I was watching an old repeat of Top Gear the other night where Clarkson drove an A8 Diesel from Lonfon-Edinburgh and Back.
He was saying that he turned the heating off to save fuel econmy, likewise he was driving on side lights in the rain rather than dipped beam.
Now I know that the air con compressor will increase fuel consumption but would you really notice extra fuel being used by using your heater?
Lights? Rear window demister? Etc?
What about using the radio?
I'm not condoning any of this, just want to know if there is a gain that is measurable. I would haev thought it would haev been too small to measure.
Thanks
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It can be measurable. On some cars on the limited end of the horsepower / torque scale the revs drop when you turn on the heated rear window, and pick up again as the ECU gives the engine more lucozade.
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basic physics you dont get owt for nowt as faraday used to say, every electrical device in operation uses electrical current , so its a drain on the battery therfore the alterrnator has to work harder to maintain the battery,s state of charge which in turn requires more power from the engine to drive the alternator
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I don't doubt that it costs, the energy in your fuel is being used on other things than driving the car forward. My question concerns is it measurable or too insignificant? I think that pushing a ton and a half of metal throught the air at 60 takes a lot of energy compared to what energy your heater takes from the cooling system and what electrical energy the heater fan consumes.
Just wondering....
PS - I guess I'm thinking all of this due to the recent hikes in the cost of diesel and the extra £35-£40/month I'm spending on it...
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Well, a 90A alternator running flat out at 50% efficiency will consume about 3 BHP....
In day-to-day driving, not noticeable.
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Well a 90A alternator running flat out at 50% efficiency will consume about 3 BHP.... In day-to-day driving not noticeable.
when you only have 60 bhp - thats 5% - it becomes noticeble.
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Well, a "60 BHP car" is unlikely to draw 90A, and no-one is likely to frive at maximum current draw 100% of the time, are they?
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would you really notice extra fuel being used by using your heater?
I doubt it, unless the thermostat is faulty and using the heater makes the engine run colder.
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... Clarkson drove an A8 Diesel ........ He was saying that he turned the heating off to save fuel ........
I wouldn't take too much notice of what Jeremy Clarkson says. He thinks that the be all and end all of acceleration is engine torque, and doesn't seem to realise that overall gearing must be taken into account as well to give a meaningful figure of torque at the driving wheels. He's a journalist, not an engineer.
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If I let the engine idle and then turn the headlights on the engine note changes as it has to work harder. So the effect is noticeable then, but perhaps not at any kind of speed.
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I recall being told on a course at Lucas many years ago; that a 45A alternator consumes 9HP at full chat - so maybe they aren't all that efficient.
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There must be something in this. BMW's new green technology includes intelligent alternator control which reduces fuel consumption. On some cars they also have regenerative breaking helping charge the battery too.
Just found this that explains a lot more:
www.greencarcongress.com/2006/09/bmw_introduces_.h...l
They claim a midsize car needs about 3.5kW!! And the quote:
"BMW found that the on-demand generation of electrical energy helps to reduce fuel consumption in the EU homologation test by approximately 4%. The driver also has access to more engine power for acceleration and dynamic driving."
So instead of switching off everything the BMW solution sounds better.
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In a sense all alternators are 'intelligent' in that output is controlled to match load. When the car is first started the alternator has to top the battery back up, so will be operating toward the top end of its output.
During steady-state cruising the output need only be quite small.
e.g. using headlights instead of sidelights - lets be generous and say 200W extra. Thats (very roughly) 1/4HP. Multiply by 4 to take account of all inefficiencies (again, very generous) and that's 1hp you would save. Not really noticeable. Something similar for HRW.
Heater fan only uses a fraction of a HP, not worth talking about.
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I reckon my 1.4 Golf does about 4mpg worse in winter; 45 as opposed to 49. Some of that will be colder starts, but as I have quite a long commute and it warms up within a mile it must also be down to the lights. Lets assume 1hp needed as suggested. Cruising at 60 or so on the M25 ( my section is usually reasonablyclear) the engine is wafting along on about 30 HP. So that makes 3% Many years ago a mate at school reckoned his Mini went faster with the lights off.
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Diesel has a calorific value of 38.60 MJ/litre or 175.45 MJ/gal.
We'll conservatively assume the A8's engine has an efficiency of 30%, and the alternator will have an efficiency of around 80%.
If we ignore sidelights etc and just assume the wattage of the dipped bulbs, lets say 120W, then that's 120/0.8/0.4 = 500W of fuel the engine consumes.
I don't know how long Clarkson took, but AA routefinder estimates about 14 hours round-trip. Using the above figure that's 25.2 MJ of fuel, or 0.14 gallons, 0.65 litres.
Based on that I really don't worry about having my lights on anymore.
Edited by mfarrow on 10/01/2008 at 17:11
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Car alternators are not very efficient - they do not contain any permant magnets so they need to use some of the energy generated to maintain a magnetic field in the coils. Car alternators are designed with cost and reliability in mind rather than efficiency.
There was link in the other thread about speed and fuel consumption - that site claims that the rear screen heater will increase consumption by 3-5%
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