Diesel throttle body?

[Johnnojack]

The Gen V and probably others have a "throttle body with a flap valve in it, just before the EGR Except I am sure it is not used for control of engine speed like a petrol engine. Most diesels don't have this part. I have only seen them on old 1950's diesel tractors which used a vacuum operated governor. So what is it for? Is it used for retardation when cruise control is on or what?

[HiTorque]

The Gen V and probably others have a "throttle body with a flap valve in it, just before the EGR Except I am sure it is not used for control of engine speed like a petrol engine. Most diesels don't have this part. I have only seen them on old 1950's diesel tractors which used a vacuum operated governor. So what is it for? Is it used for retardation when cruise control is on or what?

It's a part of EGR system that has a motor controlling the round flap. At some stage that part will be filled up with soot/gunk/**** and you will need to open it and clean. Pls read http://www.vwwatercooled.org.au/f28/...ned-27155.html

[Transporter]

... and the flap is also used for faster engine shut down, which is the main purpose of it. It is worth to remember that unlike petrol engines the diesels don't regulate the RPM with the TB flap. In short, in the diesel engine the air intake is always at maximum and the engine RPM's are regulated by controlling the amount of injected fuel.

In petrol engines the RPM's are regulated by controlling the amount of air through the intake manifold, so the accelerator is linked to the throattle body flap directly via the cable or by el. wire which connects to throttle body motor, which adjust the baffle (flap) in the TB.

[kaanage]

Can it be set (via ECU pogramming) to provided air braking (like trucks)?

[Greg Roles]

Well I don't have one anymore ( not in the intake plumbing anyway! ). It's there entirely to draw EGR into the intake by causing a restriction at idle and in normal driving, seems to open right up at high throttle. It's also that high pitched whine you can hear as you stop your car and walk away, the flap whines for about 40 seconds as it holds closed, then opens back up for next start.

The car is FAR smoother down low, as it's not creating a restriction vacuum for the EGR. Shutdown is just as rapid, but is a bit rough. Engine braking hasn't changed. Car feels far more responsive, which will be in part my entirely new intercooler plumbing, but in my opinion largely due to the removal of the biggest bottleneck in the intake, the EGR/Flap mechanisms. They are 1-3/4" on otherwise 2.5" cold side inlet anyway.

I too wondered about truck exhaust brakes, but they have specific heavier exhaust valve springs to stop them floating when an engine brake is applied - the 2.0 TDI isn't designed for it, and i won't be trying that one!

[poyta]

It's also that high pitched whine you can hear as you stop your car and walk away, the flap whines for about 40 seconds as it holds closed, then opens back up for next start.

So that's what that noise was!!

[gldgti]

just for the record -

in terms of total energy spent, theres not much difference for a diesel engine at no load or overrun(engine braking), between having the air supply cut off with a throttle flap or maximum air supply. when the piston compresses the intake charge, no matter how great the air mass is, it gets some of that energy back again on the next stroke. an intake charge with hardly any air to compress (throttle flap shut) gets almost nothing back on the next stroke, and a full charge gets quite a lot back on the next stroke. if you've ever played with a pneumatic ram you may understand what I mean. If you close all valves to the cylinder, you have an "air spring".

Anyway, my point is that the EGR flap (the piece of kit at the source of this discussion) has 2 main functions - to create a vacuum in the intake system for EGR purposes, and to aid in the shutdown of the engine when oil vapours are present in the intake system (which is normal, due to the crank case breather system). In the latter case, since the "fuel" comes through the intake air, the flap is acting very much as the throttle valve for a carburettored otto cycle engine.

I hope that helps.

PS - Older Mercedes-Benz diesels have a throttle flap for the injector pump governor system too.