dieseleux said:
Spray change with pressure, more pressure in cylinder, density of air are higher, less velocity on spray.
I agree completely.
Fix_Until_Broke said:
Scott02 - I see what you're saying but not sure that the dP across the nozzle tip varies that much and even if it does the area that it acts upon is very small relative to the area that the fuel acts upon.
SOI cylinder pressure will be ~500 PSI during startup/low load conditions (similar to a compression test). SOI cylinder pressure even at double the mass of air in the cylinder at full load might be 1000 psi. This would effectively lower the absolute pressure required to open the injector, but only by the very small area of the needle tip exposed to the cylinder pressure. I'm just winging this here, but the differential area between the annular part where the fuel acts upon and the nose area where the cylinder acts upon has to be 10:1 or more.
So even a net 500 psi increase in cylinder pressure will only lower the absolute break pressure of the injector by 50 psi at the most.
From one of my pop pressure summary sheets I have:
Pilot/Primary = 240bar/300bar = 3481psig/4351psig = 3495.7psia/4365.7psia
The pilot pop dP when pop testing is 3495.7psia/14.7psia or
237:1
The pilot pop dP in the engine is 3481psig/~500psig or
6.7:1
Surely this has more of an effect that most think it does.
But when Whitbread pop tested these nozzles,
I'm assuming that he was only looking at the spray pattern of the Primary injection.
Which my noggin tells me is even a worse situation than the Pilot injection stated above.
My reasoning is when that primary injection ignites, it's going to raise that normal cylinder pressure of 500psig up even higher. How much higher, i have no way of quantifying, but it will make the difference in dP's even worse than the above stated numbers.
...Or this is at least what my head has been telling me, and is something that has been bothering me for a very long time.
Someone smarter than me, please feel free to set me straight if i'm wrong on all this.
Comments?
EDIT: and of course this this is talking the cylinder pressures at idle, and no boost added.