Why does turning on the electrical items in a car use more petrol - surely this is rubbish ??

Question

Shatter this myth. Its written every where that if you turn your heater , heated seats and radio , wipers , headlights , etc it increases the petrol consumption.
In fact Clarkson in an econony test turned everything off - fool i think !

Air con and assisted steering are mechanically attached to the engine so i can understand them having a bearing on consumption if they are used , but not the above items.

Surely this myth is rubbish ?

The generator still spins at the same speed and has the same resistance what evers on

Intelligent replies only please , none of you guessers who just about know where to put the key it

Answer 1

the alternator is controlled by a voltage regulator.

in the need/demanding electricity the voltage regulator open a line of electrical power to magnetic field inside the alternator in the center of that field is the mass ..the mass cut the magnetic field and that is know as alternate current.the current is rectified and turned in direct current

the more the consume of electricity, the more higher the magnetic field ,the more higher the magnetic field the more the resistance,the more the resistance,the more effort from the engine ,means more gas need it.

Answer 2

The power has to come from somewhere. Aircon probably uses about one horsepower, ie about 2% of the engine's output on a typical medium car in cruise mode. Opening the windows increases drag and raises required power by up to 5%. The maximum electrical load will only be about 200 watts, less than a third of a HP and the extra fuel consumption probably won't be noticed. For a short time the power is drawn from the battery, but fairly soon the alternator has to replace this. As an aside a known performance tweak was to fit a relay to cut the alternator out when full throttle was used: claimed to shave a fraction of a second off 0-60 times.

Answer 3

Its not a myth, just you being mechanically incompetant.

Many of the answers above are quite correct, and you yourself should have been able to work out that an alternator on maximum output takes a certain amount of horsepower from the engine in order to drive it.

On certain types of low cost motor racing it is quite common to remove as much electrical load as possible, remove the normal alternator ( whose load increases with output) and fit one of the old style dynamos, geared down as well.

Formula one cars have the smallest output alternators possible to just run the required systems and keep the small battery charged.

My Landrover has a 100 amp alternator which takes about 10 hp off the engine at full load, and a relative increase in fuel consumption.

So, no myth - use the power- pay for the fuel.

Answer 4

I fail to understand why perfectly correct answers have been given bad answer ratings. Power drawn from the battery needs to be replaced. The alternator has to work harder when more power is used. For example sitting in busy town traffic the cooling fan (electric) will be required more often and when it starts up there is a noticeable drop in engine revs. The engine management system then raises the idle speed to allow the power drained from the battery to be replace by the increase in speed of the alternator. It isn't rocket science and as I said most of the answers above are perfectly correct.

Answer 5

if you were to take the alternator drive belt off and spin that alone [with say a moderately powered electric drill] in the normal direction of rotation then turn the key to on position and then turn on all electrical devices you will note a reduction in the speed of the drill motor and that it is now straining. this is due to:
1. a increase in the magnetic field of stator windings making rotor more difficult to turn.
2. the electrical loads themselves require extra effort.
3. parasitic loss of efficiency, electro-magnets make heat, the stronger the field the more heat. this is in addition to mechanical loss due to the bearings and brushes.
NOTE: on modern cars the field is controlled by the engine control module this gives a more linear charge rate and reduces amount of "field on time" thus saving a little fuel.over a long period of time whitch adds up to a lot of savings at the pump
note-2 to any nay sayers the above test can be accomplished by the average person in less than a half hour the results wont vary.

Answer 6

Well its seems the other answers hit the spot...but i wanted to add....well when you switch electrical items in the car..the load goes on the battery and then on the alternator to keep the battery voltage at the right place. this load is kinda like the alternator getting jammed and slowing down and so the engine compensates this by the idling being increased so that the alternator spins faster and gives the right amount of power..you can test this by switching on the engine of the car and switching on the headlight and then switch off the engine, you will notice a slight difference in the brightness.

Answer 7

I do not believe for a second this is a myth. Your car battery runs everything electrical inside the vehicle and in turn the battery is charged by the engine, pretty much like a dynamo on bike lights, so needless to say the more electrical stuff switched on in a car the more the battery needs to get recharged and to recharge the battery the engine needs to work a little harder therefore using more fuel.
I hope this has been helpful to you.

Answer 8

that's no myth. I had a Jaguar XK-E years ago. The alt. went out. I put another on and it wouldn't work. I had the car idling and was experimenting with some wires to the alternator. When I connected the 'correct' wires, the engine 'bog' just a little bit.
Even on my Mazda I drive now, the electric window(s) 'pull down' the rpms when the reach the top, or bottom.

Answer 9

My fuel consumption is definitely higher in winter running the heaters and in peak of summer I try not to use my air con cause that really hits the fuel consumption hard.
Its not a myth its the hard reality of owning and running a car. If you listen very carefully to the engine and watch the revs you can see them dip and recover when you turn the heating/air con on. Just as the lights dip when you turn the heater on and they then recover as the power level adjusts itself.
Its basic physics really.

Answer 10

To output more energy the alternator has more power returned to it,s field coils to increase magnetic field which in turn increases the output. This increase in magnetic clamping force requires greater power to turn the alternator hence greater fuel cost, Dyne's and Ergs are the basic units which equal Joules, 3.6 M J = 1 K/watt, and the accepted definition of a watt is work done for distance travelled. your food input is assessed in K/Jules and your work output is measured in Watts T101 Technology OU, T 281 Physics OU. In layman's terms no gain without pain! which most contributes have stated in their own words quite graphically.