Energy requirements of the vehicle

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How much energy does a vehicle use and how? 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.

But, before we continue, I want to define a few terms.

Energy is the ability to do work. A Unit of energy is defined by a few different terms. For our purposes they can be used interchangeably.
BTU (British thermal unit) is a unit of energy. 1 BTU is approximately the amount of energy in a kitchen match. BTU is commonly used to describe the energy content of a fuel.
KWh (Kilowatt-Hour) is likewise a unit of energy. 1 KWh equals 3,414 BTUs.
Joule - The metric (SI) form. There are 1055 Joules in a BTU, or 3,601,770 Joules in a KWh.

Work is the conversion of energy from one form to another (IE: Potential to kinetic). It is measured in ergs, foot-pounds, or, yes, the Joule.

Power is rate at which work is done or energy is converted (More power=faster conversion) It is measured in watts (SI) or horsepower. It can also be measured in BTUs per hour (BTU/h) One horsepower is equal to 746 watts, or 2,546 BTU/h.

Force is push. You apply force to cause acceleration. It is measured in Newton's. 1 Newton = 225 lbs of "push"

Mass - Mass is the amount of "stuff" something has. It is measured in Pounds or grams (Kilograms).

Weight. You might think this is the same as mass. It is NOT. Weight is the force that gravity exerts on a mass. This concept is important! For example, on the moon, your body will still have the same mass (amount of stuff), but your weight will be 1/6 of what it is here. In the absence of gravity, you will not have any weight, but you sill still have the same mass!

OK. Confused? It takes a while, and a bit of study, to understand the different units of measurement, and what the terms mean. Don't worry. Just understanding the conversion factors is sufficient for this discussion.

Lets think of it as a cereal box and a bowl. The box is the "gas tank". Inside it we have corn flakes. Those flakes are units of energy (Btu's or KWh). We have a bowl that will be our Mass. We accelerate that mass by filling it with corn flakes (energy). In order to fill it, we do Work by pouring the flakes out of a hole in the box. The size of that hole - how quickly we can do our work, is called power. Each time a new flake goes into the bowl, we have applied a bit of force, in the form of energy. The more flakes we put in the bowl, the larger the result (speed). I hope that helps, but, now I am hungry.

On to the discussion. 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 = ? 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.

Let me return to that 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, 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... So far we know there are basically three things that determine the amount of energy a vehicle needs. The mass (or weight) of the vehicle, (which is only important when it is changing speed or direction), friction drag from the drivetrain - predominantly the tires, and aerodynamic drag - how much the air "pushes back".

We have also learned that it does not matter how quickly the vehicle accelerates, at the end of that acceleration it will have used the same amount of energy to get there, and it will contain the same amount of kinetic energy (momentum). We learned the only thing that effects this is the mass of the vehicle.

I want to make a note right here that the above statement is not entirely true. Remember that aerodynamic drag increases at the square of the speed. While at low speeds this is negligible, as the terminal speed goes up we will have to consume an extra amount of energy in order to overcome this drag. I am going to ignore this for now as the math would only get more complex. That is simply not necessary to understand the concepts, at least for an automobile at highway speeds. A racing vehicle, on the other hand, is another story...

So, how much does it need to overcome drag? Well, that gets much, much, more complex. I also cannot write the formula in this blog (or, I don't know how). The amount of aerodynamic drag on a particular vehicle depends on the shape of the vehicle, it's frontal area, it's coefficient of drag (how slippery it is), and, of course, the exact speed it is travelling at. The math gets rather complicated, and it is different for every single different design or shape of vehicle. Since we are talking concepts here, and not a specific vehicle, we simply cannot readily figure this one out. So.

Because of that, I will offer a generic number. At 65 MPH, the average car needs about 30 horsepower to overcome drag. Not very accurate, after all, what is average? But, hey, this is concepts. Let's apply that to a practical example calculation. 30 hp equals 76,380 BTUs per hour. A gallon of gasoline contains 124,000 BTUs. A modern car converts about 22% of that (or 27,280 BTUs) to useable power at the wheels. So, 89,100/27280 equals 2.7 gallons of gasoline in one hour, or 24 MPG.

Of course, in the real, practical, world, we are more concerned about things like MPG, and the energy needed to travel a mile (or 100 miles) in a real vehicle. I will wrap up this discussion by looking at some real world examples of energy consumption. And, again, the point I want you to take away is that these energy requirements are independent of the means or efficiency of propulsion. The energy needs are the same. So, I will diverge from the theoretical discussion, and discuss what these things mean as relates to our real world use of fuels. There is not room for a detailed examination, so I will just throw some numbers out there. Again, understand the concepts!

The actual amount of fuel that is needed will depend entirely on the efficiency of the power source. A modern Internal Combustion automotive engine is approximately 28% efficient. That means only 28% of the BTUs in the gasoline are actually used to apply force to the vehicle. The rest are wasted as heat. Since that ICE vehicle needs a lot of gears - transmission, differential, there is also a significant loss of energy (due to friction) before the power gets to the wheels. Estimates of these losses are anywhere from 5% to 12%. That means, even in the best case scenario, only 23% of the BTUs in the gasoline are available to actually power the car. Many estimate this number to be as low as 15% for real world vehicles.

Compare that to an electric vehicle. Electric motors are better than 90% efficient. Since most times the motors are more directly coupled to the wheels, the frictional losses are also less. That means that maybe 85-90% of the energy stored in the batteries is available to power the vehicle. Remember, the vehicle still has the same energy requirements, and we have to put that energy into the batteries in the form of electricity. Electricity that is simply generated elsewhere.

In the world of electric vehicles, since they are very sensitive to energy consumption, the amount of energy they consume is a readily available statistic. The Tesla roadster is a real world 2700 pound car, powered solely by electricity. According to Tesla it uses.217 KWh (740 BTUs) for each mile traveled. The Chevy Volt is a near real world Plug in hybrid that weighs 3500 lbs, and reportedly uses.250 KWh (853 BTUs) per mile.

For a conventional Gasoline powered vehicle, I use my own 2005 Chevy Malibu for an example. It weighs 3700 pounds, and has gotten 24 MPG over it's life, mostly in-town. A gallon of gasoline contains 124,000 BTU's. Applying an 18% total efficiency factor (middle of the range) we can calculate that each gallon is contributing 22,320 BTU's to actually powering the vehicle. That would mean it is using 930 BTUs, or.272 KWh. per mile.

What we can see here, is that as the weight goes up, the energy requirement also goes up, but not by much. Remember, the only time that weight itself comes into play is when we accelerate. We can also make the assumption (in fact true) that the Chevy Malibu has more aerodynamic and frictional drag than either the Tesla, or the Chevy Volt, which accounts for much of it's increased need for energy.

I hope you understand from this discussion what affects the amount of energy a vehicle needs. These are basic rules of physics. It does not matter WHERE the vehicle gets it's energy from, this is the energy it needs to operate. One more time, that is an important distinction. Whether it is a gasoline, diesel, biodiesel, electric, hybrid, hydrogen, or sled dogs, any particular size, shape and weight vehicle needs a certain amount of energy put into it in order to move you and your stuff to Grandma's house. And, that, I guess, is my point.