When your common rail returns fuel to the tank, it runs it through a cooler if I'm not mistaken.

Why?

Cause hot fuel.....its hot!:eek: I have no ideal.:idea:

LATER!

When your common rail returns fuel to the tank, it runs it through a cooler if I'm not mistaken.

Why?
I think that the fuel is being used to cool the fuel regulator etc. Also the additional reduction of engine temp via another cooling source.

The fuel gets heated by several processes in the HPCR fuel system. First, the fuel drawn from the tank passes through the electronics packages of the two FICMs to cool them, then the fuel undergoes tremendous heat of compression in the injection pump and common rail, finally the return fuel has bypassed and cooled injectors that are plugged into the firing cylinders. So the fuel is heated by the system before returning and run through the fuel cooler so that the tank can be a reservoir of not only fuel but cooling liquid for the injection system.

Extremely well said Idle Chatter. Nice to find someone who really knows what is going on inside a diesel engine

Doesn'tit also have something to do with condensation in the tank, and bacteria growth?

Tom hit it on the head

I was wondering because if the viscosity of diesel fuel gets lower as it gets hot, heating the fuel might make more power. If it's used for cooling, it's probably not wise.

that was my thoughts also mcrat. you would rather have hot fuel and cool air for optimal combustion, atleast from what I know.

It's standard practice for high pressure fuel systems that use a return. Passing the fuel under high pressure heats it up and the unused fuel that goes back into the tank thru the return can get warm enough in some applications to begin to vaporize. Hot fuel in the tank bubbles introducing air into the fuel that is then pulled back to the rail resulting in inefficient combustion and other bad possiblities.

I run one on my Trans Am because it's a bigger problem with gas and the nice Weldon 2035 pump on the street. Sure I could use a pump controller to turn it down but then you're just fumbling for the power switch when you should be shifting. :)

Rick

Regarding fuel being returned to the tank, any heat energy added via compression will be lost when pressure is lost (conservation of energy law of physics). There is considerable heat added, though, from the other sources identified by Idle_Chatter and, without cooling, it would be cumulative. In an older vehicle without a fuel cooler, I've had fuel boiling in the tank. This was after hours of low rpm, low gear, low range rock crawling on a Mexican "road" on a hot day at high altitude and never over 3 mph. It's a scary sound.

'Had the same thing happen with a gasser on Independence Pass in Colorado (12,000').
Really Scary!

I was wondering because if the viscosity of diesel fuel gets lower as it gets hot, heating the fuel might make more power. If it's used for cooling, it's probably not wise.
Actually cooler fuel makes more HP. :exactly:

abear, that's true with gasoline, but perhaps diesel is different...?

I still have an old cool can with 1/2" line, maybe I could use it when adding a lift pump... ;)

Here is my understanding of why diesel fuel is cooled from working on heavy trucks. We all know diesel lubricates the fuel pump and injectors when is gets hot it loosed its lubricating ability. Also, fuel cools the pump and injectors, all systems pump more fuel than they use. That is why there is return fuel. Most trucks warn the driver if the fuel temperature goes too high. As far a power goes physics comes back into the picture. There are more molecules in a volume of cool fuel than in that same volume much hotter.

Actually hydrocarbon fuels/oils expand significantly when heated which will increase the volume and reduce density. The more dense fuel is the higher the BTU value is in relation to the volume, plus the lubricating film will be stronger as with any lubricant so cooler fuel is a win/win situation.

steel, We posted at the same time but it appears we are saying the same thing. :)

abear, that's true with gasoline, but perhaps diesel is different...?

I still have an old cool can with 1/2" line, maybe I could use it when adding a lift pump... ;)
The thing that sucks about cool cans in when you do not have ice in them, they act like a big heat sink. I ran an empty aluminum cool can one time. I squared a piston. :o:

A few sources of heat to the fuel system:

Pump inefficiency. Any pump energy that does no go into hydraulic energy, adds heat. With liquids, which do not compress much, there is no heat-of-compression per-se...like for gasses.

The fuel is used to cool the FICM. More heat there.

Most of the heat is introduced when the high pressure fuel is let down in pressure at the regulator and within the injectors through the pilot port. The injectors uses fuel energy (delta-P from supply to the pilot port/return) to help open and close. Liquids heat up when they have their pressure reduced due to all the pressure energy converted to heat. There is very little volume-expansion beuause of liquid incompressibility...so no cooling from pressure let down. Liquids behave differently from gasses.

General heat pickup via the hot engine bay...just good ol' conduction through the tubes here.

The fuel needs to be kept cool to keep it below the flashpoint, and to keep it from heating high enough to soften the tank, and other plastic bits in the system.

Warm fuel should have a smaller ignition delay due to better vaporization. Diesel does not evaporate enough for the extra enthalpy from the warm temperature to give any appreciable cooling like for gasoline.