are cars more fuel efficent during the hot weather?

fryup

My mpg has shot up over the past few days...is it because the engine reaches and maintains its ideal temperature more quickly?

its just a regular 1.1 petrol with no air-con

mike65

Engines run better in cold air as its dense but in warm weather they do reach operating temps that much quicker and hot air is "thinner" so drag is reduced

Cee-Jay-Cee

mike65 wrote: »

Engines run better in cold air as its dense but in warm weather they do reach operating temps that much quicker and hot air is "thinner" so drag is reduced

Engines also use more fuel in cold weather. Cold air is denser and so the ECU fuels accordingly. My fuel economy generally averages 9.0/100km in winter but drops to 8.4l/100km in summer for identical journeys.

GhostInTheRuins

I was wondering the same myself yesterday. I usually get 6.5l/100km per tank of fuel, but last week managed a 6.0l/100km on a tank, same kind of driving.

Matt Simis

mike65 wrote: »

Engines run better in cold air as its dense but in warm weather they do reach operating temps that much quicker and hot air is "thinner" so drag is reduced

Yes this, though a slightly more cynical interpretation is hotter air makes engines perform worse, so you get lower performance and therefore less fuel usage.

dgt

My car was noticabely down on power yesterday so yes, hot weather lowers a cars output

Henry Ford III

Do really hot road temps and sticky tarmac make any difference?

MrDerp

I've been surprised at my computed average coming down in the last month or so. It's 0.2L/100km lower than normal.

But the real difference I'm getting is on my monthly Dublin-Cork-Dublin. I typically get 9.5-10L/100km on this (average ~ 140km/h @3900rpm) and I seem to be getting <9.0L/100km now, so the comments on drag above are particularly interesting.

rovoagho

I hate the cold, except when it comes to driving. My current car is a turbo, so I'm looking forward to winter for the first time in years...

georgefalls

Henry Ford III wrote: »

Do really hot road temps and sticky tarmac make any difference?

That lets you corner a bit faster, without ending up in a ditch..:D

ironclaw

Unfortunately no, you want its as cold as possible entering the engine and as hot as possible leaving. Thermodynamics at play etc.

vandriver

I'm getting approximately 12 % better fuel efficiency compared to March,on a 1.6 hdi in city driving.

AugustusMinimus

ironclaw wrote: »

Unfortunately no, you want its as cold as possible entering the engine and as hot as possible leaving. Thermodynamics at play etc.

The hotter the exhaust, the less efficient the engine is at turning the energy in the petrol / diesel into kinetic energy.

vrusinov

It also depends on how cold is it.

At -20 C you'll spend good liter of fuel just to start your engine and warm it up to reasonable temp.

JoeA3

I'm sure those of you with aircon also have the cooling fans on full whack, at least for the first 10 minutes after you get into the car! That'll have a slight impact on economy too...

thegreatiam

its nice and sunny so people wear less clothes. You drive slightly slower to look, so you drive more efficiently hence increased mpg. ;-)

Chimaera

CJC999 wrote: »

Engines also use more fuel in cold weather. Cold air is denser and so the ECU fuels accordingly. My fuel economy generally averages 9.0/100km in winter but drops to 8.4l/100km in summer for identical journeys.

Not quite. Yes the engine will burn more fuel for a given volume of air, but it'll generate more power as a result, meaning the request for air/fuel to generate a given level of power drops compared to hot weather.

But like others have said, there are many factors at play here. Drive more, worry less

ironclaw

AugustusMinimus wrote: »

The hotter the exhaust, the less efficient the engine is at turning the energy in the petrol / diesel into kinetic energy.

The hotter it is, the hotter the fuel is burning and the more energy is being released from said fuel. That's a sweeping generalization but generally holds true. The transfer to kinetic energy is far more down to the overall engine gearing. You could have the worlds most efficient engine but won't move anywhere efficiently if your gearing is wrong.

vrusinov

I'm sure those of you with aircon also have the cooling fans on full whack, at least for the first 10 minutes after you get into the car! That'll have a slight impact on economy too...

