Governor hunting (Diesel Injection) 1

[Ibrahim M]

Hi,

Many times in tuned diesel engines, we see this phenomenon, hunting idle. 

I would like to understand the cause of this.

I know they run stiffer springs (which reduces governor sensivity, and should not increase hunting)

They also use huge injectors, with huge pumps and big fuel lines .

I can think of 2 reasons :

-Inertia of fuel, and  big nozzles cause metering imprecision and fuel may overshoot ?

-Stiff governor spring  may  mimic the engine cyclic irregularity ? ( uneven speed over a 720 cycle )

Heres a video of what i call "hunting"  (@ 1:00 min )

https://youtu.be/ohA4QAVSZlw

 

[TugboatEng]

I would guess the governor isn't hunting but the engine is misfiring due to low compression ratio and or poor fuel atomization.

 

[moon161]

Hunting is a control overshoot. Without knowledge of the control system particulars, I'm gonna guess that with the modified engine, the moving masses- crank, pistons, flywheel are the same, and that this appears at idle and smooths out under load. I think with larger injectors and so on, you basically have a higher gain on the throttle signal that is generated when you are over/under idle rpm, and the same inertia to load the throttle increase response and coast through the throttle decrease response. I suspect adding inertia to the moving parts, or decreasing the gain on the idle controller would smooth it out. I think adding lag to the response would worsen the hunting because the over/under condition will worsen, and reducing lag would improve it.

 

[gruntguru]

Adding mass to the moving parts of the governor could go either way - better or worse. Mass, damping (viscous), friction, spring-rate, linkage ratio are the variables you may be able to change. Reducing linkage ratio (proportional gain) will usually elimunate hunting but response to load and setpoint changes will suffer.

je suis charlie

 

[TugboatEng]

Usually speed droop is added to reduce hunting.