Why??

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Originally Posted by isuzurover

Umm - sure you have an engineering degree Dougal? :D

https://www.aulro.com/afvb/images/im...014/01/806.jpg
http://www.ce.utexas.edu/prof/kinnas...IFS/kinair.gif

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Air density Ben.;) Total aero drag is proportional to air density and air density decreases with temperature.
I know air thickens as it heats. But the loss of density is dominant.

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Originally Posted by isuzurover

http://upload.wikimedia.org/wikipedi...emperature.svg
http://upload.wikimedia.org/wikipedi...emperature.svg

So the higher viscosity will lead to lower Re at higher temperatures, which will reduce skin friction drag but increase wake drag (the dominant form of drag for a bluff body like a Land Rover). You cannot make a statement like that unless you have done a study of the flow around a RRC.

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That's viscosity vs pressure. I'm not seeing the application to temperature change at similar pressure.
Re is the transition between laminar and turbulent flow. We're dealing with turbulent flow.

It's not my RRC that this effect is biggest on. It's been everything I've measured the consumption on. Old corolla, petrol nissan car, diesel nissan car, VAG diesel etc.

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Originally Posted by isuzurover

Experimental studies of vehicle acceleration show a general trend of increased drag and decreased engine performance with temperature.

https://www.aulro.com/afvb/images/im...014/01/807.jpg
http://www.scielo.br/img/revistas/jb...3/a10fig02.gif

Journal of the Brazilian Society of Mechanical Sciences and Engineering - Comparison of engine power correction factors for varying atmospheric conditions

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We know engines lose performance with increasing temp. But fuel economy and performance are not the same thing.
That paper is measuring acceleration performance, not fuel consumption or drag.

All 3 of our cars (late model turbo diesel) get best economy when it's cold.

The Golf can be 10% difference between midday (35deg) and 10pm (20deg) on the same road. Rarely get below 5L/100 on the freeway if it over 30. Often see low 4s in winter at night.

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Originally Posted by harlie

All 3 of our cars (late model turbo diesel) get best economy when it's cold.

The Golf can be 10% difference between midday (35deg) and 10pm (20deg) on the same road. Rarely get below 5L/100 on the freeway if it over 30. Often see low 4s in winter at night.

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All my cars use significantly more in winter than summer. Between 1-2 litres/100km more.
Winter temp range -10C to 15C.
Summer temp range 0C to 35C.

Is your golf petrol or diesel? My tdi skoda has only got below 5 litres/100km on a 35C day.
At 15C the same trip is ~6 litres/100km averaged and I can't even get a steady 100km/h at 5 litres/100km.
Below zero it's over 7 litres/100km.

My diesel work car only broke 15km/l (6 litres/100km) once on a 550km trip that hit ~36C. Did that same trip about a dozen times in that car so I had a good baseline.
I monitor the fuel consumption of all my vehicles. In summer this one hits 14km/litre. In winter I'm lucky to get 12km/l.
Summer has the AC on almost all the time, winter has the heater on almost all the time.

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Originally Posted by Dougal

All my cars use significantly more in winter than summer. Between 1-2 litres/100km more.
Winter temp range -10C to 15C.
Summer temp range 0C to 35C.

Is your golf petrol or diesel? My tdi skoda has only got below 5 litres/100km on a 35C day.
At 15C the same trip is ~6 litres/100km averaged and I can't even get a steady 100km/h at 5 litres/100km.
Below zero it's over 7 litres/100km.

My diesel work car only broke 15km/l (6 litres/100km) once on a 550km trip that hit ~36C. Did that same trip about a dozen times in that car so I had a good baseline.
I monitor the fuel consumption of all my vehicles. In summer this one hits 14km/litre. In winter I'm lucky to get 12km/l.
Summer has the AC on almost all the time, winter has the heater on almost all the time.

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That's because every cold start it'll be running a heap of enrichment until the motors upto running temperature in winter. Lots of short run in very cold temperature will absolutely kill your fuel economy.

I've found motors always run best in really cold damp air myself :) The air is denser so you get a bigger bang for your money !

seeya,
Shane L.

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Originally Posted by DoubleChevron

That's because every cold start it'll be running a heap of enrichment until the motors upto running temperature in winter.

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No enrichment on a diesel and cold starting means nothing over a 500+km trip.

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Originally Posted by Dougal

Air density Ben.;) Total aero drag is proportional to air density and air density decreases with temperature.
I know air thickens as it heats. But the loss of density is dominant.

...

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The first graph shows kinematic viscosity ;)

Re = u.D/v (velocity*representative diameter / kinematic viscosity).

So as I said [assuming a turbulent flow regime]

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will lead to lower Re at higher temperatures, which will reduce skin friction drag but increase wake drag (the dominant form of drag for a bluff body like a Land Rover [and most non sports-cars]). You cannot make a statement like that

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As for:

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Originally Posted by Dougal

Re is the transition between laminar and turbulent flow. We're dealing with turbulent flow.

