Why does using more electrical eqipment in you car increase fuel consumption? The alternator turns regardless.

Question

Using wipers lights and aircon in a car uses more fuel. Why, when the alternator turns regardless of load?

Answer 1

The car engine generates a fixed amount of power at fixed engine rpms and at a fixed accelerator pedal setting, (i.e. a standard load). When you switch your lights etc. on then they require some of that power, albeit quite small compared to the losses in the engine/drivetrain/tyre friction etc. This power requirement puts an electrical and therefore electromechanical load on the alternator (which converts kinetic energy to electrical energy), which in turn puts a load on the engine's power output, thereby slightly reducing the power to the drivetrain. Main beam lights take 110Watts between them. Electric windows take 120Watts each, roughly, when operated. So if you switch everything on then the load can get quite high and will require extra power (slightly more throttle) from the engine if you are to maintain the same speed on the road.

Answer 2

lights, wipers, etc. will not affect fuel consumption. AIRCON however, will. This is because the compressor runs off a belt attached to the engine. This adds a load to the engine, a little like going up a hill, therefore reducing MPG.

EDIT: THE ENGINE SPEED WILL NOT INCREASE...THAT WOULD MAKE THE CAR GO FASTER BY TURNING THE HEADLIGHTS ON FFS!!!
The alternator creates a current by rotating at a speed proportional to the engine. It has two little brushes that create friction. This level of friction does NOT change the faster it rotates therefore does not change the load the more electrical devices you are using.

Answer 3

The alternator spins continuously as it has a belt that runs from the pulley of the crank. However the load or force required to turn the alternator changes from practically no load (Not charging) to high load (Charging flat battery).
The difference between full and no load is relatively low in comparison to the load required for the air condition pump.

Answer 4

You don't get something for nothing!
The alternator is spinning but the amount of work the engine has to do depends on electrical load.
With your engine idling and the ignition light out, switch on your headlights and you will hear the engine revs drop. With a modern car they will recover as the engine management system corrects the idle. You will sometimes hear a squeal when a(slightly slack) drivebelt slips just after starting as the alternator goes full output to replace the current used in starting.

Answer 5

Just because the alternator is turning don't mean the same amount of energy is going into making it turn.

Imagine you're on a bicycle. You're on the flat, and find it quite easy to maintain 10mph. Now you're going uphill, and you can still make 10mph by pedalling at the same rate, but it's much harder work: there is more resistance at the pedals.

The same goes for your alternator. Increase the electrical load, and the mechanical forces required to keep it turning will increase. Your engine will keep it turning at the same rate, but will use more fuel in the process, just as if you were going uphill.

Answer 6

All power usage will increase fuel consumption. I know a lorry driver who had two rows of lights on the tractor unit and apparently when all are switched on the engine revs drop.

Answer 7

the alternator spin all the time does not mean it charging the battery all the time. it has a voltage regulator that control charging system. each time the regulator request more voltage put out,it put a load on the alternator which cause the engine speed to increase. this is where the engine use more gas .

Answer 8

Yes it turns,But using electrics puts it under load.The load tries to slow it down the engine keeps it turning more fuel used.

Answer 9

i own a repair shop,and really it doesn't,all it does is make the alternator kick in more,which will put a little drag on the engine,but it shouldn't be noticeable in the fuel mileage on it, just about on any car made the alternator is going to kick in and out,and if it did use a little more gas it would be so little that you wouldn't notice it at all,good luck i hope this helps.

Answer 10

because of the extra load is also why people don't use indicators (and to preserve the bulb)