"Winter Diesel"? - anyone else noticed a drop in MPG?
Discussion
This is not the thread I though I'd ever make on Pistonheads, but here we are so let's just go with it. 
Has anyone else noticed a drop in MPG over the past week or two? I'm down 5-8mpg on average, doing the same trips in the same conditions.
Granted the car does take a little while longer to get up to operating temps, but when discussing this with a mate he suggested that fuel companies change diesel slightly during winter, and that could be the cause of the decreased fuel economy.
I noticed this most last night coming home from Hull, cruise set at 80 leptons would normally net me a 48-50mpg average. Last night it was 42!! Yikes.
So this winter diesel thing - myth or not?
Has anyone else noticed a drop in MPG over the past week or two? I'm down 5-8mpg on average, doing the same trips in the same conditions.
Granted the car does take a little while longer to get up to operating temps, but when discussing this with a mate he suggested that fuel companies change diesel slightly during winter, and that could be the cause of the decreased fuel economy.
I noticed this most last night coming home from Hull, cruise set at 80 leptons would normally net me a 48-50mpg average. Last night it was 42!! Yikes.
So this winter diesel thing - myth or not?
Depends on the biofuel / ethanol content. There definitely is a difference between winter and summer fuel though. Just struggling to remember the details at the moment.
Eta - https://en.m.wikipedia.org/wiki/Winter_diesel_fuel
Danny, if this for your Audi its the least of your worries, i've seen that last vid
Eta - https://en.m.wikipedia.org/wiki/Winter_diesel_fuel
Danny, if this for your Audi its the least of your worries, i've seen that last vid
Edited by KarlMac on Monday 7th November 10:10
My car is petrol but I've noticed a drop in mpg over the last week on my trip counter. Where it usually can read in high 30s on my usual 7 mile trip, it's now reading 33 to 35.
When i measure it properly the car has always done 30 to 31mpg tank to tank. Next fill up I'm expecting maybe 27 to 28.
I'm putting it down to shirt journeys, cold weather, and a heavy right foot at times.
When i measure it properly the car has always done 30 to 31mpg tank to tank. Next fill up I'm expecting maybe 27 to 28.
I'm putting it down to shirt journeys, cold weather, and a heavy right foot at times.
Slight changes in fuel itself, denser air (good for engine, bad for aero!), higher hotel locds (lights on, heater/blower on full, heater screens and seats), changes in traffic, different weather, all sorts of stuff can affect fuel economy over the seasons.
It generally takes a while longer to come up to operating temp aswell, as the engine block is cold soacked over night, plus when you turn it on at 7.30am the first thing you do is turn the heating on the demist. It can take a fair while to come up to running temp fully, especially on big diesel engines with iron blocks.
dannyDC2 said:
So this winter diesel thing - myth or not?
It's not a myth that the fuel mixes change seasonally, but even if you were running on fuel you bought in summer, you'd be seeing lower economy figures anyway.Colder temperatures mean denser air, which is harder for your car to shove out of the way = more fuel needed for the same speed.
Oil will take longer to warm up (engine/gearbox/diff) causing higher resistance for longer = more fuel needed for same performance.
It generally rains more/is wet on the ground more in winter = more rolling resistance from tyres pumping water off the floor.
Your lights will be on more during the shorter days/longer nights, as will your heater/heated seats.
Your car will quite likely have a higher battery drain just from being cold too, so the BMS will charge it more, meaning more fuel needed.
Most folk will run aircon if they have it, to demist the screen. That needs power, which comes from the engine.
Of the list above, the slow warmup/dense air/wet ground issues are the biggest. The others are smaller.
I noticed an indicated drop in MPG indicated by the car on Saturday. Went to pick up my daughter so a decent motorway run, the car (E320 CDI) indicated 46mpg where as I'd normally see 48-50mpg on the same run.
That said, if I work it out properly it still managed a 47mpg run so can't complain. I can imagine it's the short runs where the engine takes that bit longer to warm up that will mean a drop in efficiency though.
That said, if I work it out properly it still managed a 47mpg run so can't complain. I can imagine it's the short runs where the engine takes that bit longer to warm up that will mean a drop in efficiency though.
I've always wondered, how much does using the heater effect the engine warmup time? Naturally, the first thing you reach for when you get into a cold car is the heating control. You feel that air flow gradually warming up as the coolant heats up, and of course, the oil temperature lags some way behind. How much cooling effect does using the heater have? It's drawing heat from the cooling circuit, but does this cause any net loss to the overall coolant temperature, and does that slow the overall engine warmup down?
Loyly said:
I've always wondered, how much does using the heater effect the engine warmup time? Naturally, the first thing you reach for when you get into a cold car is the heating control. You feel that air flow gradually warming up as the coolant heats up, and of course, the oil temperature lags some way behind. How much cooling effect does using the heater have? It's drawing heat from the cooling circuit, but does this cause any net loss to the overall coolant temperature, and does that slow the overall engine warmup down?
Depends where the heater takes its feed from. If the heater matrix is connected to part of the cooling system after the thermostat then you won't get any heat until the thermostat opens anyway.If the heater matrix is connected to the coolant circuit before the thermostat, then yes, of course the heater will be robbing some energy from the coolant which would otherwise have contributed to a warmer engine. Yes, it would slow engine warm up, and increase engine cool down.... how much would depend on a great many factors.
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