Electrical devices and mpg

daveharnett

All of the 500 million 'how to increase your fuel efficiency' articles of the last six months all mention turning off unnecessary electrical devices.

My question is how does this effect your fuel efficiency?

All the car's electrics are powered by the battery, which is in turn recharged by the alternator, yes? That being the case, does the alternator have some way to reduce drag on the belt/engine when the battery is full?

L1011

Sit in your car idling, listen to the engine note
Click on the full high beams, listen to the engine note - it goes up.

More load on the alternator means more resistance, which puts more load on the engine

Cookie_Monster

turn the steering full lock, good example of how the revs will drop momentarily before the enging can compensate for the higher power demand

Zube

daveharnett wrote: »

does the alternator have some way to reduce drag on the belt/engine when the battery is full?

Yes, the more current you pull from the alternator, the harder it is to turn. It's a "laws of physics" thing.

Cookie_Monster

Zube wrote: »

It's a "laws of physics" thing.

pesky physics, ruins the fun for everyone it does

voxpop

yeah i get an extra 6 mpg by not steering much and keeping the headlights off :pac:

Zube

daveharnett wrote: »

All of the 500 million 'how to increase your fuel efficiency' articles of the last six months all mention turning off unnecessary electrical devices.

Careful, though, you often read that you should turn off the air-con, but opening windows has a much worse effect on your mpg at speed.

E92

Zube wrote: »

Careful, though, you often read that you should turn off the air-con, but opening windows has a much worse effect on your mpg at speed.

+1. Air conditiong is more fuel efficient than open windows once you hit about 40 mph.

It's to do with the fact that speed is proportional to the cube of air resistance.

leex

Read somewhere before that aircon typically cuts mpg by 10%.

towel401

voxpop wrote: »

yeah i get an extra 6 mpg by not steering much and keeping the headlights off :pac:

power steering ftl

it's only for lil girls anyway

Del2005

leex wrote: »

Read somewhere before that aircon typically cuts mpg by 10%.

My last car had aircon and it didn't use 10% more fuel, it was marginally more.

Driving at speed with the windows open is worst, all the turbulence inside the car is adding to the wind resistance of the car.

I think some of the fuel saving people don't live in the real world. By saying don't drive with aircon they are trying to kill us off with heat stroke:eek:

The easiest way to save fuel is too make sure your tyres are inflated correctly and don't carry any unnecessary weight in the car. And if you pay a bit of attention and drive smoother you'll save even more.

Mr.David

[quote=E92;57599709
It's to do with the fact that speed is proportional to the cube of air resistance.[/quote]


No its the square not the cube.

F=0.5 p V^2 Cd A

Sorry to be a nerd :P

Zith

leex wrote: »

Read somewhere before that aircon typically cuts mpg by 10%.

Older air con units used to be belt driven which is pretty inefficient. Recent units have their own electric motor driven from the battery which actually works out more efficiently.

blackbox

Quote:
Originally Posted by E92;57599709
It's to do with the fact that speed is proportional to the [B
cube[/b] of air resistance.

Mr.David wrote: »

No its the square not the cube.

F=0.5 p V^2 Cd A

Sorry to be a nerd :P

[NERDMODE]

Actually, it's the other way around - the air resistance is proportional to the square of the speed (formula is correct)

/[NERDMODE]