My Octy 3 had its first MOT today and my tester is a friendly type and invited me to have look underneath the car when it was up on the ramps. He pointed out a device mid way along the exhaust pipe and said this was an exhaust retarder. When you take your foot off the accelerator the retarder closes off the exhaust gas flow and turns the engine into a compressor slowing down the the vehicle. Usually a diesel engine has very "engine braking" compared to a petrol one and the lack of engine braking combined with relatively low gearing would mean very little slowing of the vehicle and increased use of the brakes. This device provides a braking effect via the engine and reduced wear on the normal brakes.

 

Apparently these are relatively common on big trucks particularly to stop brakes overheating on long downhill runs but are now being used on some diesel cars too.

 

This was a new one on me and so I thought I'd pass it on. Maybe you all knew about these so apologies for wasting your time lol

 

Bob

I think the primary purpose of the flap may be different to that suggested. Post #7 here suggests it's related to EGR function, albeit they're discussing a VW rather than a Skoda: http://forums.tdiclub.com/showthread.php?t=275066

Thanks Wino,

 

Always good to learn about these things. When I asked my MOT man for more detail he clearly was not that familiar with them on cars but had seen them on trucks.

Looks like an expensive part if it does go wrong but the VW ones do seem easy to remove and clean out so if I need to hopefully the one on my Octy will be as straightforward.

 

BRs

 

Bob

6 hours ago, Wino said:

I think the primary purpose of the flap may be different to that suggested. Post #7 here suggests it's related to EGR function, albeit they're discussing a VW rather than a Skoda: http://forums.tdiclub.com/showthread.php?t=275066

The bit about it creating back pressure for the EGR to function when the car is on boost is wrong. EGR only ever engages when a car is cruising and the engine is under very little load. 

 

EGR partially fills your cylinder with inert gas, effectively making the cylinder smaller, a smaller cylinder volume means you can use less fuel and still keep the fuel ratio from getting too lean. 

 

If you're on boost you're trying to make power so you want as much fuel and air in the cylinder as possible so the EGR valve will be closed. 

 

A exhaust brake like his MOT guy suggested makes more sense. 

Edited 29 September, 20187 yr by ian_feel_keepin_it_reel

Sounds like your info may relate to petrol engine EGR set-ups.

Main purpose in diesels is NOx reduction by lowering peak temperatures, which you may well want to do at operating conditions other than very light load.

 

 

The difference with diesels is you can lean out the mixture without risk of pre-ignition, but leaning out raises cylinder temps. 

 

If you're on boost and you need to lower the cylinder temp you just rich up the mixture. 

 

If you made an engine that used EGR when under load you might as well just make a smaller capacity engine. 

Think you need to study diesel combustion a little more.

8 minutes ago, ian_feel_keepin_it_reel said:

The difference with diesels is you can lean out the mixture without risk of pre-ignition, but leaning out raises cylinder temps. 

 

If you're on boost and you need to lower the cylinder temp you just rich up the mixture. 

 

If you made an engine that used EGR when under load you might as well just make a smaller capacity engine. 

http://bioage.typepad.com/.a/6a00d8341c4fbe53ef01a5118b9f0c970c-popup

 

Part of the EGR

 

Full article here. http://www.greencarcongress.com/2014/03/20140322-ea288.html

Edited 29 September, 20187 yr by SuperbTWM

6 minutes ago, SuperbTWM said:

http://bioage.typepad.com/.a/6a00d8341c4fbe53ef01a5118b9f0c970c-popup

 

Part of the EGR

 

Full article here. http://www.greencarcongress.com/2014/03/20140322-ea288.html

Well... I stand corrected.

1 minute ago, ian_feel_keepin_it_reel said:

Well... I stand corrected.

 I'm not sure under what load parameters its working but I'm fairly sure the only circumstance its inhibited is when the engine is at full throttle. But i'm not 100%

 

To be honest the only EGR system I have worked on was the old PD system where you just had a valve on the intake and then on later models the pneumatically operated anti shudder flap got replaced with a motorised one which would close the intake up which would then force the engine to suck more egr gasses through.

 

Its getting ridiculous the amount of extra crap in the engine bay to go wrong now

1 minute ago, SuperbTWM said:

 It's getting ridiculous the amount of extra crap in the engine bay to go wrong now

Damn straight, particulate filters on petrol engines now. 

 

I can't wait until electric vehicles are more affordable.