Fueling at an idle...

[TORQUE THIS]

Do injectors fuel differently at an idle? Does a stock injector flow the same as a larger injector? What if a fueling box is added to the system? Does it change anything?

 

[nickleinonen]

the injectors fuel less at idle. on a hydromechanical system, the rack [on an inline pump, rotarys are more complicated] shortens the effective stroke on the plunger so it only injects fuel until the spill port is uncovered. on electronic injectors, it works similarly, but there is an electronic solonoid that controls the opening and closing of the spill port. to be honest, i am not quite sure how it works on the 98. 5-2002 cummins engines... never learned or researched that before...

 

[Bigsaint]

I'm not sure how a larger aftermarket injector wouldn't be flowing slightly more than the OEM injectors, even when at idle ? I'll leave that one for the experts as I am not sure how much more.



The electronic fueling boxes add fuel when under boost, so my best guess is no extra fuel while at idle.



Scott W.

 

[A Johnson]

This question is best answered in generalities.

At an idle the injectors fuel the same as far as the mechanical aspects go. The nozzles start to open at a pressure predetermined by the bias spring. As the fuel flows into the cylinder, pressure continues to rise as the injection pump pushes the fuel thru the nozzle. When the volume decreases, pressure decreases below the preset amount, the nozzle closes.

The governor (mechanical or electronic) adjusts the amount of fuel according to demanded engine rpm.



At idle the commanded rpm is easy to achieve so the bigger injectors really don't have much effect.



In a WOT situation the fuel pump is trying to maintain or get to the upper RPM limit so it tries to put as much fuel in the engine as it can to get up to the top rpm, and bigger injectors can increase the fuel flow to help achieve this.



The guys with the P7100 pumps increase fuel volume with the rack stop plate which allows the delivery valves to open further pushing more fuel thru the same size injector.



The twenty four valve guys get injectors with a higher flow rate which allows more fuel of the same pressure thru the nozzle. The fueling boxes can be thought of as an electronic rack stop plate.



A stock injector doesn't flow the same as a larger injector, the larger injector may have a lower or higher popoff pressure, more or bigger holes in the nozzle, different spray angle etc.

If pop off pressure is lower in the larger injector, timing will be advanced somewhat and end of injection will be later. (again speaking in generalities)



The amount of fuel may be less to maintain idle depending on the amount of advance that has taken effect.



Basically it takes a given amount of fuel to maintain idle rpm and for all practical purposes at idle the injectors fuel about the same amount regardless of WOT flow.



I'm not really a fuel system expert but this is the way I percieve it to be.



I'm sure some one will correct me with any major points I missed.

A Johnson

 

[drees1]

Good explaination!!