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[Tony H]

I saw a t.v. programme last week about reducing fuel bills. Something the "expert" said was that if you reduced the electrical load i.e turned off the heated rear window, heated seats etc this would improve fuel consumption. How? I always thought that the mechanical load of an alternator was constant no matter what the power demand  :-/ please correct me if I'm wrong and enlighten me

[Taxi_Driver]

I saw a t.v. programme last week about reducing fuel bills. Something the "expert" said was that if you reduced the electrical load i.e turned off the heated rear window, heated seats etc this would improve fuel consumption. How? I always thought that the mechanical load of an alternator was constant no matter what the power demand  :-/ please correct me if I'm wrong and enlighten me

That would be nice.......but not true im afraid....

Looking at it simply wot goes out must = wot goes in.

So load an alternator more with electrical gadgets and the engine has to work harder to turn it.......hence you use more petrol.

Or even simpler 'there aint no such thing as a free lunch'    

[kris9128]

cant see it making much difference mate to be honest.  :-?
if you want to cut your fuel bill either drive like your granddad or get a smaller car  

[Lazydocker]

Switching off the electrical load does make a difference, it's just very small!

The only sure way to reduce your fuel usage is to either learn how to drive more economically or walk!

[m1ct]

Switching off the electrical load does make a difference, it's just very small!

The only sure way to reduce your fuel usage is to either learn how to drive more economically or walk!

Indeed, what a shame so few drivers know how to drive a car economically and with common sense. I am sick of following people up a slight hill with speed limit of 50 going only 35mph. They try to `tackle the mountain` in the fifth gear causing frustration to others behind them and as soon as you overtake them, you get either the middle finger or full beam+fogs (quite effective at dusk lol)
btw if it wasn`t for tight bends, i would overtake them straight away, obviously  

[Tony H]

Switching off the electrical load does make a difference, it's just very small!

The only sure way to reduce your fuel usage is to either learn how to drive more economically or walk!

But how? :-/

[JamesV6CDX]

If cruising at 3500rpm say, the alternator will be turning hell for leather, so under those conditions I can't see it needing more engine power to cope with electrical load?

[Entwood]

When spinning under no electrical load (disconnected) the only load wil be the mechanical ones of bearing drag and air resistance of the cooling fins. These are fixed and purely dependant on RPM

When connected to an electrical circuit, the additional loads of overcoming the magnetic field in order to produce electricity, and a thing called "back EMF" ( ElectroMotive Force) come into play ...  back EMF is the resistance of a circuit to actually doing anything .. ( think of it as the alternator having to push all those electrons down a wire when they don't want to move from their present steady state)

as a very simple example ...

open circuit alternator has no electrical work to do. Switch on 1  x 15watt bulb .. it must now produce 15 watts of energy ... switch on another .. it must now produce 2 x 15 = 30 watts  and so on and so on.

NOW  746 watts = 1 horse power !!!!   so .. the amount of load needed to actually feel a difference is beyond the capabilities of most car electrical systems !!

Theoretically it will make a difference .. but in practice you'll never see it


HTH

[Chopsdad]

It must be very small  

I travelled 100 miles  - A road & M6 - both heated seats on (wife's on level 5), climate running 2 levels - her's fighting for heat and mine for cold, all lights on + wipers.  The road was waterlogged but I drove at 60 mph & 72 mph and got home to read 35.4 mpg.

Never had better than 31.5 in the dry on that run before - perhaps it's the new exhaust.  Perhaps it's me driving smoother. But it's deffo not adversely affected by the electric draw  

[Lazydocker]

Driving economically doesn't mean driving slowly! This is what most people fail to grasp. I certainly don't hang about, in fact when my mate supplied me 2 new tyres of a brand i've never heard of he asked me to keep him informed with how they perform because he knows how i drive!

Economy is directly linked to smooth driving... That's why i regularly see high 20's on the mid (which i have checked several times on brim to brim fills and proved to underread slightly) doing fairly short journeys!

