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Old 02-04-2010, 02:28 PM   #1 (permalink)
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Diesel engine braking puzzle

Help I am baffled! I put a domelight switch into the fuel cut-off circuit of my Diesel Rabbit hoping to create a button I could push to increase engine braking by going to zero fuel flow. So far, in the few times I've tried it, when I let go of the button fuel flow did not start again. In each case, the engine immediatly refired with the ignition switch. The fuel cut off solenoid is about 6.2 ohms, and the switch varies from about .3 to 1.0 ohms depending on how you let go of it. If you just ease it off the resistance is higher, than if you let go quickly. I used about 12 feet of 14Ga. 19 strand wire. The circuit diagram just shows the solenoid on a staright track from circuit 15 to ground. Is circuit 15 on a current limiting resister that is bypassed while cranking? Can anybody help me with this? I am convinced I can boost my mileage if I can get this to work. The car runs down a level road at 27 mph at the idle fuel flow, so I don't get much engine braking and the brakes get worked more than they should have to. You can turn off the igition to coast but when you turn the ignition back on the poor glow plug timer gets turned on so that's not a good way to go.

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Old 02-04-2010, 06:07 PM   #2 (permalink)
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Progress to report

After running a test looking at fuel cutoff solenoid voltage during cranking vs running I could see no evidence of two levels of voltage. So I took it out for another test. This time I let the switch snap shut, and got a lot better results. It worked most of the time, So I guess it will work after all, I just need a better switch.

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Old 02-05-2010, 02:01 AM   #3 (permalink)
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Could you give some details about your engine (year, type, etc.)?
I read somewhere that turning off the fuel pump while engine braking in gear can cause bad things, but that may be for the hitech commonrail DI turbodiesels. As for older types (your case, I believe), it may be OK. Not sure though.
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Old 02-05-2010, 02:27 AM   #4 (permalink)
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Fuel cut off

1981 VW Rabbit NA Bosch rotary pump. I have no clue as to why cutting the fuel flow off could cause any damage. This is how the engine is normally controlled. My most recent tests using a rocker switch, were 100% successesful. I wanted a momentary interupt switch, but the one I tried closed with too much variability in the resistance to act reliably. Hecho in Mexico.
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Old 02-05-2010, 12:43 PM   #5 (permalink)
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Rotary injection pumps are lubricated by fuel. You need to be absolutely sure that shutting off the fuel solenoid doesn't affect lubrication of the pump. Your switch would not be an issue if you were shutting the engine off with it, but if your goal is compression braking, I'd do more research. Spinning a dry rotary injection pump is something you will regret. You might want to talk to a diesel shop that rebuilds Bosch rotary pumps.
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Old 02-06-2010, 12:22 AM   #6 (permalink)
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Thanks for the caution Fumes

Thanks Fumes but this action certainly does not leave the pump dry. If it did it would have to be reprimed and that is not the case. It does not cut off the supply of fuel to the pump, rather it spills the output pressure. So the load on the pump is vastly reduced and the lubrication situation is improved. But since I am only 99.99% sure of this I will check it out and I appreciate you giving me a caution. I drove to Tucson and back today and was hoping to improve on my 60.6 mpg trip last time, but I guess the replacement of the Gear oil with Pennzoil Synchromesh, removal of the mudflaps, and the addition of the engine braking switch were not enough to make up for the fact that I drove quite a bit faster even though I was drafting semis most of the time. Driveing around Tucson looking at houses for hours didn't help, nor did the fact that I had to run the lights coming back because it was dark. Oh well live and learn, I got 55.2 mpg. Some real time instrumentation would help me avoid some of the problems but I have not figured out how to implement that. I think the injector spill flow returns to the tank so a fuel flow meter would only give an indication, not a true measurement.
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Old 02-06-2010, 01:59 AM   #7 (permalink)
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Quote:
Originally Posted by Crazyrabbit View Post
Thanks Fumes but this action certainly does not leave the pump dry. If it did it would have to be reprimed and that is not the case. It does not cut off the supply of fuel to the pump, rather it spills the output pressure. So the load on the pump is vastly reduced and the lubrication situation is improved. But since I am only 99.99% sure of this I will check it out and I appreciate you giving me a caution. I drove to Tucson and back today and was hoping to improve on my 60.6 mpg trip last time, but I guess the replacement of the Gear oil with Pennzoil Synchromesh, removal of the mudflaps, and the addition of the engine braking switch were not enough to make up for the fact that I drove quite a bit faster even though I was drafting semis most of the time. Driveing around Tucson looking at houses for hours didn't help, nor did the fact that I had to run the lights coming back because it was dark. Oh well live and learn, I got 55.2 mpg. Some real time instrumentation would help me avoid some of the problems but I have not figured out how to implement that. I think the injector spill flow returns to the tank so a fuel flow meter would only give an indication, not a true measurement.
2 flow meters - one on the input, one on the return.

SentraSE-R has a thread right now where he's giving away some Radio Shack momentary contact switches if you send him a SASE. You might check it out if you need a momentary switch.
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Old 02-06-2010, 02:44 AM   #8 (permalink)
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Old school rules

I went to radio shack and could not find the momentary disconnect function I was looking for in other than a .5 Amp sub-miniature switch. The Pep-Boys dome light switch I tried was all over the map for on resistance 1.2 to .3 ohms or so. That made the turn on of the fuel cut-off solenoid unreliable. I used an old Volvo rocker switch I had that showed a consistant less than 0.2 ohms closed and that has worked great so far.

For the two meter theory, How about two rotameters, a stopwatch and an Abacus? Who needs electronics?
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Old 02-06-2010, 03:29 AM   #9 (permalink)
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Quote:
Originally Posted by Crazyrabbit View Post
I went to radio shack and could not find the momentary disconnect function I was looking for in other than a .5 Amp sub-miniature switch. The Pep-Boys dome light switch I tried was all over the map for on resistance 1.2 to .3 ohms or so. That made the turn on of the fuel cut-off solenoid unreliable. I used an old Volvo rocker switch I had that showed a consistant less than 0.2 ohms closed and that has worked great so far.

For the two meter theory, How about two rotameters, a stopwatch and an Abacus? Who needs electronics?
If that's what makes you happy, I suppose it would be fine.

Personally, I'd just take both figures and subtract the return from the input. I guess I can see how that would be too much of a pain, though.
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Old 02-06-2010, 08:49 PM   #10 (permalink)
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So I'm a little confused here... you are trying to increase engine braking, correct? A tried and true method of this is to use an exhaust brake. I suppose you could install a throttle somewhere in the intake tract and close that to produce engine braking, but I'm not sure if there would be any consequences to doing that (aside from introducing an obstruction to the intake tract).

If you are trying to cut fuel completely while coasting I would use caution. As stated before, injector pumps are lubed by fuel. While the injector pump may stay primed on the low pressure side, the high pressure side might be starved of lube. The fuel cut solenoid drops the collar separating the high from the low side. I'm not sure if this just lets the fuel bypass and go back to the tank via the return line or if the lift pump is still running and slowly pressurizing the low side causing leaks from the seals. Hopefully someone with experience with VW IDI engines will chime in and confirm or deny this.

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