3 The World‎ > ‎1 Issues‎ > ‎Energy‎ > ‎

2 Transport

Transport currently uses 36% of all UK energy consumption and 26% being accounted for by road transport.

Where does transport energy go?

Lets take a look at my fiat Punto with its 1200cc engine which can deliver 44Kw or 58 horse power. For those that don't know 750w is a horse power and it is literally about the maximum power of a horse. A person is about a third of that.

The car weighs only 860Kg (Its a low power car).
We need a few conversions here 30mph is 13.44 meters per second
70mph is 31.36 meters per second.

What is the kinetic energy of my car with two passengers (80Kg each) at 30mph, it is;

0.5 x ( 860 + 2 x 80 ) x 13.44 x 13.44 = 92,123 Joules

That's enough energy to run a 100w light bulb for 921 Seconds or 15 minutes.
In fact my car with its 44Kw engine could run 440 100w light bulbs when I have my foot flat to the floor.
I can also work out that I can accelerate to 30mph in approx 2.1 seconds just by working out 92123 / 44000 but lets not show off here.
(60mph takes four times the energy so that is 8.4 seconds)
The point is that every time stop at a set of traffic lights I waste enough energy to run a 100w light for quarter of an hour!

Now lets try to work out how much energy is used due to air resistance and friction etc. i.e. to keep rolling along at 30mph what does it take. The best way to find out is to look at how quickly the car slows down around 30mph. That's an experiment I can do. How long does it take to go from 35mph to 25mph if I just press in the clutch so no energy is delivered to the wheels.

First I work out the energy at 35mph is about 125KJ and at 25mph is about 63KJ so we loose 62KJ over that time actually. In practice it takes 10sec to slow down this much. so that is 6200J a second or 62 100w light bulbs. OK its approximate because energy loss is faster at higher speed but its good enough.

Next question how much of that loss is wind resistance?

Well the force of wind resistance goes up by the square of velocity and drag power by the cube.

Rolling resistance is a pretty constant force mostly depends on the weight of the car so power is proportional to velocity?

From this we can work out what's what by looking at the slow down rates at different speeds.
By the way the car engine is a heat engine and has a theoretical efficiency limit determined by the temp it ends up at about 35% and another 5% is lost in the gears and transmission.

What can we do to reduce that wasted energy?

First don't put obstructions in front of moving traffic. It would appear that obstructing traffic has been the main objective of transport departments across the country for the last 20 years supposedly to make public transport more appealing.
Second and most obvious is reduce the weight of the car. Use carbon fibre rather than steel for bodywork, reduce the weight of the engine its self which if we need less power is quite possible.
At the end of the day to move these two people 160Kg around we are actually moving another 860Kg.
What is the big mistake governments make? Car manufacturers have been looking at car efficiency for years. They don't need to be told how to be more fuel efficient they just need to be told to be more fuel efficient. Tax Fuel.

Rickshaws in London

A few years ago the streets of London were graced with rickshaws. What genius do we have to thank for that. I saw one the other day with "I have a zero carbon footprint" written across the back of it and a long traffic queue behind him. No doubt what ever he saved he made up for in the delays he caused.
No what if we took the rickshaw, which currently has a human engine of one third of a horse power that is 250w and added to it a little electric motor of say four times that 1Kw or just over one horse power. It might be able to get around without obstructing all that traffic and still with a motor only one 44th of my Fiat Punto. That has got to be good.
Sorry its not allowed!

But as you cry yourself to sleep, remember, its all about appearing to save energy, not about actually doing it.




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