50.4 MPG

(05-11-2012 02:09pm)PhilSkill Wrote:  As Engery systems cannot be 100% efficient, The more load you draw from the car's system the more resistance the alternator has against turning, therefore more engine power is required to turn the alternator, therefore using power that could have been used to drive the wheels and so costing you mpg.

Thanks, I'm still not clear on this though. I thought that the alternator's output was constant (or at least linked to the engine speed), and not related to the actual demand on the battery. In the same way that a bike light dynamo works, if you've got the dynamo engaged it doesn't make any difference to how hard it is to pedal whether you've got the light turned on, turned off, or even got several lights rigged up to it. I thought the alternator was simply a dynamo, but connected to a battery rather than direct to the lights etc?

The magnetic field in the stator windings increases with the current draw in the car, this magnetic field acts as a resistance to the Magnetic field being created by the driven windings.

When you use the Heated windscreen the current draw may be a good few amps at 12V and as the Battery is also likely to be charging and drawing amps along with everything else, those amps will be coming from the altenator, and thus causing resistance to movement.

Your bike generator will likely be fairly inefficient and the effect of the lights current draw will likely hardly notice.

Draw enough current through your altenator and it will act as an electronic brake! (Or more likely burn out the windings )

(05-11-2012 02:58pm)PhilSkill Wrote:  The magnetic field in the stator windings increases with the current draw in the car, this magnetic field acts as a resistance to the Magnetic field being created by the driven windings.

When you use the Heated windscreen the current draw may be a good few amps at 12V and as the Battery is also likely to be charging and drawing amps along with everything else, those amps will be coming from the altenator, and thus causing resistance to movement.

Your bike generator will likely be fairly inefficient and the effect of the lights current draw will likely hardly notice.

Draw enough current through your altenator and it will act as an electronic brake! (Or more likely burn out the windings )

Ahh, thanks for the good explanation!

To help put this into context I've done a quick calculation based on the things that are normally used on a drive to work in the morning using 90% of fuse rating to give running current and I've only included the big consumers not all the sensors etc.

So if using the below

Power steering
Heater
Blower
Heated front screen
Heated rear screen
Heated seats
Radio
Amplifier
Heated steering wheel

This gives a total current draw for these items of 355.5 amps. Your alternator has to supply this plus enough to charge the battery after starting, glow plugs (these have a 60A fuse) etc. 355.5 Amps equates to a power consumption of 4266watts or just over 2 kettles on during your journey.

A 190bhp engine is 141kw so our 4.2 kw for our toys to keep us warm uses 3% of the engine power plus whatever it uses to charge the battery and supply the various sensors and processors that are running.

Hope this rough calculation helps and it is just that a rough calculation.

Also Alternators are around 60% efficient so from engine power your 4.2kw loading on Alternator requires 7Kw of engine power, now hopefully the fuses are'nt that closely rated but yes 4% of 40mpg is 1.6 mpg just to run your toys...

I didn't spec the kettle though, so will I be ok?

(06-11-2012 12:14am)NightFox Wrote:  I didn't spec the kettle though, so will I be ok?

Yes, As long as you dont want Tea!

On Top of that I learned that per 100watt the fuel consumption increases by 0.1 Liter.
This will make almost 0.5liter/100km or (hopefully I am not wrong) = 1.4 mpg !

That's why I haven't ordered the heated stearing wheel.

There's a difference between the British mpg and the USA mpg isn't there??


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Yup. Miles are the same, but gallons are not. US gallon is 1.2009 of a UK gallon. so in the US you use more gallons per mile than we do in the UK.