pango1in
16-08-2013, 12:28 PM
Given the 37 page long thread on fuel economy on the 2.0 TDI engine in the Mk7 Golf, I thought I would summarise the thread so that people with the same issues didn’t have to spend an hour going through all the posts. It’s worth a read if you have a TDI Golf. Well done if you get to the end!
The problem people were having seems to be two-fold.
Firstly, the mpg achieved by the 2.0 TDI engine is well below the published consumption figures (see www.honestjohn.co.uk (http://www.honestjohn.co.uk/)). The published figures are 56.5/68.9/78.5. Personally, I’m achieving around 40mpg in town and 53mpg on the motorway, sometimes more if I have a tailwind… For me, motorway driving achieves 77% of combined and 67% of extra-urban. It is interesting that I haven't exceeded the Urban figure on any journeys (something I would take for granted should happen). I have tracked my overall consumption on my Fuelly page, below. I have done nearly 3,000 miles to date.
In comparison, I used to own a BMW 120d (new in 2010). The combined consumption was around 60mpg with extra-urban being around 70mpg. From very early on, driving the car at 70mph on the motorway with cruise control on would give me around 58-60mpg – very close to the combined figure. This was a more powerful car too. I understand that I will never achieve published figures, that’s not my point. However, I would expect my Golf to get significantly closer to the published figures than I’m currently getting.
Secondly, several people have noted that the DPF undergoesan “active regen” at frequent intervals. Reading the DPF sticky on this forum,which has been verified by a VW technician I have spoken to, there are several levels of engine intervention when the soot level in the DPF increases. Below 45% full, the DPF is left to try and burn off the soot passively, without engine intervention, so called “passive regen”. A VW technician confirmed that for passive regen to take place, the DPF should be operating between 350C and 500C in order to catalyse the soot it collects.
The first level of active regen stage occurs at about 45-50% full. At this point, the engine takes steps to artificially increase the exhaust temperature for around 10-15 minutes to ensure that the soot is burned off. It does this by injecting fuel into the cylinders during the exhaust stroke in the engine. The unburned fuel will be fed into the exhaust as the cylinder empties. This fuel will reach the DPF and burn, raising its temperature to 600C. At 600C, the soot doesn’t last long and it burnt off pretty quickly. The process also uses a fair amount of fuel (which links it into the first issue!), and the poor fuel economy encountered during the active regen seems to persist for some time after the active regen takes place (at least it does for me).
If the DPF doesn’t have enough time to complete one of these active regen cycles then the DPF will continue to fill and you will get a warning light. You are then instructed in the manual to drive the car around for 15 minutes to allow the car to be able to carry out one of these cycles uninterrupted. If anyone manages to avoid doing this… then you need to re-read the manual and make sure you have a friendly relationship with your garage…
Now, the issue that people were having seemed to be that thea first stage active regen was happening more than expected. It was initially suggested that this could be for several reasons:
· More obviously, driving it to the nearby shops and back each day is not going to heat the engine up. If you journey is not long enough, nothing in the engine is going to get up to temperature. You will use more fuel anyway, and soot will build up in the DPF without being passivelyburned off.
· You are driving downhill wherever you go. Sounds silly, but when you drive downhill you use far less fuel than if you are going uphill. Again, things aren’t going to get up to temperature or temperatures are going to fall (specifically exhaust temperatures) when the engine isn’t under load.
· You are stopping and starting all the time. Every time you stop, the exhaust temperature will drop stopping the catalytic process. Even more so if you stop/start when you pull up at a junction.
My commute is 20 miles each way, 11 of which are on DC/Motorway, the rest are in fairly free flowing urban roads, with a couple of nasty bottlenecks. I would have thought that this would have been long enough to get everything up to temperature on the engine, with the motorway being a good constant drive without any stops (usually!). I was surprised to find that I have had about 10 active regens during my short time with the car – mainly noted by higher idling revs when pulling off of the motorway (two of these were verified by VCDS). Some were during my commute and some were straight afterlong motorway journeys (about 200 miles at a good speed, no jams). My expectation would be that probably my commute, and definitely the long journeys, would allow the car to warm up and for the exhaust to reach themagical 350C-500C to allow passive regeneration and avoid any active engine interventions…
This didn’t appear to be the case.
