KEY POINTS:
We all drive cars, but I wonder how many of us actually know how the engine works, even at a basic level. Considering nearly all road cars are four-strokes, I'll begin with them.
First there's the piston (which is inside a cylinder and is attached to a crankshaft via a connecting rod) at the top of its stroke with all the valves closed. The gases in the small space left between the top of the piston and the head are now fully compressed and the spark plug has fired and ignited the fuel-air mixture. The explosion of expanded gases forces the piston down towards the bottom of the cylinder and rotates the crankshaft. This is called the power stroke.
Just before the piston reaches the bottom of its stroke the exhaust valves begin to open. By now the fuel-air mixture has been burned and the cylinder pressure begins to push the burned mixture into the exhaust port where it starts to enter the exhaust system. After the piston passes bottom-dead-centre, it begins to rise.
Now the exhaust stroke begins. This forces the remainder of the burned gasses out of the cylinder to make room for new air and gas. While the piston is moving toward the top of the cylinder the exhaust valve quickly opens, goes through maximum lift and begins to close before the piston can touch it.
Just before the piston reaches the top of the cylinder, the intake valve is not fully closed. The exhaust stroke of the piston has pushed out just about all of the burned mixture, and as the piston approaches the top the intake valve begins to slowly open. Here begins a siphon or "scavenge" effect in the chamber.
The rush of gases into the exhaust port will draw in the start of the intake charge. This is how the engine flushes out all of the spent fuel. Some of the new gas actually escapes into the exhaust. Some of the unburned gas is ignited in the exhaust system and can be seen, mostly in race cars, as flames exiting the tail pipe.
Once the piston passes through top-dead-centre and starts back down, the intake charge is pulled in quickly so the exhaust valve must close at precisely the right point, after the top, in order to keep any burned gas from re-entering. This area around top-dead-centre with both valves open is referred to as "overlap". This is one of the most critical moments in the cycle and all points must be positioned correctly with the top-dead-centre of the piston to avoid any damage to the valves.
The exhaust valve is now closed and just after the piston starts its downward stroke the intake valve is opening. This is called the intake stroke, where the engine "breathes" and the cylinder fills with another charge of the fuel-air mixture.
The intake valve reaches its maximum lift at some defined point (usually about 106 degrees) after top-dead-centre. This is called the intake centreline, which refers to where the cam has been installed in the engine in relation to the crankshaft.
The piston goes all the way to the bottom and as it starts back up again the intake valve starts closing. The closing point of the intake valve will determine where the cylinder actually begins to build pressure, as it's now into the compression stroke.
When the mixture has all been taken in and the valves are now both closed, the piston begins to rise, compressing the mixture. This is where the engine builds power. Just prior to the piston reaching the top of the cylinder, the spark plug fires and we are ready to start again.
The engine cycle described is typical of all four-stroke engines. Another article will explain the roll of the camshaft and how it opens and closes the valves in time with the rising and falling of the piston.
