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Diesel consumption


woolybanana
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Yes and for a number of reasons.

1.  Your driving style will/should change and you could end up with a proportionately larger amount of shorter journeys

2.  The fuel additives that stop waxing reduce the efficiency

3.  You are running more ancilliaries that consume fuel - heat lights etc.

4.  And if like me you have to drive up the A7 into the Mistral blowing at 60-110kpm against you, your consumption will go up from a normal 7-7,5l/100km to 10-11,5l/100km - I was gobsmacked!

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Yeah but no but yeah but no!!

All of the above is true for both petrol and diesel engines however the diesel combustion cycle is far less affected by a cold intake charge than a petrol engine will be.

A petrol engine will run a significantly richer mixture untill the engine is a full operating temperature, once a diesel has run for a few seconds the combustion chamber is at operating temperature and their is virtually no enrichment taking place, granted it takes a long time for either engine to warm up in these conditions and extra fuel is used to create that extra heat, internal combusion engines are heat engines.

This morning I set off up the hill from my house and it was minus 8 degrees, I checked the fuel computer and after 400 yards it was registering 32mpg, a petrol engine would have been in the low single figures, my engine never warms up for my current journeys, only twice a week when I go the 20 miles or so to the swimming pool yet it is still returning 55MPG overall, there is no perceptible difference in my consumption to that in the summer, and I am a real anorak about even 1/2 an MPG.

The economy on short journeys was what convinced me to switch to diesel.

I dont usually notice the econmoy dropping when driving against the wind but then I drive relatively slowly so the efects are minimised, however when I tow my big trailer (far too big in fact) it is very very noticeable, sometimes I cannot reach 70KM/H and I dont even realise that it is windy, the combination is very sensitive to gusts and will snake easily yet I am unaware of the very moderate wind not even showing on the treetops but the fuel economy and cruising speed tell their own story.

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Technically - Diesel engines will use more fuel in winter due to the specification its made to in winter. From October onwards refineries make the diesel to a lower CFPP spec (cold filter plugging point) ie mostly -15 where in summer its made to -5 cfpp so in winter the composition is different due to addition of an additive and distillation range to achieve -15 cfpp however as most ulsd is blended with 5% of fame producing (bio-diesel) you have to run to a much lower cfpp -17 before fame is added to get back to -15 after addition. So basically the reduction in mpg is due to changing the distillation range to meet winter grade fuel .

In layman's terms a chemical is added at the refinery during the winter to reduce the viscosity of the diesel fuel to stop it blocking the injectors etc. The colder it is the thicker the diesel fuel would become without the additive. The side effect of adding the additive is that you have to put your food down more to get the same acceleration and speed which means you burn more fuel. You may be surprised to know you are not alone in asking this question, one only has to type the question in to Google.

That is also coupled to the colder the air the more dense it becomes so the more diesel is required. The same goes for petrol.

Hope that helps. 

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If you hadnt added the bit at the end in laymans terms I would have sworn that was not you but your nemesis posting Quillan [6]

I have always thought that the only reason you clash it that you are so alike [:P]

Still dont understand what you have posted though, I know it has an additive to reduce the viscosity but dont see whey this increases fuel consumption except for perhaps in on liter there are a few CC's less of combustible diesel.

Regarding the air density, yes a greater quantity of air will be aspirated and the injectors will add more fuel to compensate but the engine will run much more efficiently as a result and the fuel economy will increase, on a petrol engine significantly so, diesels probably less so as a percentage.

The only time where the consumption would increase would be for a constant no load throttle opening  where the revs would increase, even if you were to drive at a constant throttle opening the vehicle would go faster and travel further thus bringing the MPg to equilibrium.

In my opinion and speaking as a MPG anorak the winter conditions do not make a perceptible difference to diesel fuel economy, that is to say on a modern turbocharged diesel but a massive one to a petrol engined vehicle, of course some people drive around at high revs in 1st gear when snow is forecast so they will see a difference, others may drive slower than normal and may get better economy.

Woolybanana if you really are seeing a significant increase in consumption for the same journey pattern then maybe there is a problem, perhaps get the ECU faut codes read?

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[quote user="Chancer"]

Still dont understand what you have posted though, I know it has an additive to reduce the viscosity but dont see whey this increases fuel consumption except for perhaps in on liter there are a few CC's less of combustible diesel.

[/quote]

I forgot to add a bit to the 'laymans bit' to explain why, sorry about that.

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When using petrol, would E10 make a difference?

We avoid the stuff now anyway, as we found consumption in our Prius went way down when using it, but used to put it in the Saab regularly on our way north/south, as it was on our route back on to the autoroute after an overnight stop, and it didn't make any difference in that whatever time of year. But I wondered if it would make a difference generally because of its composition.

The Prius definitely uses more petrol in cold weather, whatever length of run we're on. But as we get such good mpg compared with the Saab and Porsche, it mostly seems wonderfully frugal as well as giving a good ride.

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E10 has 3% roughly less energy per litre, so your consumption will always be higher with E10 all other things being equal.  This actually makes E10 uneconomic at current price differentials, so buying E10 is a commitment to trying to solve global warming and not a saving to the purse.
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I've found that adding Millers EcoMax or similar helps (not only in winter but all year round). As far as I understand, in winter the additives which alter the viscosity are, as noted, going to have an effect on the combustion and therefore economy but the Millers also boosts the cetane (diesel equivalent of octane) rating by a few points (depending on the concentration you use) offsetting the effect.

If you get what I mean :)
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It's also worth mentioning that when a car is really cold then a certain amount of extra energy needs to be expended to turn the engine, gearbox, axles etc which will have greater friction due to increased viscosity of lubricant and a reduction of tolerance due to thermal contraction.
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You beat me too it!

I have just returned from the swimming pool, about 15 miles away, the engine was at running temp but judging by the creaking and groaning from the suspension and steering not much else was.

There is a hill that I drive up and coast down, always the same speed at the top, the 50 km/h limit, at the bottom I am always up to 65mph (sorry to change units) and its a bit of a white knuckle ride to coast around the bends without throttle control/engine braking, tonight there was no wind  and I was 10mph slower!

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[quote user="JK"]It's also worth mentioning that when a car is really cold then a certain amount of extra energy needs to be expended to turn the engine, gearbox, axles etc which will have greater friction due to increased viscosity of lubricant and a reduction of tolerance due to thermal contraction.[/quote]

Same with me [:)]

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