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You need to go and read a fluid mechanics textbook. Unless you are travelling at a velocity at which mach number is relevant then Re is the best dimensionless number describing flow behaviour. If you think Re is just for determining if flow is laminar or turbulent then you are showing your lack of understanding.

How about you go and find a single study which supports your annecdotal claims???

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Originally Posted by isuzurover

You need to go and read a fluid mechanics textbook. Unless you are travelling at a velocity at which mach number is relevant then Re is the best dimensionless number describing flow behaviour. If you think Re is just for determining if flow is laminar or turbulent then you are showing your lack of understanding.

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I have a few fluid mechanics texts. They all boil form drag down to:
F= Cd*A*1/2*rho*velocity^2

You think these are wrong?

Rho being air density. The air density at 30C is ~10% less than at 0C.

You'll notice that viscosity isn't referred to. It's impact is small enough that for the normal temperature range, it's ignored.

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Originally Posted by isuzurover

How about you go and find a single study which supports your annecdotal claims???

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Here's a good one, it is using fluid drag to measure density and thus temperature of the upper atmosphere:
http://deepblue.lib.umich.edu/bitstr...#39;sequence=5

You'll find that same formula from above on page 8 (eqn 12). He doesn't care about viscosity either. It's not significant.
He does however note the drag coefficient changing by roughly a factor of 2 at supersonic speeds.

Cummins publication page 30: http://cumminsengines.com/uploads/do...whitepaper.pdf

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Originally Posted by cummins

Ambient Temperature
Air becomes more dense as temperatures drop, which increases air resistance.
For every 10°F drop in temperature, aerodynamic drag increases by 2%.
Thus, fuel efficiency will drop by 1%.
Overall, fuel economy tends to be higher in the summer than the winter. According to North American Truckload Fleet Data, driving in the summer increases fuel
mileage by 8 to 12% over driving in the winter months
.

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Nasa drag equation: https://www.grc.nasa.gov/www/k-12/airplane/drageq.html
Again the same.

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Originally Posted by Dougal

No enrichment on a diesel and cold starting means nothing over a 500+km trip.

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Yeah there is ... On the old diesels I've had it's been a wax filled type device with a cable (like a choke cable) that runs back to the injection pump and moves the cold start/run enrichment device.

However if your doing 500km trips, cold starts will make bugger all difference. It's the 5km trips that kill the fuel economy as your always running full enrichment.

Cold running enrichment in deisels is still nothing like the "choke" in an old petrol motor though.

seeya,
Shane L.

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Originally Posted by DoubleChevron

Yeah there is ... On the old diesels I've had it's been a wax filled type device with a cable (like a choke cable) that runs back to the injection pump and moves the cold start/run enrichment device.

However if your doing 500km trips, cold starts will make bugger all difference. It's the 5km trips that kill the fuel economy as your always running full enrichment.

Cold running enrichment in deisels is still nothing like the "choke" in an old petrol motor though.

seeya,
Shane L.

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I think that's a cummins thing. Called KSB or similar. It's rumoured to advance the timing until the wax melts, but I've never driven one.

My Isuzu has excess fuel starting, but only until it's actually started. My work car has a pretty simple EFI system with only one timing map. My other tdi car though has a different injection timing map for every 10C of water temperature. For maximum smoothness and minimim noise/smoke throughout the cycle.
Pretty clever really.

Drag on a car is extremely complex, and in almost all cases will be a mix of skin friction drag AND bluff body/wake drag.

We are talking about a comparatively tiny difference in density and viscosity, while holding everything else equal. The overall REAL difference in drag will be tiny (either way).

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A body moving through a fluid experiences a drag force, which is usually divided into two components: frictional drag, and pressure drag. Frictional drag comes from friction between the fluid and the surfaces over which it is flowing. This friction is associated with the development of boundary layers, and it scales with Reynolds number as we have seen above. Pressure drag comes from the eddying motions that are set up in the fluid by the passage of the body. This drag is associated with the formation of a wake, which can be readily seen behind a passing boat, and it is usually less sensitive to Reynolds number than the frictional drag. Formally, both types of drag are due to viscosity (if the body was moving through an an inviscid fluid there would be no drag at all),

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Drag of Blunt Bodies and Streamlined Bodies

All the stuff you have posted above is far too oversimplified for the small difference we are talking about here. [Edit] or relevant much more to trucks and planes than cars and 4x4s.

Any difference in vehicle performance or efficiency across the span of temperature is 99.9% due to combustion efficiency, rather than drag.

EDIT: you obviously need to read up on skin friction drag:
http://en.wikipedia.org/wiki/Parasit...#Skin_friction

EDIT 2:
A passenger car/van/4x4 is much closer in shape to a sphere than a flat plate (or a truck).
Show me which part of the attached curve is not dependant on Re (and therefore viscosity).
https://www.aulro.com/afvb/images/im...014/01/780.jpg
http://www.aerospaceweb.org/question.../drag-disk.jpg