Puzzle Corner

Quantum-man · Post by **Quantum-man** » Sun Dec 27, 2009 9:46 pm

So whilst some of those who would be top brain here are obviously busy eating turkey I shall take this time for a quick interlude.

Firstly I digress:
I think this program reached the US in the '70's. Alas both presenters have now died this year...
http://www.youtube.com/watch?v=WCYKGei2fyY

Anyway now to ''talk turkey" [Not the Aussy Lesbian mother's guide ]
Recently on another forum not a stone's throw away, I made a bold statement re injector construction

"The requirement to use the washer style shims to allow fuel leak-off is an Urban Legend" possibly originating from the Mercedes injector experts.:

Amazingly none of the resident boffins challenged my statement...

So, in the interests of killing the myth off so that experimenters will feel free to make their own simple shim adjusters from sprung steel, here's the proof:

Take one standard spare injector.
Split the body.
Take the top half, and gently remove the spring.
Typically, the shim will stay up inside the top, stuck in with the diesel.
Tapping the top gently often releases the shim.
Inspecting it, it is basically a flat washer in profile.
Looking up inside the body top, it is clear that there is a machined face with a dimple in the middle. The dimple is NOT a passageway.
Clearly the machined face is what the washer sticks to, so the leakage fuel does not pass between shim and body face.
Closer inspection reveals a machined recess all round the circumference. This is about 2mm x 2mm [rough guess.] Hold body up to light/computer screen and the position of the 2 leakoff barbs will become apparent.
Now replace shim using the spring and then again remove the spring and also peer back down. The shim will have centralised and the light will be seen STILL shining through the barbs.

How then does the diesel get from inside the spring to the outside? Well initially the gaps between the coils allow seepage.
I'm assuming that the pintle lift at maximum actually shut the coils together at full opening.
So how does the diesel escape?
If you now inspect the coil ends it can be seen that the ends are both machined flat. [Some folks grind this down for adjustment purposes] Look carefully at the very end of the coil and it can be seen that there is a notch out that allows the diesel to pass.
It is the same either end, so unless both ends are ground away, the diesel will simply pass through the two gaps.

Case proved

Ah, just to tidy things up.. Why then is the shim holed?
Well if the shim was solid, then it would be harder to change them out when callibrating by shim change. The dimple in the middle may just be a legacy from construction, but, I feel it is useful as a pivot point for an 'L' shaped shim remover that can hook the shim in the middle and flick it out.

Quantum-man · Post by **Quantum-man** » Sun Dec 27, 2009 9:55 pm

TonyB wrote:OK, let's think about this a little. When the injection pressure rises to the point where the injector "pops off" the fuel is injected into the cylinder. So for the dynamic timing of the pump to advance, the injection pressure would have to reach that point sooner in the cycle, therefore popping off the injector sooner.
If the washer under the timing access bolt on the back of the pump were missing, the volume of the chamber where the fuel is compressed by the piston would be reduced slightly. The injector pop-off pressure would be reached earlier in the piston's stroke, therefore advancing the dynamic timing slightly.

TonyB
Jacksonville, FL
81 Dasher 1.6na

Tony whilst writing my interlude, you solved it.

For some reason my pump is lacking the washer. It doesn't leak though, when gently nipped up.
My bolt head looks slightly strange. I'll compare it with one on another pump when I get round to it....
The washer being copper actually flattens slightly for each timing measurement, unless renewed.

Would you have solved it without my hints?

TonyB · Post by **TonyB** » Sun Dec 27, 2009 10:49 pm

All the hints helped. The only thing you do when you measure the static timing is remove that bolt. Tonight I was staring at a picture of the internals of the pump & realized that the bolt formed one wall of the chamber where the fuel is pressurized. Aahhaa! Good puzzler!

TonyB
Jacksonville, FL
81 Dasher 1.6na

rabbit_man · Post by **rabbit_man** » Mon Dec 28, 2009 1:17 am

Now that would have to mean that fuel is compressible.

If there is a washer under the bolt then that area that had been filled with the bolt is now fuel, so for that to change your timing....the fuel would have to squish slightly. That really confused me

I would've thought that it wouldn't matter whether it's fuel or a bolt taking up that area.

Quantum-man · Post by **Quantum-man** » Mon Dec 28, 2009 3:27 am

rabbit_man wrote:Now that would have to mean that fuel is compressible.

