Naked Science Forum
General Science => General Science => Topic started by: Adrian on 12/02/2009 14:30:01
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Adrian asked the Naked Scientists:
Hi again,
If I travel 100 miles by car, and I have a 2 litres bottle of water in my car, how much petrol do I use more than without the bottle?
Many thanks,
Adrian
What do you think?
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The difference in fuel consumption would be so slight, it would be virtually impossible to measure.
Make it 30 litres of water (ie 30 kgs) and you might see a slight increase in fuel consumption.
Commercial vehicles see a marked increase in fuel consumption when fully laden, but for the greater part, the increase in consumption will occur during acceleration.
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Don't you "just" need to work out the energy required to move a 2kg object horizontally for 100 miles, then work out how much fuel that corresponds to? I guess that you are going to have to make some assumptions about mean velocity and acceleration though.
In formula one, it is often qouted that 1 kg of fuel can affect the lap time by several tenths of a second per lap, so the final effect might actually be quite large.
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A high performance F1 car must weigh 605 kgs, so most have to carry ballast of around 150 kgs. Because these cars are subject to constant acceleration and deceleration, every extra kilo makes a difference. But for those of us who drive in the 'road friendly' manner, and even some of the ignoramuses who try to emulate Stirling Moss, driving a car of the average 1000 - 1800+ kgs with an extra 2 kgs would not make any significant difference.
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I also depends on your driving style, traffic, route, etc. Hard acceleration and breaking require more power when the vehicle is heavier. Repeated doing this due to traffic and route has additional effects. Less mass is better but no idea how to calculate this.
Don just wrote pretty much the same while I was typing this.
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Don't you "just" need to work out the energy required to move a 2kg object horizontally for 100 miles,
That's easy to work out but not helpful. The answer is zero.
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Exactly:
W = s.F
=0 when s (displacement vector) is at right angles to F (force vector).
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Only to be nitpicking:...at constant speed. (If you have to accelerate of course you need work).
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As a general rule, I expect extra weight will increase rolling resistance losses in the tyres (and maybe wheel bearings) - which will affect even motion at constant velocity. Extra weight might also slightly affect the height/angle of the car and thus have a small aerodynamic effect (especially at high speed).
But as others have said, the main difference is that it will take more energy to accelerate the vehicle - which becomes more significant for stop-start journeys.
Even so, if the car plus driver has a mass somewhere around 1300kg, an extra 2kg would be expected to be pretty insignificant (somewhere around 0.15%).
In general, fuel economy can be maximised by
-adopting a 'relaxed' thinking-ahead driving style where you can regulate your speed by lifting your foot off the accelerator rather than using the brake all the time
- don't sit right on the tail of the car in front
- anticipate lights, junctions, people pulling out of sideroads eg. and lift the foot off the accelerator as soon as you know you will need (or likely need) to slow down within the next 15 seconds.
-choosing routes where you are not stopping and starting and changing gear every 20 seconds
-not travelling much faster than 60mph (as aerodynamic drag gets much worse)
Driving safely comes above all else, and you absolutely should brake when needed.
The concept is more to use the accelerator more sparingly so you only have to brake for the unexpected rather than the expected.
Realistically you will not be liked by other drivers if you slow down early enough to glide to a perfect stop all the time, so you will still use the brake - but hopefully only complete the stop rather than to take you all the way from 50 down to 20mph for example. (You can travel something like half a mile on the flat on 5th gear with no foot on the gas going from 55mph to 20mph.)