Large heatsink? under floor

[KeithCheetham]

 Does anyone know what this large diecast object is on the underside of my fabia, about under the drivers seat. I have tried looking through 7-Zap but cannot identify it. Nothing is wrong with the car, just curious.

[corndog]

Fuel cooler

[RichardatWakefield]

Hi Keith,

Yes, as corndog says, it is a fuel cooler.

As I understand it, a proportion of the fuel coming up from the tank, through the fuel filter, then going through the Unit Injectors is deliberately "leaked off" back down to the fuel tank in order to carry away excess heat, the air-cooled unit you've pictured does this job.  The unit injectors combine the function of high pressure fuel pump and fuel injector.  Pressurising the fuel will heat it up, as I see it this leaked-off fuel carries away excess heat and the little radiator cools the fuel then returns it to your tank.

 

As I envisage it, even when you lift off the accelerator and go on the overrun (nil fuel is injected), the camshaft will carry on compressing fuel, and generating heat, my guess is this fuel bypass system is vital to keeping the injection system at a comfortable working temperature.

 

My 1.4tdi PD Fabia Mk 1 had one, I'm guessing the current 1.9tdi PD Roomster parked outside also has one underneath.

PD is the German "pumpe duse" or "Dosing pump", their description of the Electronic Unit Injectors used on these engines, the extra loading exerted on the camshaft lobes by these units is the reason our PD engines have a special engine oil grade specified.

 

Hope that helps!

 

BTW, diesels I was working on with canal holiday boats way back when had a leak-off system, as did a later Direct Injection Rover Maestro 2.0di, although fuel pressurisation back then was done by an engine driven pump separate from the injectors.  The current Common Rail (C.R.) injection systems also use a standalone high pressure fuel pump, which temporarily stores the pressurised fuel in a cast-metal tube or "rail", from which the injectors can draw highly-pressurised fuel as dictated by the engine's electronic control unit, the "brain" which assesses masses of info from loads of sensors, then allows the injectors to draw exactly the volume and timing of fuel to ensure optimum combustion to deliver clean, quiet, economical running.

[TMB]

Edited 28 December, 2018 by TMB

[KeithCheetham]

 Thanks for the updates, having run a 56 plate 1.4TDI from Nov 10 until my current 1.9TDI was suprised that I had not see it before I did a front wheel bearing recently.

[sepulchrave]

It's an expansion cooler as noted, fuel compressed in the unit injectors will be very hot indeed and could combust if exposed to air when it gets back to the tank or even soften plastic parts.

[Xylynx]

Does anyone know the reason why some 1.4 TDI have this cooler and some don't? Was the design changed to allow the system to tolerate higher fuel temperatures?