Greetings: Here is a question for stick owners. On a PD4104, 6-71 I am wondering if the the fuel pressure is diverted back to the tank when the fuel rack is placed in a no fuel position. Also: During hot summer days can you coast down long grades with the rack off without damaging the injectors. Seams possible for a short time. Thanks from dcburd!
the fuel is always circulating back to the tank....
that's how a Detroit works....
the injector is bathed in flowing fuel to lube it, to cool it and to meter it vis the pushrod and tappet...
the rack position via the helix just says how much the piston gets
The rack is in no-fuel quite often while driving. Any time you have your foot off the pedal, it goes to no-fuel except at idle. Coasting down a hill is a good example, but so is lifting off the throttle in traffic, while shifting, just revving the engine up and letting it drop to idle, etc. It doesn't hurt anything in the least, and it's good for fuel economy. Any engine with jake brakes and a buffer switch will only turn on the jake brake when the engine is in no fuel. As Eagle notes, the majority of fuel pumped into the engine is always returned to the tank, only a smallish portion ever gets injected into the engine. Most is used to cool and lubricate the injectors.
brian
FWIW the return fuel (spill rate) to the tank is around .7 gpm on a 6L71
Quote from: luvrbus on December 20, 2015, 02:22:17 AM
FWIW the return fuel (spill rate) to the tank is around .7 gpm on a 6L71
Neat number to know! If you happened to be driving at 60 mph, or 1 mile per minute, and be getting 10 mpg while doing it, the injectors would be putting 0.1 gpm into the engine, compared to .7 gpm returned to the tank.
Brian
Manually pulling out on that old fashioned "PULL TO STOP ... PUSH TO START" lever also was a silly but effective way to let a very cold 2 stroke Detroit with MUI with good starting batteries spin without fueling, thus warming up the cylinders/chambers.
Then after about 10 seconds, quickly push in the lever to the start position. Just another old trick probably now forgotten on how to help an old Detroit start ... when it does not want do. We were taught this in 1972 at the Fire Academy. Seemed to work.
I know that trick, learned it here!
Brian
Used that trick myself on my mechanical 6v92 bus. Only I had to actually hold the fuel lever in at the back of the engine :)
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I've looked at this topic a few times. The one item that caught my eye was the part of the original question regarding coasting down long grades. I may have misinterpreted the phrasing -- but for safety sake, we always want to go down a grade in the same gear we'd use to climb the grade, using the engine retardation to keep the bus under control.
I know that wasn't the point of the question, but I can see a new bus owner reading this, and thinking that going down in neutral would save fuel. That money could then be needed for the ambulance and hospital bills. I've been on some grades in my car where brake overheating is a concern. In heavier vehicles, control of brake effectiveness is crucial. The phrase "pucker factor" comes to mind.
Arthur
Actually-if you have a cold start on a mechanical Detroit, start from the engine compartment. Turn on ignition, push in the stop lever on the governor and hold it. Then crank the engine. Count to 5 then slowly release it (over the next 5 seconds) and the engine will just come up to speed. This works because as you push the governor to stop, the injectors retard timing which aides in starting. I've used it several times and have not had to use ether. Good Luck, TomC
Without ether, in the Arctic... you'd be waiting till spring... ???
Funny how some people say "go down in the same gear you go up in" , that rarely holds true. many 7 and 8 percent grades going up I'm down to 1st gear and 19 mph. The same grade going down easily in 3rd gear with the jakes on and never touch the brakes. now, if it's super winding roads theb that's a different story.
Please allow me to expand on what Runcutter said. One of the reasons to keep it in the same gear is heavy duty truck/bus transmissions do not have syncros so you just can't slam it into any gear you want at any time. This is why we were trained to stay in the same gear as it alone will hold you at the same speed you climbed the grade in without jakes or brakes.
If you ever drove a heavy truck or even a bus and it 'popped' out of gear, you would be amazed/scared how fast you accelerate. Brakes along won't stop you and will quickly overheat. Game over.
Sure, if you have jakes or a retarder all the better. The main idea is to keep your speed down to where the brakes will remain effective at all times. So as Arthur said, descending in neutral is a death wish.
Drive Safe. and Merry Christmas!
I agree with the idea and theory with a manual transmission but not with a automatic
Stupid snotty comment that I want to take back, sorry.
Brian
Clifford I agree. I should have clarified my post a little better. my bus is an auto, were it a manual I would most certainly have a different point of view.
Great information! I am studying up on the 6-71 rack. In summary the rack goes to no fuel during normal deceleration with varying return supplies as little as .1 gpm. The governor holds idle. The statement regarding coasting then slipping out of gear with a rack in no fuel got my attention. I am not into thrillers! Thanks!
Where does the .1 come from,the spill rate on 6L71 you test the pump with is .9 gpm @ any fuel not injected is returned to the tank.
Clifford, I came up with the .1 gpm as what the engine actually burns, if you got 10 mpg at 60 mph. It's probably wrong, I did it in my head and my head ain't all that good any more... 10 mpg at 60 mph is 6 gallons per hour, or 0.1 gallons per minute. OK, having done it again with a calculator it's entirely possible I was right... What I wanted to know was the ratio between what was burned vs what was returned to the fuel tank, because I thought that most of the fuel was actually bypassed back to the tank. Now all I have to do is get 10 mpg, but I felt that a nice little gm might get that.
Brian
So if the .1 gpm burned at 60 mph is right, and you buy fuel for $3.00 per gallon, then you spend 30 cents a minute to drive the bus, which is obvioulsy 30 cents a mile, or $18 an hour. Which is about what I always figured.
Brian
Quote from: dcburd on December 23, 2015, 04:33:13 AM
Great information! I am studying up on the 6-71 rack. In summary the rack goes to no fuel during normal deceleration with varying return supplies as little as .1 gpm. The governor holds idle. The statement regarding coasting then slipping out of gear with a rack in no fuel got my attention. I am not into thrillers! Thanks!
No Brian this is the .1 I was referring to the return
Ok, to make it simple, just put it in neutral going down hill and shut it off. No fuel consumption, no fuel return to the tank, and you gain speed. My grand father used to do that, but he had manual steering and manual brakes in his car.
JC
Quote from: lostagain on December 23, 2015, 11:15:07 AM
Ok, to make it simple, just put it in neutral going down hill and shut it off. No fuel consumption, no fuel return to the tank, and you gain speed. My grand father used to do that, but he had manual steering and manual brakes in his car.
JC
.... was he kicking and screaming ?
Nope, grinnin'!