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October 12, 2006

Cars are so, so energy inefficient

Filed under: advocacy, General — schmadzie @ 10:54 pm

The following snip is from the summary of a CSIRO report on the future of cars. An interesting read if you have an interest in sustainable transport.

To me the killer fact is only 13% of fuel energy is used to propel current vehicles. Pretty outrageous when you think about it. Sure, less cars, more public transport, more active transport, better designed communities etc are essential but cars are going to be around for a while so why not make them better.

Be interested to know the energy efficiency of a bike. Must be pretty high. Any ideas anyone?

From the CSIRO:

Fuel cells could be a long-term solution to transport-related greenhouse emissions, but in the meantime car makers could do a lot more to improve vehicle fuel use, says CSIRO’s low emissions transport leader David Lamb. “Today’s typical car wastes most of the energy it gets from the fuel,” Lamb told the NRMA summit (see Truss: consumer demand should be the basis for alternative fuels take-up). “Only about 13% of the energy is used to propel the vehicle,” he said. “The rest is wasted in friction, noise, heat, braking and mechanical losses.” Car makers “already know” many technologies that could improve fuel efficiency, but they all cost more, he said. “They would have supplied them already except that fuel was so cheap. Engine improvements will arrive quite quickly now that fuel is more expensive, but they will cost about 10% more than today’s car.” Over the next six or seven years engines will generally get smaller and accommodate 5% or 10% biofuels, Lamb said.,,.html



  1. Hello,

    you mentioned interest in efficiency of bikes – depends what you define as the efficiency. the efficiency of cars may be 13% if you look at the energy that is used by combusting the fuel and the kinetic energy achieved by the car. However, there is a more relevant way to look at it. The task was not to move the car, it was to move the stuff inside the car. So if you define a goal, like to move an 80kg person 10km, then you can make meaningful statements on efficiency. For a car to move the 80kg person, it needs to also move its 1000kg hulk. This means that to start with, efficiency is limited to 80/1080 = 7.4%. Then you take into account other inefficiencies, like the conversion of petrol to motion in a combustion engine (30-40%), efficiency of drive chain and gear box (~80%) and you could get to around 2%. On a 20kg bike, you start with 80/100 = 80%. You can estimate the drive chain efficiency 80-98%, conversion of human energy to motive energy ~85% and you can include conversion of food energy to human energy as well ~25% and get to around 14%.

    But then, who cares how much energy was in the food (besides elite athletes), as long as you have enough to eat? Or what does the energy content of the petrol matter anyway? Also, i haven’t taken into account the energy needed to lay down a road that can take a 1000kg vehicle. So another way to look at it is: what does it cost to move an 80kg person 10km with a bike and a car? With a bike, not much, just cost of bike and repairs over its lifetime. (nothing at the nunnery). With a car, petrol, road taxes, car purchase, car maintenance, petrol station infrastructure, value of land paved by road… pretty high.

    If you want some practical numbers, riding from Central to UNSW in 14 minutes takes about 40Wh of energy to cover the 5.5km. Ask someone who drives how much petrol that takes, i guess about half a litre. That’s about 5000Wh.


    Comment by bikeclub — December 10, 2006 @ 8:50 pm

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