Puzzled by why I was getting so many active regens, I hooked up my car to a Ross-Tech VCDS system (apparently this could invalidate my warranty if the software fails as a result of using the VCDS – be warned). I logged two types of journeys. First was an up and down on a 3 mile stretch of a DC with the car all warmed up. I logged several parameters (DPF inlet and outlet temp, speed, fuel consumption, oil temp, soot mass etc.) at 1 second intervals. On that first run, I found out the following:
· At a constant speed the DPF didn’t seem to everget over 350C.
· The DPF temperature drops like a stone when you stop.
· Hard acceleration is the best thing for getting the DPF temperature up.
· Going down a hill causes a drop too.
· Driving in a lower gear at higher revs seems to make NO DIFFERENCE to the DPF temperature (going against prior perceived wisdom on how to clear a DPF??)
Speed and DPF temperature for that run are shown in the first graph (see the end of the post). The final 3 runs were in 5th, 4th and 3rdgear respectively.
I was disturbed by the fact that under normal driving, within the speed limits, the DPF wasn’t getting up to operating temperature. I could also see that the soot mass was steadily increasing through the journeyand not being burnt off, which makes sense if the DPF wasn’t even reaching 350C.
The next day, I took the car out for a longer journey. A 40mile DC/motorway (A21/M25) round trip at a constant 75mph on the speedo. On the way out, I noticed that DPF temperature was very high, at about 600C. I pulled over and saw that the car was undergoing an active regen. This lasted about 13 minutes in total, during which the soot mass in the DPF dropped from 22g to 3g. This took most of the outward journey and finished as I turned around (coincidence?).On the return trip, I had the same problem as before – the DPF’s temperature didn’t really increase past 350C. The only time it did was going up a very steep hill on the A21. Disappointing – see the second graph.
Following a long conversation with a VW technician withtheir customer services, I can confirm the following:
· The temperature at which the DPF burns soot off is between 350-500C.
· The fact that the DPF wasn’t reaching/only just reached 350C under motorway conditions is normal.
· Active regens heat the DPF up to 600-650C.
· The active regens (without warning lights) are normal. This is just one of the ways that the engine manages the DPF temperature and soot load.
· The technician stated that the active regen was actually part of the passive regen system, and not anything out of the ordinary.
· My fuel economy is low, but should continue to improve over the next 2-3k miles.
Hopefully this will help to clear up any questions people may have. I can’t help but be disappointed that the DPF won’t reach a good operating temperature on its own. I am also very disappointed with the fuel economy given the journeys I do – I expected more. Knowing this, I may even have gone for the petrol version.
2196221963
The problem people were having seems to be two-fold.
Firstly, the mpg achieved by the 2.0 TDI engine is well below the published consumption figures (see www.honestjohn.co.uk (http://www.honestjohn.co.uk/)). The published figures are 56.5/68.9/78.5. Personally, I’m achieving around 40mpg in town and 53mpg on the motorway, sometimes more if I have a tailwind… For me, motorway driving achieves 77% of combined and 67% of extra-urban. It is interesting that I haven't exceeded the Urban figure on any journeys (something I would take for granted should happen). I have tracked my overall consumption on my Fuelly page, below. I have done nearly 3,000 miles to date.
In comparison, I used to own a BMW 120d (new in 2010). The combined consumption was around 60mpg with extra-urban being around 70mpg. From very early on, driving the car at 70mph on the motorway with cruise control on would give me around 58-60mpg – very close to the combined figure. This was a more powerful car too. I understand that I will never achieve published figures, that’s not my point. However, I would expect my Golf to get significantly closer to the published figures than I’m currently getting.
Secondly, several people have noted that the DPF undergoesan “active regen” at frequent intervals. Reading the DPF sticky on this forum,which has been verified by a VW technician I have spoken to, there are several levels of engine intervention when the soot level in the DPF increases. Below 45% full, the DPF is left to try and burn off the soot passively, without engine intervention, so called “passive regen”. A VW technician confirmed that for passive regen to take place, the DPF should be operating between 350C and 500C in order to catalyse the soot it collects.
The first level of active regen stage occurs at about 45-50% full. At this point, the engine takes steps to artificially increase the exhaust temperature for around 10-15 minutes to ensure that the soot is burned off. It does this by injecting fuel into the cylinders during the exhaust stroke in the engine. The unburned fuel will be fed into the exhaust as the cylinder empties. This fuel will reach the DPF and burn, raising its temperature to 600C. At 600C, the soot doesn’t last long and it burnt off pretty quickly. The process also uses a fair amount of fuel (which links it into the first issue!), and the poor fuel economy encountered during the active regen seems to persist for some time after the active regen takes place (at least it does for me).
