Prob asked before but fuel consitency

Daff_Duck · 20 Feb 2012 at 15:15

Hope the title is not too misleading, but seems a bit of a topic of debate in our office lately seeing as the price of diesel is rocketing. I was in the local garage and I was saying my MPG had fallen rapidly over the last few months . When asked where I filled up (Tesco) he said that he had quite a few people in saying more or less the same thing there MPG was going down the pan and it maybe that Tesco/Asda were somewhat "diluting" there fuel?

Is this something that is a "common" practice or is it all just hearsay?

rodenal · 20 Feb 2012 at 15:17

It all comes from the same source, but with different additives. So it's unlikely at best

Duke · 20 Feb 2012 at 15:24

You generally get less MPG during the winter.

Firestar_3x · 20 Feb 2012 at 15:26

Inferior crude at the Tescos refinery tbh.

shoes · 20 Feb 2012 at 15:42

I get about 5mpg more from optimax than I do from shell's normal unleaded - is this normal?

ci_newman · 20 Feb 2012 at 15:56

shoes said:
I get about 5mpg more from optimax than I do from shell's normal unleaded - is this normal?

Yes, because they are different fuels with different RON ratings. 5mpg probably doesn't off-set the increased purchase cost though.

crinkleshoes · 20 Feb 2012 at 16:12

ci_newman said:
Yes, because they are different fuels with different RON ratings. 5mpg probably doesn't off-set the increased purchase cost though.

5mpg... yes it would offset the purchase cost, but is physically impossible just from an improvement in RON

crinkleshoes · 20 Feb 2012 at 16:14

Duke said:
You generally get less MPG during the winter.

illogical statement is illogical

cooler air = more efficient burning = more power per explosion = more MPG :confused:

But there are other factors such as road surface (albeit generally less grip which equals more fuel efficiency), people driving like numpties meaning you have to speed up and slow down more, added resistance during motorway cruises due to rain etc...

lordrobs · 20 Feb 2012 at 16:55

crinkleshoes said:
illogical statement is illogical

cooler air = more efficient burning = more power per explosion = more MPG

But there are other factors such as road surface (albeit generally less grip which equals more fuel efficiency), people driving like numpties meaning you have to speed up and slow down more, added resistance during motorway cruises due to rain etc...

You will only get more power if the ECU adapts to the denser air by adding more fuel. The main reason that economy will be lower is that the car will be running for longer before the engine is warm and therefore at its most efficient.

crinkleshoes · 20 Feb 2012 at 17:02

lordrobs said:
You will only get more power if the ECU adapts to the denser air by adding more fuel. The main reason that economy will be lower is that the car will be running for longer before the engine is warm and therefore at its most efficient.

Of course... merely a comment on how his post didn't make sense without clarification.

and for semantics... it's *less* fuel...

the ECU would need to add less due to the reduction in knock capability thanks to the cooler air.

I'd be curious to do some real testing on this as basic physics knowledge will tell you that a cooler engine will warm up faster.

Simple throught experiment... will a cup of coffee stay warmer for longer if you put the *cold* milk in straight away or after 5 minutes?

Put the milk in first... is the answer.

Or... you will get ice cubes faster by putting boiling water in the freezer than you do if you put room temperature water in...

Rate of change of temperature is directly related to the initial difference in temperatures.

The combusting fuel will be at nearly the same temperature during winter or summer... the question is, do the other factors really add up to that.

In the most simple reference frame, an engine would warm up quicker in winter than it will in summer.

there are so many variables involved though...

Uriel · 20 Feb 2012 at 17:10

lordrobs said:
You will only get more power if the ECU adapts to the denser air by adding more fuel. The main reason that economy will be lower is that the car will be running for longer before the engine is warm and therefore at its most efficient.

