Interesting, thanks. From PH: "Apparently the new pistons use a gudgeon pin that has a larger diameter, hence the new rods".
Changing the ring locations and sizes on their own wouldn't require new rods unless the gudgeon pin position or size need changing.
http://www.pistonheads.com/gassing/topic.asp?h=5&f=86&t=1482527&i=20&mid=0&nmt=
I had a look at some photos of the TFSI pistons. They are a bit like bottle caps! There's almost nothing to them in terms of depth. If Audi even marginally increased the size of the ring gaps and ring lands, it would I think push the location of gudgeon pin slightly downwards (assuming Top Dead Centre is fixed). Yes I'd already read about the larger pin. I can't quite figure out why they did this unless maybe this reduces the tendency of these wide & flat pistons to slap. Whatever the reason, new con rods would be needed.
I was thinking about all this in the bath this morning and I have an idea about how all this might have come about. These are just the private musings of a man with too little to do...
Go back to the early noughties. The crude oil price is high and rising. Fuel economy (and to a lesser extent emissions) is driving car buyers decisions in show rooms. VAG sort of have the economy angle covered in Europe with diesel but what about the profitable prestige markets in the US & newly emerging China which are predominantly gasoline driven? VAG's answer is TFSI. It's built on three economy based pillars; Direct injection, low friction (thin) rings & low viscosity oil. It also has a decent turbo to provide the oomph premium car buyers want. VAG are aware of the inlet valve deposit problem with DI engines but their engine development program (mainly thrash & trash engine testing) doesn't flag this up as a serious issue. They are also probably aware of the problems with using a low friction ring pack but they get around this with their infamous 'some oil consumption is normal for this class of engine' blurb. So they launch and it's a hit with prestige car buyers, even in Europe.
But what Audi have missed in their testing is protocols is how things interact and how his engine is susceptible to Death By Normal Driving. The fuel efficient ring pack is an inefficient cylinder sealer. I suspect it's primary flaw is poor sealing during the atomised fuel injection/compression stroke (that is BEFORE ignition). When the engine & engine oil are both cold, I can envisage how this would drop a lot of atomised unburned petrol into the sump where it condenses into the oil. When the engine gets hotter the gasoline evaporates off as it's supposed to do. However being a chemical engineer, I can see how this would cause something called extractive distillation of the low viscosity, relatively high volatility engine oil. The light evaporating petrol 'pulls' a tiny amount of the engine oil into the gaseous phase. Having a lot of hot blow-by gas, lots of purge air and the engine under vacuum all feed the extractive distillation process. Now what Audi should have done was cool and compress the blow-by to knock out the heaviest component (oil) so it would be taken care of in the oil separator. But they don't. The hot gaseous mix goes through the oil separator untouched. However in the cold intake system, the gaseous oil instantly condenses particularly on cold surfaces. Some survives however until it hits the inlet valves. I suspect the conversion of oil to crud is complex but it's clear from the many photographs on the forums that it happens. That crud on the backs of valves, is not a product of combustion, it was once oil. Having said that, I'm guessing the bulk of the now condensed oil gets fed through to the cylinders and is burned. However oil doesn't burn like gasoline. It burns badly especially in liquid form and especially at low combustion temperatures. Have you see all those pictures of dirty, black spark plugs pulled from TFSI engines? That's soot from burned oil. Ditto the crud on the piston facing valve surfaces. And hot soot can be abrasive. Thin piston rings that positively allow a high degree of passage plus hot soot is definitely a recipe for ring wear.
Then it all goes belly up because ring wear allows more fuel dilution which creates more extractive distillation, etc, etc.
Then something new and sudden can happen. Deposits on the back of the inlet valves can build up to a degree where a bit falls off and is sucked into the cylinder. If you're lucky, it bounces out the exhaust valve. If you are unlucky, it gets trapped in the First Land and starts scoring the bore (which of course leads to yet more fuel dilution). I have trawled a lot of the forums now and I've read several comments which go..'my oil consumption was okay until... and then it started to increase' and 'I pulled the head and all of the bores are okay apart from cylinder x which is badly scored'. The falling lump of coke theory fits nicely with these comments.
It's only my personal opinion but I suspect that had VAG, from the outset, used an ultra low volatility engine oil in these TFSI engines or put in place something to cool the blow-by before the oil separator, they might just have got away without serious problems. Such are the benefits of hindsight.
If the answer to all this is VAG putting in pistons with a proper, more old-fashioned ring pack (with thicker rings) then maybe this problem is put to bed. I have read that some OEMs like Toyota & Hyundai/Kia have DI petrol engines running in the US without problems. I do wonder whether they too figured out where VAG went wrong and took corrective action before launching their engines.
I think I'm sort of done with this now. I only involved myself out of old professional curiosity. I think I might now take the wife out for a drive in my cheap but ultra-reliable Picanto!
Cheers guys.
