11.1 Lesson 11 Introduction

Welcome to Lesson 11

Transportation is a major part of everyday life. It helps people travel to work, school, and stores, and it also moves food, products, and raw materials across the country and around the world. Because transportation is so important, it also uses a large amount of energy.

In the United States, most transportation energy still comes from petroleum-based fuels such as gasoline, diesel, and jet fuel. However, transportation is changing. Today, we also see hybrid vehicles, electric vehicles, biofuels, natural gas vehicles, and other advanced transportation technologies. At the same time, different types of transportation—such as cars, trucks, trains, ships, barges, and airplanes—use energy in different ways and have different impacts on efficiency, cost, and emissions.

In this lesson, you will explore the major types of transportation, the fuels they use, and how energy is converted to move people and goods. You will also examine vehicle efficiency, cost to drive, and how transportation choices affect energy use in the United States.

Lesson Objectives:

Identify the major types of transportation, including light-duty vehicles, trucks, rail, water transportation, and aircraft.
Explain how internal combustion engines convert fuel into motion through the four-stroke cycle.
Compare gasoline, diesel, hybrid, plug-in hybrid, and electric vehicles based on how they operate and the fuels they use.
Describe several alternative and advanced transportation fuels, including biodiesel, ethanol, natural gas, hydrogen, and propane.
Explain the purpose of vehicle efficiency standards such as CAFE standards.
Calculate or compare vehicle cost to drive using fuel price and fuel efficiency information.

mxw142 Wed, 07/15/2026 - 23:39

11.2 Energy Use for Transportation

As we have seen nearly a third of total energy use in the US falls into the transportation sector. This amounts to 28.2 Quadrillion BTUS of energy in 2025, which is 28,200,000,000,000,000 BTU!!.

Flowchart showing U.S. energy consumption by source and sector for 2025. The text description below describes the graphic in detail. — U.S. Enegry Consumption by Score and Sector, 2025

According to the U.S. Energy Information Administration, petroleum products supplied about 89% of total U.S. transportation energy use in 2025. Petroleum includes fuels such as gasoline, diesel, and jet fuel. This means that most transportation in the United States still depends heavily on oil-based fuels.

The same source reports that biofuels contributed about 6% of transportation energy use in 2025, and most of that biofuel was blended with petroleum fuels such as gasoline, diesel, and jet fuel. Natural gas accounted for about 5%, and nearly all of that use was connected to natural gas pipeline compressors rather than everyday passenger vehicles. Electricity used by mass transit systems was less than 1% of total transportation energy use.

These numbers are helpful because they show that, even though electric vehicles receive a lot of attention, the U.S. transportation system is still mostly powered by petroleum.They also show that changing transportation energy use is a big challenge because it involves millions of vehicles, many different industries, and several kinds of infrastructure.

Gasoline is still the largest transportation fuel in the United States. On an energy basis, gasoline accounted for about 52% of total transportation energy consumption in 2025, while distillate fuels such as diesel accounted for about 22%, and jet fuel accounted for about 13%.

This helps explain why transportation is such an important topic in an energy class. When we talk about reducing fuel use, lowering emissions, or improving efficiency, we are often talking about reducing dependence on gasoline, diesel, and jet fuel.

Pie chart showing U.S. transportation energy sources for 2025. The text description below describes the graphic in detail. — U.S. Transportation Energy Sources, 2025

mxw142 Wed, 07/15/2026 - 23:40

11.3 Types of Transportation

When many people think about transportation, they usually picture light-duty vehicles such as cars, SUVs, vans, and pickup trucks. These are the vehicles most people use in everyday life to get to school, work, stores, and other activities. Because they are so common, light-duty vehicles make up a large share of transportation energy use in the United States. In fact, they account for about 52% of total U.S. transportation energy use.

However, transportation includes much more than personal vehicles. A complete understanding of transportation must also include the systems that move goods, raw materials, and large numbers of people from place to place. These systems are essential to the economy and to daily life, even if we do not always notice them.

Light-Duty Vehicles

Light-duty vehicles are used mainly to transport people. This category includes:

Cars
Sport utility vehicles (SUVs)
Minivans
Small pickup trucks

These vehicles are common because they are convenient and flexible. People can use them for commuting, shopping, and travel. Most light-duty vehicles have traditionally used gasoline, but today they may also be powered by diesel, electricity, or hybrid systems.

Because so many people rely on these vehicles every day, they play a major role in energy demand, fuel consumption, and greenhouse gas emissions.

Medium- and Heavy-Duty Transportation

Not all transportation is about moving people. A large part of the transportation system is used to move products and materials. This includes trucks that deliver food, furniture, clothing, building supplies, and online orders.

Examples of medium- and heavy-duty transportation include:

Delivery vans
Box trucks
Semi-trucks and tractor-trailers
Dump trucks
Buses

These vehicles are especially important because they connect manufacturers, warehouses, stores, and customers. Many of them use diesel fuel because diesel engines are powerful and efficient for carrying heavy loads. However, some buses and trucks are beginning to use electricity, hydrogen, or other alternative fuels.

Rail Transportation

Trains are another important form of transportation. Rail systems can move both people and freight.

Passenger rail moves people between cities or within urban areas.
Freight rail moves goods such as coal, grain, chemicals, vehicles, and shipping containers.

Rail transportation is often more energy-efficient than moving the same amount of cargo by truck, especially over long distances. This is one reason trains are important in the broader energy and transportation system.

Water Transportation

Transportation by water is also an important part of the economy. Ships and barges move large amounts of cargo across oceans, rivers, and inland waterways.

Examples include transporting:

Oil and natural gas products
Agricultural products such as corn and soybeans
Coal and other raw materials
Consumer goods in shipping containers

Water transportation can be very efficient for moving heavy cargo over long distances. Barges, for example, can carry a large amount of material using relatively less energy per ton of cargo compared with trucks.

Air Transportation

We also need to consider air travel. Aircraft move both passengers and cargo quickly over long distances. Air travel is especially important for:

Business and personal travel
Emergency and medical transport
Shipping high-value or time-sensitive goods

Although air transportation is very fast, it usually requires more energy per passenger or per pound of cargo than trains or ships. For this reason, aviation is an important area of study when discussing transportation energy use and efforts to reduce emissions.

Transportation Energy Sources in the United States

In an energy class, it is not enough to know what types of vehicles we use. We also need to know what fuels power them.

