How do lights use more petrol?

ste_bla · 10 Mar 2008 at 01:10

Ive always been hold that using lights, heater radio etc uses electrical more power than when off, which i get. However I dont get how they cause you to use more petrol?

Surely your engine will still run at the same rpm etc as there would not be any extra load on the engine (as the alternator belt is always there)?

Can anyone explain in simple terms how it works?

Skeeter · 10 Mar 2008 at 01:13

power for electrical devices comes from the battery, which is charged by the alternator, driven by the engin, powered by fuel.

Think about it, the only fuel you put in your car is petrol... everything that requires energry to run must get that energry directly or indirectly from the petrol.

Conscript · 10 Mar 2008 at 01:15

Skeeter said:
power for electrical devices comes from the battery, which is charged by the alternator, driven by the engin, powered by fuel.

Think about it, the only fuel you put in your car is petrol... everything that requires energry to run must get that energry directly or indirectly from the petrol.

But I think his question is, how can the load on the alternator increase fuel consumption as it is driven at a constant speed?

I know it must do, but I don't have the know how about electrical systems to undertsand fully, I must admit.

ste_bla · 10 Mar 2008 at 01:17

Conscript said:
But I think his question is, how can the load on the alternator increase fuel consumption as it is driven at a constant speed?

Yup thats what i wanted to now just wasnt sure how to put it!

Psycrow · 10 Mar 2008 at 01:35

Loading an electrical generator creates whats known as back torque. The greater the current drawn from the alternator, the greater the shaft torque required. The extra energy comes from the engine and thus uses more fuel.

MagicBoy · 10 Mar 2008 at 01:39

The more electrical load on the alternator the more difficult it is to drive. It converts mechanical into electrical energy - basically the reverse process to an electric motor.

Clarkey · 10 Mar 2008 at 02:42

because with more load on the engine you need more throttle opening to maintain the same rpm. Exactly the same as going uphill you need to give it more throttle to maintain your speed (or rpm). More throttle opening = more air = more fuel.

MikeHunt79 · 10 Mar 2008 at 02:53

Try this...

Leave your engine running idle.

watch your rpm gauge. Now turn on your lights, full beam, heater maximum, wipers, etc. and I can assure you that the rpm will have dropped due to the extra load (unless you have a seriously powerful engine)...

The thing is, 750W is around 1hp, so I wouldn't lose sleep over it... The power drawn by lights and other electrical gizmos is next to nothing really compared to the power needed to move forwards. It will probably 1-2 mpg difference at the most.