Lighter car vs heavier car, energy efficiency

Question

I typically assumed in vehicle energy efficiency debates, the importance always in propulsion- to reduce vehicle mass. This usually implies high energy mass density fuel, like hydrogen fuel cells, gasoline, super capacitors or something similar. Interestingly, lithium ion is not so good.

But then I realize we operate in atmosphere. So it seems in real world it could be plausible that heavier is better. I guess the idea is that energy is stored as momentum. And perhaps improving the vehicles momentum, reduces the parasitic, constant loss of energy due to air drag. So then there would be two competing constraints, to reduce mass, and also increase mass, and then with additional things. Such as the idea, fewer topology per volume, may reduce mass further. Kind of like say 8 small engines vs 1 giant cylinder. The topology is optimized with 1 cylinder.

So my question is how does mass reduce air drag energy loss if it even does? Perhaps it relates to ballistic coefficients?

One supporting observation to support asking this question would be that larger tanker ships are very efficient then smaller ones. Making analogous the issue with a ship in water rather then a car in air.

Answer 1

Mass has nothing to do with aerodynamic drag, except in the trivial sense that larger vehicles are heavier and have more drag- but not because they are heavier, only because they are larger.

When cars are made lighter, they become more economical because it requires less work to accelerate them to speed and less work to drive them up hills.

When cars are made more compact, they become more economical because they create less drag, and hence require less work to push them through the air at constant speed.

A vehicle like a tanker has a volumetric capacity (which makes money) that scales as the cube of its linear dimensions, while its hull area (which creates drag) scales as the square of its linear dimension. This means that the most efficient way to make tankers is to make them as large as possible.

Answer 2

To Niels' answer I will add a couple points.

At low speeds engine friction is the dominant mechanism for energy loss. At higher speeds it is atmospheric drag.

Air has mass. For a car to pass, air must be pushed out of the way. It must be accelerated in front of the car, flow around the car, fill in the void behind, and come to rest after the car has passed. If the cross sectional area of the car is $A$, the volume of air displaced in time $t$ is roughly $Avt$. The speed of the air flow is proportional to the speed of the car. The kinetic energy needed to move air is $1/2 mv^2 \approx 1/2(\rho Avt) v^2 \propto v^3t$. Thus the power lost to drag $\propto Av^3$.

This is why the size and shape of the car matters. It determines how much air is pushed around.