DPF / Short journeys debate

[Pulsedriver]

Hey folks, trying to discuss with my colleague about the whole 'dpf regen' thing, my car is a 2014 BMW F30 330d and his is a 2012 BMW F30 320d we both do our fair share of short journeys, as well as a long drive / blast down the motorway etc.

The debate began now when he mentioned switching the car on for a short moment can damage it, for example, to move your car to another space / move it out the way if you are blocking another colleague in and they are leaving before you etc and he purposefully never moves his car / parks it miles away just so he doesn't have to 'turn it on then off seconds later'
Now this is not something I have ever thought of as damaging, maybe I am wrong...

Same applies to the shops, if he is popping in to collect a takeaway and has his other half in the car, he will leave the engine running for 2, 3, 5 mins rather than switch it off - this is bizarre in my view but would be interested to hear views on whether he is correct, I believe it is more the 'dpf' side of things rather than engine damage, as apparently if it was a petrol car he wouldn't worry at all about doing it...

 

[Dis86]

v.short on/off engine running can lead to fuel wash.

 

[vanpeebles]

Wear and tear on the engine due to oil not being up to pressure. You can also flood some modern engines by moving them very a short distance

 

[jpaul]

You can also flood some modern engines by moving them very a short distance

Indeed - I never move petrol BM short distances, when it's cold otherwise may not be able to re-start easily;
a friend, even called out the AA on a petrol Yaris, that they couldn't re-start.
Not sure if you can damage/poison the catalyst after flooding.

...if the cars warm ie. take-away scenario . no issue

 

[Mercenary Keyboard Warrior]

I thought most cars tried to DPF regen when the engine is already up to temp, and after deciding its due. The problems come when doing lots of short or slow journeys as they may well, especially in winter, not get to the target parameters. Eventually you get to the forced regen point and it will start adding fuel to increase temps.

A few short journeys are not going to cause any massive issues to the DPF, only those same journeys if the DPF is desperate to start a regen and not repeatedly getting the opportunity.

Certainly fuel wash or oil dilution are going to be more of a risk when repeatedly starting an engine and running for very short periods, but any car with variable service will detect this and take it into account with the servicing.

Obviously there is a short period right at the start as you crank the engine until the oil gets to pressure where there will be reduced oi pressure.

 

[DereksDontRun]

could it also be due to open loop vs closed look fuel lookups? afaik, when the car is cold it uses an open loop fuel trim which is usually rich, and switches to closed once it's up to temp. So v short journeys could mean it's never switching over to closed loop, and thus always running rich. Meaning more/unburnt fuel is getting into the exhaust/cat???

 

[Frozennova]

could it also be due to open loop vs closed look fuel lookups? afaik, when the car is cold it uses an open loop fuel trim which is usually rich, and switches to closed once it's up to temp. So v short journeys could mean it's never switching over to closed loop, and thus always running rich. Meaning more/unburnt fuel is getting into the exhaust/cat???

You can't get away with that with emissions anymore, modern stuff has typically switched to full time closed loop within 5-10 seconds of running.

Heck, even my modified car on an aftermarket ECU is full time closed loop ~25 seconds after starting regardless of temperature

 

[Howard]

I agree with the switching on for a few seconds thing, I try to avoid it if possible.

 

[Semple]

Purely from a DPF perspective, this...

I thought most cars tried to DPF regen when the engine is already up to temp, and after deciding its due. The problems come when doing lots of short or slow journeys as they may well, especially in winter, not get to the target parameters. Eventually you get to the forced regen point and it will start adding fuel to increase temps.

If you're out doing short journeys during a time where the DPF isn't regenerating, then there will be no issue. The problem starts to build up when people regularly use the car for short journeys and keep interrupting DPF regeneration. The car is clever enough that if it is interrupted it will attempt regeneration next time the engine is on, the problem with a lot of people using diesels is that the engine is rarely on for long enough for it to perform a full regeneration, eventually leading to a blocked DPF requiring a forced regen.

 

[sandys]

the problem with a lot of people using diesels is that the engine is rarely on for long enough for it to perform a full regeneration, eventually leading to a blocked DPF requiring a forced regen.

This is the issue we have with ours, we have diesel for towcar but 95% of the time it just does shops and school, we are always having DPF light on, I have to take the car on a big run to clear it, pain in the butt, we are replacing with a petrol despite diesel being a better tow engine due to pick up from low revs.

 

[Rroff]

Have to say (probably jinxing myself now) I've never had problems (other than EGR) with vehicles only used for short runs including any DPF issues - what I have found is that big engine diesels with a lot of shorter stop/start journeys seems to result in EGR valve problems :s

I do have the vehicle(s) serviced on schedule with a DPF check/service as part of that though.