Bar graph showing U.S. transportation energy use by mode and type for 2025, with light trucks at 33%, followed by cars and motorcycles at 19%, and other trucks at 23%. — U.S. Transportation Energy Use by Mode and Type, 2025

U.S. Transportation Energy Use by Mode and Type
Mode/Type	Perecent Usage
Light Trucks	33
Cars and Motorcycles	19
Other rucks	23
Aircraft	12
Boats and Ships	4
Pipeline Fuel	3
Trains and Buses	3
Military (all modes)	2
Lubricants	<1

mxw142 Wed, 07/15/2026 - 23:41

11.4 Internal Combustion Engines

Much of the transportation sector relies on internal combustion engines (ICEs). These engines use a mixture of fuel and air that is ignited inside the engine to move a piston. In this process, the chemical energy stored in the fuel is converted into mechanical energy to move the vehicle and thermal energy as heat.

Most internal combustion engines operate using a four-stroke cycle, which includes the following steps:

Intake – The engine takes in a mixture of air and fuel.
Compression – The piston compresses the air-fuel mixture.
Combustion (Power) – The mixture ignites, creating a small explosion that pushes the piston down and produces power.
Exhaust – The engine pushes out the waste gases created during combustion.

Illustration of a four-stroke cycle engine showing the stages of Intake, Compression, Power, and Exhaust with labeled sections and arrows indicating airflow and piston movement. — Four Stroke Cycle Engine

There are two main types of internal combustion engines, spark-ignition (gasoline engines) and compression ignition (diesel engines).

Watch this video by Shannon Odell from TEDed:

This is what happens when you hit the gas (6:04)

This is what happens when you hit the gas

Transcript: This is what happens when you hit the gas (6:04)

In 2015, two men drove a Volkswagen across the continental United States on just over 100 gallons of fuel. Their 81-mile-per-gallon performance doubled the car’s estimated fuel rating, and set the record for the lowest fuel consumption ride of a diesel car. The duo call themselves hypermilers and are experts in techniques that maximize a car’s fuel efficiency, such as the pulse-and-glide.

In the pulse, drivers accelerate slowly until they’re traveling slightly above their intended speed. They then slowly release the throttle and glide, until they’re slightly below, and repeat. To understand why this strategy saves fuel, we first need to unpack what exactly is going on beneath a car's hood.

Internal Combustion

Non-electric cars run on internal combustion engines, or ICEs. Cars are often advertised as sporting a 4-, 6-, or 8-cylinder engine, which refers to this device's main components. Within each of these cylinders is a piston, which moves up and down, spinning a bar known as a crankshaft, effectively converting linear motion into a rotary motion that can drive the wheels. What powers these pistons’ movements is what gives these engines their namesake: combustion.

As the piston lowers, air and fuel are sprayed into the cylinder’s chamber. Then as the piston rises, this air and fuel mixture is compressed. In gasoline engines, a spark is introduced, igniting the gas. In diesel engines, the compression alone creates a mini explosion. This combustion causes an immediate increase in temperature and pressure, propelling the piston down, as it starts the cycle again.

The gas pedal controls the amount of air and subsequent fuel released into the chamber. The more fuel in the chamber, the more powerful the combustion, making the crankshaft rotate faster. Driving down the highway, ICE cars spark thousands of blasts per minute. But explosion-power driving is pretty inefficient, as much of the energy generated is lost to heat. In fact, only 16 to 25% goes towards moving the wheels. These explosions also create CO2, and ICE engines produce 15% of the total global carbon emissions.

The pulse-and-glide can increase efficiency for two reasons. First, when accelerating to higher speeds during the pulse, the engine works at a higher efficiency compared to traveling at a constant lower speed. And second, modern car engines shut off fuel injection or idle, when decelerating. Meaning that as the car glides, the wheels are driven by inertial energy, rather than combustion, ultimately saving fuel.

But even at their peak performance, ICE hypermilers can’t compete with the true champion of fuel efficiency rides: the electric vehicle.

Electric Vehicles

Many EVs run on induction motors, which have two main parts: a stator and a rotor. The stator is a series of rings, with copper wires wrapped around it. By conducting electricity at variable rates, these wires create a rotating magnetic field. This field induces the rotor with electrical current, causing it to spin, and driving the motion of the wheels.

For EVs, pressing on the accelerator changes the frequency of current driven into the wires of the stator, in turn increasing the rate at which the rotor spins. By utilizing battery power rather than gasoline, 65 to 69% of the energy consumed by EVs goes directly to moving the wheels. And since EVs don't create explosions, fewer parts are needed below the hood. While a typical ICE vehicle has over 2,000 moving parts to help contain, cool, and maintain combustion, a typical EV has about 20.

EVs are completely changing the hypermiling game as drivers compete to travel the farthest on the fewest kilowatt-hours. And records will likely only get more impressive, as the design of EV motors allows for the introduction of innovative energy-saving devices. For example, most EVs utilize regenerative braking, where energy normally lost to friction is conserved. As the car slows, the electric motor operates in reverse, capturing the vehicle’s kinetic energy to recharge the battery. Some companies are even equipping EVs with rooftop solar panels, further increasing their range.

Since they don’t burn fuel, EVs have zero tailpipe emissions. That’s not to say they’re always carbon neutral. EVs require regular charging of their batteries, meaning their emission profile is only as clean as the electric utility they plug into. So as global grids continue to shift towards renewable sources, EVs are also becoming greener, making them an even more attractive, hyper-efficient option.

Credit: This is what happens when you hit the gas - Shannon Odell. TEDed. Accessed July 15, 2026.

mxw142 Wed, 07/15/2026 - 23:42

11.4.1 Gasoline (Spark Ignition) Engines

Gasoline engines are probably the most common type of internal combustion engine you see in everyday life. In the United States, most passenger vehicles use gasoline as their main fuel.

Gasoline engines are called spark-ignition engines because the combustion step begins when a spark plug ignites the compressed air-fuel mixture inside the engine. This small explosion pushes the piston downward and produces power.

Gasoline is made of short-chain hydrocarbons refined from petroleum. It has a relatively low boiling point, around 85°C (185°F), which is lower than the boiling point of water. Gasoline is also highly volatile, which means it evaporates easily. You may notice this when fueling a car or filling a lawn mower on a hot day. The smell you notice comes from gasoline vapors entering the air as some of the hydrocarbons evaporate.

