Diesel fuel cooling

[white exec]

A recent post threw up that this vehicle
C5 5 DOOR SALOON 2.0 HDi 110 (DW10ATED)
was fitted with a diesel fuel cooler - #9 in the diagram...

I'm used to the idea of diesel fuel needing warming in cold conditions, but the requirement for cooling was something I just hadn't realised.

Googling the issue produced this, which is very informative:
http://forums.tdiclub.com/showthread.php?t=20401

[GiveMeABreak]

To answer your question the fuel cooler is on the return line Chris.

Given the high temperatures of the diesel in an HDi can be as high as 36,000 PSI for newer Diesel engines, it has to be cooled before it goes back into the plastic tank.

[white exec]

The article quotes anything up to 3kW of heat being returned from a modern HP diesel, so I can appreciate the need to remove some of that, as opposed to half that on previous generations of injection.

It also quotes the most efficient fuel temperature for diesel being 45ºC, as the best trade-off for fuel density, lubricity and viscosity. That might explain why many diesel cars run so much more smoothly and with more available power on colder days, and on hot days before full under-bonnet temperatures have been reached. On these colder days, and after start-up-overfuelling has backed off, not only is the fuel cooler (more dense), but so is the density of the air.

Some other web articles explain that fuel coolers are installed in the return line not just to protect the plastic tanks, but also because the rate of removal of heat is higher when the contents of the cooler are higher. Placing the cooler in the supply line would bring far less to the party in terms of heat removal.

All interesting stuff.