreases detonation tendency. But power gains would be small.

 

[rbattelle]

I need to think more carefully before I speak. I think I'm wrong about the increase in fuel rail pressure not increasing mass flow rate. While it's true that flow through an orifice has limited mass flow rate, that's for compressible flow. I think if we consider flow through the injector incompressible, then mass flow rate will indeed be proportional to drive pressure.

 

[rbattelle]

I apologize, I'm completely wrong about this. For incompressible flow through an orifice, mass flow rate is proportional to drive pressure.



I'm sorry if I have confused anyone. In the future I'll think before I speak.

 

[RankRam]

Wow :-{}

 

[jponder]

I think you should report yourself for being combative

 

[RDietze]

wow, rbattelle, your OWN post



i was going to chime in something "smart-alec" but it looked like you were having too much fun arguing with yourself

... . but two smart alec's beat me to the punch



thanks for the laugh guys!

 

[rbattelle]

Ah, there's nothing quite like arguing with youself. I was close to kicking my own arse for being a doofus!

 

[RankRam]

LMAO

 

[CIverson]

I dont' know what all your talking about up there, but when I had a gasser I understood it as the injector being open for a specified period of time by the computer, and if you raise the pressure more fuel gets squirted into the cylinder than at a lower pressure even though the injector is open for the same amount of time.

 

[KLockliear]

In the end, you got it.

 

[krabman]

LMAO.

 

[jgillott]

Originally posted by CIverson

I dont' know what all your talking about up there, but when I had a gasser I understood it as the injector being open for a specified period of time by the computer, and if you raise the pressure more fuel gets squirted into the cylinder than at a lower pressure even though the injector is open for the same amount of time.

Couldn't have said it better. With the time being held constant, the volume will increase as pressure increases.



Justin

 

[Vaughn MacKenzie]

Soooooo, rbattelle, what happens to compressible and noncompressible substances' flow rates through an orfice when subjected to a vacuum??



hehehehe



Vaughn



ps Please don't try thinking about that one with a keyboard in front of you . . .

 

[jponder]

why do you guys keep interupting rbatelle while he's having a discussion with himself! Get some manners!



Okay RB> Continue on

 

[rbattelle]

It's so hard to talk to yourself when others keep interrupting! Maybe I should make my own bulletin board where I can just have lengthy discussions with myself!



Vaughn,



If you're referring to a vacuum on the downstream-side of the orifice, in the case of incompressible flow Bernoullis equation provides a simple relationship between area ratio, pressure differential, and velocity.



For a compressible flow through an orifice, once the flow is choked (M=1 at the throat), mass flow is constant regardless of pressure downstream of the orifice.



I suspect you're older, wiser, and more educated than me, so if I've mis-spoken to myself again, feel free to correct me!

 

[SBashaw]

Sooooooo,... ... ... basiclly... ... ... ..... , it just makes its go faster!

 

[KBennett]

Vaughn, wait for rbattelle to answer himself or you will be arguing to more than one personality

 

[rbattelle]

:-laf

 

[Vaughn MacKenzie]

ROFLMAO

Right, Kbennet . . . rbattelle, I'm not stopping you from re-evaluating your response