Gasoline is sold based on its octane rating. In the United States, gasoline is usually sold as regular, midgrade, and premium. These values can vary somewhat by state, but regular gasoline is usually 87 octane, midgrade is around 89 octane, and premium is typically 91 to 93 octane.

Octane ratings are based on two reference fuels:

Iso-octane (C₈H₁₈), which is assigned a value of 100.

Text description of the Iso-octane image.
Ball-and-stick model of an iso-octane molecule showing a branched hydrocarbon structure composed of eight carbon atoms and eighteen hydrogen atoms. The central carbon chain has three methyl branches, creating a compact, irregular shape. Carbon atoms are typically shown as dark gray or black spheres connected by rods, with smaller white hydrogen atoms attached around the outside.
Credit: Moleview
Note: You can use your mouse to rotate and zoom in on the Iso-octane molecule.
Normal heptane (C₇H₁₆), which is assigned a value of 0.

Text description of the Hectane image.
Ball-and-stick model of a heptane molecule showing a straight-chain hydrocarbon structure composed of seven carbon atoms and sixteen hydrogen atoms. The carbon atoms form a zigzag chain, with smaller hydrogen atoms attached around the outside. Carbon atoms are typically shown as dark gray or black spheres connected by rods, while hydrogen atoms are shown as smaller white spheres.
Credit: Moleview
Note: You can use your mouse to rotate and zoom in on the Heptane molecule.

Fuels with more branched hydrocarbon chains tend to have higher octane ratings, while fuels with straight-chain hydrocarbons tend to have lower octane ratings. Some fuels can even have octane values above 100. For example, ethanol (C₂H₅OH) has an octane rating of about 113 and is often blended into gasoline. The fuel you buy is a mixture of many different types of compounds. The octane rating (87, 89, 93) is the average mix of all of the compounds found in that fuel.

mxw142 Wed, 07/15/2026 - 23:43

11.4.2 Diesel (Compression Ignition) Engines

Diesel engines are different from gasoline engines in one very important way: they do not use a spark plug. Instead, diesel engines use compression ignition, also called autoignition.

In a diesel engine, air is compressed inside the cylinder until it becomes very hot. Then diesel fuel is injected into that hot, high-pressure air. Because the air is so hot, the fuel ignites on its own. This combustion pushes the piston downward and produces work.

Diesel engines usually operate at higher compression ratios than gasoline engines, which helps them ignite the fuel without a spark. This is one reason diesel engines are often more efficient than gasoline engines.

Because diesel engines ignite fuel differently, diesel fuel also has different chemical properties than gasoline. Diesel fuel generally contains longer hydrocarbon chains and tends to work better with straighter-chain molecules, while gasoline performs better with more branched molecules.

Diesel fuel is rated using a cetane number. The cetane number measures how easily and quickly diesel fuel ignites under compression. In general, a higher cetane number means the fuel ignites more readily, which can improve engine starting and combustion performance.

Cetane ratings are based on reference fuels, including:

Cetane (C₁₆H₃₄), which has a cetane number of 100

Text description of the Cetane image.
Ball-and-stick model of a cetane molecule (C₁₆H₃₄) showing a straight-chain hydrocarbon structure composed of sixteen carbon atoms and thirty-four hydrogen atoms. The carbon atoms form a long zigzag chain, with smaller hydrogen atoms attached around the outside. Carbon atoms are typically shown as dark gray or black spheres connected by rods, while hydrogen atoms are shown as smaller white spheres.
Credit: Moleview
Note: You can use your mouse to rotate and zoom in on the Iso-octane molecule.
Iso-cetane or 2,2,4,4,6,8,8-heptamethylnonane, which has a cetane number of 15

Text description of the Iso-cetane image.
Ball-and-stick model of a 2,2,4,4,6,8,8-heptamethylnonane molecule showing a highly branched hydrocarbon structure. The molecule consists of a nine-carbon main chain with seven methyl groups attached at multiple positions, creating a compact, irregular shape. Carbon atoms are typically shown as dark gray or black spheres connected by rods, while smaller white spheres represent hydrogen atoms attached around the outside. The extensive branching gives the molecule a wider, more clustered appearance than a straight-chain hydrocarbon of similar size.
Credit: Moleview
Note: You can use your mouse to rotate and zoom in on the Iso-octane molecule.

In simple terms, the cetane number tells us how easily diesel fuel will ignite in a diesel engine. This is different from octane rating in gasoline, which measures how well a fuel resists igniting too early.

Petrol (Gasoline) versus Diesel - Don’t try this at home chemical combustion video.

What's the Difference Between Petrol & Diesel? | Bang Goes The Theory | Earth Science (3:51)

What's the Difference Between Petrol & Diesel?

Transcript: What's the Difference Between Petrol & Diesel? (3:51)

Nowadays most cars on the road fall into one of two categories: either petrol or diesel. Now, from the shape of these vehicles on the outside, you wouldn't know which is which, but on the inside their engines just don't work in the same way, and the net result of that is what finally emerges from their exhaust pipes is also significantly different. All but the very latest diesels give off at least twice the toxic nitrogen dioxide and 10 or 20 times the dangerous soot particles that petrol cars do. So why is there such a big difference? It really comes down to the fuel itself.

Petrol and Diesel, they may look the same and their pumps sit side by side on the garage forecourt, but there is a significant difference between the two of them, and to show you I need a bowl of each and a box of matches. By the way, danger alert: do not not try this at home.

Now petrol is a highly volatile fuel. That means it evaporates easily, so Petrol gas effectively comes up and mixes with the air, and so catches fire readily. But diesel, made of longer heavier molecules, gives off fewer fumes and is actually quite difficult to light. Now because of this big difference in flammability between the two fuels, the engines work in fundamentally different ways.

This is my petrol engine. It's very simple. It's just a single cylinder, and into it I've got a nice fitting piston that can slide up and down. The way the fuel gets introduced into the cylinder, because petrol vaporizes so readily, it actually gets injected into a warm chamber just prior to the cylinder. And then, as the Piston Rises, it sucks in a mixture of petrol Vapors and air, and because they're both gases essentially, they can mix very very intimately. So when you introduce a spark, you get quite a nice explosion. And you'll see on that explosion it's a very blue flame, and that blue means that there's very few soot particles in there. It's clean burning.

