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mikefossl · 15-03-2013, 01:43

Approximately how much fuel flows back to the tank when a diesel engine is running at 1/2 throttle?

daddle · 15-03-2013, 04:01

Quote:

Originally Posted by mikefossl

Approximately how much fuel flows back to the tank when a diesel engine is running at 1/2 throttle?

1. Depends entirely on the engine type.
2. Diesels usually have a fuel or speed control, not a throttle.
3. Might be more proportional to RPM rather than throttle. Might be constant on some engines.
4. The small boat diesels I have worked on returned maybe 10-20 liters per hour to the tank at most speeds. Far more that they consume.

mikefossl · 16-03-2013, 17:20

So the reason I'm asking is that I'm adding a secondary fuel tank which has only two connections. One is obviously for the fuel fill, the second for the draw. Is there any reason not to plumb the engine fuel return back to the fuel draw? Does it need to go back to the tank?

daddle · 16-03-2013, 17:36

Best if it goes to the tank as it then clears any air that gets in. If it just goes round and round the air may accumulate enough to eventually cause a problem. There may be a heating issue as well ??

Good idea to go to the bottom of the tank too because it some cases air bubbles can work their way to the engine while it is shut down. But a low loop in the hose should do the same function.

Can you fit an inlet to the access plate? Or tap one into the side of the fill tube?

Kettlewell · 16-03-2013, 17:40

I'm not a diesel expert, but I don't think it is a good idea. For one thing, I haven't seen a traditional marine diesel that does this, so there must be some good reasons. I would imagine that hot diesel might not be so good for the injection pump, and I wonder if the loop would create some sort of fuel block. But, as I said, I am not certain about this.

NewMoon · 16-03-2013, 17:58

If I understand correctly, the cooler diesel from the tank is what lubricates and also keeps the injectors cool enough - tight clearances there, and a lot of potential friction and wear.

I'd say it ought to go back to the tank it's being drawn from, or you could be surprised how much the level of diesel shifts from one tank to another. We've set the valves such that the return went to a different tank, and it made quite a difference - we no longer do that.

bobconnie · 16-03-2013, 19:23

Most of the time you would want the fuel to return to the tank it came from. But at times you might wish to return the fuel to another tank, to change the trim of your boat of course this depends on the placement of your fuel tanks. Ive always liked day tank setups, as then if placed right they have no trim affects on a boat ! just my 2 cents

markpierce · 16-03-2013, 19:43

The recommended practice is to return fuel to the same tank it was taken from.

mikefossl · 17-03-2013, 02:07

Quote:

Originally Posted by daddle

Best if it goes to the tank as it then clears any air that gets in. If it just goes round and round the air may accumulate enough to eventually cause a problem. There may be a heating issue as well ??

Good idea to go to the bottom of the tank too because it some cases air bubbles can work their way to the engine while it is shut down. But a low loop in the hose should do the same function.

Can you fit an inlet to the access plate? Or tap one into the side of the fill tube?

Tapping into the fill tube. Inspired!

Thanks.