I've been over and back with VAG here in Ireland lately regarding the emissions update. It's been slow and not hugely productive so far. I have asked specifically for information on the CFGB engine (2.0 170bhp) and 6-speed DSG transmission.

 

The response I got looked like it had never been near an engineer and that whoever wrote it was mostly copy/pasting from press material. They continue to assert that there are no adverse effects from the update.

 

They confirmed that the VCA in the UK handled the approval process for Skoda but declined to supply any information related to the approval. I will be pursuing an FoI enquiry with the VCA in the coming days. If anyone here has already done so and would like to share the information, please do so here. A quick scan through the list of completed FoI requests shows quite a few related to the TDI emissions scandal.

 

What they have said (that I haven't seen confirmed in press material) is the following:

 

• EGR usage is increased during certain operating conditions
• Soot generation is increased, again during some operating conditions; consequently DPF regeneration will be more frequent
• There is an additional injection event during the combustion cycle; overall the amount of fuel injected per cycle is not changed, just the timing and duration of injection events
• Affected vehicles fitted with the 7-speed DSG get a DSG remap as well as the engine remap. They say the main change there is to drop from 7th to 6th under some load conditions. (IIRC only the 1.6 TDI got this transmission)

 

I had additionally looked for power/torque curves comparing before and after; CO2 emissions per NEDC cycle as a result of the update; specifics of the changes to EGR and fuel injection; whether there are any changes to be made to the 6-speed wet clutch DSG; and confirmation that any such changes won't brick the transmission (a bunch of US owners ended up with bricked DSG controllers as a result of the update there). These were not included in their original response; I'm waiting for them to get back to me since I replied to their letter. I'm not that hopeful of a response but we'll see.

 

The increased soot production is likely deliberate: in passive regeneration, the additional soot loading will provide fuel for continuous NOx reduction, thus lowering NOx output. They could get a bit of extra soot by injecting a small amount of fuel late in the combustion event. At that point it won't have sufficient time to burn off cleanly, and will leave soot behind.

 

Without knowing under what conditions the changes have been made, it's hard to conclude exactly how updated vehicles will be affected. I have a feeling vehicles used mainly over short distances will be worse off, with higher DPF loading and less chance of successful regeneration happening. Cars that live on the motorway are probably not going to see much, if any, problems here as they will be passively regenerating the DPF all the time.

 

More EGR and higher soot loading could also lead to greater intake clogging, particularly on cars that are driven lightly all the time. I suspect the recent TPI to realign the EGR connection to the inlet manifold has something to do with this.

More soot implies poorer economy under the conditions where this change applies, since less of the fuel is being turned into heat (and therefore work). Without knowing the operating conditions under which this occurs, it's anyone's guess as to what this will look like in the real world.

 

I need to do some reading on the implications of more EGR, especially if it reduces the oxygen/fuel ratio to near stoichiometric.

 

For now my call on this is that it's probably not as bad as the naysayers and tinfoil hat brigade want to think it is, but it's not the panacea that VAG are claiming it to be either. VAG have hurt their credibility within the enthusiast community badly by keeping this information under wraps, but I suspect a large percentage of owners don't pay enough attention to how their cars drive to notice or care about the effects of the update and will happily drive away until their PCP runs out and they sign up for the next new car.

" They continue to assert that there are no adverse effects from the update. "

 

We'll they're hardly going to put it in writing if it is detrimental.

and they're unlikely to want to dredge up documentation and test results for an individual member of the public. FOI doesn't apply here they can, should they wish, tell you to take a hike.

Edited 6 December, 20178 yr by Aspman

12 hours ago, chimaera said:

• EGR usage is increased during certain operating conditions
• Soot generation is increased, again during some operating conditions; consequently DPF regeneration will be more frequent

Doesn't the second of those automatically occur whenever the first is occurring?

I thought more soot was inevitable with more EGR percentage (because of the lower peak temperatures and less complete combustion)? Rather than being something they might deliberately be trying to achieve.

13 minutes ago, Wino said:

Doesn't the second of those automatically occur whenever the first is occurring?

I thought more soot was inevitable with more EGR percentage (because of the lower peak temperatures and less complete combustion)? Rather than being something they might deliberately be trying to achieve.

