Category: Automobiles

With the car world being pushed towards higher efficiencies, the question “What causes fuel/electricity consumption in a car?” arose in my mind. The answer was easy: Weight and Aerodynamics. Lets talk about weight today.

Energy Consumption

In terms of the physics of it, it doesn’t matter which fuel is used or if the car is electric or gasoline powered; If we calculate the energy required, we can use whichever fuel we like to supply that energy. Once the energy consumption of the vehicle is ascertained, the fuel can be chosen based on the environmental impact, availability, cost and other factors.

Kinetic energy at a constant speed is a function of mass and velocity. City driving involves a lot of acceleration and braking as well though and every time we brake, we’ve got to accelerate again and energy consumption is proportional to the square of acceleration.

With the velocity being a function of speed limits and acceleration being a function of driving habits of the user, the vehicle design majorly only affects the weight in this equation. Let’s look at the co-relation between weight and fuel consumption of various cars using an available dataset:

Vehicles weighing approximately 2000lbs lie around the 30mpg mark in this dataset. When we look at the cars weighing around 4000lbs, the mpg drops to the 15mpg area. Of course there’s other factors at play too, most important of them being the aerodynamic drag which is a function of frontal area and drag-coefficient. And although the frontal area is highly correlated to the size of the vehicle which lends even more support to this thesis, let’s ignore that for a minute.

Electric vs Gasoline

I love electric cars and their amazing torque characteristics. And the electric drivetrain itself is actually lighter than a traditional IC engine. The thing that literally weighs electric cars down is the battery. And while there’s very little reliable data on how much more an electric car weighs compared to a traditional ICE car, an approximation can be had using platforms offering both electric and ICE drivetrains. The Hyundai Kona electric for example weighs about 335kg or about 750lbs more than its gasoline counterpart for the same trim package. So we’ve added 25% extra mass to a car, which has increased its energy consumption by at least the same percentage.

To paint a simpler picture, imagine increasing the car driving population of the world by 25%.

What’s ideal?

To really be green, we need to be driving lighter cars. If electric cars are greener, then we need to be driving electric cars that are lighter than their ICE counterparts. Unfortunately this seems to be a bit of a technical challenge right now.

Japan, having gone through austerity once, really has it figured out in my opinion. The best selling vehicle in the USA is the Ford F150 weighing 4400lbs and having a fuel consumption rating of 16mpg. In contrast, the best selling car in Japan is the Honda N-Box weighing 2100lbs with a fuel consumption rating of 54mpg.

That’s effectively reducing the emissions and fuel consumption footprint by at least 70% for the same distance driven. What’s the impact of the current set of laws again? Which law encourages lightweight cars?

Laws and Regulations

Unfortunately, with emission standards being relatively loose for SUVs and Crossovers, automakers are incentivizing customers to switch to these vehicles. This is backwards compared to what is ideal. Who’s even making these laws?

The ideal laws would incentivize lighter cars with lower taxes and discouraging heavier cars by adding taxes to the heaviest of cars. Does something like this exist? Yes, Japan again with its Kei car laws.

What do I do?

Buy lighter cars. You’ll save money and probably have more fun while driving a more agile car. In the words of Colin Chapman: Simplify and add lightness.

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