Glow plugs

[Jlol]

Scanned my A3 1.9tdie, not sure if the glow plugs need replacing or is it an electrical issue.

[Steve Q]

Could well be the glow plugs but I do find it odd in some ways they've all gone simultaneously. 

[Jlol]

Will be taking it to the mechanic when I'm free but the car runs completely fine

[Steve Q]

Glad it runs fine 🙂 the symptoms of failing glow plugs is that the car will struggle to start in cold mornings. 

[Stevey Y]

5 hours ago, Steve Q said:

Could well be the glow plugs but I do find it odd in some ways they've all gone simultaneously. 

Hi Steve multiple owners or high milage may well explain, that and its age when you consider that the suggested change intervals are 60k, they don't struggle to start in the cold mostly because of the insane pressure from the fuel rail.

Steve.

[Steve Q]

3 hours ago, Stevey Y said:

Hi Steve multiple owners or high milage may well explain, that and its age when you consider that the suggested change intervals are 60k, they don't struggle to start in the cold mostly because of the insane pressure from the fuel rail.

Steve.

Thanks Steve, I've learnt something new. When I had mine changed I only had 2 working out of the 6. 

As you know the service interval it makes me wonder if it's been missed, and if so what else has been missed. 

[cliffcoggin]

3 hours ago, Stevey Y said:

 they don't struggle to start in the cold mostly because of the insane pressure from the fuel rail.

Steve.

I am struggling to understand how a high fuel pressure makes starting easier. Diesels rely on compression of the fuel/air mixture to raise the temperature high enough for auto ignition. I understand glow plugs can supply extra heat in low temperature conditions but how does fuel pressure at the injectors come into it?

[Stevey Y]

23 hours ago, cliffcoggin said:

Steve.

I am struggling to understand how a high fuel pressure makes starting easier. Diesels rely on compression of the fuel/air mixture to raise the temperature high enough for auto ignition. I understand glow plugs can supply extra heat in low temperature conditions but how does fuel pressure at the injectors come into it?

Hi Cliff, struggle no more, the fuel rail pressure at idle [warm] is about 650psi, [cold] its about 850psi this helps atomisation of the fuel when its fed into the injectors, when you consider the Venturi in the injectors is less than four microns, a human hair is six microns, fuel fed in under that pressure will almost instantly atomise and become highly combustible on compression, in effect you dont really need glow plugs they just add an element of warmth that makes the swirl vapour even more combustable, the problem really becomes a problem when you add a DPF to the build, most diesels equipped with these need to achieve a temperature of at least 600 degrees to regenerate especially when most Audi diesels are post injection which means the injection process happens later this is called retardation and the fuel droplets are cooler, this is where the plugs come in and super heat the colder fuel thus making it combust at the front of the DPF core and start burning the soot and unburnt fuel from the core itself, so in essence if they did not have such high pressure in the rail the whole process is chemically impossible.

Steve.

[cliffcoggin]

Ah, so better atomisation is the key. That makes sense and is something I should have thought of for myself. Thanks for the explanation.