Now a diesel engine has to work differently because diesel won't readily vaporize into a gas to make it flammable. It's injected into the air as a kind of fine Mist, a sort of High Press aerosol. Now that really does catch fire. So I'll spray a fine mist of diesel droplets into my cylinder. There you go. There's a big difference. The flame is bright yellow, and that yellowness indicates that there are glowing soot particles in there. And you can quite readily see the smoke coming off it, and indeed the soot, and that's the problem because in the operation of an engine there really isn't time for all those droplets of liquid to burn completely.

So what happens is some of it just gets reduced down to particles of carbon soot, and that means that the exhaust system of a diesel engine has a bigger job to do.

Credit: What's the Difference Between Petrrol and Diesel? BBC Earth Science. YouTube. Accessed July 15, 2026.

mxw142 Wed, 07/15/2026 - 23:44

11.5 Hybrid and Electric Vehicles

Today, many vehicles use electricity in addition to or instead of gasoline. These vehicles are designed to improve fuel efficiency, reduce emissions, and lower operating costs. The three main types are hybrid vehicles, plug-in hybrid vehicles, and electric vehicles.

Hybrid Vehicles

A hybrid vehicle uses both a gasoline-powered internal combustion engine and an electric battery. The battery helps power the vehicle, but a hybrid cannot be plugged in to charge. Instead, the battery is recharged through regenerative braking and by the engine during normal driving.

Regenerative braking is a system that captures some of the energy normally lost when the vehicle slows down and sends that energy back to the battery. Because the battery helps the engine, the vehicle uses less gasoline than a traditional gasoline-powered vehicle. This makes hybrid vehicles more fuel-efficient than standard internal combustion engine vehicles.

Plug-In- Hybrid Vehicles

A plug-in hybrid electric vehicle, or PHEV, also uses both a gasoline engine and an electric battery. However, a PHEV has a larger battery than a standard hybrid. This allows the vehicle to drive on electricity alone for a limited distance, usually about 10 to 30 miles, before the gasoline engine is needed.

Unlike a regular hybrid, a plug-in hybrid can be charged by plugging it into an electrical outlet or charging station. This makes PHEVs a good choice for drivers with short daily commutes, especially if the commute is within the vehicle’s electric-only range. For longer trips, the gasoline engine provides flexibility because the driver can refuel at a gas station as usual.

Electric Vehicles

Electric vehicles (EVs) run entirely on electricity. They do not have a gasoline engine, and they produce no tailpipe emissions because there is no combustion happening in the vehicle. Because electric vehicles convert electrical energy directly to mechanical energy to turn the wheels, the are more efficient than internal combustion engines. EVs are recharged by plugging them into the electrical grid.

Because electric vehicles have fewer moving parts than traditional gasoline-powered vehicles, they usually require less maintenance. For example, they do not need oil changes and have fewer engine-related parts that can wear out. However, EVs are often heavier than gasoline vehicles because of their large battery packs.

Try it yourself:

The Alternative Fuel Data Center provides helpful information about how hybrid vehicles, plug-in hybrid electric vehicles (PHEVs), and electric vehicles (EVs) work. Explore the site to learn about the main parts and systems in each type of vehicle.

Visit: The U.S. Department of Energy page How Do Plug-In Hybrid Electric Cars Work?

As you explore, look for: the battery, the electric motor, the gasoline engine, the fuel tank and the charging port.

Think about how these parts are needed in a hybrid, a plug-in hybrid, and an electric vehicle.

Where is the nearest EV fueling station to you?

Many EV drivers experience range anxiety, especially on long trips. Range anxiety is the worry that the vehicle may run low on battery power before reaching a charging station, or that a charger may not be available or working.

To help with this, drivers can use apps and websites to find charging stations and plan their routes.

Use the Department of Energy Alternative Fueling Station Locator to find the nearest charging station or fast charger:

Alternative Fueling Station Locator - Nearest

You can also use PlugShare’s Trip Planner to map out a road trip and see whether the trip could be completed in an electric vehicle:

PlugShare’s Trip Planner

mxw142 Thu, 07/16/2026 - 14:46

11.6 Alternative Fueled Vehicles

In addition to gasoline, diesel, hybrid, and electric vehicles, there are several other alternative and advanced fuel vehicles on the market. These vehicles are not as common as gasoline or diesel vehicles, but they are still commercially available and important to mention as they may become more common in the future.

These fuels are often studied because they may reduce petroleum use, lower some emissions, or provide more fuel choices for drivers and fleets.

Biodiesel

Biodiesel is a fuel that can be made from vegetable oils, animal fats, or recycled cooking oil, such as used fryer oil from restaurants. It can be used in some diesel engines as an alternative to petroleum diesel.

Biodiesel is usually sold as a blend with petroleum diesel. For example:

B20 = 20% biodiesel and 80% petroleum diesel
B100 = 100% biodiesel

Many diesel vehicles can use lower biodiesel blends, such as B5 or B20, without major changes. Higher blends may require engine modifications or manufacturer approval.

Biodiesel is often described as a cleaner-burning fuel than petroleum diesel. It can also improve the lubricity of the fuel, which means it helps reduce friction between engine parts. Some drivers even say biodiesel exhaust smells a little like fried food.

Ethanol

Ethanol is an alcohol-based fuel with the chemical formula CH₃CH₂OH. In the United States, ethanol is commonly made by fermenting corn. In other countries, such as Brazil, it is often made from sugar cane.

Text description of the Ethanol image.

Ball-and-stick model of an ethanol molecule (CH₃CH₂OH) showing a simple organic structure composed of two carbon atoms, six hydrogen atoms, and one oxygen atom. The two carbon atoms are connected in a short chain, with an oxygen atom attached to one end and a hydrogen atom bonded to the oxygen, forming a hydroxyl group. Carbon atoms are typically shown as dark gray or black spheres, hydrogen atoms as smaller white spheres, and the oxygen atom as a red sphere.

Credit: Moleview
Note: You can use your mouse to rotate and zoom in on the Iso-octane molecule.

Ethanol is blended into most gasoline sold in the United States. The most common blend is:

E10 = 10% ethanol and 90% gasoline

Some vehicles, called flex-fuel vehicles, can use higher ethanol blends such as E85.

Because ethanol contains oxygen, it can help gasoline burn more completely in the engine. This can reduce carbon monoxide and some other pollutants.

