Does air con really eat your fuel?

pwllbwdr

Strider590 wrote: »

It I think depends on whether the aircon is driven from the auxiliaries belt or not, if it's fully electric....

Like for example EPAS (Electronic Power Assisted Steering) which uses an electric motor instead of the belt driven hydraulic pump system used in old school PAS.

Anything extra that the engine has to physically move, will always have an effect on fuel consumption... Electrically driven devices have very little negative effect.

Erm I don't think so. AC requires power to drive it. If that is supplied electrically then the alternator will be applying a load to the engine sufficient to generate that power (torquexangular velocity). In comparison, the energy required to drive a power steering system is tiny.

philnicandamy

To be honest I had a 1.6 Note SE auto for 3yrs the MPG was poor at best and thats without using the A/C...best I drove round town? 25-27MPG

Hammyman

philnicandamy wrote: »

To be honest I had a 1.6 Note SE auto for 3yrs the MPG was poor at best and thats without using the A/C...best I drove round town? 25-27MPG

If all you are doing is short journeys around town then of course you will be getting that MPG.

Strider590

pwllbwdr wrote: »

Erm I don't think so. AC requires power to drive it. If that is supplied electrically then the alternator will be applying a load to the engine sufficient to generate that power (torquexangular velocity). In comparison, the energy required to drive a power steering system is tiny.

Except the alternator over produces more than enough power to cope with additional electrical load and applying addition electrical load does not cause any addition physical load on the engine. Therefore no additional fuel is being used.
A faulty alternator might cause a slight misfire if it can no longer cope with the live feed to the ingition coils, but your battery would have died by then and these days your cars ECU would probably drop into "limp home mode" if the effect was that serious.

Older power steering and AC used a pump, but later EPAS is driven by a 3 or 4 phase motor from an artificially generated supply, the need for hydraulic pumps was removed. One of the major reasons for the change to EPAS was to increase fuel efficiency, reliability and the other was to provide an adaptive system that could be used in different makes/models of cars with a simple reprogramming procedure, thereby reduces volume production costs.
I worked very closely with this technology before the recession hit us.

On this subject of electrical drain affecting fuel economy.... I need to point out that people who think they save money by driving around on their parking lights at night time, or in fog/snow, instead of using the headlamps have their logic completely wrong!!! :mad:

jrrowleyws

Strider590 wrote: »

On this subject of electrical drain affecting fuel economy.... I need to point out that people who think they save money by driving around on their parking lights at night time, or in fog/snow, instead of using the headlamps have their logic completely wrong!!! :mad:

thats my pet hate too! TBH, I dont think they are trying to save fuel, more just ignorance.

Mark_Hewitt

SusieWinwood wrote: »

Further to my thread the other week, we are almost certainly buying a Nissan Note 1.6SE Automatic this week.

It comes with air con and I asked the sales chap does it really eat fuel? He said well if it does say 30mpg without it on, if you put it on it might do 29. It's this just salesman talk or does it really not use a lot?

Sounds about right. It *does* use a little bit more fuel, but nothing you should be massively concerned about. Personally I leave my A/C on all the time.

Gloomendoom

Strider590 wrote: »

Except the alternator over produces more than enough power to cope with additional electrical load and applying addition electrical load does not cause any addition physical load on the engine. Therefore no additional fuel is being used.
A faulty alternator might cause a slight misfire if it can no longer cope with the live feed to the ingition coils, but your battery would have died by then and these days your cars ECU would probably drop into "limp home mode" if the effect was that serious.

Older power steering and AC used a pump, but later EPAS is driven by a 3 or 4 phase motor from an artificially generated supply, the need for hydraulic pumps was removed. One of the major reasons for the change to EPAS was to increase fuel efficiency, reliability and the other was to provide an adaptive system that could be used in different makes/models of cars with a simple reprogramming procedure, thereby reduces volume production costs.
I worked very closely with this technology before the recession hit us.

On this subject of electrical drain affecting fuel economy.... I need to point out that people who think they save money by driving around on their parking lights at night time, or in fog/snow, instead of using the headlamps have their logic completely wrong!!! :mad:

So somehow automotive electrical systems are exempt from the laws of physics?

I don't think so!

If aircon takes 3hp to drive it, it doesn't matter if that power is provided by an electric motor or a mechanical linkage to the engine, ultimately that 3hp is generated by burning fuel.

Strider590

Mark_Hewitt wrote: »

Sounds about right. It *does* use a little bit more fuel, but nothing you should be massively concerned about. Personally I leave my A/C on all the time.

Yep.... and AC prefers to be left turned on, if unused for extended periods it can fail.... The cost of repairing it is bound to be far in excess of the cost of the extra fuel you may have used by leaving it turned on.

Gloomendoom

Yes, AC is best run at least once a week, even in the winter. Five minutes should be enough to keep everything in working order.

withabix

I leave mine on all the time.