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andy1741

Why is it bad to use diesels for short trips?

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People always say diesels are for people who do the mileage and should not be bought for short trips. What exactly is harmful for diesel cars by doing short trips. I mean short trips are bad for any car really.

The obvious answer is the DPF & EGR from my knowledge. This will get blocked with soot because the driver wont be doing motorway runs to clear it. That is obvious but what if the DPF is removed or the car doesn't have one (for example MK2.5 1.8tdci). Also imagine the EGR blanked too. What damage is being done to the engine then by doing short trips? Anymore so than a petrol even?

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2 minutes ago, andy1741 said:

People always say diesels are for people who do the mileage and should not be bought for short trips. What exactly is harmful for diesel cars by doing short trips. I mean short trips are bad for any car really.

The obvious answer is the DPF & EGR from my knowledge. This will get blocked with soot because the driver wont be doing motorway runs to clear it. That is obvious but what if the DPF is removed or the car doesn't have one (for example MK2.5 1.8tdci). Also imagine the EGR blanked too. What damage is being done to the engine then by doing short trips? Anymore so than a petrol even?

They are the biggest issues, so it won't be as bad without those components, but you also have more soot in the cylinder so things like injectors can get coked up.

Also diesels are slower to warm up, so the engine never spends much time with up to temp oil lubricating all components fully, exhaust doesn't get up to temp to dry it out properly, etc.

Same issue on a petrol car if the trips are short, but it tends to be less of a problem, due to faster warm up and cleaner running engines.

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2 minutes ago, alexp999 said:

They are the biggest issues, so it won't be as bad without those components, but you also have more soot in the cylinder so things like injectors can get coked up.

Is this really an issue? I have absolutely no idea but I would have assumed soot or not, the vacuum in the cylinder from the exhaust stroke would remove everything from the cylinder? 

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2 minutes ago, andy1741 said:

Is this really an issue? I have absolutely no idea but I would have assumed soot or not, the vacuum in the cylinder from the exhaust stroke would remove everything from the cylinder? 

In a perfect theoretical world it would, but it doesn't unfortunately.

A cold running diesel engine produces more soot due to incomplete combustion, some of that is deposited on things like the injectors in the cylinder.

A hot (up to temp) diesel engine with the regular high rev blasts will help to keep them clear.

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Ok that makes sense but without an EGR and DPF are these issues negligible or as big of a deal as people make them out to be?

I ask this because in Ireland, the motor tax rules changed to be measured from engine cc to Co2 emissions in 2008. Since then there has been a massive influx of diesel cars. I'm just interested as diesel cars seem to only fit a very select group of people. Yet they seem to be way more common than just a small group of customers. Are the issues with using a diesel just magnified or are people's car's dying everywhere? 

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It's just something to be aware of if you don't want problems in the long run.

If you do a longer spirited journey one a week/fortnight/month (depending on number of short journeys), you'll hopefully keep it clear.

But ONLY short journeys, will cause problems. As you say even on petrols, but it tends to be worse on diesels, due to slow warm up time, thicker oils and more soot.

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1 minute ago, alexp999 said:

But ONLY short journeys, will cause problems. As you say even on petrols, but it tends to be worse on diesels, due to slow warm up time, thicker oils and more soot.

Ok this makes more sense. I always got the impression that there was something about diesels that made it just so much worse but in reality I assume its the combination of a couple of factors of how diesel engines work that make them worse for short trips than petrols.

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19 hours ago, andy1741 said:

People always say diesels are for people who do the mileage and should not be bought for short trips. What exactly is harmful for diesel cars by doing short trips. I mean short trips are bad for any car really.

The obvious answer is the DPF & EGR from my knowledge. This will get blocked with soot because the driver wont be doing motorway runs to clear it. That is obvious but what if the DPF is removed or the car doesn't have one (for example MK2.5 1.8tdci). Also imagine the EGR blanked too. What damage is being done to the engine then by doing short trips? Anymore so than a petrol even?

Apart from any technical reasons, short trips tends to equal lower mileage, and for the increased price of a diesel, you'll never reap the financial benefits of the better economy.  

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32 minutes ago, Guzzilazz said:

Apart from any technical reasons, short trips tends to equal lower mileage, and for the increased price of a diesel, you'll never reap the financial benefits of the better economy.  

Very true in general but I feel this is incorrect in certain countries, one being Ireland, where the significant decrease in road tax can easily match the increased price of a diesel. But as far as I'm aware this is not he case in the UK.

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One point to bear in mind is that, due to the slow warm-up, diesels take a proportionately bigger mpg hit than petrol on short journeys.

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3 hours ago, mjt said:

One point to bear in mind is that, due to the slow warm-up, diesels take a proportionately bigger mpg hit than petrol on short journeys.

...not in my experience... My SEAT Exeo which I had for 7 years would average over 40mpg on my station trip every day... The 1.0 Ecoboost needs longer to start becoming efficient, even though the Ford is near operating temp, and the Exeo had barely got off cold...

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2 minutes ago, Guzzilazz said:

...not in my experience... My SEAT Exeo which I had for 7 years would average over 40mpg on my station trip every day... The 1.0 Ecoboost needs longer to start becoming efficient, even though the Ford is near operating temp, and the Exeo had barely got off cold...

40MPG is low for a diesel, and proportionally a bigger drop from optimal than a petrol would be.

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I chose my words carefully, deliberately using 'proportionately', because whilst 40mpg is a decent value it's still a lot less than the same engine would give on a long steady run and a bigger difference than you'd get with a petrol. Our old Mk1.5 1.8TDCi used to give around 45-48 mpg up to around 50 miles locally, but on a holiday run up to Scotland it went up to 64mpg.

