ALT-1 How do four Stroke Engines Work
Most of the vehicles on the road today are powered by four cycle or four stroke gasoline engines. To explain how this type of engine works you need to understand some of the components and terms used.
The engine block is a piece of cast metal into which large holes or bores are drilled, called cylinders. Most engines today have in four, six or eight cylinders, though there are some five and twelve cylinder engines out there also.
The pistons are cylindrical and made to fit into the cylinders. The diameter of the piston is slightly smaller than the cylinder to allow for the movement of the piston up and down in the cylinder. To hold the piston centered in the cylinder there are three rings fitted around the outside of it. The top two are for sealing the cylinder, preventing the expanding gasses from the combustion of the fuel/air mixture from leaking around the sides of the piston. The bottom ring scrapes excess oil off the cylinder wall, preventing it from leaking up into the cylinder.
The pistons are connected to the engines crankshaft with connecting rods. As the pistons move up and down in the cylinders the connecting rods cause the crankshaft to turn. It is this turning of the crankshaft that supplies power to move the vehicle.
There are also at least two valves, more than two in many of today's modern engines. One, the intake valve, opens to allow the fuel air mixture to enter the cylinder. The other, exhaust valve, is to allow the gasses created by combustion to escape. These valves open and close in conjunction with the cylinder moving up and down to create the four strokes.
During the intake stroke the piston begins to move downward in the cylinder creating a low pressure area in the cylinder. At the same time the intake valve opens and normal atmospheric air pressure pushes air down through the intake manifold and into the cylinder.
If the engine uses a carburetor to create the fuel/air mixture, the carburetor will be mounted on top of the intake manifold. As the air flows through carburetor it pulls gasoline out of a small tube. The fuel atomizes as it comes out of the tube creating a fuel air mist. This mist travels through the intake manifold passages and into the cylinder to be burned.
Most engines today use fuel injection to create the fuel/air mist. In this case the air comes into the intake manifold and flows through the manifold passages without the fuel. Small devices called fuel injectors spray the proper amount of fuel into the air stream either just before it enters the cylinder or the injector sprays the fuel directly into the cylinder.
The piston continues down in the cylinder until it reaches the bottom of this intake stroke. The piston then begins to move upward beginning the next stroke, called the compression stroke. The intake valve closes as the cylinder travels up, creating a virtually airtight compartment and trapping the fuel/air mixture. As the piston moves up it compresses the fuel/air mixture in the top of the cylinder.
You may have heard talk about compression ratios, numbers like eight to one or ten to one. This indicates a ratio of how much the air is compressed. To understand this think about the cylinder that holds 10 cubic inches of air when the piston is at its lowest point as the piston travels up the cylinder it begins to compress that air until it reaches its maximum compression. In the case of a ten to one compression ratio that ten cubic inches of air would be compressed to one cubic inch. Normally the higher the compression ratio the more power an engine produces. However increasing the compression ratio causes other problems such as pinging or detonation.
As it reaches the top of this stroke an electrical spark jumps two electrodes on a spark plug that extends into the cylinder. This spark ignites the fuel/air mixture causing it to burn. Many people believe this fuel/air mixture explodes, but engine designers go to great lengths to prevent the fuel from exploding. If watched in slow motion a person can actually watch the flame travel through the cylinder as it burns. As the mixture burns it expands forcing the piston back down the cylinder. When the piston reaches the bottom it has completed the third stroke.
As the piston begins to move back up the cylinder the exhaust valve opens and the rising piston forces the burnt gasses out of the cylinder and into the exhaust manifold, eventually to exit out the exhaust pipe at the back of the car. Then the piston has now traveled through all four strokes and begins the process over again, thousands of times a minute, the faster the car is going the more times per minute the engine must go through all four strokes.
The process I have described is the basics of how this type of engine works. There are many other process and variables that occur during the four strokes that are designed to pull more power with fewer harmful emissions. Engine designers and engineers are constantly making adjustments to all aspects of the modern vehicle engine in an effort to make it more efficient and burn less fuel.
