11.4 Internal Combustion Engines
11.4 Internal Combustion EnginesMuch 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.
11.4.1 Gasoline (Spark Ignition) Engines
11.4.1 Gasoline (Spark Ignition) EnginesGasoline 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.
11.4.2 Diesel (Compression Ignition) Engines
11.4.2 Diesel (Compression Ignition) EnginesDiesel 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)
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.