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Will cars ever be energy efficient?

Will there be a time in the near future when fuel for cars will be energy efficient, environmentally friendly and available at the pumps?
St Peters Primary School in South Lanarkshire (age 5-14)

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2 Responses

  1. Cars are already energy efficient. The main problem is that they are too heavy! An adult human weighs perhaps 70 kg, but a car typically weighs 1000 kg.

    So all work done by the engine accelerating and braking the car does is mainly accelerating the car and only 10% of the energy is used on the payload.

    Making lightweight cars is possible, but they cost more. Generally lowering the speed of transport would help keep lightweight cars safe.

  2. The internal combustion engine is actually already pretty efficient, and couples that with high power to weight and volume ratios that makes it ideal for transport applications. I suspect what is hidden behind the question is a lack of knowledge about the Second Law of Thermodynamics.

    Everyone knows the First Law, which is in effect the Law of Conservation of Energy. If this was the only physical law governing energy then we would have neither energy problems nor any carbon-footprint issues! However, the Second Law places restrictions on the inter-convertibility of different kinds of energy and on our ability to convert certain kinds of energy to work – that is the source of all our problems but we cannot simply wish it away. One consequence of it is that when we convert certain sources of energy (such as hydrocarbon fuels, which are very energy dense – orders of magnitude more so than electrical batteries – and therefore great for transport applications) into useful work that conversion involves heat and the Second Law restricts the amount of heat that can be converted into work. This is actually what we mean by “efficiency” which is the ratio of the useful work we get (to propel our vehicles, etc) divided by the chemical energy in principle available in the fuel.

    The energy not used for work is not “lost”, but unfortunately it is in the form of heat dumped into the environment, and that is now causing us problems (even if we develop biofuels to replace our dwindling mineral hydrocarbon resources).

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