roythebus wrote:
It would seem a common thing for garages to take cars home as "road tests", maybe a mile round the block won't give the same length of run to get to operating temperature.
Not sure how far you'd need to drive to get engine/exhaust temperature up to an adequate temperature for passive reneration, but on a cool day, from cold my oil might take 10 miles to get to its normal temperature in mixed driving. Which is a lot further than what I'd assumed with just the more normal coolant temperature guage, but since I've had an oil temperature guage, have always assumed that the engine takes a bit longer to warm up than I thought previously when relying on coolant temperature (on the other hand, when I spoke to a VW technician about it, he thought the oil temperature might be lagging the actual engine temperature a bit).
Anyway, I'm not entirely convinced that *passive* regeneration is adequte to burn off any soot. Saw one test that monitored soot accumulation on a motorway run, and the higher exhaust temperatures didn't seem to make any difference - it just seemed to keep accumulating as it would if the engine and exhaust were still cold.
Which is maybe why I've often been on quite long runs, and when coming to a halt suddenly realise the thing is going through the *active* regeneration process
Well remember when I'd only had the car a few weeks, came off the motorway after a longish drive on a hot summer day, and wondered why the engine seemed to be running a bit different - subsequently found out that that was the *active* regeneration process.
Dunno if everyone is aware of it, and the process varies from car to car, but these are the telltale signs for me. Happens maybe every couple of hundred miles, and lasts about ten minutes, I'd guess.
Anyway, signs and symtoms are:
(1) Engine sounds different. Most noticeable when decelerating and accelerating, changing gear etc, at a roundabout, say. Probably wouldn't notice at a steady motorway speed. And things like traffic noise, my heavy metal CDs etc might make it more difficult to hear.
(2) Idle speed has increased slightly. To confirm (1), I'll let it coast for a couple of seconds when changing gear, and the rev counter will be reading very slightly higher than the normal idle speed. Maybe just 100 rpm or so, but still noticeable.
(3) Burning smell. Never noticed that in the car, but there is a slight burning smell noticeable if I stop the car and get out. Easily missable, though. Have read elsewhere about people experiencing very strong burning smells, even inside the car, but certainly not my experience.
(4) Cooling fan operating. The only time I think I've ever heard my cooling fan is when the dpf is regenerating and I kill the engine. After a couple of seconds the cooling fan will come on, but goes off again just as quickly. Again, I've read of much more obvious cooling fan activity during regeneration, but certainly not on my VW.
(5) Increased fuel consumption. Well that *would* happen if extra diesel is being injected to increase the exhaust system temperature
Never really monitored that myself, but one claim I read the other day was by a car owner who said his fuel consumption doubled during active regeneration
I suspect that's a slight exaggeration, but in any case the increased fuel consumption will only happen during the ten minutes or so of the regeneration process.
So those are the signs to look out for on the more general guides (like the AA) which apply across all manufacturers, but of course they won't all be exactly the same, and each model and engine will be at least slighly different.
Another thing I've noticed on the VW, though, is that the engine oil temperature will be slightly higher than normal - I'm guessing about five degrees or so.
Of course, if anyone has an oil temerature guage they'll know it varies depending on things like the external temperature and engine load. So going down a long hill on a cold winter's day, mine would show maybe 85C. Climbing a long motorway incline on a warm summer day might show 110C. So maybe add on another 5C to that if the dpf is actively regenerating.
There are other one-off signs as well. I recall when the car was still reasonably new and it was snowing outside. Lots of steam started rising from the bonnet, and I thought there was a problem of some sort. But I think the dpf had been regenerating, and the heat from the process had made the bonnet quite hot, turning the wet snow into steam
But, as I said, I don't like to interrupt it when it's going through an active regeneration, so if I'm nearing home, and the engine sounds a bit different, I'll let it coast for a couple of seconds, and if it's idling higher than normal I'll drive for an extra couple of miles to let the process complete.
But if the dpf warning light comes on, a lot of people think a 20-minute motorway run at highish revs will clear the soot, and this is because of the *passive* regeneration process, ie because of the higher temperatures generated on a long run in normal driving.
But I suspect *active* regeneration will be the main factor at work. Problem is that with short trips etc the exhaust temperature won't get high enough for either passive or active regeneration.
Which is why, I think, the car often seems to wait until I'm driving home to start actively regenerating - if I'm pottering around in town, idling on the rank etc, it won't start the active regeneration. But when I drive home five miles or so, and the exhaust gets reasonably hot, it'll start actively regenerating. But home isn't far enough away to finish the process, so I'll end up driving a couple of miles extra to complete the procedure
Of course, if I didn't have the five-mile drive home at the end of the night, and did the same pottering about town every night, the filter would eventually get sufficiently clogged up for the warning light to come on, then I'd have to go for a good drive anyway.