How much energy does a vehicle use and why?
A vehicle (your car) uses an enormous amount of energy to get us and our stuff around. Some seem to be confused about what those energy requirements are, or how that energy is used. So I thought I would try to clear the air on that subject. I will try to stay away from excessive math. I will also ignore some small details. They are not important to your understanding. My purpose is to explain the concepts. To keep the length under control, this article is in two parts. First we will look at what affects the energy needs of the vehicle. Then, in part two, we will look at the actual numbers, and real world implications. Ready?
There are only four things that effect how much energy is necessary to move your vehicle. The first is the weight (mass, or amount of stuff) of the vehicle. This is important only when we need to change the speed – IE: Accelerate or decelerate. As we shall see though, it is very important for that.
Two others factors are the frictional losses, mostly due to tires, and the aerodynamic drag – how much the air is pushing against the vehicle.
Finally, factor number four, gravity. Of course it takes more energy to go up a hill (fighting gravity) than it does to go down one (making use of gravity). However, since Gravity would unnecessarily complicate this discussion, we are going to pretend the world is flat, which will then allow us ignore it. With that out of the way, we are left with three things that determine your vehicles energy needs – Mass, Friction, and aerodynamic Drag. So, let’s take a closer look.
First up is the vehicle mass. It takes certain quantity of energy to accelerate a given mass to a given speed. That energy is actually converted from the mechanical energy your car engine (or motor, or squirrel) produces into the kinetic energy, (or momentum), that is then stored in the mass of the vehicle.
Interestingly, once we are at a steady speed, the mass does not matter (remember, the world is flat, we are ignoring gravity!). Once accelerated, and at a that steady speed, the vehicle’s mass possesses all the energy that was put into it while accelerating as kinetic energy – also known as momentum. And, it will continue to travel at that speed using no further energy, unless acted upon by another force. Indeed this is how our interplanetary spacecraft operate, it is how our space probes go on for years without needing any fuel. Unfortunately for us though, here on earth there are two other forces that exist to complicate our lives.
So, next up we have friction: Friction is always present (there is no perfect system). In a car, in addition to the bearings and gears, the primary source of friction is the tires against the roadway. Now, this is can be a good thing because that friction, going by another name – traction, is what keeps your vehicle going in the right direction! But, when considering energy, as long as the vehicle is moving it is having to overcome that friction, using some amount of energy in the process. Friction is always trying to slow your car down by removing kinetic energy from it. That means we have to continually add that energy back to maintain the same speed.
And, the last factor is aerodynamic drag. Any time your vehicle is moving, air is pushing back, trying to slow it down by removing the kinetic energy from it. Again, the same as for friction, we have to continually add energy to the vehicle to maintain a constant speed. This is literally the amount of energy needed to “shove” the air out of the way.
A quick fun factoid: Aerodynamic drag increases with the square of the velocity, however, the power (force) needed to overcome aerodynamic drag increases with the cube of the velocity. So, if you double the speed, there is four times as much air pressure pushing against you. To overcome that drag will require eight times the power (2x2x2). A car going 50 mph may require only 10 horsepower to overcome aerodynamic drag, but that same car at 100 mph requires 80 hp!
Another fun fact. As you can ascertain, when travelling at a fixed speed, the mass (weight) of the vehicle is of little concern. A heavier vehicle will cause more frictional losses (tires are squished more), but, if they are the same size as far as aerodynamic drag is concerned, it doesn’t matter if your vehicle weighs 2000 pounds, or 200,000 pounds it will take the same amount of energy to keep it going. Once in motion, it stays in motion until an outside force acts on it – in our case, friction and aerodynamic drag. This is the reason railroads are incredibly efficient. For their weight, they have very little frontal area (aerodynamic drag), and since the wheels are steel, on steel tracks, they have much less friction than a road vehicle with rubber tires. Of course, a train also does not start and stop very much.
There it is. These are essentially the ONLY things that effect the amount of energy your vehicle needs! (Note, for the physicists, I did say essentially!). Once again, they are: The energy needed for acceleration, and the energy necessary to overcome mechanical friction and aerodynamic drag.
As to how much energy. Well, there is a very fixed requirement for the amount of energy needed to accelerate a mass. Issac Newton told us so! The larger the mass (more weight) the more is needed. An important point here: It does not matter how quickly you accelerate. The amount of energy needed and consumed to accelerate a specific mass to a specific velocity is the same. This is true whether you impart it quickly to get to a certain speed, or if you take your time to attain the same speed. In the end, the same mass at the same speed will contain the same amount of kinetic energy (momentum). The formula is kinetic energy = ½ times mass times velocity squared. And, note that the velocity is squared. So, to go twice as fast will require adding four times as much energy to the mass.
Now, you are probably wondering about Power. Specifically Horsepower. What role does it play? Well, we know that to accelerate a specific mass to a specific speed, we need to impart a specific amount of energy into it. Is that specific? If we have “more power” defined as the capacity to do work, we can impart that energy to our mass faster. That means we will achieve our velocity quicker. But, we have used the same amount of energy. We have simply put that energy into our mass quicker, and for a shorter period of time.
Think of a box of cereal. The box is our energy source (gas tank). Inside that box are individual corn flakes. Each flake is a BTU of energy. In order to achieve a certain speed we need to transfer all of the flakes inside that box into our bowl (our vehicle). If we make a small hole in the top of the box, we can pour a certain amount of flakes through that hole. With a small hole, it may take us 30 seconds to pour them all into the bowl. If we make the hole twice as big, we can pour them out twice as fast, and achieve our objective (speed to breakfast) twice as fast. Well, you’ve hopefully figured it out by now. The hole is our engine, and the size of the hole is the power that engine has (HP-Hole Power). Bigger hole (power), quicker result. But, note, we have transferred the same exact amount of flakes (energy from the gas tank) to achieve our goal, no matter the size of the engine (hole). We just did it quicker!
OK. Hopefully you now have some idea of the factors that affect your vehicle’s energy needs.
But, before we go on to part two, just for fun, lets figure out how much energy we need to accelerate a typical 3000 pound (1360 Kilogram) car to 65 Mph (29 Meters per second). We will use the formula for momentum. Once again, it is: Momentum = the mass (in kilograms) times the square of the velocity (in meters per second), divided by 2. The result is the number of Joules needed.
29 squared is 841 times 1360 = 1143760/2 = 571,880 Joules. As we will learn in part two there are 1055 Joules in a BTU. So, we used 542 BTUs of energy to accelerate the car. Doesn’t sound like much? Well, we will get into that in part two. Stay tuned!