However, ethanol contains less energy per gallon than gasoline. This means a vehicle will usually travel fewer miles on a gallon of ethanol than on a gallon of pure gasoline.

Ethanol has less energy per gallon than petroleum derived gasoline, which means you will be able to drive less miles on one gallon of ethanol versus on gallon of gasoline.

Natural Gas

As discussed earlier in the course, natural gas is a fossil fuel made mostly of methane, CH₄.

Natural gas vehicles are designed to use either:

Compressed natural gas (CNG)
Liquefied natural gas (LNG)

CNG is stored at very high pressure, usually around 3,600 psi. LNG is natural gas that has been cooled to about -260°F so that it becomes a liquid.

Natural gas vehicles are often used in fleet applications, such as buses, delivery vehicles, and municipal vehicles. They work especially well when vehicles return to a central location each day for fueling.

Even though natural gas pipelines are common in many parts of the United States, CNG and LNG fueling stations are much less common than gasoline stations. This is one reason natural gas vehicles are often used in centrally managed fleets rather than by individual drivers.

Propane

Propane, also called liquefied petroleum gas (LPG), is another alternative transportation fuel. It is used in some fleet vehicles, school buses, delivery trucks, and service vehicles.

Propane is C₃H₈

Text description of the Propane image.
Ball-and-stick model of a propane molecule (C₃H₈) showing a straight-chain hydrocarbon structure composed of three carbon atoms and eight hydrogen atoms. The carbon atoms are connected in a short chain, with hydrogen atoms attached around the outside. Carbon atoms are typically shown as dark gray or black spheres connected by rods, while hydrogen atoms are shown as smaller white spheres. Propane is a three-carbon alkane with the molecular formula C₃H₈.
Credit: Moleview
Note: You can use your mouse to rotate and zoom in on the Iso-octane molecule.

Propane vehicles can produce lower emissions of some pollutants compared with conventional gasoline vehicles. Propane is also widely used in areas where fleet managers want a fuel that is easy to store and transport.

Like natural gas, propane is most common in fleet settings where vehicles can refuel at a central location. It is less common for personal passenger vehicles.

Hydrogen

Hydrogen is the simplest element on the periodic table with just one electron and proton. Hydrogen can also be used as transportation fuel. Some vehicles use hydrogen in a fuel cell, which produces electricity to power an electric motor. These vehicles are called fuel cell electric vehicles (FCEVs).

Hydrogen fuel cell vehicles do not burn gasoline or diesel. Instead, the fuel cell combines hydrogen with oxygen from the air to produce electricity. The main byproduct is water vapor.

Hydrogen vehicles can be refueled more quickly than many battery electric vehicles, but hydrogen fueling stations are still very limited in most areas. Because of this, hydrogen vehicles are not yet common for most drivers.

mxw142 Thu, 07/16/2026 - 14:47

11.7 Vehicle Efficiency Standards

Most internal combustion engine (ICE) vehicles measure fuel efficiency in miles per gallon (mpg). A higher mpg value means the vehicle can travel farther using one gallon of fuel.

Today, fuel efficiency is an important part of vehicle design, but that was not always the case. Before the oil crisis of the 1970s, fuel economy was not a major concern for many automakers or consumers. In fact, fuel efficiency had declined over time. The 1908 Model T got about 21 mpg, while the average American car in 1973 got only about 12 mpg.

Because of rising oil prices and concerns about dependence on foreign oil, Congress passed the first Corporate Average Fuel Economy (CAFE) standards in 1975.

What are the CAFE Standards?

CAFE standards were created to improve the fuel economy of cars and light trucks sold in the United States. The goal was to encourage automakers to build more fuel-efficient vehicles and reduce overall fuel use.

The word corporate average is important. CAFE standards apply to the average fuel economy of all the vehicles a manufacturer sells, not to every single vehicle by itself. This means an automaker can still sell some less-efficient vehicles as long as those are balanced by more-efficient models in the rest of the fleet.

For example, a company might sell:

many smaller, fuel-efficient cars
some larger vehicles with lower fuel economy

If the average for the full fleet meets the standard, the company is in compliance.

Important Notes

CAFE standards originally focused on cars and light trucks. They did not apply in the same way to heavy-duty vehicles. Some trucks used for work purposes were treated differently from passenger cars, which is one reason truck efficiency rules developed differently over time.

CAFE Timeline

1978: The first CAFE standard began at 18 mpg
1990: The standard gradually increased to 27.5 mpg
1990 to 2010: The standard stayed at 27.5 mpg
2011 to 2020: Standards increased significantly, rising from about 30 mpg to 42.4 mpg

These changes pushed automakers to improve engines, reduce vehicle weight, and develop hybrid and electric vehicles.

Interactive Data

The chart below from the Alternative Fuels Data Center shows historical fuel economy data and trends in vehicle efficiency from 1978 with estimates into the future. Use it to explore how fuel economy has changed over time.

The Vehicle Fuel Efficiency (CAFE) Requirements by Year graphic with descriptive text is available from the U.S. Department of Energy's Office of Critical Minerals and Energy Innovation site.

How Corporate Average Fuel Economy Works

CAFE standards are based on the average fuel economy of a company’s fleet.

For example, suppose a manufacturer sells:

1,100 cars that get 50 mpg
400 cars that get 22 mpg

The average fleet fuel economy would be:

$\frac{(1100 \times 50) + (400 \times 22)}{1100 + 400} = \frac{5500 + 8800}{1500} = 42.5 mpg$

So even though not every vehicle gets more than 42 mpg, the company’s average fleet fuel economy is 42.5 mpg.

Electric Vehicles and CAFE Standards

Hybrid vehicles, plug-in hybrid vehicles, and electric vehicles have helped manufacturers improve their fleet fuel economy. Because these vehicles use less gasoline, or no gasoline at all, they raise the average efficiency of the vehicles a company sells.

This means automakers can improve their CAFE performance by offering more:

hybrid vehicles
plug-in hybrid vehicles
electric vehicles

As a result, fuel economy standards have played an important role in encouraging the growth of advanced vehicle technologies.

mxw142 Wed, 07/15/2026 - 23:47

11.8 Efficiency Per Passenger Mile

The chart below compares different types of transportation using the same unit: gasoline gallon equivalents per passenger mile. This makes it possible to compare transportation modes on a common basis, even when they use different energy sources such as gasoline, diesel, or electricity.

