MK7 GTD - Real Life MPG

[monkeyhanger]

Well, today was cold, around 2 degrees celsius, and with 75 on the dual carriageway my commute came out at 38 - and I had my first regen interruption today too, ignition off, lock, and the fan going mental.

Hopefully this isn't my first regen (230 miles on the clock) and others have been happening during the drive?

First regen at 250 miles - sounds about right - most likely it'll be smellier than subsequent regens as that heat peels away some traces of protective oils etc from the exhaust system.

I drove a bit harder today than I have been for a week and got up to 45mpg for my 12 mile commute at 4C, compared to 42/43 for the rest of the week. Driving it a little harder early on does have its advantages of a quicker warm-up (not by much) and better mpg for a longer part of the journey.

[AlanH]

I often wonder what effect relative humidity has on fuel consumption. Is a dry, low humidity day better than a damp, humid day?

[Gordor]

I am putting full load on my engine at least once every trip...thinking it will stop the regen....result is better MPH, no regen and big smile

[dippy_x]

5.5 degrees ambient temp.  52.3mpg on my 11 mile trip home from office.  Bizarre I tell you!

[monkeyhanger]

I often wonder what effect relative humidity has on fuel consumption. Is a dry, low humidity day better than a damp, humid day?

Humid and warm are better conditions for optimum mpg (more so warm). Humid air is less dense than dry air, and warm air is less dense than cold air at any given pressure. As air warms up, it expands, making it less dense in an open system (the atmosphere), or a higher pressure in a closed system (like the engine's cylinders). Moist air is less dense than dry air at any given temperature or pressure because water has a lower molecular weight than either Oxygen or Nitrogen. At 25C, 100% saturated air is about 1% less dense than dry air.

Overall, 100% saturated air at 25C is approx 9% less dense than 100% saturated air at 0C (not that 0C air can hold much water), but the moisture rather than the temp only contributes about 1/10 of that 9%.

So in the summer, with 9% less dense air, that's a hell of a lot less drag on the car as it pushes through the air. Also, being maybe 25C warmer in the summer than in the worst of winter, warm-up time from a cold start could be reduced by as much as 3 miles - on a 12 mile journey like my commute, that's the difference between 4 miles of the journey running optimally and 7 miles of the journey running optimally.

Humidity has a positive effect, but only just - maybe 1%

The air being less dense has no effect on the engine's oxygen requirements for efficient combustion, air is drawn into the engine greatly in excess.

[monkeyhanger]

I am putting full load on my engine at least once every trip...thinking it will stop the regen....result is better MPH, no regen and big smile

I find it best to put that load on quite early in the trip and get the engine up to temp quicker.

Just going to work - if traffic permits me to get up to 80mph on the dual carriageway, I'll hit 46/47mpg for my trip and reach 90C by 7.5/8 miles.

Do the same journey and traffic only allows me to maintain 60/65mph on the dual carriageway and i'm looking at 43mpg for my trip and reach 90C by 9 miles.

The longer the commute, the less relevant the warm up phase is to your mpg.

[Gordor]

I am putting full load on my engine at least once every trip...thinking it will stop the regen....result is better MPH, no regen and big smile

I find it best to put that load on quite early in the trip and get the engine up to temp quicker.

Just going to work - if traffic permits me to get up to 80mph on the dual carriageway, I'll hit 46/47mpg for my trip and reach 90C by 7.5/8 miles.

Do the same journey and traffic only allows me to maintain 60/65mph on the dual carriageway and i'm looking at 43mpg for my trip and reach 90C by 9 miles.

The longer the commute, the less relevant the warm up phase is to your mpg.

You're loading before the engine is up to temperature MH?

[monkeyhanger]

You're loading before the engine is up to temperature MH?

When the water is up to temp I do (well 3/4 throttle max, until oil is to temp). I'd be more concerned about getting the oil temp right up if we weren't using fully synthetic oil that doesn't have a drastic change in properties such as viscosity over it's operational temperature range.

Don't forget also that diesel fuel itself is lubricative, so you can push a diesel car harder early on than I would do with a petrol engine without risking excessive engine wear.

[monkeyhanger]

2400 miles past 1st service and my dad's mpg has not gone up in the slightest, compared to pre-service. He's still getting a consistent 47mpg (better than me, but his commute is quite a bit longer and he does far more miles in the line of duty at work).

[Gordor]

Maybe we are going to have to admit that we are stuck with poor and random mpg?