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MG MGF Technical - Electric load affects milage? NFC

Having a discussion down the pub yesterday about getting the best mileage as per Clarkson on telly the other night.
The bit we couldn't figure is that if the alternator is always working, how does using the lights etc. and even aircon degrade the mpg you get?
Just curious.
Ddraig

>The bit we couldn't figure is that if the alternator is always working

Walk across the office.
now walk across the office pushing a filing cabnet

Both times you are walking accross the office, the first you are only overcoming your resistive losses (like the bearings in the alternator), the second time you are doing other work as well (pushing amps thru a bulb)
Will Munns

Absolutely. As an aside, I really didn't think Clarkson's "adventure" was that impressive. OK, it was a twin-turbo - but they won't even spin up if you keep the revs down. And yes it was a big V8 - but it was a diesel and 40mpg isn't exactly stellar. I get 45mpg out of the F on a run driving normally!

Tim.
Tim Jenner

800 miles from a tank ain't bad, I get about 200 miles, but then with much abuse and BMW baiting, rounderbouts with hard acceleration and an 'average' speed of 70, with the roof down I got _down_ to 40 MPG - Just think of how far I could have got driving sedatly!
Will Munns

Of course without the fan a normal car will not use any less fuel with the heater on - bloody aircon!
Will Munns

800 miles from a tank's not bad but the tank was huge! Forget what he said now, but it was something like 20 gallons...
Tim Jenner

yes, mine is 20 litres!
Will Munns

My old Passat GT16v Estate was super slippery aerodynamically, usually giving about 38MPG on a daily 60 mile round trip. Come the big bad fuel protests and I brimmed it at the start and got an amazing 48MPG with all that pussyfooting about so 40MPG out of that even more modern Audi is not that good. Maybe going faster would have demonstrated better efficiencies over a petrol. I suspect that friction in the V8 and quattro drive train is representing a fair amount of the fuel consumption - my Passat had hardly any friction as it had done over 200K miles !!
tony

Conservation of energy. Theres only so much energy in the petrol and if you convert some of it to light, heat (heater), electric to power wipers, and so on, you'll get less energy for moving the car.

I cant remember the science, but it must take more energy to turn the alternator when there is an electrical load on it.
Russell Parslow

Put simply - power out means some power in!
tony

AirCon is not driven by the alternator / electrics (though its fans may be) - the condensor is driven directly from the engine, and can take significant power to drive it.
Something like a HRW (heated rear window) can take about 1/20th horsepower - doesn't sound much, and I'd be surprised if it was really measurable (it's usually on a timer anyway)
Leaving all windows up is a good idea - but why didn't he check his tyre pressures at the start of each day ? it's essential for good economy.
I agree that it wasn't that impressive - my old Carlton could get over 600 miles from a tank of petrol, without playing any silly games (like not taking breaks).
That Porsche though really was impressive (and I'm no fan of Porsche)
Steve

>AirCon is not driven by the alternator / electrics
>(though its fans may be)

Most Aircon systems run a radiator fan continiously whilst in use!

>the condensor is driven directly from the engine

but like the alternator can be 'unloaded' so the drag is reduced to almost nothing.
Will Munns

Power to run headlights on dip beam = 110 Watts
Add another 20 watts for the side lights = 130 Watts

746 Watts in a Horse Power

130/746=0.174 HP

Not a lot! But aircon can use a a lot more.

Bruce
Bruce Caldwell

>Add another 20 watts for the side lights

A little more than that, think of all the lamps inside the car (buttons/gauges etc), each swallowing a watt or so but point taken!

I think the alternator can draw max 65Amp ~= 1k watt, but as already said, the main draw for aircon is not thru the alt, but thru its own compressor
Will Munns

Have you ever observed the alternator slow down when at engine idle, you turn on lots of stuff? Sudden cold downpour of heavy rain so on go wipers at top speed, heated rear screen, washer motors and screen blower. If you listen and watch at such times, you'll see the engine revs drop noticeably as the alternator gets loaded with demands like that.

As they sometimes say up Norf ... "Owt for nowt"... and down south.. :O))
John McFeely

Engine note changes, but there is only the merest blip on the tacho, because the ECU compensates by flowing more air/fuel
Will Munns

>> The bit we couldn't figure is that if the alternator is always working, how does using the lights etc. and even aircon degrade the mpg you get? <<

Can't argue with what the folks say Ddraig, but what is often not fully appreciated is that as the alternator produces more power, the harder it is to turn.

Unlike electrical generators you may have come across elsewhere that use an 'earth magnet' of permanently magnetised ferrous metal, the alternator uses an electromagnet instead.

To increase the power generated, it increases the strength of magnetic field that makes it that much harder to turn the metal wire turnings inside the field.

Thus, for increasing electrical load placed upon the alternator, the harder it is to turn the alternator pulley, and thus the more work - and therefore fuel - is expended...
Rob Bell

>Unlike electrical generators you may have come across
>elsewhere that use an 'earth magnet' of permanently
>magnetised ferrous metal, the alternator uses an
>electromagnet instead.

>To increase the power generated, it increases the
>strength of magnetic field that makes it that much
>harder to turn the metal wire turnings inside the field.

You're a doctor right ;-)

the magnetic field generated by the outer coils (instead of the earth magnet) is the same, but the spinning coils press back against it with more force as the current is loaded on, so in this way no diffrent to a normal generator.

<physics>
If you take a coil of wire and force a current thru it it generates a magnetic field, a coil nearby will feel this field and if possible push electrons thru itself in sympathy (if possible because we are loading the coil with a switch and a light), if the switch is off then no current can flow, so the coil behaves as if it were not there, if we close the switch then electrons flow thru the lamp, switch, and importantly coil.

Because current is flowing in the lamp coil it puts up a magnetic field, this pushes against the field created by the other coil, the bigger the lamp load (more lights) the more current that flows in the lamp coil, so the bigger the push back. the push back is called 'back emf'.

the problem we have now is that magneric effects are transient, the setting up of the magnetic field takes a few ms, so the light will only blink once. The solution is to vary the magnetic field, In mains this happens as a result of alternating current, reversing the voltage and as a result the field 100 times a second. In the alternator it is done by moving the lamp coil thru the field created by the magnet (in this case electro-magnet).

The force with which the magnet repells the back emf is the effort that needs to be overcome by the engine, so as the current in the lamp side of the alternator grows, the back emf grows and the higher the load on the pully.
Will Munns

OK, and how does EPAS work? ;-)

As for aircon, isn't that more to do with the additional load of powering a walloping great compressor too?
Cannonball Bob

>> You're a doctor right ;-) <<

That's okay Will - I defy you to explain Starling's law or gas diffusion in lung ;o) ;o) ;o)

Any way, wrong magnetic field, but correct principle ;o) LOL
Rob Bell

>That's okay Will - I defy you to explain Starling's law
I'd much rather not - at least if I keep quiet about what I don't know people will just assume I know everything ;-)

>Any way, wrong magnetic field, but correct principle

Quite, all this Newtons law stuff is quite elegent in the way that everything automagicly balances out, the way that you had phrased it sounded like there was some sort of feedback loop in the alternator, the truth is much more elegent in simplicity.
Will Munns

LOL Will! ;o)
Rob Bell

"at least if I keep quiet about what I don't know people will just assume I know everything ;-)"

Better to say nothing and be thought a fool than speak and remove all doubt ;-)
Cannonball Bob

This thread was discussed between 01/06/2004 and 02/06/2004

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