RE: Driven: Volkswagen Polo Blue GT

[Gizmoish]

As most others have said, it does sound good.

Can't help wondering what will happen in 5-7 years time though when these are deep in the secondhand market - will cylinder deactivation be so commonplace (like stop/start is already) that everyone will understand, or will people test drive, think it's something wrong with the engine and not buy it?

[va1o]

Love it, as I've been saying since Geneva I think I might buy one when it comes out.

[julian64]

Not understanding this.

In bluemotion mode the inlet and exhaust valves remain closed by a movement of the camshaft.

Surely they want all the valves on that cylinder to remain open, otherwise those two cyliders are in a constant state of compressing and expanding the trapped air causing a vast degree of work to take place.

Probably why it only improves fuel consumption by 20%.

There are obviously reasons why they can't leave them open and take all that work away, the primary one would be punping uncombusted air into the exhaust, but surely there would be a better way.

[kambites]

julian64 said:

Surely they want all the valves on that cylinder to remain open, otherwise those two cyliders are in a constant state of compressing and expanding the trapped air causing a vast degree of work to take place.

Can't do that unless you've got a non-interference engine - the pistons would hit the valves.

How much energy do you actually lose anyway? You'll get back most of the energy from compressing the air when it decompresses again on the "power stroke". You only lose what manages to escape into the block as heat while it's compressed, which given that the air should start out cold and the block will be hot, wont be much.

Edited by kambites on Tuesday 10th July 14:40

[Gizmoish]

kambites said:

julian64 said:

Surely they want all the valves on that cylinder to remain open, otherwise those two cyliders are in a constant state of compressing and expanding the trapped air causing a vast degree of work to take place.

Can't do that unless you've got a non-interference engine - the pistons would hit the valves.

How much energy do you actually lose anyway? You'll get back most of the energy from compressing the air when it decompresses again on the "power stroke". You only lose what manages to escape into the block as heat while it's compressed, which given that the air should start out cold and the block will be hot, wont be much.

Edited by kambites on Tuesday 10th July 14:40

I assume that he was assuming the valves open outwards.

[kambites]

Gizmoish said:

I assume that he was assuming the valves open outwards.

I've never seen an engine whose valves open outwards? Has it ever been done?

[julian64]

kambites said:

Gizmoish said:

I assume that he was assuming the valves open outwards.

I've never seen an engine whose valves open outwards? Has it ever been done?

Not sure if serious.

[kambites]

julian64 said:

kambites said:

Gizmoish said:

I assume that he was assuming the valves open outwards.

I've never seen an engine whose valves open outwards? Has it ever been done?

Not sure if serious.

Entirely. Educate me.

[julian64]

kambites said:

julian64 said:

kambites said:

Gizmoish said:

I assume that he was assuming the valves open outwards.

I've never seen an engine whose valves open outwards? Has it ever been done?

Not sure if serious.

Entirely. Educate me.

Obviously the internet doesn't come across too well. In saying the valves should be open rather than closed I didn't mean they should be hitting on the pistons. I meant that the design should open the cylinder to the air rather than enclose it to get rid of the obvious wasted energy. I wasn't suggesting a mechanism, let alone the obvious daft mechanism attributed.

[kambites]

julian64 said:

Obviously the internet doesn't come across too well. In saying the valves should be open rather than closed I didn't mean they should be hitting on the pistons. I meant that the design should open the cylinder to the air rather than enclose it to get rid of the obvious wasted energy. I wasn't suggesting a mechanism, let alone the obvious daft mechanism attributed.

Good, so I'm not going mad.

I'm not convinced you'd save all that much by not compressing the air though - I suppose it depends on exactly how hot the air gets when you compress it compared to the block's temperature. I'd have thought you'd get back well over 90% of the energy that you put into compressing the air?

[abarber]

This and the triple engined Focus make great alternative to the taxi engined snores.

It might not be light, but Polo's are the size of a MK3 Golf of a few years ago. Inside, space isn't much different to a MK4 Golf.

[anonymous-user]

julian64 said:

Not understanding this.

In bluemotion mode the inlet and exhaust valves remain closed by a movement of the camshaft.

Surely they want all the valves on that cylinder to remain open, otherwise those two cyliders are in a constant state of compressing and expanding the trapped air causing a vast degree of work to take place.

Probably why it only improves fuel consumption by 20%.

There are obviously reasons why they can't leave them open and take all that work away, the primary one would be punping uncombusted air into the exhaust, but surely there would be a better way.

I spoke to a bloke from Audi at Goodwood FoS this year who explained the cylinder deactivation on one of their V8s to us. He said that the valves opened (not sure if he meant both inlet and exhaust) to create effectively an air damper to stabalise the engine when 4 of the 8 cylnders are switched off.

But to me it sounds like closing the valves would create the air damper effect. The explanation of the Polo here makes sense in a way, even if it is the complete opposite of what I was told by this Audi guy.

[iain1970]

Good to see tht by the end of the first page, PHers rise to the old clichés of "How much?" and "How heavy?".

It's 2012, not 1985 FFS.

[n4aat]

I thought the old CLK AMGs could turn off 4 cylinders.

How have they got on? My uncle still has an X reg still going strong.

[greekph]

Ok, without having looked much into how this cylinder deactivation works I have a question that arises naturally:

Suppose you are driving along on a quiet open road with little traffic and the conditions for the cylinder deactivation to work are fulfilled. This means that the engine could potentially be running with cylinders deactivated for hours on end. So, you have 2 cylinders that are running practically cold, without any combustion in them.

I am no engineer, but I see this as a source of trouble in the long run, with glazed cylinders, worn piston rings and all sorts of bad things.


As an aside, GM were doing it with V8s in the US a while ago. I think the Chrysler/Dodge Hemi from around 2003 has this feature too.

[molineux1980]

I like this alot. But way too pricey for me to be able to afford, until second hand 5 years in the future. Surely these new generation , low capaicity high output engines will cause problems down the line?

[the-photographer]

molineux1980 said:

I like this alot. But way too pricey for me to be able to afford, until second hand 5 years in the future. Surely these new generation , low capaicity high output engines will cause problems down the line?

The Euro4 versions of VW super/turbo engines (140/150/170ps) have generally been doing fine since their launch around 2006 (subject to the low volumes they sold). But the Euro5 versions have had some serious problems, time will tell.

[va1o]

molineux1980 said:

Surely these new generation , low capaicity high output engines will cause problems down the line?

Well it was this time 6-years ago now that VW launched the twincharged 170PS Golf GT 1.4 TSI and they've generally been OK. Interestingly the early cars seem to fair better than the later EU5 compliant versions which suffer from oil consumption woes. On the whole they are generally OK and no worse than the equivalent DPF fitted diesel.

[Mouse1903]

Brilliant car, faster than the Mk5 Polo GTI and almost double the economy. The new gen of Polo is the size of a Mk3 Golf so would actually make a good family car too imo

[HBFS]

I have a bit of an issue with mpg claims

I have the Fabia vRS 1.4 TSI 180 DSG. (Manufacturers combined 45mpg, extra urban 55mpg)

Real world economy for me is 32mpg.
I got ~51 mpg this week on long motorway runs but had to drive like a saint, very rarley going above 60mph (Mostly 52-56)

It's not just me either, I post and browse Brisk Skoda a fair bit and a lot of people have their mpg in sig. Not sure I've ever seen anyone getting 36mpg + with a mk2 Fabia vRS

So I predict real world econmy for this will be 42-47mpg combined.