What the **** is a droop screw????and what does it do????..
Unlike most gasoline engines, all diesel engines have a governor. The governor does two main things: 1) it sets the maximum speed the engine can turn, and 2) it forces the engine to turn at a fixed speed, determined by the pedal position, regardless of load... in other words if the pedal is at 1/2 full, the engine will always spin at, let's say, 1700RPM.... Well assuming it has enough horse power...
The fuel adjustment screw (smoke), sets the absolute maximum amount of fuel you can squirt into the combustion chamber under full throttle, full load conditions. The idle screw sets the minimum speed the engine can turn, and the flyweights have a spring that sets the maximum speed.... but something is still missing from this model.... stability.
Governors are always a compromise.
Let's assume that the pedal is 1/2 way depressed, and the engine is turning 1700RPM, and the load is suddenly increased. The RPMs are going to drop, or droop, and the governor is going to increase the fuel in an effort to bring the speed back up.
How much fuel should the governor give the engine to do this?
If it gives the engine full fuel, the engine will take off, overshooting the 1700RPM desired speed, and then cut the fuel back (because it is going too fast) and undershoot, and then give it full fuel again and overshoot, and .... Well, it will start doing a very annoying thing called hunting (or surging).
If, on the other hand, it only gives the engine a tiny amount of fuel, it won't start hunting, but it will never get back to the set speed of 1700RPM... it will be a wimp.
This is where the drop, or droop, adjustment comes into play. It allows the mechanic to setup the engine's governor to best match the particular application for the engine.
If the drop/droop is set too tight (won't let the engine drop in speed), the engine will always overshoot and then undershoot the set speed when the load changes, eventually (hopefully) settling in on the desired speed. If it is set too loose, it will never get back to the set speed when the load increases.... the gutless wonder syndrome.
Our droop/drop screws are probably always set correctly when the injection pump leaves the pump stand at the depot (I hope!), but at some point in the life of the engine, there was a change order to disable the Fuel Density Compensators (FDC) that gave an emergency one-size-fits-all setting for both the fuel (smoke) screw, and the drop/droop screw to compensate for disabling the FDC. The problem is it is easy to adjust the smoke screw, but you have to take things apart to set the drop/droop screw... so human nature being what it is, it probably didn't get done on some of our trucks.... or if it did, one-size-fits-all solutions are generally one-size-fits-none solutions anyway. There is a reason for the injection pump stand.
In review, the idle speed screw sets the minimum set RPM for the governor, the fuel (smoke) screw sets the maximum amount of fuel charge ever set by the governor, and the droop/drop screw sets how much to let the governor reduce the engine speed when a load is applied, in the name of stability.
-Chuck