Not necessarily. The effect of EGR is to dilute the available oxygen within the intake air, and make more of the gas inert. Part of the reason NOx is a big problem with diesel engines is the fact that they typically operate with a fuel air ratio that is a good deal lower than stoichiometric, so there's lots of spare oxygen available. With the conditions of temperature and pressure that exist in a diesel engine, there's plenty of scope for nitrogen to react with the spare oxygen and form NOx. EGR reduces the excess of oxygen while still leaving it dispersed enough to allow combustion to proceed; less spare oxygen means less NOx.

 

EGR can also improve fuel economy since there's a bunch of gas available to do work once it's heated by combusted fuel (same sort of idea as the turbofan engine in aircraft). Turn up EGR and inject less fuel, and you get more efficient running at low loads, particularly with turbocharging.

 

The upper limit for EGR rate is the point where oxygen concentration drops below stoichiometric with fuel, and soot is guaranteed as you approach that. If EGR is set so that there's still enough oxygen available soot formation shouldn't be a huge issue. As I said above though, I suspect they've jacked up soot formation a bit to provide something for NOx to react with on the way out so it turns back into nitrogen gas.

@chimaera; thanks, that's useful clarity beyond my previous understanding.

 

Have you watched this, if so how 'spot on' or otherwise would you say it is?

 

44 minutes ago, Wino said:

@chimaera; thanks, that's useful clarity beyond my previous understanding.

 

Have you watched this, if so how 'spot on' or otherwise would you say it is?

 

Just watched it through there.

 

He's wide of the mark on a few things: there are EU5 engines with SCR, and EU6 without. SCR is not required for NOx control by either standard, but it will make it a lot easier to achieve. The downside is the expense of it. One of the reasons VAG went for the defeat device initially was the cost of installing SCR on each vehicle, compounded at the time by the fact that it was covered by a Daimler Benz patent, and required any other manufacturer using it to pay a royalty to DB.

 

I'd like to see him supply evidence of the relationship between EGR rate and soot production. He presents it as 'common sense' which is always dangerous ground, especially if you're going to call out the other side for lack of evidence. Having read up on it, it's mostly right, but it's an oversimplification.* At low engine loads it's unlikely to cause significant particulate emissions.

 

Refinement is an issue. The most efficient and least NOx producing injection pattern also happens to be the noisiest. No matter what way you slice that cake you're in trouble.

 

I don't buy the line that an engine with 66 % blockage of the EGR valve should not be affected by the update. An EGR valve that blocked is a problem no matter what map the engine is running, and it's going to be having a detrimental effect on performance. I would argue that in the longer term, higher EGR flow will probably reduce EGR blockage since most blockage problems are caused by insufficient gas flow to prevent material getting stuck.

 

I'm with him on VAG's obstinacy in relation to the problems. As I said in the OP, I think they've caused themselves a lot of reputational damage with their approach to the update. The revelation of the defeat device in the first place caused them a lot of trouble, and they've done little to redress that since.

 

In the US, the first-generation common rails sold there have had a partial fix approved, since it's been found that they're not capable of being made 100 % compliant with US regulations without seriously damaging performance. The EPA there allowed a partial fix and the payment of a fine per vehicle because of this. I think VAG would have gotten traction for this idea among owners and dealers here if they had been more open about the issues. As it stands, everyone is ****ed off at them and not willing to give them any room.

 

I feel sorry for dealers caught in the middle of this: they're on the front line dealing with a problem not of their making, trying to keep customers happy while also toeing the company line so that they don't lose their franchise (and their business).

 

 

 

*For a more thorough explanation see "Introduction to Internal Combustion Engines" by Richard Stone, pg. 261 of the third edition.

Sorry, I forgot to thank you again for the above response until just now, and the reference for further reading,  much appreciated.

 

A belated question about your previous post, after I've thought about it more. When you said this:

 

On ‎06‎/‎12‎/‎2017 at 12:28, chimaera said:

EGR can also improve fuel economy since there's a bunch of gas available to do work once it's heated by combusted fuel

 

That's only the case for a petrol engine isn't it? The cylinders are always filled  on a diesel, aren't they, whether with normal air or a blend of air and recirculated exhaust? Not a rarefied situation like a throttled petrol engine? What am I missing?