If there is a washer under the bolt then that area that had been filled with the bolt is now fuel, so for that to change your timing....the fuel would have to squish slightly. That really confused me

I would've thought that it wouldn't matter whether it's fuel or a bolt taking up that area.

Diesel IS compressable. ....1% per 100bar.
If anyone thinks that the diesel's high pressure is limited to the 150bar of the injector, they are most certainly mistaken

TonyB · Post by **TonyB** » Mon Dec 28, 2009 11:17 pm

Now that would have to mean that fuel is compressible.

If there is a washer under the bolt then that area that had been filled with the bolt is now fuel, so for that to change your timing....the fuel would have to squish slightly.

Think of pumping up a car tire vs. pumping up a bicycle tire. The smaller tire will reach 30psi sooner than the large tire will. Yes, air is much more compressible than fuel and we're pumping up a smaller area than a tire, but the same principle applies.
If there is no washer under the bolt, then that area in which the fuel is pressurized is smaller. That means the pressure will reach the injector pop-off pressure sooner and so the injection of fuel will start sooner.

TonyB
Jacksonville, FL
81 Dasher 1.6na

TylerDurden · Post by **TylerDurden** » Wed Dec 30, 2009 9:48 am

So, why regard the timing advance as dynamic?

Seems more like a static advance, since it does not vary during operation.

Quantum-man · Post by **Quantum-man** » Wed Dec 30, 2009 10:44 am

TylerDurden wrote:So, why regard the timing advance as dynamic?

Seems more like a static advance, since it does not vary during operation.

Every time I use the dial gauge I get the same reading off the top of the pump piston [assuming I don't use the little copper washer when taking this reading]
If I take up the option of using my diesel strobe light, which uses the pulse of diesel from the injector fuel line to 'strobe' which gives a dynamic reading, then this will change over time due to the crushing of the copper washer.
In my case I don't seem to have a copper washer at all, so that the high pressure chamber is smaller than most, so the 'capacitor effect' is reduced (remember 1% volume reduction /100bar of pressure may be significant when the capacitor volume is significant compared to the fuel line bore coupled with the injector volume.

It now becomes apparent just how important those one-way valves at the start of the fuel lines are...

TylerDurden · Post by **TylerDurden** » Wed Dec 30, 2009 11:11 am

The change in displacement due to a missing washer seems insignificant to timing; considering the total volume of pressurized fuel includes the pressure-chamber, plunger bores & recesses, head passages, delivery valves, injector lines and injectors. They all are open to each other, above break pressure.

Quantum-man · Post by **Quantum-man** » Wed Dec 30, 2009 9:42 pm

I don't have the exact figures, but I think the effect would be noticable, because
1) The loss of a washer would be over a mm. Piston thrust is only 2.3mm.

2) Because of the oneway valves at the pump, they maintain a degree of pre compression up the capilliary tube and into the injector.
If it would stop raining, and I can find a washer, I could do a simple practical test.

In any event the puzzle wasn't concerned with the size of the effect, but merely that there was an effect

wolf_walker · Post by **wolf_walker** » Wed Dec 30, 2009 10:45 pm

Good one.

I'd be quite interested to here a with and without reading from your advance measurement light.

TylerDurden · Post by **TylerDurden** » Thu Dec 31, 2009 12:03 am

The pressure remaining in the lines will be close to the injectors' break-pressure.

Once the pressure in the high-pressure chamber exceeds the line pressure, the delivery valves are forced open, making a continuous path from chamber to injector, as well as to the cutoff bore.

So, while the stroke of the plunger is only 2.3mm the total volume of pressurized fuel is much greater than the bore of the chamber and the +/- 1mm X 6mm displacement change.

Post by **Fatmobile** » Thu Dec 31, 2009 1:34 am

Seems like it would reach breaking pressure and open the delivery valves earlier with the smaller chamber.

TylerDurden · Post by **TylerDurden** » Thu Dec 31, 2009 9:08 am

Absolutely.

The extended puzzle is, how much... A few sec^-6?

Would the thermal expansion of the lines & other components be more significant?

Also consider the bolt is modified: the threads are flatted on one side to allow fuel to reach the copper washer, so the high-pressure chamber is larger than just the plunger bore.

Post by **Fatmobile** » Thu Dec 31, 2009 11:48 am

I don't think the lines would have any effect on when the chamber reaches the breaking pressure,..
until it reaches that pressure they aren't even in the picture.

Definately an enlightening topic.