Fuel return design - PeachParts Mercedes-Benz Forum

bgkast · #1 01-29-2006, 05:31 PM

I am wondering why diesel engines use a fuel return system? I have read that it is because the diesel warms up when running through the IP and warm diesel does not lubricate the IP properly so the return line allows the diesel to cool before running through the system again. Can anybody confirm this? Thanks, -Bryan

DieselAddict · #2 01-29-2006, 10:29 PM

Doesn't every vehicle have some sort of a fuel return system? The fuel or injection pump always needs to pump more fuel than is necessary so that you don't get fuel starvation when suddenly more fuel is needed. The excess is returned to the tank. This does have the effect of warming the fuel, but that's beneficial. It keeps the fuel from gelling when driving in very cold weather and warm fuel ignites easier than cold fuel. I don't think the temperature of the fuel has a significant effect on its lubricity.

Ken300D · #3 01-29-2006, 10:42 PM

It would be interesting to pull the return line sometime where it comes back to the fuel filter bracket just to see how much is being returned.

On other diesel engines I have seen, these return lines last a lot longer than those on the MB engines. Yet, they seem to be the braided rubber type.

Hmmmmm...... Maybe a length of Yanmar or John Deere return line might be higher quality?

Ken300D

Ara T. · #4 01-29-2006, 10:58 PM

My return line is 20 years old and seems to be doing fine, how much more durable to you want it to be?

bgkast · #5 01-29-2006, 11:59 PM

Well I am trying to decide if I should use a looped system or a full return system for my Veg oil conversion. Many Veg oil users loop the return from the IP back to just before the lift pump. It seems to work for them, but there must me a good reason to have a return line back to the tank...

Addicted · #6 01-30-2006, 12:16 AM

wrong forum,
here but since I just answered to you on the other post I thought I would say something here. If you look at the pictures on the forum I put up for your WVO question you can see I run a looped system. Seems to work fine.

Interesting question though. The only reason I use is looped system isI don't have to worry about contaminating my diesel tank with WVO every time I switch back.

P.E.Haiges · #7 01-30-2006, 12:30 AM

bgcast,

Three reasons why a fuel return system is used are:

1. The injectors leak a small amount of fuel and it must go somewhere.

2. It allows the fuel system to bleed itself.

3. The fuel pressure in the IP should be constant, so a release valve used to do this and the overflow fuel must go somewhere.

P E H

pawoSD · #8 01-30-2006, 01:37 AM

These engines return quite a bit of fuel, anyone who's done a purge with a bottle has seen the amount, its probably around a couple gallons per minute or so, there's quite a bit of flow going on. How would a fuel loop instead of a return even work, I imagine that'd cause some big problems, pressure in the loop would keep getting higher, or the pump would be damaged, or lines would blow off, or something, it needs to be equalized by being returned to the source, thats why it seems to be the going trend for wvo users to use a 6 port electronic valve to select fuel sources.....

Addicted · #9 01-30-2006, 02:15 AM

Well here is my guess and I wish I could draw a picture. But the fuel being looped back down the hose has less fuel than it did after it left the pump. The pump pulling the same amount all the time simply pulls that fuel back along with new fuel from the tank. Its hard to see on my other post, but you can see the 6 port pollack I have on there.

All I know for sure is that I've been using the closed loop system for about
6k and it seems to work.

Hope someone can correctly answer this soon. The system makes sense to me in either closed or looped returns.

bgkast · #10 01-30-2006, 02:15 AM

Thanks for all of the replies. I know that many people have used looped systems successfully but I am thinking I will add a return line into my system design. The only way I can figure out to do it and have a separate VO and diesel filter and keep the stock arrangement of the diesel filter between the lift and injection pumps is to use 2 6 port valves as in the attached diagram. It's a bit complicated, but it will allow me to keep the diesel system set up as the factory designed it.