Fun fact: I've seen research with conclusion there's more economical to use aircon rather than opening windows on high speeds.

Jesus.

Oh sh*te, don't say that lads. My mpg is bad enough as it is!

Renegade Mechanic

They are indeed. CJC999 is right. Hot air, being less dense, carries less oxygen which is picked up by the O2 sensor. The ecu reduces the fuelling to keep the proper air/ fuel ratio. It costs you some power but that only applies for things like overtaking. And as mentioned hotter air is easier to slip through. Also at motorway speeds or anything above 50mph air con is more efficient than open window.

bmstuff

My consumption went up actually because the car is too slow now because of the heat so I have to drive harder
14.8mpg to 14.4 now

Renegade Mechanic

bmstuff wrote: »

My consumption went up actually because the car is too slow now because of the heat so I have to drive harder
14.8mpg to 14.4 now

Ban Ki Moon just called, The UN need to borrow a civilian Hummer for covert ops!

bmstuff

Renegade Mechanic wrote: »

Ban Ki Moon just called, The UN need to borrow a civilian Hummer for covert ops!

I have been there already, I let the young fellas looking after that now. Tesco here I come...:D

Renegade Mechanic

bmstuff wrote: »

I have been there already, I let the young fellas looking after that now. Tesco here I come...:D

Try not to crush any cars parking now. Or the store itself!:)

bmstuff

Renegade Mechanic wrote: »

Try not to crush any cars parking now. Or the store itself!:)

That's fine it is only a bimmer, a Nissan is more solid, ask me how I know...not

Voodoomelon

My consumption has improved by about 8-10%. Car is noticeably more sluggish though.

kona

The fuels density changes along with the airs density. It's 30 degrees max, the ECU will have fuel trim tables to compensate for changes. Air inlet temp, maf sensors will let the ECU know what volume of 02 is in the cylinder, the injector will inject the fuel . Essentially less O2 by volume means less fuel by volume needed to burn at 14.7:1 . Then your not using choke , everything is warm quicker.

larchill

MrDerp wrote: »

I've been surprised at my computed average coming down in the last month or so. It's 0.2L/100km lower than normal.

But the real difference I'm getting is on my monthly Dublin-Cork-Dublin. I typically get 9.5-10L/100km on this (average ~ 140km/h @3900rpm) and I seem to be getting <9.0L/100km now, so the comments on drag above are particularly interesting.

9.0l/km:eek: - I'm getting almost ½ that @ 4.7/4.8l/km with my 2.0Tid A4 on my daily Drogheda - Dublin run:P You must have a 5.0 V8 or something!

Renegade Mechanic

bmstuff wrote: »

That's fine it is only a bimmer, a Nissan is more solid, ask me how I know...not

Ah the name makes sense! And I'm pretty sure where youre sailing there so wont quiz you on it.

MrDerp

larchill wrote: »

9.0l/km:eek: - I'm getting almost ½ that @ 4.7/4.8l/km with my 2.0Tid A4 on my daily Drogheda - Dublin run:P You must have a 5.0 V8 or something!

Nope. Just the joys of driving a 5 speed petrol at 140km/h for 250km :pac:

challengemaster

People being happier because of the weather also helps, tend to become more placid while driving = better mpg

Mr Mike

I think the biggest factor for the increased economy is that the engine gets to normal operating temperature faster because of the higher starting temperature.

The oil is much less viscous in these temperatures so the engine has less drag.

My car economy went from 5.6 to 5.2 L/100km in the last week (1.3 petrol).

ironclaw

MrDerp wrote: »

Nope. Just the joys of driving a 5 speed petrol at 140km/h for 250km :pac:

Are you getting 4.8L/ km or 4.8L/100km?

Because its the inverse of MPG, 10L/100km is far worse than 4.8L/100km.

4.8L/100km is 58MPG which is pretty amazing to be fair.

Matt Simis

Mr Mike wrote: »

I think the biggest factor for the increased economy is that the engine gets to normal operating temperature faster because of the higher starting temperature.

