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.
Text description of the Four Stroke Cycle Engine image.
The image illustrates a four-stroke cycle engine, consisting of four distinct sections. Each section depicts a stage of the engine’s operation: Intake, Compression, Power, and Exhaust.
- Intake: The leftmost section shows an intake valve where air (represented by an arrow) is entering a blue cylinder, with a green area indicating the volume of air or fuel mixture. The piston is positioned at the bottom of its stroke.
- Compression: The next section is labeled "Compression" and features the piston raised to compress the air-fuel mixture inside the blue cylinder. The valve is closed, and the internal chamber is shown in green, indicating its sealed environment.
- Power: In the third section, labeled "Power," the piston is at the top of its stroke after combustion has occurred, shown by a red area indicating high pressure. Arrows indicate the direction of force generated by the combustion process.
- Exhaust: The rightmost section shows the piston moving downwards, pushing out exhaust gases (depicted by an arrow) through the exhaust valve. The cylinder is colored orange to indicate the exhaust phase.
The sections are numbered one through four at the bottom in corresponding colors, enhancing clarity of each stage.
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)
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.