Plus denser air = more wind resistance, therefore greater power required to travel at high speed.

fish · 20 Feb 2012 at 17:49

Surely the increased friction of the system due to the cooler temperatures(requiring extra enerygy from the fuel) and the fact that the cooler engine is not isolated from it's environment (thus heat is not totaly transfered to the engine as it must warm the surrounding environment also), along with the fact that the exhaust gasses which are expelled from the system are hot not cold (leading to further losses)would lead to a cooler engine warming up more slowly. In experiments i have carried out on the viscosity of a liquid varying with temperature the cooler liquid required more time and energy to get to the final temperature of the same liquid which was warmer initially. due to losses my observations show entropy to work well in one direction (reduction in temperature) as the environmet is a mitigating factor.

Hope that makes sense. (it does in my head but not easy to convey written down)

crinkleshoes · 20 Feb 2012 at 20:55

Good points, but then we'd also have to consider how that increased friction causes extra heat in itself.

Maccy · 20 Feb 2012 at 21:01

I was wondering this earlier on my drive home actually - my MPG has been pretty poor the past few months and my driving hasn't changed at all. Always fill up using super unleaded at Sainsburys.

crinkleshoes · 20 Feb 2012 at 21:09

To comment on the OP, I too noticed a drop in fuel economy with supermarket fuel. I got a 10p/litre off voucher from Tesco, so filled up.

Got only 180-190 from a tank, where I normally get 240-260.

It was a one off, I normally go to Shell and have had pretty consistent fuel economy from their fuel.

Deep · 20 Feb 2012 at 21:30

According to my Fuel Log in the Summer months I was doing about 44-46mpg, last few months has been down to about 38-40 mpg

fish · 20 Feb 2012 at 21:43

crinkleshoes said:
Good points, but then we'd also have to consider how that increased friction causes extra heat in itself.

The extra heat introduced to a system by friction will use more fuel to overcome the additional friction (the heat energy must come from somewhere). Coupled with the cooler environment means there will be a constant cooling factor working on an engine in cooler weather, couple this with the heater matrix also taking heat from the system (people use the heater in the winter but rarely in the summer, useles you are my missus and it is always cold) then even when everything is up to temperature it will still require more fuel to maintain the temperature of the engine in cooler conditions. I would liken it to boiling a kettle at ambient room temperature vs boiling a kettle in a freezer, there will be more energy consumed by the kettle in the freezer due to environmental conditions (the freezer may be a little colder than winter air but not by much when wind chill is factored in)

Just to add in the summer i get 52mpg from my focus in the winter i get around 47mpg

Simon · 20 Feb 2012 at 22:04

crinkleshoes said:
Of course... merely a comment on how his post didn't make sense without clarification.

and for semantics... it's *less* fuel... the ECU would need to add less due to the reduction in knock capability thanks to the cooler air.

I'd be curious to do some real testing on this as basic physics knowledge will tell you that a cooler engine will warm up faster.

Simple throught experiment... will a cup of coffee stay warmer for longer if you put the *cold* milk in straight away or after 5 minutes?

Put the milk in first... is the answer.

Or... you will get ice cubes faster by putting boiling water in the freezer than you do if you put room temperature water in...

Rate of change of temperature is directly related to the initial difference in temperatures.

The combusting fuel will be at nearly the same temperature during winter or summer... the question is, do the other factors really add up to that.

In the most simple reference frame, an engine would warm up quicker in winter than it will in summer.

there are so many variables involved though...

My head hurts.

Surely its obvious a colder engine takes longer to warm up because it has more energy required to warm up.

Colder oil is also thicker increasing drag.

crinkleshoes · 20 Feb 2012 at 22:55

Simon said:
Surely its obvious a colder engine takes longer to warm up because it has more energy required to warm up.

That's not the reason - the rate of change is more relevant than the total amount of energy required.

Exactly the same reason boiling water freezes faster and luke warm liquids stay warmer for longer than boiling liquids

... very simple physics

When it comes to engines though, a lot more complicated - I think it's interesting how many different factors there are.

BinnsY · 20 Feb 2012 at 23:00

I think something that gets forgotten when it comes to the MPG dip in winter is the simple fact that its cold and dark. Over the last month or so I've had to have my heater / heated seats and at least the side lights on pretty much every trip. All of these like to sap away at my precious petrol