Thread: 4bd1/4bd1T and fuel consumption

Originally Posted by Dougal

Fuel is governed to the load on the engine. Your max fuel screw only matters when your foot is hard down.

ah yes, the Engineer Vs the Pilot.

Originally Posted by flagg

Those figures are almost enough to get me to turn back the fuel screw.........




...... Nahhh ! :-p

Originally Posted by flagg

Surely the IP supplying more fuel would cause you to use more fuel? (Or is this an other moment where I find myself asking the stupid questions so that others don't need too)

Further to what Dougal said:

I don't know why people want to call it a 'fuel screw'. It is a stop that limits the maximum load that can be placed on the engine. It is called the 'maximum load set bolt' or I call it the 'maximum load adjustment screw'.

You might like to think of load increase as increase torque, acceleration (increase speed), or increase road grade/slope.

The term fuel screw probably has lead you to assume, incorrectly, that adjusting this screw in or out adjusts the rate of fuel injection, much like adjusting jets in a carburetor. Wipe any such notion from your mind.

When driving, you push the accelerator (no throttle in a diesel) down to increase the load. The 'governor' responds by allowing the 'control rack' to move further in the 'increase fuel' direction. If you push your right foot so much that the governor butts up to the full load adjustment screw, it has gone as far as it can go.

When the engine speed increases, the load decreases, or you lift your right foot, the 'governor' will pull the 'control rack' back in the 'reduce fuel' direction.

If you wind the 'full load adjustment screw' out some more, the 'governor' can then allow the 'control rack' to travel further in the 'increase fuel' direction, providing your right foot has told the governor that is what you want it to do.

At low engine rpm's, the VE (volumetric efficiency) of the engine is higher, so it has more air available to burn more fuel. However at low rpm, the efficiency of the fuel injection pump is lower, and provides less fuel.

At high engine rpm's the VE is lower, so there is less air, but the fuel injection pump is more efficient and supplies more fuel.

With a turbocharged diesel, the turbo produces less boost at low engine rpm's and more at higher rpm's.

The stock governor is set to allow for the above situation, it compensates the full load fuel rate across the range of engine rpm's so the control rack moves further in the increase fuel direction at low rpm and less at high engine rpms.

When you wind the full load adjustment screw out, you have increased how far the control rack can travel at full load, by the same amount across the entire rpm range. The adjustment is not proportional to the stock full load settings.

I should also add.
When I found and fixed my fuelling problem about 6 years ago (boost compensator diaphragm was worn to holes) my travel times particularly in the mountain passes dropped markedly.
But my fuel consumption didn't change.

Being able to pull higher gears on hills is beneficial for fuel economy. Previously I had to use lower gears and thus more engine turns per hill and more fuel.
The savings I made pulling higher gears nicely offset the slightly higher speeds.