Can someone explain how a car engine works?
In: Technology
You have a contained explosion in the cylinders from sparking a mixture of petrol and oxygen that explosion or expansion drives a piston up with enough of these moving in sequence you provide power to the wheels to produce motion.
* The engine has little tubes inside.
* Inside each tube is a plunger with a pole on it.
* The tube is filled with gasoline and air.
* A spark plug inside the tube sparks and causes the gasoline to explode.
* The force of the explosion pushes the plunger up which pushes the pole.
* The pole it attached to a crank.
* When the pole is pushed it, it turns the crank.
* The crank is connected through a bunch of gears etc to the wheels.
* This makes the wheels move.
Suck, squeeze, bang, blow.
You ingest (suck) and compress (squeeze) a mixture of fuel and air in a confined space, you ignite it (bang) and allow the high-pressure gases to push against a piston, converting thermal energy to mechanical work to turn the wheels, then you expel (blow) the gases out of the cylinder so as to reset the cyclic system and allow the process to happen again.
Throw multiple piston-cylinders together into one engine block, and have that engine go through the suck-squeeze-bang-blow process thousands of times per minute per cylinder.
First, the chemistry:
Gasoline is made up of long molecule chains called “hydrocarbons.” These are typically long chains of carbon with hydrogen around the edges. One common hydrocarbon you’ve probably heard of is C8H18, which is called “Octane” because it has eight carbon atoms.
When atoms connect to each other, the bonds they form store energy. When these bonds are broken, this energy is released as heat. However, stable molecules like hydrocarbon will only break down if two conditions are met.
First, you need other atoms around that the broken parts can bind to form smaller molecules. For hydrocarbon, oxygen works really well, because both carbon and hydrogen like to bond to oxygen. Where can we get oxygen? From the air! This is why gasoline produces both water (H2O) and carbon dioxide (CO2, the greenhouse gas.) It can also produce carbon monoxide (CO) which is poisonous and deadly. Car engines are designed to minimize carbon monoxide emissions but even the most efficient and well tuned car will produce some.
Just putting hydrocarbons next to oxygen will not cause a reaction, because both are stable and happy just as they are. They need a spark to get going. Think of this like a sled at the top of a big hill sitting in a shallow depression. You have to push the sled uphill a little to get to the downhill part, but once you do, the sled will go the rest of the hill by itself. An electrical spark is a very easy and reliable way to cause gasoline to react with oxygen.
This works best if we “aerosol” the gasoline: turn it into lots of little floating droplets in oxygen-rich air. If you get the right fuel-air mix, a tiny spark is enough to ignite it and create a very rapid reaction that gives off lots of heat (exothermic) and creates lots of gasses. We call that an “explosion.”
Next the engineering:
Now that we know that gas+air+spark creates an explosion that releases energy, the question becomes, how can we design a machine which converts this energy into something useful, and does so safely and reliably?
First, we have to only use a small amount of gas and air at a time to create a small, control explosion. Then we want to contain it, which we do by putting it in a long, hollow tube. We want one end of the cylinder to be able to move: as the fuel-air mix expands inside the tube, the moving end will be pushed out in a straight line. In a car engine, the hollow tube is usually called a “cylinder” (because of its shape) and is drilled into a strong, heavy piece of metal called the “engine block.” The moving end of the cylinder is called the “piston.”
Next we need a source of gas and fuel. These are provided by the “fuel line” and “air intake” respectively. They are mixed in something called a “fuel injection system” (in modern cars) or a “carburetor” (in older cars.) A carburetor is simply a tube with some valves designed to create the exact right fuel-air mix, as well as spray the fuel into tiny droplets. A fuel injection system is the same thing but with more computers.
The last ingredient is the “spark plug” which creates the spark to create the explosion. The only other thing hooked up to the cylinder is the exhaust, which removes the water vapor and carbon dioxide after each explosion.
We start with the piston down so there’s only a small space inside. We inject the fuel and air mixed by the carburetor, then when we have enough the spark plug makes a spark and ignites it. This starts pushing the piston out. After it expands enough and we’ve captured all the energy from the explosion, the piston passes the exhaust and the gasses are sucked out. Then a spring pushes the piston back down, and the cycle starts over.
If we put a level on the piston, we can now capture some of the energy released as useful work. But there are a handful of practical problems to work out. First, the energy is released in bursts, not evenly. This is solved by having four or more cylinders in one engine (4, 6, 8 being the most common for cars.) These are staggered so that while one is pushing, the others are releasing exhaust and resetting.
Second, we’re creating straight line motion, while what we want to a rotating motion to turn the wheels. This is is accomplished with a “camshaft.” Each piston has a metal rod which pushes against an egg shaped wheel attached to a central shaft. The pushes are off center and staggered so that each piston pushes the egg shaped wheel around in the same direction when it is their turn to push. The rotating motion of the camshaft can then be transmitted to along the driveshaft and axes to the wheels.
An interesting question is where do we get the electricity for the spark plugs? The solution is very clever: we get it from the engine itself! By hooking a small electric generator to the engine itself, we can borrow a fraction of its power to charge the car battery, which can then be used to power the spark plugs (and incidentally other electrics inside the car.)
The final problem is that setting off lots of little explosions is creating a lot of waste heat. We need a “cooling system.” Most cars use a liquid cooling system where tubes pass through the engine block close to the cylinders. This liquid heats up and is pumped away to a radiator where it can dissipate safely before being pumped back into the engine block in a closed loop.
Car engines can seem pretty complicated, and they are, but at heart they’re simply tiny explosion machines with all the other stuff being fairly straightforward solutions that arise from creating dozens of tiny explosions every second. They look complicated because all these different systems and parts (fuel, air intake, exhaust, electrical, cooling system) are crammed into a tiny space, but each system is largely independent and can be understood one part at a time.
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