The oil is much less viscous in these temperatures so the engine has less drag.

My car economy went from 5.6 to 5.2 L/100km in the last week (1.3 petrol).

This would only matter on short journeys and have virtually no effect proper commuting. Also Oil Temp is realistically not hotter in this weather than in 10c weather as the Oil temp is mostly a function of the combustion process which operates at vastly higher temps. While we may think its much hotter out, to an engine or exhaust its a minor difference.. temp is relative.

Antenna

This time of year, and with this weather, we will have the lowest usage of headlights - less alternator load, which also has an impact on fuel economy.

Tallon

My MPG has gone down since the good weather!

Is it the Air Con on all the time?

Voodoomelon

Air con will certainly hit your MPG, especially with smaller engines. I have mine on all year round so probably the reason i've seen a significant improvement.

ironclaw

Antenna wrote: »

This time of year, and with this weather, we will have the lowest usage of headlights - less alternator load, which also has an impact on fuel economy.

Temperature, Fuel quality, driver style, onboard electronics would play a 10 times greater role that using your headlights. Your talking about 10 amps of load and thats with full beams on. Dips would be negligible on the overall circuit. The engine as a whole is well able to cope with this plus a whole lot more i.e. Overhead.

To inform someone that fuel economy is adversely affected, to the point where you could save by not using them, by headlight usage is extremely bad advice from a technical and safety standpoint.

Antenna

ironclaw wrote: »

Temperature, Fuel quality, driver style, onboard electronics would play a 10 times greater role that using your headlights. Your talking about 10 amps of load and thats with full beams on. Dips would be negligible on the overall circuit. The engine as a whole is well able to cope with this plus a whole lot more i.e. Overhead.

To inform someone that fuel economy is adversely affected, to the point where you could save by not using them, by headlight usage is extremely bad advice from a technical and safety standpoint.

I was not giving "advice" that people switch off headlights to conserve fuel.

This thread was not about how to save fuel, but about why there can be better fuel "efficiency" in hot weather, i.e. the summer and its reasonable to say electrical (alternator) load is a contributory reason why.

The vast majority of motorists here do not have headlights on during fine summer days.

What I said 3 posts back is surely fair comment ?

referring to a VW manual for bulb wattages from some years ago....

Putting on Dips - these are 55 Watts each (H7 bulbs) , both give 110Watts. The sidelights also come on (5W each X2 = 10W),
Tail Lights 10W each X2 = 20W
Number plate Lights 5W each = 10W
(I'm not including small bulbs on dashboard which also come on)

Total with Dips on = 150 Watts . Alternator is at best maybe 70% efficient - so around 220W of extra Kinetic energy taken by alternator.

Now turning on Full beams (the Dips, and the other bulbs or course remain on ) H1 bulbs 55W each. so 260W for all the bulbs. Alternator approx. 70% efficient so about 370 Watts of Kinetic energy taken by alternator

So the approx. 220 W (Dips) or 370W (Full beam) of Kinetic energy has to come from the engine. It doesn't come out of thin air.

And you are understating the load with full beams on, talking about 10 amps (Amps X Voltage = power)

There are also other extra electrical loads during winter - Demister user (which many people then forget to switch off when not required), and other electric heaters in some cars.
Glow plugs also in diesel engines, which may be significant if someone was making frequent short journeys in cold winter weather compared to doing so in summer.


note that the AA has the following on the web:

Turn it off – Any electrical load increases fuel consumption, so turn off your heated rear windscreen, demister blowers and headlights, when you don't need them.

from

http://www.theaa.com/motoring_advice/fuels-and-environment/drive-smart.html
and

http://www.theaa.ie/AA/Motoring-advice/Driving-advice/Eco-Driving-Advice.aspx

Jesus.

Jesus man that comment was 2 and a half years ago. Even I couldn't bring something that dead back to life

Special Circumstances

Wow talk about playing a long game in an argument. 2 years and it's still burning away at ya?