For many drivers, taking a look under the hood of a car can be intimidating. There’s usually the engine sitting in a tangle of metal, hoses, belts and wire or a large plastic cover that hides all that stuff. In either case, it can be hard to figure out what’s going on when you turn the key. Yet the principles behind a typical internal combustion engine are pretty simple. Leonardo da Vinci had them worked out in 1506.
Now, it did take more than 370 years for automotive technology to catch up with da Vinci’s ideas. By 1879, however, everything had come together in the world’s first practical self-propelled vehicle, complete with an internal combustion engine designed by the car’s inventor, Karl Benz.
We’ll skip that history lesson today, though, to focus on the basics.
What Is an Internal Combustion Engine?
Let’s start with the difference between an internal combustion engine and an external combustion engine. As you might guess, both rely on the combustion process for energy, which is a fancy way of saying that they burn fuel. This is worth pointing out because some engines don’t. Although not suitable for today’s cars, engines powered by compressed air, spring mechanisms and even solar energy also exist.
As for combustion engines, it’s where the combustion occurs that separates the types. An internal combustion engine simply means the fuel is being burned inside of the engine itself. In a typical car or truck, that happens in the cylinders to drive the pistons, as described below. But many rocket engines and turbines are considered internal combustion engines as well.
With an external combustion engine, the fuel is burned in a separate, external chamber. Here the steam engine is a prime example. Water is heated in a tank to make the steam, which then has to get into the cylinder to do its work. The key advantage: It’s a lot easier to control the combustion process in a large outside tank than in the tight spaces of a cylinder. In fact, that was one of the key hurdles in turning da Vinci’s internal combustion engine into reality. Thomas Savery had patented the simpler steam engine by 1698, nearly 200 years before Benz’s internal combustion unit.
How Does an Internal Combustion Engine Work?
At its most basic, a typical internal combustion engine is designed to change the up-and-down motions of its pistons into the circular motion of the car’s wheels. To accomplish this, each individual piston has a circular head that fits the exact circumference of the cylinder, which is literally a cylinder carved out of the engine block. The piston is attached by a connecting rod at the bottom to the engine’s crankshaft. When the cylinder heads move up and down in the cylinders, the connecting rods pull up and push down on the crankshaft to make it spin. Of course, the process is more of a “down and up” than “up and down.”
It begins when you turn on the ignition of a car. That uses electricity from the vehicle’s battery to get the pistons moving. When a given piston slides down its cylinder, valves open so that fuel and air rush into the empty space. After the piston moves down as far as it can, the connecting rod rotates around the crankshaft and the piston goes back up. Because the cylinder valves are closed at this time, the air-fuel mixture is squeezed into the increasingly smaller space between the cylinder head and the top of the cylinder. That’s also where the spark plug is. The spark ignites the compressed fuel and air, and the expanding gases push the piston back down into the cylinder. This provides the force to the connecting rod to move the crankshaft.
The crankshaft spins, the cylinder valves open and the piston pushes out the exhaust gases as it rises again in the cylinder. From there, the cycle continues.
Bonus Question: What’s the Difference between a Four-stroke Engine and a Two-stroke Engine?
You may have noticed that there were four steps included in that explanation. With that in mind, these engines also are called “four-stroke” or “four-cycle” engines. The separate piston movements are the intake stroke, compression stroke, power stroke and exhaust stroke.
Less common nowadays, but still used with some chainsaws and similar tools, are two-stroke engines. These combine the different cycles as follows: A piston upstroke draws fuel and air into the bottom of the cylinder, below the piston head. When the piston moves downward, the fuel-air mixture is compressed below the piston head and, during the same stroke, pushed around the piston into growing space at the top of the cylinder. This also forces out any leftover exhaust gases at the same time.
When the piston moves up, compression again takes place at the top of cylinder, even as more fuel and air are sucked in at the bottom. The spark plug ignites, and it drives the piston down to spin the crankshaft. But again, the downward movement has two effects. It’s also compressing a new fuel-air mix into the space below the piston.
Since these engines make power every two strokes, not every four, the result can be more bang for the buck than with a four-stroke engine of the same size. On the other hand, two-stroke engines aren’t as efficient or durable as four-stroke engines, and they are a bit trickier to fuel. If you forget to mix the gasoline and oil first, you can forget about the engine working, too.