This article is intended as a very basic overview of how the combustion process works in a typical 4-cycle engine, and why timing plays such an important roll. Although the entire process can get fairly technical and involves many systems, here we will focus only on what is happening inside the cylinders and the related components.
4-stroke engines (also called 4-cycle engines) are so named because each piston makes 4 strokes (or movements) during one power cycle. Note that this is actually two revolutions of the crankshaft. The highly simplified picture below shows a typical 4-stroke cylinder, with the important components labelled:
The following drawings and paragraphs describe the basics of what is happening during each stroke.
a). Intake valve opens
b). Piston moves downward and ECU fires the injector to inject fuel into the air stream
c). Air/fuel mixture is drawn (or forced) into the cylinder. When the piston is as far down as it can go, it is said to be at
BDC (Bottom Dead Center)
a). Intake valve closes
b). Piston moves upward, compressing the air/fuel mixture in the sealed cylinder. When the piston is as high up as it can go, it is said to be at
TDC (Top Dead Center)
a). The spark plug is fired
b). The air/fuel ignites and burns rapidly, forcing the piston downward as the hot gas expands
c). Power from the piston is transferred to the rotating crankshaft
a). Exhaust valve opens
b). Piston moves upward, forcing combustion leftovers out of the cylinder
c). Exhaust valve closes, and intake valve opens as the piston starts moving back down to repeat the cycle
Each cylinder goes through this process separately, but they overlap. For example, as the #1 cylinder's piston is moving downward during the power stroke, #4's piston is moving downward on the intake stroke, #2's piston is moving upward on the exhaust stroke, and #3 is moving upward on the compression stroke. This continuous cycle is what sends the "pulses" of energy to the crankshaft that get converted to continuous torque.
Note: Don't confuse the plug wire numbering on the coil packs (driver's side of intake manifold) with the firing order of the cylinders. On the 1G 4G63, the coil outputs are 4-1-2-3, but the actual sequence that the plugs fire in is 1-3-4-2. While looking at the front of the car, the cylinders are numbered from 1 to 4, with #1 on the driver's side.
Although the concept is simple, in reality there is much more to it (of course!). For one thing, the exact times that the intake and exhaust valves are either open or closed don't line up exactly with TDC or BDC. This can be both good and bad, depending on how it's adjusted and what the goals of the engine are. Second, the spark plug doesn't fire when the piston is exactly at TDC; it actually fires a little earlier or later, to allow the flame in the cylinder to burn more efficiently.
This is a good place for one of those "Ahhh...now I get it!" moments :-).
There are two completely different types of "timing" going on here. The ECU (Engine Control Unit) determines when to fire the spark plugs in order to get the most efficient fuel burn. Since the ECU knows where the crankshaft and therefore the pistons are at any given time (from the CAS), it can determine how far ahead of TDC it can send a pulse to a spark plug. This is called timing advance, and is what all the fuss in the forums is about. For example, "Set your base timing to 5* BTDC" means that you want the ECU to fire each spark plug when it's respective piston is 5 degrees before reaching top dead center. In other words, the crankshaft still has to rotate 5 more degrees for a piston that is about to fire is truly at the top of it's stroke. When you tune and "add or subtract timing", you are simply adjusting when the ECU fires the plugs relative to where each piston is in it's cylinder.
The second type of timing is mechanical timing, which links the crankshaft (and therefore the pistons) with the valves. This is what your timing belt, cam gears, and cams do. Their sole purpose is to make sure that the intake and exhaust valves open and close when each piston is in the right spot in the cylinder. When you hear people mention adjustable cam gears, aftermarket cams, or "degreeing the cams", this is what they are talking about. "Bigger" cams have differently shaped lobes that affect when the valves open and how long they stay open, and this has a big impact on how air flows through the motor. Adjustable cam gears allow some fine tuning of the valves by changing the relationship of the timing belt to the cams; in other words, they move the opening/closing range of the valves forward or backwards in time, relative to the pistons.
Our 4G63 and similar motors are known as 'Interference" motors. This is because the valves share space in the cylinders with the pistons. If the mechanical timing described above is out of sync for some reason, such as a broken or incorrectly installed timing belt, then unfortunately the valves and pistons will try to occupy this shared space at the same time... which results in bent valves and trashed pistons. This is why it's so important to make sure that the timing belt is installed correctly, and that it stays in top condition.
A full discussion of engine compression and combustion can get pretty deep, and I purposely kept away from the more complicated details in this article. The point here was to enlighten beginners on how the combustion process works.
There are tons of good links on the internet if you want to learn more; for example:
Compression ratio - Wikipedia, the free encyclopedia.
