Internal combustion engines! We absolutely love them. We love the sounds, the smells, and the vibration. Us enthusiasts just can’t leave them alone. We love talking about them, learning about them, taking them apart, fixing them, modifying them, finding ways to make more power or make them go further on the same amount of fuel. But how do they actually work?
Welcome to Launch Control. Let’s get into engines and how they work.
Let’s start with some basic parts. Now, in this article, we’re covering conventional piston-powered, reciprocating-type fuel burners that you’ll find in nearly every car and truck on the road today. That means an engine that burns a liquid or vaporized fuel (in this case, gasoline or “petrol” for our friends across the pond) on the inside and transforms vertical and horizontal energy from the pistons into rotational energy around the crankshaft.
We’ll get into other types of engines, like the Wankel (or rotary), and electric engines (or motors) in the future. But, when it comes to our old friend, the reciprocating piston engine, there are really just a few main parts we need to know to understand the basics of what’s going on under the hood.
Blocks, heads, pistons, crankshafts, valvetrains, intake and exhaust manifolds, and connecting rods. You’ve probably heard all these terms before. Let’s explore what these parts are, where they live, and how they all work together to give us that sweet, sweet sensation of turning fuel into thrust. Let’s start at the bottom.
The block is what the entire engine is built upon. It’s the big chunk of metal that houses the pistons and crankshaft. With a straight, inline, flat, or “V” configuration, the block will have a hole for each piston (called cylinders) and enough space built in to house the crankshaft and all of the other parts that bolt the two together.
Engine manufacturers spend an enormous amount of time finding just the right blend of secret herbs and spices to make the block strong enough to take the abuse that’s bound to be thrown at it yet light enough that your car doesn’t drive like it’s got a boat anchor attached to the hood.
So, we’ve got a crankshaft attached to a piston assembly via a piston rod. And we can see how we get things spinning by moving the pistons up and down.
But how do we actually get the pistons going up and down? Well, that’s where the cylinder head comes in. The cylinder head caps off the cylinders with an airtight seal. It houses the valvetrain and spark plug (or “glow plug” for you diesel guys) and provides the air a path in and out of the engine. And, by using a couple of valves (you need at least two, one for the intake and one for the exhaust), we can control airflow in and out of that cylinder.
So, we take a couple of valves, throw a few springs on them to make them stay shut, and problem solved, right? Wrong. We’ve got to be able to control the opening and closing of those valves with crazy precision to get this all to work properly. And we do that with the use of a camshaft.
The camshaft is mechanically connected to the crankshaft, typically with a belt or chain (gears are sometimes used too, but that’s usually for very high-performance engines or large diesel engines). It’s timed to open and close the valves of each cylinder when that cylinder’s piston is at a certain point in its revolution cycle. See where I’m going with this?
We can break the combustion process into four simple stages: intake, compression, ignition, exhaust. But, hold on, if there are four jobs that need to get done and we can only get one of those done every time the piston moves up or down, doesn’t that mean the crankshaft has to do two complete revolutions for every one time the piston actually pushes itself down?
Well, yes. That’s exactly what it means. And when you’ve only got one cylinder, you can imagine how it may not be the smoothest and most efficient operation possible. That’s why your Honda Civic buzzes at idle and your pushmower makes your hands go numb.
Now, there are some things we can do to cut down on the vibration, like adding weights to the crankshaft to help keep up rotational momentum between firing events. And we get that done with the use of a flywheel out back and a harmonic balancer in the front. And we can even go so far as to add balance shafts, which are literally additional spinning weights designed to counteract the forces naturally made by the engine. But, ultimately, if you want a smoother engine, you need more combustion cycles happening more of the time. Go search “V12 coin test” if you’ve never seen it.
Side note: the flywheel also gives us a really convenient way to use a starter motor. Since the flywheel is large, we can just add teeth to the outside of it and use a small gear on the starter motor itself to give it a huge mechanical advantage. Using a small motor to get the engine spinning is much better than the way it used to be done.
So, we’ve got pistons pumping and valves opening and closing. The piston is drawn down by the crankshaft. And, with the intake valve opened and the exhaust valve closed, fresh oxygen-rich air with just the right amount of fuel is pulled in. As the piston gets to the bottom of its stroke, the intake valve closes.