Edited 9 December, 20178 yr by Wino

19 hours ago, Wino said:

That's only the case for a petrol engine isn't it? The cylinders are always filled  on a diesel, aren't they, whether with normal air or a blend of air and recirculated exhaust? Not a rarefied situation like a throttled petrol engine? What am I missing?

 

It can be for any engine, depending on load requirements. If we go back to basics, the underlying principle in any engine is that heating a bunch of gas causes it to expand, and that expansion can be harnessed to do work.

 

The heat can be internal or external and is usually provided by combustion, hence the terms internal combustion engine or external combustion engine. ICEs include petrol and diesel otto engines and derivatives of that cycle as well as gas turbine engines; ECEs would be reciprocating steam engines (as used on rail locomotives), the Stirling engine and steam turbines. You might notice a pattern here: most engines fall into reciprocating (otto, Stirling, steam) or turbine (gas or steam) classes.

 

In a reciprocating engine, the expansion of the gas inside or outside the cylinder drives a piston down, and a crankshaft turns that into rotation; in a turbine, the expansion of the gas turns the turbine which generates thrust (in a jet engine) or shaft power (in steam or gas turbine applications).

 

All this is by way of highlighting that the working fluid doesn't have to be oxidising or combustible: steam certainly doesn't burn, and the Stirling engine uses air alone as the working fluid.

 

So to the question at hand, in a reciprocating ICE, combustion of some quantity of fuel heats the air/fuel/combustion products mixture making it expand and drive the piston down. For a given power requirement only so much heat is needed, so the ideal situation is only to burn enoughfuel  to get that amount of heat, and that amount of fuel needs just enough oxygen: any spare oxygen tends to at least partially get turned into NOx.

 

For the expansion process, a certain amount of gas has to be heated by a certain amount: what that gas contains and where the heat comes from doesn't matter. More gas expanded results in more power. One way we commonly do this is through the use of a turbocharger which packs a greater mass of gas into the cylinder during the intake process: this works on two fronts, by providing more gas for expansion, and providing more oxygen so that more fuel can be burned. Another way we can do this, especially when power demand is low, is through using EGR to provide a mostly inert working fluid. This works better if the exhaust gas is cooled as part of the recirculation process.

 

This suits a petrol engine a little better than diesel, since it needs an air/fuel ratio that's richer than stoichiometric, so providing more overhead for displacing air with exhaust gas. In a diesel engine (or a petrol that's using lean burning technology) we need an excess of oxygen in the intake air, so we can't make as much use of EGR. It's still useful even in that situation though. By using a greater portion of inert gas to do work, we can displace some of the fuel that would otherwise be needed, so economy can improve.

 

The reality of applying these ideas is rather more complicated than I've outlined here, and there's no guarantee that increasing EGR will improve economy all the time: the whole thing is a subtle mix of different effects, some of which enhance each other and others cancel each other. Understanding and applying those effects is how engine designers earn their money

 

In the context of the emissions update, I would imagine the very fine points of the combustion process were probably not explored that much since VAG had a solution (the defeat device) which didn't require them to optimise quite so much. For the update, I reckon they delved much deeper into that space, plus modelling tools and computational power have advanced notably in the last 8-10 years, allowing greater exploitation of the intricacies of the process.

 

On the lower powered engines especially that's probably allowed them close most if not all of the gap in performance that reducing NOx should have created. The higher specific output of the 170 bhp engine probably leaves less room for improvement, and may account for the issues some owners have reported.

 

A side note on soot: it's carbon so potentially combustible when it enters the cylinder as part of recirculated gas. However it can also serve as nucleation sites for more and/or larger soot particles, particularly in a port injected petrol engine where the fuel and air have mixed prior to entry into the cylinder. Soot formation generally will be driven by the spray conditions from the injector: typically a soot particle will result from a single droplet of fuel. Smaller droplets will give rise to finer soot particles should conditions favour soot formation; the caveat is that smaller particles are more likely to completely combust since there's a greater surface area to volume ratio, so soot formation is less likely.