How To - Understand Fuel Tanks

[Mike Noc]

Originally Posted by T-Cut

I thought the HP leakback is always hot?

TC

Yes I was thinking it's always hotter than the fuel in the tank but on a freezing cold day at first start up the fuel will be at ambient temperature. So the heat transfer is pretty much constant and warm fuel will become hot fuel, but cold fuel will only become warm.

It will pick up heat as it goes through the LP circuit, but it is starting at a very low temperature so instead of introducing more cold thick fuel into the circuit the thermostatic valve restricts fuel to the cooler and the returning fuel goes round again by joining the outlet pipe from the ITP where the loss to the HP pump can be replaced.

As the fuel comes up to temperature so the thermostatic valve opens more and allows hot fuel back to the tank.

I don't know this is happening, there is no description to cover it, but looking at the circuit and the thermostatic valve it looks like it would operate this way.

Whether the thermostatic valve closes completely, or merely restricts the flow I don't know, but the way it is piped if the fuel isn't going through the cooler then it must join the outlet from the ITP which is the supply to the UBP.

Mike

[Mike Noc]

Originally Posted by StuBlake13

What a valuable thread Great insights to be gleaned here and being new to diesel it has me intrigued as I've never heard of this Quarter Tank Syndrome and other than this thread and one regarding Dodge vehicles there is nothing more Google can offer me. Can someone elaborate as to where this term is derived from?

Hi Stu. On the diesel engined cars, if the intank fuel pump packs up the car will run fine until it gets down to around a qaurter of a tank of fuel. It will then stop with the under bonnet pump sucking air.

This seems to happen more on a motorway or other long straight roads which could be due to the fuel not sloshing over the central part of the saddle tank.

That's the Quarter Tank Syndrome, and if we can understand what is going on in the tank and low pressure side of the fuel circuit then we will find out exactly what causes it.

All good fun!

Mike

[HarryM1BYT]

Originally Posted by Mike Noc

As the fuel comes up to temperature so the thermostatic valve opens more and allows hot fuel back to the tank.

I don't know this is happening, there is no description to cover it, but looking at the circuit and the thermostatic valve it looks like it would operate this way.

Whether the thermostatic valve closes completely, or merely restricts the flow I don't know, but the way it is piped if the fuel isn't going through the cooler then it must join the outlet from the ITP which is the supply to the UBP.

Mike

That is not the way I would expect it to work. Surplus fuel has to be returned to the tank, irrespective of temperature, so I would expect the thermostat to simply determine whether the fuel returns via the cooler or not via the cooler. I've looked at it, but even the the pipewark around the cooler is quite complex.

The point of the cooler is to prevent the fuel temperature rising above a certain temperature and melting the tank.

[Mike Noc]

Originally Posted by HarryM1BYT

That is not the way I would expect it to work. Surplus fuel has to be returned to the tank, irrespective of temperature, so I would expect the thermostat to simply determine whether the fuel returns via the cooler or not via the cooler. I've looked at it, but even the the pipewark around the cooler is quite complex.

The point of the cooler is to prevent the fuel temperature rising above a certain temperature and melting the tank.

Yep I'd expect it to go that way too but looking at the pipework there isn't a path for the fuel to bypass the cooler and go back to the tank.

If it doesn't go through the cooler it joins the outlet of the ITP. There is a small restrictor that leads back to the tank, but that also connects to the ITP outlet, so I'm assuming not much would get past it or too much of the ITP outlet fuel would go that way too.

I do think the thermostatic valve's job is to stabilise the fuel temperature, and there is a benefit in doing this as it warms the fuel in very cold climates and makes it less viscous.

If they just wanted to ensure the fuel didn't get so hot it melted the tank why not send the fuel through the fuel cooler full time and do away with the thermostatic valve?

Mike

[T-Cut]

If the fuel needs to be at a certain (warm) temperature for efficient LP pumping, would there not be a fuel pre-heater system to allow completely cold starting to work?

TC

[Mike Noc]

Don't think it needs to be a minimum temperature - I'd liken it to the thermostat in the cooling system. The engine will run cold, but the coolant thermostat ensures it gets up to operating temperature quickly so it can run more efficiently.

Mike

[T-Cut]

I'm getting confused over the temperature gradients around the tank. I can't understand why the fuel in the tank has to be warmed up. I guess I need to read more.

TC

[Mike Noc]

Originally Posted by T-Cut

I'm getting confused over the temperature gradients around the tank. I can't understand why the fuel in the tank has to be warmed up. I guess I need to read more.

TC

I might be barking up the wrong tree but looking at the way the return line is piped up if the fuel doesn't go through the cooler it can't get back into the tank so goes round again so to speak.

The only reason I can think of for doing it this way is to get the fuel up to a minimum temperature at first start up, even if its only needed in frozen climates, and then regulate the temperature using the thermostatic valve to ensure the fuel doesn't get too hot.

Mike

[Mike Noc]

Originally Posted by T-Cut

I'm getting confused over the temperature gradients around the tank. I can't understand why the fuel in the tank has to be warmed up. I guess I need to read more.

TC

Had a good trawl through the net to see if fuel is warmed in a common rail diesel system and didn't come up with anything until I found this:

http://www.christiantena.pwp.blueyon...uelsupply.html

TBH I can't see why the circuit is piped up the way it is if it isn't to make it capable of warming up the supply fuel, even if only at first start up in freezing conditions, as well as cooling the hot fuel returning to the tank once everything has warmed up.

Mike

[T-Cut]

Originally Posted by Mike Noc

TBH I can't see why the circuit is piped up the way it is if it isn't to make it capable of warming up the supply fuel, even if only at first start up in freezing conditions, as well as cooling the hot fuel returning to the tank once everything has warmed up.

In the interests of science, I've just forked out £40 for the Xpart Manual. I hope this contains enough detail to establish what goes on.

EDIT: Reading your link (which refers to pre-heating as well as cooling) and your earlier stuff, the MGR system could be seen as a way of controlling the temperature of circulating fuel. After all, the cooler is there to cool and if there's a bypass, the implication is that fuel doesn't always need cooling.

TC