If the DPF doesn’t have enough time to complete one of these active regen cycles then the DPF will continue to fill and you will get a warning light. You are then instructed in the manual to drive the car around for 15 minutes to allow the car to be able to carry out one of these cycles uninterrupted. If anyone manages to avoid doing this… then you need to re-read the manual and make sure you have a friendly relationship with your garage…
Now, the issue that people were having seemed to be that thea first stage active regen was happening more than expected. It was initially suggested that this could be for several reasons:
· More obviously, driving it to the nearby shops and back each day is not going to heat the engine up. If you journey is not long enough, nothing in the engine is going to get up to temperature. You will use more fuel anyway, and soot will build up in the DPF without being passivelyburned off.
· You are driving downhill wherever you go. Sounds silly, but when you drive downhill you use far less fuel than if you are going uphill. Again, things aren’t going to get up to temperature or temperatures are going to fall (specifically exhaust temperatures) when the engine isn’t under load.
· You are stopping and starting all the time. Every time you stop, the exhaust temperature will drop stopping the catalytic process. Even more so if you stop/start when you pull up at a junction.
My commute is 20 miles each way, 11 of which are on DC/Motorway, the rest are in fairly free flowing urban roads, with a couple of nasty bottlenecks. I would have thought that this would have been long enough to get everything up to temperature on the engine, with the motorway being a good constant drive without any stops (usually!). I was surprised to find that I have had about 10 active regens during my short time with the car – mainly noted by higher idling revs when pulling off of the motorway (two of these were verified by VCDS). Some were during my commute and some were straight afterlong motorway journeys (about 200 miles at a good speed, no jams). My expectation would be that probably my commute, and definitely the long journeys, would allow the car to warm up and for the exhaust to reach themagical 350C-500C to allow passive regeneration and avoid any active engine interventions…
This didn’t appear to be the case.
Puzzled by why I was getting so many active regens, I hooked up my car to a Ross-Tech VCDS system (apparently this could invalidate my warranty if the software fails as a result of using the VCDS – be warned). I logged two types of journeys. First was an up and down on a 3 mile stretch of a DC with the car all warmed up. I logged several parameters (DPF inlet and outlet temp, speed, fuel consumption, oil temp, soot mass etc.) at 1 second intervals. On that first run, I found out the following:
· At a constant speed the DPF didn’t seem to everget over 350C.
· The DPF temperature drops like a stone when you stop.
· Hard acceleration is the best thing for getting the DPF temperature up.
· Going down a hill causes a drop too.
· Driving in a lower gear at higher revs seems to make NO DIFFERENCE to the DPF temperature (going against prior perceived wisdom on how to clear a DPF??)
Speed and DPF temperature for that run are shown in the first graph (see the end of the post). The final 3 runs were in 5th, 4th and 3rdgear respectively.
I was disturbed by the fact that under normal driving, within the speed limits, the DPF wasn’t getting up to operating temperature. I could also see that the soot mass was steadily increasing through the journeyand not being burnt off, which makes sense if the DPF wasn’t even reaching 350C.
The next day, I took the car out for a longer journey. A 40mile DC/motorway (A21/M25) round trip at a constant 75mph on the speedo. On the way out, I noticed that DPF temperature was very high, at about 600C. I pulled over and saw that the car was undergoing an active regen. This lasted about 13 minutes in total, during which the soot mass in the DPF dropped from 22g to 3g. This took most of the outward journey and finished as I turned around (coincidence?).On the return trip, I had the same problem as before – the DPF’s temperature didn’t really increase past 350C. The only time it did was going up a very steep hill on the A21. Disappointing – see the second graph.
Following a long conversation with a VW technician withtheir customer services, I can confirm the following:
· The temperature at which the DPF burns soot off is between 350-500C.
· The fact that the DPF wasn’t reaching/only just reached 350C under motorway conditions is normal.
· Active regens heat the DPF up to 600-650C.
· The active regens (without warning lights) are normal. This is just one of the ways that the engine manages the DPF temperature and soot load.
· The technician stated that the active regen was actually part of the passive regen system, and not anything out of the ordinary.
· My fuel economy is low, but should continue to improve over the next 2-3k miles.
Hopefully this will help to clear up any questions people may have. I can’t help but be disappointed that the DPF won’t reach a good operating temperature on its own. I am also very disappointed with the fuel economy given the journeys I do – I expected more. Knowing this, I may even have gone for the petrol version.
2196221963