Well, mechanically, It’s a big air pump. A 4.0L engine, for example, is designed to move 4 liters of air during a full cycle – that’s under ideal conditions, by the way. If you were to put a balloon over the exhaust and catch the air, you’d get 4 liters of it each time the crankshaft spins 720 degrees.
So we know about the 720 degrees of crankshaft rotation, but what else is going on?
Two turns of the crankshaft will carry every piston through four strokes, but we’re going to focus on what happens in just one cylinder, which is the smooth bore in which the piston travels.
A piston generally has three grooves around its crown, and specially hardened rings fit in those grooves. The piston is attached to its connecting rod by a polished and hardened pin, called a ‘wrist pin.’
As the crankshaft is turned, either by the starter or by the power strokes of other pistons, the piston we’re focusing on is drawn down into the cylinder to pull the air and fuel in (we call that concoction the ‘mix’), then the piston goes back up to squeeze the air, gets a hit from the explosion of air and fuel that happens when the spark plug fires, then drives the crankshaft around as it goes back down.
The piston is connected with a special rod to the crankshaft, which actually drives the piston during three of its four strokes. The crankshaft, camshaft, and everything else in the engine is oiled, either with pressurized oil or splash oil.
That’s a whole different system. Each cylinder has a companion cylinder – we’ll talk more about that in a minute.
There are usually two compression rings that ride in the top grooves machined into the piston (those rings are missing in the picture, but their grooves are visible) and one pair of oil scraper rings that ride in the bottom groove along with a separator.
The oil control rings are a different material than the 4 compression rings and are designed to squeegee almost all the oil from the cylinder walls as the piston moves down. The green area you see outside the cylinder walls is where the engine coolant flows, because those cylinder walls get really hot.
So what controls the flow of the air? Well, the first place the air goes after it leaves the air filter is a throttle plate you control with your foot when you apply the accelerator.
Air movement through an engine is controlled by parts called ‘valves,’ that are shaped sort of like big nails (see photo), but they’re mounted with the head down and they move to open ports that allow air to enter and leave their particular cylinder. The one shown is an intake valve – the piston in the photo is at top dead center (TDC) and can be seen right below the valve. The white area is where air comes in.
Valves are held against their seats by strong springs attached to their stems, and they’re opened by egg-shaped lobes called ‘cams’ on a shaft that spins at half the speed of the crankshaft. The shape of those egg-shaped lobes determines how much the valves open and how long they stay open, which has a great impact on the way the engine idles as well as on how much horsepower and torque it produces.
The camshaft’s primary job is to opens the valves, and it is driven by the crankshaft via gears, a chain, or a belt.
During the intake stroke when the piston is traveling down and air is entering the cylinder, the appropriately named intake valve is open. It closes when the piston has traveled as low as it will go.
The piston rises into the cylinder (driven by its connecting rod, which is clamped around a smooth pin on the crankshaft), squeezing the air and fuel mix, and right before it reaches the top (usually about 10 degrees of crankshaft rotation Before Top Dead Center, or BTDC), the ignition system fires the spark plug, which ignites the ‘mix’ of air and atomized fuel. That starts a controlled explosion that slaps the piston pretty hard right as it’s starting back down on its power stroke. This all happens really fast.
5 If this process is working right, all the fire in the cylinder has gone out by the time the crankshaft has moved 24 degrees past top dead center on that cylinder’s travel. The cam sensor tells the Powertrain Control Module (PCM) when this 24 degree mark has passed so the PCM will know when to safely deliver the next fuel spray.
Now, about those companion cylinders: Engines with even numbers of cylinders will have two of their pistons moving together – For instance, on a four cylinder engine, pistons 1 and 4 are at TDC simultaneously while cylinders 2 and 3 are at BDC (Bottom Dead Center). A hundred and eighty degrees of crankshaft rotation later, 2 and 3 are at TDC and 1 and 4 are at the bottom of their travel.
Why is this significant? Well, you have to understand that to work on an engine. The harmonic balancer is a weighted wheel on the front of an engine (the pulley that drives the belts is generally either bolted to it or is a part of it), and that weighted wheel is marked so you can find Top Dead Center on cylinder number 1.
The problem is that if you aren’t aware of where the camshaft is (remember, it turns only one round for every two rounds of the crankshaft) is, you might have number 1 cylinder on TDC exhaust instead of compression. When installing the ignition distributor on engines that have one, you have to know whether you’re on #1 TDC compression or TDC exhaust. That’s not too hard to figure out if you pull the number one spark plug, put a cork in the spark plug hole, and slowly turn the engine in the direction that it runs until it pops the cork out of the hole. That’ll happen while the #1 cylinder is approaching TDC compression, so the next time the zero mark on the balancer lines up after the cork pops out of the number one spark plug hole, you’ve found the spot you’re looking for.