I'll be interested to see what economy i get tomorrow with the pikey house on the back!

just my opinion and experiences, but i give the boy racers a run for their money but do it without treating the pedals like switches and get good economy!

[Tony H]

When spinning under no electrical load (disconnected) the only load wil be the mechanical ones of bearing drag and air resistance of the cooling fins. These are fixed and purely dependant on RPM

When connected to an electrical circuit, the additional loads of overcoming the magnetic field in order to produce electricity, and a thing called "back EMF" ( ElectroMotive Force) come into play ...  back EMF is the resistance of a circuit to actually doing anything .. ( think of it as the alternator having to push all those electrons down a wire when they don't want to move from their present steady state)

as a very simple example ...

open circuit alternator has no electrical work to do. Switch on 1  x 15watt bulb .. it must now produce 15 watts of energy ... switch on another .. it must now produce 2 x 15 = 30 watts  and so on and so on.

NOW  746 watts = 1 horse power !!!!   so .. the amount of load needed to actually feel a difference is beyond the capabilities of most car electrical systems !!

Theoretically it will make a difference .. but in practice you'll never see it


HTH

So if I'm right what your saying is the more demand on the alternator the higher the magnetic resistance the alternator creates :-/

[Entwood]

When spinning under no electrical load (disconnected) the only load wil be the mechanical ones of bearing drag and air resistance of the cooling fins. These are fixed and purely dependant on RPM

When connected to an electrical circuit, the additional loads of overcoming the magnetic field in order to produce electricity, and a thing called "back EMF" ( ElectroMotive Force) come into play ...  back EMF is the resistance of a circuit to actually doing anything .. ( think of it as the alternator having to push all those electrons down a wire when they don't want to move from their present steady state)

as a very simple example ...

open circuit alternator has no electrical work to do. Switch on 1  x 15watt bulb .. it must now produce 15 watts of energy ... switch on another .. it must now produce 2 x 15 = 30 watts  and so on and so on.

NOW  746 watts = 1 horse power !!!!   so .. the amount of load needed to actually feel a difference is beyond the capabilities of most car electrical systems !!

Theoretically it will make a difference .. but in practice you'll never see it


HTH

So if I'm right what your saying is the more demand on the alternator the higher the magnetic resistance the alternator creates :-/

Sort of ... the higher the load the more work the alternator has to do, the electricity is produced by cutting a magnetic field with a coil, as the load increases the regulator increases the strength of the magnetic field, so the alternator has to work harder to force the coil through that magnetic field.

Remember that electricity is only a form of energy, and energy cannot be made or lost, only converted from one form to another, so when you produce energy from an alternator you are converting mechanical energy (rotation) into electricity. The energy for that rotation comes from the engine, which is converting stored energy (petrol) into rotational energy very inefficiently - look how much is wasted in heat etc  

The point is this is happening at a very low level .. if you had all your lights on, and your heated windscreen, and your wipers, I doubt you would be using even 300 watts, less than 1/3 of a horse power, and your car produces ( well mine does !! ) 217 BHP, so it is 1/600 ths of your load.. you would use more fuel in 1 acceleration to 30 mph than the alternator would use in a year !!

The calculations on Air Con power useage is very similar BTW  

[HNO3GOH]

But if you want to use the heated seat and stay warm in the winter and still save on fuel then put gaffa tape on the gap around the headlights and the gap where the bonnet meets the wing and gaps in the bumper  
I think i will go now

[Kevin Wood]

Sort of ... the higher the load the more work the alternator has to do

Absolutely. The alternator converts mechanical energy into electrical energy. Switch on the heated rear window whilst watching the revs at idle and you'll probably notice a slight dip. Engine is being more heavily loaded, and the speed drops a little before the ECU responds and gives it more throttle (hence more fuel).

However, it's a bit like nut munchers saying that unplugging your mobile phone charger at night will reduce your electricity bills and reduce CO2 emissions. Technically it's true but if your oven's on for more than 10 seconds a week it pales into insignificance.

Kevin

[Tony H]

Thanks for clarifying that every one it's clear now   however as suspected the "expert" was talking relative b@llocks regarding fuel saving