A passenger mile means moving one passenger one mile. Looking at efficiency this way helps us compare how much energy is used to move people by car, bus, train, or airplane.

The Average Per-Passenger Fuel Economy by Travel Mode graphic with descriptive text is available from the U.S. Department of Energy's Office of Critical Minerals and Energy Innovation site.

Looking at the chart above, transit rail appears to be one of the most efficient ways to move passengers. Rail systems are often efficient because trains can carry many people at once, experience relatively low rolling resistance, and in many cases use electricity.

Air travel may also appear more efficient than some people expect on a per passenger mile basis. Although airplanes use a large amount of energy overall, many flights carry a high number of passengers. When that energy use is divided among many riders, the energy use per passenger mile can be lower than expected.

You may be surprised that transit buses can have relatively low efficiency on a per passenger mile basis. One reason is that buses often run fixed routes and schedules whether they are full or nearly empty. If only a small number of people are riding, the energy used by the bus is divided among fewer passengers, which lowers efficiency per passenger mile.

Demand response vehicles, such as taxis and ride-share vehicles like Uber or Lyft, can also be less efficient. These vehicles may need to travel extra distance to pick up passengers, which increases the amount of energy used for each trip.

Key Takeaways

Transportation efficiency is not just about the vehicle itself, but it also depends on how many passengers are riding, how far the vehicle travels, whether the vehicle runs full or mostly empty and what type of fuel it uses.

mxw142 Wed, 07/15/2026 - 23:47

11.9 Cost to Drive

An important question to consider when purchasing a vehicle is how much it costs to drive. In general, cost to drive depends on two main factors:

the price of fuel
the fuel efficiency of the vehicle

For vehicles with internal combustion engines (ICEs), fuel efficiency is usually listed in miles per gallon (MPG). This information is provided by the manufacturer, but your actual fuel economy may be different depending on how and where you drive.

For example, driving in heavy traffic, making short trips, or driving aggressively can lower fuel efficiency.

Tips for Improving Fuel Efficiency

The U.S. Department of Energy provides several tips for improving fuel economy. Two of the most important are:

Avoid rapid acceleration
Avoid hard braking and speeding

These aggressive driving habits use more fuel and reduce efficiency. By driving more smoothly, some drivers can improve their fuel economy by about 10% to 40%. This is true for many types of vehicles and is one of the easiest ways to reduce fuel costs.

Example one:
How much does it cost to drive a gas car?

Assume your vehicle gets 25MPG and gasoline costs $3.75/gallon. How much does it cost you to drive per mile?

Hint: Remember back to Lesson 2: When we learned about conversions. We can consider our 25 miles/gallon and $3.75/gallon just more conversions.

We want our answer to have units of $/mile, so we need to find a way to get the gallons to cancel out.

$(\frac{$ 3.75}{gallon}) \times (\frac{gallon}{25 miles}) = (\frac{$ 3.75}{25 miles}) = \frac{$ 0.15}{mile}$

It costs about 15 cents per mile driven in this situation.

Example two:
How much does it cost to drive an electric car?

An electric vehicle requires 30 kWh to drive 100 miles. If you charge in your home, where electricity cost $0.18/kWh, how much does it cost to drive per mile?

Once again, we can look at the givens just like conversion factors. In this case the fuel efficiency of an EV is given as 30kWh/100 miles.

Remember we are looking at solving for $/mile again, so follow the units.

$\frac{$ 0.18}{kWh} \times \frac{30 kWh}{100 miles} \frac{$ 0.18 \times 30}{100 miles} \frac{$ 5.40}{100 miles} \frac{$ 0.054}{mile}$

= ~ $0.054/ mile or about 5.4 cents per mile to drive electric

Example 3:
Real life comparison EV versus Gasoline

You live in an off-campus apartment 5 miles from campus. You drive the 10-mile round trip 4 days a week, plus another 200 miles a week for weekend errands, hanging out with friends, and part-time job commuting. Assume gasoline is $4.25/gallon in your area and electricity costs $0.16/kWh. Compare the cost to drive per month via gasoline powered vehicle that gets 30 MPG and electricity in a car that gets 32 kWh/100 miles.

Estimated monthly driving: 40 mile/week to class = 160 miles/month +800 miles/month

Estimated ~ 1000 miles/month ** (rounded up from 960)

Cost to drive a gas car for a month ($/month)

$(\frac{1000 miles}{month}) \times (\frac{1 gallon}{30 miles}) \times (\frac{$ 4.25}{1 gallon}) = (\frac{1000}{month}) \times (\frac{1}{30}) \times (\frac{$ 4.25}{1}) = (\frac{1000 \times 1 \times $ 4.25}{30 \times 1}) / month = (\frac{$ 4250}{30}) / month = $ 141.67 / month$

It will cost $141.67/month to drive a gas car.

Cost to drive an electric car for a month ($/month)

$(\frac{1000 miles}{month}) \times (\frac{32 kWh}{100 miles}) \times (\frac{$ 0.16}{1 kWh}) = (\frac{1000}{month}) \times (\frac{32}{100}) \times (\frac{$ 0.16}{1}) = (\frac{1000 \times 32 \times $ 0.16}{100 \times 1}) / month = (\frac{$ 5120}{100}) / month = $ 51.20 / month$

It will cost $51.20/month to drive an electric car.

The monthly savings of driving an electric car versus gasoline in this scenario is $141.67-$51.20, or $90.47/ month. Remember this depends on how much you drive and the cost of fuel. If you charge at a fast-charging station, prices would be significantly higher to drive electric.

mxw142 Wed, 07/15/2026 - 23:48

11.10 Future of Transportation

Transportation is changing quickly. For more than a century, most cars, trucks, ships, and airplanes have depended on petroleum fuels such as gasoline, diesel, and jet fuel. Today, engineers, governments, and companies are working on new ways to move people and goods using less fuel and producing fewer emissions.

One important idea to remember is that the future of transportation is not just about cars. Passenger vehicles matter, but the transportation sector also includes trucks, buses, trains, ships, and airplanes. Each part of the system has different energy needs, so the future will likely include more than one solution.

Electric Vehicles

Electric vehicles, or EVs, are likely to be a major part of the future of transportation. EVs run on electricity stored in batteries, which means they do not burn gasoline or diesel in the vehicle itself.

