The Basics of Internal Combustion
Open the hood of your car and look in. If it was built in the past 15 years, you'll be confronted with a sea of black plastic. These covers hide the engine, making its inner workings invisible. So what exactly goes on when the ignition key is turned?
The ignition is a switch that lets electric current into the starter motor. This has a small gear mounted on one end that engages with gear teeth on the outside of the flywheel, so the starter gets the flywheel turning.
The flywheel is bolted to the end of the crankshaft, which runs the length of the engine. To understand the function of the crankshaft, picture a door handle. Pushing down at the end makes the handle rotate. If there wasn't a stop, it would be possible to push the handle around in a full circle. Well, a crankshaft is like a series of door handles joined together in pairs. This produces a series of levers, or "cranks," that can be pushed down to rotate the shaft.
So what pushes down on the crank?
That's the job of the pistons. The pistons slide up and down in cylinders and are joined to the cranks by connecting rods. Above the piston is a space called the combustion chamber. This is where a mixture of gas and air is burnt. "Burnt" implies that it happens gradually, but the actual combustion process is more like an explosion, and as the mixture explodes it expands rapidly. This creates a force that pushes the piston down and turns the crankshaft.
This draws the piston down, but when the crank gets to the six o'clock position, it changes direction and starts to rise up in the cylinder again. However, the combustion chamber is full of burnt gases resulting from the explosion. These need to be pushed out to make room for the next charge of gas and air.
At this point a valve opens at the top of the combustion chamber. As the piston rises, it pushes the burnt gases out of the chamber, and they flow into the exhaust. Then, as the piston reaches the top of its stroke, the exhaust valve closes, and an inlet valve opens to let in the next charge.
Now the piston begins to descend, and this creates a suction that pulls in the mixture of gas and air. At the bottom of the stroke, the inlet valve closes, and as the piston rises up once more it starts to squeeze the mixture into an ever smaller volume. This compression continues until, when the piston is at the top of its stroke, a spark is created. This ignites the mixture, creating the explosion that pushes the piston back down, and the cycle is complete.
Astute readers will have noticed that the piston actually makes two cycles, (720 degrees of rotation,) for each push down on the crank. This sequence is often described as "suck, squeeze, bang, blow," and is the four-stroke (two up and two down) combustion cycle. If there was only a single cylinder in the engine, the "bang" or power stroke would need enough force to spin the crank for two whole revolutions. This would make the rotation speed rather uneven, so most engines use multiple cylinders and stagger the power strokes to create a smoother motion.
In a four cylinder engine this is achieved by having one power stroke for every 180 degrees of rotation. A V8 engine will have one power stroke for every 90 degrees of rotation, which is why engines with more cylinders are inherently smoother.
So, having covered the basics of the combustion cycle, there are two other aspects of a car engine that need some explanation. These concern the opening and closing of the two valves and the timing of the ignition spark. These have to be synchronized with the motion of the pistons, otherwise the engine will not run.
The timing of valve opening and closing is set by a camshaft. This looks like a smaller version of the crankshaft, but in this case the crank action is achieved by cams, which push the valves open. Each valve only needs to open once every two revolutions of the crankshaft, so the camshaft runs at half engine speed. In many engines a chain is used to connect the two, with the speed reduction being achieved by different size sprockets. (You can see the same thing on a bicycle.) Other engines use a belt and pulley to drive the camshaft.
The ignition spark is created when high-voltage electricity jumps the gap in a spark plug. (This is exactly the same as lightning but on a smaller scale.) The high voltage is created in an ignition coil, which converts the low voltage used by the car into the higher energy needed for the spark. In older engines this high voltage is sent out to the spark plugs by a distributor driven by the camshaft. However, modern engines have a computer that decides exactly when each spark plug should fire.
Car engines can seem complicated and mysterious, but the basic principles are quite straightforward. At their heart it's just "suck, squeeze, bang, blow," combined with timing of valves and spark to ensure gas and air are drawn in, ignited, and blown out at just the right moments.
