Mpg dropped considerably in the past week...

[bl52krz]

Cyrano, the answer is a qualified yes.My son works for a well known oil company, and even he did not believe me when I said your best bet if you have oil fired central heating is to try and keep the tank/s filled up in the winter time because you get more fuel for your money. To prove me wrong, fill a spare fuel container with diesel, then tighten the top down very tight. When the temperature gets up by around ten degrees, gently undo the top and watch the diesel pour out the top. Another thing with petrol, never fill the tank right up to the top otherwise as the temperature rises it will spill out of the tank. I have just remembered a statement by a USA person involved with statistics who said he always filled up in the morning because you always get more fuel then than afternoon,normally,because the temperature was lower..

[T-Cut]

Originally Posted by bl52krz

Cyrano, the answer is a qualified yes.

Surely the expansion coefficient of fuel (which is tiny) will have a miniscule effect on fuel consumption compared with the large variation in running temperature seen in diesels? If it had, the fuel producers would take whatever steps necessary to avoid you getting more than your money's worth in cold weather. I know fuel pumps are calibrated/certified accurate at 20C or whatever, but the economic effect of delivery temperature must be negligible. The fuel actually entering the engine doesn't vary much, it's always pretty hot and the backflow from the injectors warms the tank contents over a long drive. That would alter the fuel consumption in the other direction. It's an interesting thought anyway.

TC

[slovcan]

Originally Posted by T-Cut

Surely the expansion coefficient of fuel (which is tiny) will have a miniscule effect on fuel consumption compared with the large variation in running temperature seen in diesels? If it had, the fuel producers would take whatever steps necessary to avoid you getting more than your money's worth in cold weather. I know fuel pumps are calibrated/certified accurate at 20C or whatever, but the economic effect of delivery temperature must be negligible. The fuel actually entering the engine doesn't vary much, it's always pretty hot and the backflow from the injectors warms the tank contents over a long drive. That would alter the fuel consumption in the other direction. It's an interesting thought anyway.

TC

Over here they do just that. The pumps are calibrated to compensate for temp differences.

https://www.ic.gc.ca/eic/site/mc-mc....g/lm04344.html

[T-Cut]

Originally Posted by slovcan

Over here they do just that. The pumps are calibrated to compensate for temp differences.

Interesting, it's something I've never thought about. I guess maybe because the ambient temperature variation in the UK is never that extreme. It is however being implemented here according to Wikipedia:

Gasolene Metrology
Temperature compensation is common at the wholesale transaction level in the United States and most other countries. At the retail consumer level, Canada has converted to automatic temperature compensation, and the UK is undergoing conversion, but the United States has not. Automatic temperature compensation, known as Standard Temperature Accounting in the UK, may add a tiny amount of additional uncertainty to the measurement, about 0.1%.

But I doubt this effect alters fuel consumption to the extent reported by the OP.

TC

[hogweed]

Just to chuck my 2p-worth in, I moved home from Gloucestershire to Antrim last year, and have noticed a fairly drastic increase in consumption. I was getting about 43-44 fairly consistently for years in England, with mixed driving – here it’s around 39-40.

Only difference I can see is that here it is always, ALWAYS, farquing freezing. I mean, obviously the oil takes longer to heat up, so is stiffer etc, but I wonder if diesel engines are less efficient in the cold – from my M3-owning days, I know that petrol engines are supposed to go better in the cold, as the air is denser, therefore higher effective compression, and bigger bangs…

[HarryM1BYT]

Originally Posted by hogweed

I know that petrol engines are supposed to go better in the cold, as the air is denser, therefore higher effective compression, and bigger bangs…

Both diesel and petrol run better with colder more dense air, but the diesel becomes much less efficient in colder weather, due to the long warm up times.

The 0.1% would be unnoticeable as a mpg difference. At a base of 50mpg, it would be around 50.2mpg.

[klarzy]

Not as simple as a bad MAF sensor causing over fueling.. ????

[grivas]

Originally Posted by bl52krz

I get around 46-48 mpg in the summer just doing reasonably short and middle distances. 50-52 on m/way journeys. Winter time, I have just got 37 mpg doing short journeys. I.E. 1to 3 miles at a time. Fbh works very well but of course burns diesel when working. Lately it has been running all the time on short journeys because of the ambient of around -2 to -7. So lower mpg in winter is normal for other reasons also, like fuel ‘shrinkage ‘.

So what is 'fuel shrinkage' never heard of it.

[grivas]

Originally Posted by Pete_901

Mg zt-t cdti. Was averaging 47 mpg now I can barely get it to go over 35 mpg? Usually get about 230 miles for £30. This week I’ve got about 150 miles tops. Any ideas/commen problems?

All things being equal as it were, the first thing you should check is the running temperature, as this will affect the fuel consumption considerably. A poorly performing thermostat, which is not uncommon in the diesel cars if this has never been serviced!! will be your most likely cause of poor economy, if you have a FBH fitted, then this run more or less continuously and the overall result will be poor economy.

[T-Cut]

In simplistic terms, heat energy generated by combustion needs to be used for driving the pistons. All heat absorbed by the surroundings in warming up the engine is wasted. Diesels need heavy construction and therefore have high mass. This makes the warm up period protracted compared with spark ignition engine, but once hot they're more efficient.Clearly, the colder the engine initially, the longer it will take to get to optimal efficiency. Some diesels never become warm enough no matter how long they're running. Ideally, no heat should be transferred to the surrounding mass. That's why heat engines run most efficiently at high temperatures. For piston engines, the upper limit is relatively low because of the characteristics of petrol and diesel fuels, lube oils and low cost metallurgy. They have 20-40% efficiency (petrol-diesel resp.). Other engine types are more efficient because they run hotter. Gas turbines for example run extremely hot, even above the melting point of the turbine blades. These can achieve 98+% efficiency.

TC