Many countries are increasing EV adoption, but Norway has become one of the clearest examples of how quickly the transition can happen. In 2025, 95.9% of all new passenger cars registered in Norway were fully electric. In December 2025, the EV share of new car sales in Norway reached 97.6%.

Norway’s progress did not happen by accident. Long-term policies, charging infrastructure, and tax incentives helped make electric cars a practical choice for many drivers. Norway’s government has also connected transportation planning to larger climate goals and reductions in road-transport emissions.

Electrifying Trucks

Passenger cars are only one part of transportation. A large amount of energy is also used to move goods by truck. Because of this, the future of transportation also includes medium- and heavy-duty vehicles

Norway has set an especially ambitious goal for trucks. Reports on Norway’s policy state that, beginning in 2030, all new trucks are expected to be either zero-emission vehicles or vehicles powered by biogas. This is important because trucks are harder to electrify than passenger cars. Heavy trucks need large batteries, powerful charging equipment, and enough range to move cargo efficiently

At the same time, electric truck technology is improving. Governments and companies are investing in chargers, vehicle development, and demonstration projects to show that electric trucks can work in real-world settings

Sustainable Aviation Fuel

Air travel is one of the hardest parts of transportation to change because airplanes need fuels with high energy density. Batteries work well for many cars and buses, but they are much harder to use for large commercial aircraft over long distances.

Because of this, one important idea for the future of aviation is sustainable aviation fuel, often called SAF. The U.S. Department of Energy describes SAF as an aviation fuel made from biomass and waste resources that can provide performance like petroleum-based jet fuel. The Department of Energy also explains that SAF can be made from feedstocks such as corn grain, oil seeds, algae, fats, oils, greases, agricultural residues, forestry residues, and municipal solid waste

Mass Transit

Another major part of the future of transportation is mass transit. In many cases, the cleanest transportation option is not just changing the fuel in a vehicle but also moving more people at once.

Public transit already helps reduce carbon emissions by shifting some travel away from personal cars. Transit systems are also beginning to electrify their fleets. For example, the Massachusetts Bay Transportation Authority states that it plans to convert its entire bus fleet to battery-electric buses by 2040

Electric buses can lower greenhouse gas emissions compared with buses powered by fossil fuels, but transit agencies also face challenges. They need charging infrastructure, updated maintenance facilities, and operating plans that keep buses in service while batteries are charged. This shows that the future of transportation is not just about buying new vehicles. It also requires new infrastructure and planning.

High speed rail is very common in Europe and Asia and may some day have a role in the US. As we have seen, rail is the most efficient method for transporting people. Japan has been running high speed rail for over 50 years. The bullet train travels about 120 miles/hour transporting more than 400,000 people a day. Technology exists to get lots of people from place to place quickly and efficiently.

What is next in the future of transport is not entirely certain. New developments are constantly underway including drones for delivery and possibly human transport in congested cities.

See the First-of-its-Kind 'Air Taxi' That's Set to Revolutionize Travel (4:08)

See the First-of-its-Kind 'Air Taxi' That's Set to Revolutionize Travel

Transcript: See the First-of-its-Kind 'Air Taxi' That's Set to Revolutionize Travel (4:08)

[Sam Brock]
You're looking at liftoff of an aircraft from the future. Except, it's primed to smash expectations in the present from San Francisco to New York to Osaka, Japan, where flight demos have already stoked a major buzz around this electric powered air taxi, created by California-based Jobi that combines a verticaled accent with a mid-air transition to fixed wing flying. All through six ultra-efficient motors.

It looks like a helicopter, it looks like a plane. What is it?

[Buddy Denim]
Our lift systems all tilt forward within the cells. Now, he benefit of that, we look like a helicopter when we take off, but thenwe transition to wing-born flight.

[Sam Brock]
Lead demonstration test pilot Buddy Denim.

[Buddy Denim]
100%, you're looking at the future of aviation here.

[Sam Brock]
Took us around the plane he acknowledges gives off Jetson vibes. Even surprising ATC controllers on a recent flight.

[Air traffic controller 1]
What is that thing?

[Air traffic controller 2]
It's a Joby.

[Sam Brock]
But rest assured, is real.

Inside the aircraft, there's more than 50 different computers, all talking to one another. The pilot is still the conductor of this operation, but the computers are what's doing the heavy lifting.

[Buddy Denim]
It's one of the first few airplanes in the history where the pilot controls are not hooked to a control service on the airplane. They go to the flight control computer. The pilot actually asks for exactly what he or she wants to fly.

[Sam Brock]
The company's CEO, Joe Ben Bevert, says the air taxi which holds up to four passengers and can fly around 140 miles per hour is not meant for the 1 percenters, but for any passengers looking for a ten-minute lift to the airport.

[Joe Ben Bevert]
In terms of the price point, our target is to be competitive with ground transportation over the time. The electric propulsion on our aircraft makes these aircraft dramatically more efficient.

[Sam Brock]
Joby acquired helicopter company Blade last year and in the process, acquired it infrastructure and routes, like this one from New York's JFK to the West Side Highway in the city. But there's a big difference in acoustic footprint.

With an electric motor, there's no roar of an engine like on this helicopter. According to Joby, the air taxis are 100 times quieter, so subtle that people on the ground aren't even going to hear it.

[Buddy Denim]
We could have a conversation in a cafe, you would never hear the airplane come over.

[Sam Brock]
The FAA could grant approval in the coming year as part of a sweeping program in 12 states to streamline new technologies like these called EV-TOM.

[Sean Duffey - U.S. Secretary of Transportation]
It's going to give us different regional mobility. Where we live, where we work can fundamentally change with an EVITAL.

[Sam Brock]
The tech being explored by a number of companies that could be coming to a city near you.

[Joe Ben Bevert]
This is a game changer for our industry. Very excited about rolling out service in New York, Florida, and Texas. This is a really exciting moment for the launch of this next mode of transportation.

[Sam Brock]
So many of you at home probably wondering, what's the range on these air taxis. Right now, it's about 100 miles. Perfect for back and forth to the airport. What they do is they charge it in intervals, as passengers are getting on and off, since the battery never gets low. Also, Joby has a partnership with Uber, which means you can theoretically, at some point, order either on your Uber or Joby app, get the ride to the heliport, take the air-taxi from the heliport to the airport. They call that seamless multi-modal.

Sounds like it could be a fantastic move here for consumers. We'll find out in a matter of months.

[Savannah Guthrie]
Well.

