Economics 201: Thermodynamics and efficiency

 Thermodynamics and economics: Why a car is the most expensive mode of transport.

A typical gasoline engine in a car runs at about 25% efficiency. That is, the energy in 1 out of every 4 litres (or gallons) of fuel in the tank does “useful work”. The energy is mostly converted to waste heat, but some is wasted in "internal losses", such friction in the engine and transmission, pumping coolant etc. (Efficiency in the lab can be much higher, but we’re talking real-world here, not lab conditions).

The “useful work” consists of moving the car. So about 1 of 4 litres of fuel is used to move the car and its driver. Let’s assume the car plus driver weighs a tonne (1000kg), of which the driver weighs 90kg. Since a litre is 1,000ml, the system burns 1ml of fuel per kg of weight. Of this, 90ml will be used to move the driver. The rest (910ml) is used to move the car. So out of a total of  4,000ml of fuel, 90 ml is used to move the driver. That’s approximately 1/4%.

Therefore: If the fuel costs $1/litre, you’ve spent $4 to move yourself, of which 1 cent's worth of fuel moves you, 99 cents’ worth moves the car, and $3 pays for internal losses and waste heat. One can scavenge some of that waste heat to warm the cabin in winter, so it’s not entirely wasted.

The above calculation ignores the effect of speed, because speed increases fuel consumption overall. However, since with increasing speed an increasing fraction of the energy is used to push air out of the way, the fraction used to move the driver decreases. In other words, at most 1/4% of the fuel moves the driver down the highway.

I’ve also ignored the effect of passengers, stuff in the trunk, etc, since those merely increase total weight and hence total fuel consumption. The amount of available useful work will still be about 25% of the energy in the fuel. The fractions for moving the car and moving the people in it will change somewhat: a loaded car will transport people and their gear a little more efficiently than a nearly empty one.

But however you tweak the scenario, using a car to transport people is appallingly wasteful.