 

[robj20]

Depends on the car as well and the systems it uses.
My Mondeo will happily regen at idle, never had a light on or any trouble and do plenty of short trips under 5 miles.

 

[kitfit1]

Got a BMW 635 D Sport here, can't say i have ever had a regen issue with it even when using it for continuous short journeys. It's done 72,000 miles now as well, so if it was going to be an issue i would have thought it would have happened by now.

 

[sideways14a]

DPF and short journeys = crap.
Either do a big hashing once a week or two or get a petrol.

 

[kitfit1]

DPF and short journeys = crap.
Either do a big hashing once a week or two or get a petrol.

Mine always gets a long blast every weekend, maybe the reason i have never had a regen issue.

 

[Drumroll]

One thing I think is really weird: the A Class W177, once started the regeneration, takes way longer than any other diesel I've used before, despite staying at 70mph, so not a speed/change of rpm issue. Another one is in the event I'm unable to carry on driving to allow it to complete, even after 30 minutes or so, the regeneration will start again, but I think is because the conditions for a regeneration still present. Won't be any delay. Will kick in as soon as I start the engine again.
Before that, the Mazda CX5, when the ambient temperature was mild, once started, the rpm would jump straight to 2k range and keep for a while. It was a way to accelerate the conditions for a regen, but despite this, the regents were much more often than the ones I have now. And even doing this, wasn't always before a regen. Would do as many times as I start the car, regardless of long journeys, or short journeys.
Before it was one every 300-400 miles, but not as long as the Mercedes, but the Mercedes does one every 600-700 miles.
Starting the engine for a very short period is bad, but not being a daily thing, I don't believe it will be a problem.
I would be more worried about the very long and inappropriate oil changes recommend by BMW than a short start of the engine.
So far, changing oil every year/10k miles and using good diesel, never had any problems.
My partner's car just clicked 60k miles. Emission test is very close to when it was new.
True is most of the complex systems used now require more attention, but if you're the kind of person having talks about DPF and all, you're fine. Problem is the people who wants to drive a 2 tonnes behemoth just for school run, are frightened by the running costs of a petrol to cope with it, go for a diesel, and then pest the dealer about oil changes because of contamination, as the only regeneration happening is the active ones, which never complete.

 

[Drumroll]

Mine always gets a long blast every weekend, maybe the reason i have never had a regen issue.

True. Even if it's not guaranteed the long run will be exactly when the regeneration needs to happen, there's a good chance to help.
Other issue is hybrids. Lots of people not driving enough to keep charge in the starting battery, being unable to even start the car. Seems like a petrol or a fully electric are better for short distances than anything else, the only problem being the price of an electric for lower miles, so a petrol is the way to go.
I'm just curious about how the new filters fitted to few new petrol engines will work. If requires the same treatment as the DPF, that would be chaos.

 

[jpaul]

The wear&tear, as said, maybe a strong reason to avoid 30s journeys.

Castrol used to advertise 75% of engine wear during warm up, so - what does that translate into ?
if you have two cars both with 50k miles but one has 1/2 as many cold starts, that one will have 1/2 the wear (or so ?, anyone good at man maths);
so even if the emissions are in check, mpg might be going south and reliability, for the next (post pcp) owner;
(the diesels going to be taking longer to warm up too, so more wear, and the dpf has a limited number of life cycles)

seems a pity as a buyer you cannot demand to hook up on the OBD interface and get that data plus the number of regens.

 

[Frozennova]

True. Even if it's not guaranteed the long run will be exactly when the regeneration needs to happen, there's a good chance to help.
Other issue is hybrids. Lots of people not driving enough to keep charge in the starting battery, being unable to even start the car. Seems like a petrol or a fully electric are better for short distances than anything else, the only problem being the price of an electric for lower miles, so a petrol is the way to go.
I'm just curious about how the new filters fitted to few new petrol engines will work. If requires the same treatment as the DPF, that would be chaos.

Almost entirely a non-issue for petrols, EGTs on average are higher than diesel so passive regeneration will be far more frequent, petrols produce less particulates so the GPF will see lower soot loading and it's also far easier to generate EGTs hot enough on a petrol engine to regen a GPF

 

[Drumroll]

Almost entirely a non-issue for petrols, EGTs on average are higher than diesel so passive regeneration will be far more frequent, petrols produce less particulates so the GPF will see lower soot loading and it's also far easier to generate EGTs hot enough on a petrol engine to regen a GPF

I did notice how fast the petrol engines are up to optimum temperature, but if any special measure is required, people driving 2 or 3k miles per year, 1 mile journey are in for a shock. And those are the ones who tends to keep their cars for 10+ years, as the mileage won't be a factor, so the only way to justify the depreciation is to keep the car as long as possible.