[Sam Brock]
Back to you guys.

[Savannah Guthrie]
We give that explanation five stars.

[Carson Daily]
He's selling me, he's selling me on it.

[Savannah Guthrie]
I mean.

[Carson Daily]
Joby bought Blade. Joby's got to deal with the Uber.

[Savannah Guthrie]
Yeah.

[Carson Daily]
Sounds like they are.

[Savannah Guthrie]
But won't all of the traffic then just move to the sky? You look up and it's all these,

[Carson Daily]
Eventually

[Savannah Guthrie]
I don't love that.

[Craig Melvin]
There's a lot more space up there.

[Savannah Guthrie]
But I love innovation.

[Craig Melvin]
Would you try it out? Would you do it?

[Al Roker]
Absolutely. Why not? Sure. I mean, at least once.

[Craig Melvin]
Okay.

[Al Roker]
I still haven't got into one of those Waymo's yet. So, I don't know, we'll see.

Hey, thanks for watching and don't forget you can catch the Today Show every morning on NBC or take Today when you're on the go, just foolloew the Today podcast on Apple Podcast, Spotify, or wherever you listen.

Credit: See the First-of-its-Kind 'Air Taxi' That's Set to Revolutionize Travel.Today. YouTube. Accessed July 15, 2026.

mxw142 Wed, 07/15/2026 - 23:48

11.11 Conclusion

Transportation is one of the largest energy-using sectors in the United States, and it affects nearly every part of daily life. People depend on transportation to travel to school, work, and stores, while businesses rely on it to move food, products, and raw materials. Because transportation is so important, understanding how it uses energy helps us better understand the larger energy system.

In this lesson, you learned that transportation includes much more than passenger cars. It also includes trucks, trains, ships, barges, buses, and airplanes. Each type of transportation has a different purpose, uses different amounts of energy, and may rely on different fuels or technologies.

You also explored how internal combustion engines work and why gasoline and diesel engines are different. Gasoline engines use spark ignition, while diesel engines use compression ignition. These engines have powered transportation for many years, but they still depend heavily on petroleum-based fuels.

At the same time, transportation is changing. Hybrid vehicles, plug-in hybrid vehicles, and electric vehicles are becoming more common. Other fuels, such as biodiesel, ethanol, natural gas, hydrogen, and propane, also provide alternatives in some situations. These technologies may help reduce petroleum use, lower some emissions, and improve efficiency, but each one also has limits related to cost, infrastructure, range, or fuel availability.

Another important idea from this lesson is that efficiency matters. Vehicle efficiency affects how much fuel is used, how much it costs to drive, and how much energy is required to move people or goods. Efficiency can be measured in different ways, such as miles per gallon, cost per mile, or energy use per passenger mile. The most efficient choice is not always just a different car—it may also involve moving more people at once through rail or public transit.

You also learned that the future of transportation will probably involve more than one solution. Passenger cars may continue shifting toward electricity, but trucks, aircraft, ships, and mass transit may use different technologies depending on their energy needs. Electric vehicles, sustainable aviation fuels, improved transit systems, and cleaner freight transportation are all likely to play a role.

Key Takeaways

Transportation is a major part of U.S. energy use.
Most transportation in the United States still depends on petroleum fuels such as gasoline, diesel, and jet fuel.
Transportation includes cars, trucks, trains, ships, barges, buses, and aircraft.
Gasoline and diesel engines work differently and use different types of fuel.
Hybrid, plug-in hybrid, and electric vehicles are important parts of the changing transportation system.
Alternative fuels can reduce petroleum use in some vehicles and fleets.
Efficiency and cost to drive are important when comparing transportation choices.
The future of transportation will likely include a mix of technologies, not just one single solution.

Final Thought

Transportation is not just about getting from one place to another. It is also about energy, technology, infrastructure, cost, and environmental impact. As transportation systems continue to change, understanding these connections will help you make better decisions as a driver, consumer, voter, and energy citizen.

Reflection Questions

Why is transportation such an important topic in an energy class?
What is the difference between gasoline, diesel, hybrid, and electric vehicles?
Why might the future of transportation require more than one technology?
How does vehicle efficiency affect the cost of driving?
Which type of transportation do you think will change the most in the next 20 years, and why?

mxw142 Wed, 07/15/2026 - 23:49

Lesson 11: Transportation

11.1 Lesson 11 Introduction

Welcome to Lesson 11

Lesson Objectives:

11.2 Energy Use for Transportation

Resources to Go Deeper

11.3 Types of Transportation

Light-Duty Vehicles

Medium- and Heavy-Duty Transportation

Rail Transportation

Water Transportation

Air Transportation

For more information

11.4 Internal Combustion Engines

This is what happens when you hit the gas (6:04)

11.4.1 Gasoline (Spark Ignition) Engines

What is an Octane number?

11.4.2 Diesel (Compression Ignition) Engines

What is a Centane number?

What's the Difference Between Petrol & Diesel? | Bang Goes The Theory | Earth Science (3:51)

11.5 Hybrid and Electric Vehicles

Hybrid Vehicles

Plug-In- Hybrid Vehicles

Electric Vehicles

Try it yourself:

Where is the nearest EV fueling station to you?

Try It Yourself

11.6 Alternative Fueled Vehicles

Biodiesel

Ethanol

Natural Gas

Propane

Hydrogen

11.7 Vehicle Efficiency Standards

What are the CAFE Standards?

Important Notes

CAFE Timeline

Interactive Data

How Corporate Average Fuel Economy Works

Electric Vehicles and CAFE Standards

11.8 Efficiency Per Passenger Mile

Key Takeaways

11.9 Cost to Drive

Tips for Improving Fuel Efficiency

Example one: How much does it cost to drive a gas car?

Example two:How much does it cost to drive an electric car?

Example 3: Real life comparison EV versus Gasoline

Cost to drive a gas car for a month ($/month)

Cost to drive an electric car for a month ($/month)

11.10 Future of Transportation

Electric Vehicles

Electrifying Trucks

Sustainable Aviation Fuel

Mass Transit

See the First-of-its-Kind 'Air Taxi' That's Set to Revolutionize Travel (4:08)

11.11 Conclusion

Key Takeaways

Final Thought

Reflection Questions

Example one:
How much does it cost to drive a gas car?

Example two:
How much does it cost to drive an electric car?

Example 3:
Real life comparison EV versus Gasoline