Lights on uses 5% more fuel

[Ian D Smith]

I was driving last week and on Radio 2 there was a guy explaining the benefits of driving with lights on at all times. I always do it in the Seven but only in poor light in the Tintop.

 

The guy said it use about 5% more fuel. Is this true? I can't understand why? Is there additional resistance in the alternator when charging?

 

Any experts out there?

[Stationary M25 Traveller]

In Dubai, the locals don't use nights at night as they reckon it wastes petrol. This ... in a state where you can get a tank full of fuel for £6, including attended service and a window wash.

 

I'm talking about cars, trucks, buses, mopeds - and on a 7-lane (each way) highway too !!

 

Has to be seen to be believed !

 

7 related photos

[Sheds Moderator]

Lessee now. 2 sides - 5W each. 2 dip lights, 55W each. 2 tails, 5W each. Skip the dash lights. 130W total. Say the alternator is 75% efficient, it needs to generate c. 190W, say 200W. That's just over 1/4 (brake)horsepower. If 1/4 bhp is 5% of average output then average output is 5bhp, enough to propel a moped up to about 45mph. I don't know what speed a car would attain with 5bhp available.

 

Whoever did these calculations is no engineer or physicist, probably the same guy who suggested that a mobile phone charger cost £27 a quarter to run. Assuming the thing consumes as much disconnected as it does connected the real figure is about 1% of this unless your utility supplier is Shylock.

[Peter Carmichael]

55 watts per front dipped.

5 watts per side light

5 watts per rear light

 

All times 2.

130 watts. Alternator is about 90% efficient. i.e. 144 watts.

 

A horsepower is 746 watts. In order for 5% more fuel to be used, the average power requirement for general driving would have to be 3.87 horsepower. Fag packet assumptions on load factor suggest this may be typical of a car running at 40mph-ish constantly with no acceleration.

 

Lots of assumptions in that. Using those assumptions, the lights-on scenario would be 0.84% at a constant 70mph.

 

Luckily for the green-conscious, the percentage becomes disappearingly small when applied to Chelsea tractors. On my recent run across france, for which I have reasonable fuel consumption figures, this might have been 0.37%.

[Peter Carmichael]

Some different assumptions, but the same method...

 

 

Just too slow, I guess.

[Sheds Moderator]

😬 some pretty similar numbers in there. The conclusions are the same.

 

Oh, and in answer to the original question, yes a loaded alternator is harder to turn than one with no load. For proof, leave the engine idling and turn on the lights. You will hear the engine faler and slow down slightly as the load increases.

[Jason Plato]

did they mention the power consumption of having the radio on 😬 😬

 

Taffia rear gunner

 

[susser]

VBH the other day was wittering about the new electic Smart. One of her comparison tests was "miles per £"; she put the lights and other electrical loads on in the petrol model and suggested that "this won't make much difference with the petrol model but will make more difference to the electric one"

Sorry VBH but you're talking out of you pretty little a55.

The electrical power (say 200W) used by a petrol engined car will be generated by an alternator which has, as we've already discussed, losses, which is turned by an infernal combustion engine which has much larger losses.

The electric Smart will use this power from it's batteries directly, with tiny losses.

OK, I realise she was talking "miles per £" and not "miles per Joule"

I reckon.

 

[Sheds Moderator]

I don't think for one instant that VBH understands the first thing about energy conversion in cars or anything else, I think, Susser, that you are flattering her. I nthink she's reading from a script that a researcher has written, or someone from Smart's marketing dept. Asking them to understand the kind of fag packet calculations made by me or Peter at the start of the thread is akin to asking me to run the country, I might have a go at it but I wouldn't understand the first thing that was going on.

 

VBH may be pretty but Pallab Gosht (sp?), BBC Science Correspondent, she is not.

[Stationary M25 Traveller]

asking me to run the country, I might have a go at it but I wouldn't understand the first thing that was going on.

Ideal qualification for working in HMG or being PM - you're in

 

7 related photos

 

Edited by - Stationary M25 Traveller on 6 Mar 2008 11:42:25

[Jason Plato]

pretty little a55

 

my ass is smaller than hers !

 

Taffia rear gunner

 

[susser]

VBH !!

If you're reading this, take no notice of that Dave J. You're welcome to plonk that (slightly bigger than his) A55 in my lap anytime.

BOSS. Alright, so I was flattering her, but now you've gone and blown it.

 

It was not so bad as the time that Penny Mallory (on the used car roadshow) said. "So tell me. What is the difference between a straight four and a vee eight"

I didn't hear what came next as I was reduced to a gibbering wreck on the floor.