Then, as the crankshaft works its way around, it pushes the piston back up, squeezing or compressing that fuel-air mixture. With the piston right at the top, we channel our inner Zeus and spray some lightning across the spark plug. The fuel-air mixture combusts and expands, which forces the piston back down, spinning the crankshaft. The exhaust valve opens, the crankshaft pushes the piston back up to expel the used-up fuel and oxygen, and the process starts all over again.
We aren’t getting into the ignition components in this article, I just have too much to say about them to squeeze it into this article. For now, just understand that on all conventional spark-ignition engines, there is a system in place that delivers energy to the spark plug and forces an electrical arc to take place that ignites the fuel-air mixture. We’ll talk all about it very soon, I promise.
With this basic understanding, you can close your eyes while going down the road and imagine how exactly that symphony plays that you’re conducting with your right foot. Seriously, do not actually do that. Keep your eyes on the road, people.
The really impressive part is that this process is happening 500 times for every 1,000 RPMs the engine is turning per cylinder, per minute. So, when you’re wide-open throttle in your mom’s Pacifica and you’re approaching that 6,000 RPM limit to prove to all the ladies how cool you are, those six pistons are screaming at 3,000 explosions per minute, or 50 complete burns per second. See, with the right perspective, even a minivan can be cool.
But, how do we keep all those metal parts rubbing together smoothly and not overheating? That’s where engine oil comes in. Engine oil is literally the lifeblood of the engine, and without it, you’re looking at an explosion.
Engine oil lives down in the sump (or pan) mounted underneath the block and gets pumped up through the inside of the crankshaft where it can lubricate the crank’s main bearings and cylinder rod bearings. It then makes its way through the oil filter up into the cylinder head where it can lube the camshafts and valvetrain.
The oil spraying out of the sides of the main and rod bearings does double duty as it coats the cylinder walls, helping create a perfect seal and keeping things sliding smoothly. What keeps the oil from seeping up into the combustion chamber and burning off? Piston rings!
Typical engines will use three rings: two compression rings and one oil control ring. Those rings expand during the ignition cycle, keeping the burning fuel and air mixture on top of the piston and scraping excess oil off the cylinder walls. They then relax on their way back up to help the engine spin as freely as possible and give that all-important oil a chance to keep some distance between the piston and the cylinder itself.
We’ll get into oils in a future article, but that’s about all I want to say about it here. And as far as which oil you should be using, well, I don’t have a thick enough flame-retardant suit for that.
Obviously, the air and fuel mixture isn’t drawn directly into the cylinder head straight from the outside world, we’ve got to filter that air and tune how it flows to make it efficient. Also, we have to have some way of slowing that air down, otherwise, the engine would just rev to the moon the second you started it, providing there was enough fuel to keep up.
And the exhaust gasses don’t just blow right out the side either. So, we use manifolds to control how the air flows in and out of the engine itself. We’ve talked a lot about air, but how do we sneak fuel into the party? Injectors!
Now, some of you might say, “What about carburetors?” I promise that we’ll get into the different types of fuel delivery systems in an upcoming article. But, to keep things simple, we’re just going to go on ahead and use the injector for this example.
For fuel to ignite and burn, it’s got to be mixed with oxygen first. Since the fuel is a liquid, oxygen atoms can only bump up against the fuel molecules on the outermost layer. To make fuel burn as efficiently as possible, we need to take a big stream of liquid fuel and stratify it into a bunch of tiny droplets. Basically, turn it into a vapor or mist. The finer the fuel droplets, the quicker and more completely we can get them to burn. And gas is expensive these days. So, the finer the better.
In a multi-port fuel-injected engine, we’ve got one fuel injector per cylinder and that injector points directly at the intake valve. Just as the valve starts to open, the injector fires that fine mist of fuel, which mixes with the oxygen in the air as it all rushes into the combustion chamber.
Different types of fuel injection systems include mechanical injection, CIS (or “Continuous Injection“), throttle body injection, and direct injection. Let us know if you want to see how those systems function down in the comments below. There we have it. Everything you need to know about how an engine works.