However, it may well be that the 1.0L Ecoboost, with it's iron block, doesn't warm up as fast as aluminium-engined petrols.

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1 minute ago, mjt said:

I chose my words carefully, deliberately using 'proportionately', because whilst 40mpg is a decent value it's still a lot less than the same engine would give on a long steady run and a bigger difference than you'd get with a petrol. Our old Mk1.5 1.8TDCi used to give around 45-48 mpg up to around 50 miles locally, but on a holiday run up to Scotland it went up to 64mpg.

However, it may well be that the 1.0L Ecoboost, with it's iron block, doesn't warm up as fast as aluminium-engined petrols.

It's the opposite, the iron block speeds up warm-up time.

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On 16/01/2017 at 2:44 PM, andy1741 said:

What exactly is harmful for diesel cars by doing short trips.

Something that should be mentioned is that Diesels run lean. Very lean at idle.

With a normal petrol engine, the stoichiometric mix means a very high water content in the exhaust gas. Something like 44 parts Nitrogen, 8 parts CO2 and 6 parts water vapour, by volume, for full combustion.

The Ecoboost petrol engine can run lean, though I have no idea if and when this option is used.

Petrol exhausts corrode rapidly from the inside, due to this water, made worse by any NOx, which can form nitric acid. This will a big problem on short journeys where the exhaust stays cool. Diesel exhausts can last much longer, with minimal internal corrosion, as there is usually a period of lean running (idle or low power) to flush out the water before switching off.

Water in the oil can be a problem for petrol engines used for short runs, for the same reason.

I would say that without a DPF there is not much in it. If buying secondhand, the initial capital cost is not so much higher, and Diesels on short runs still remain more frugal on fuel than petrol on the same short runs.

Long oil warm up time, and long cabin warm up time (!) are the main disadvantages of Diesel that I can think of, in this case. Some Diesels have electric heaters in the coolant system to cure this. As the warmed coolant also goes to the "oil cooler", which also works as an oil warmer from cold, the oil will benefit too.

It is the DPF that ruins Diesel for many people. DPFs mean more CO2 for the whole planet in exchange for a tiny reduction in one specific type of possible pollutant in crowed, congested cities. But then the rule makers live in cities!

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3 hours ago, alexp999 said:

40MPG is low for a diesel, and proportionally a bigger drop from optimal than a petrol would be.

My Exeo on average in normal running did about 43 mpg.  On a long trip (Cornwall and back) it would indicate up to 51...  My Ecoboost shows about 35-37 for the 40MPG equivalent despite the temperature getting well up the gauge, where the Exeo wold have just started to move.  On a run, the Focus looks like it may get well over 50, so in comparison it's MUCH worse. Don't get me wrong, I love the Focus, but the MPG figures are SOOOO different from my experience with other makes...

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20 hours ago, Tdci-Peter said:

Long oil warm up time, and long cabin warm up time (!) are the main disadvantages of Diesel that I can think of, in this case. Some Diesels have electric heaters in the coolant system to cure this. As the warmed coolant also goes to the "oil cooler", which also works as an oil warmer from cold, the oil will benefit too.

Brilliant post and well summed up. In this example I was assuming the EGR was blanked but where do you think the EGR falls into this? Does the EGR clog up significantly faster by doing mainly short trips?

20 hours ago, Tdci-Peter said:

It is the DPF that ruins Diesel for many people. DPFs mean more CO2 for the whole planet in exchange for a tiny reduction in one specific type of possible pollutant in crowed, congested cities. But then the rule makers live in cities!

Great point!

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25 minutes ago, andy1741 said:

Does the EGR clog up significantly faster by doing mainly short trips?

Perhaps.

While cold, there is likely to be more smoke in the exhaust, though on my car, the EGR stays closed for a short while after starting, when the smoke is at its worst. But if the engine makes smoke under heavy load, this may be more of a problem on longer runs, accelerating up to motorway speed for example.

I do wonder how much of the gunk that restricts inlet manifolds comes from the exhaust, via the EGR, and how much is oil from the crankcase breather system. Probably the worst thing is a mix of the two, the oil makes the smoke stick to the walls, and then hardens. I annually clean out and inspect the oil separator on my car, to try to keep this source of gunk down. Not sure if it makes much difference, but it makes me feel better.biggrin.png

I think the perils of EGRs are a bit over-rated. My car has done 155k miles, mostly with the EGR in use. I have had it blanked for about 2 years on occasions, though it is unblanked and back in use now. The car still pulls ok, and will climb the steepest dual carriageway hills around here at well over 70 (Not that I would :whistling1:!), in 5th gear. So there must be some airway left open!

My EGR closes under heavy load with substantial turbo boost, and at high RPM (over 2500rpm). So the valve itself should have no impact on max power, provided it does close ok and there is still enough airway open to work. Turbo engines are less sensitive to airway restrictions than naturally aspirated engines, just look at all the quite small hoses from turbo, via intercooler to inlet manifold it has to cope with.

The main problems I have had with the 1.8TDCI EGR have been electrical, bad contacts inside the actuator, quite simply.

BTW, the coolant heater I wrote about does not seem to be an option on the Focus, that can have an electric air heater for the cabin, or a fuel fired coolant heater for real Arctic conditions when even anti-freeze would turn to jelly. It was one out of a Jag X type that I saw advertised, and was tempted to try. Haven't done so yet though.

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