4 Stroke and 2 Stoke Engine Theory

A four stroke and two stroke engines are very different but both have unique and ideal purposes. Let’s start with the similarities, they both use fuel and a piston, to produce rotational energy and have about the same parts. Next let’s take a look at some differences; the four cycle engine has four cycles when it operates. The two stroke engine obviously has two cycles. Now lets dive into each one of the engines and explore what their uses are and what some of their parts are too.

The four stroke engine is called that because, obviously it has four stages. These stages are intake, compression, power stroke and exhaust stroke. Let’s take a closer look at each one. The intake stroke is when fuel (usually gasoline or diesel mixed with air) is put in the cylinder. The cylinder is where the explosion takes place. Intake happens when the piston moves down creating suction, then a valve lifts, allowing fuel in the cylinder. The compression cycle happens next, this is the step where the piston goes back up the cylinder, causing immense pressure to build. Next is the power stroke, this is the cycle that powers the machine. (Top dead center or TDC is when the piston is at the very top of the cylinder) At TDC the spark plug ignites the compressed fuel/air mixture and the explosion drives the piston violently down. Finally the exhaust stroke happens, after the piston reaches BDC or bottom dead center of the cylinder, a different valve opens and when the piston goes back up the open valve allows the used fuel to escape. Then the process happens all over again.

I know you’re thinking, how can all these things time out perfectly when the piston is rotating upwards of 2-9 thousand R.P.M.s, or how can it not blow up, or even how does the piston keep rotating? Well it all very simple first of all, the crankshaft is the axle where the piston rotates around. Off of the crankshaft, there are gears and pulleys and belts linked to everything. So basically you can’t move anything without moving a different part, which intern moves another part. Now, why doesn’t the engine simply blow up? Well, every bolt on the engine has a specific tightness so that the vibration will not shake it loose. Also, the piston, valves, crankshaft and all the bearings are all lubricated with oil. Oil keeps everything somewhat cool and very lubricated. What keeps the piston in motion? The flywheel does this. The flywheel is essentially a weight that rotates on the crankshaft. This works on the principle of Newton’s 3rd Law of Motion. And finally, what keeps all these things in place? Simple, it is called an engine block. The block holds only the main parts.

Now, let’s look at the two stroke engine. The main difference between the two stroke and four stroke engine is that the two stroke engine has only two cycles; as compared to the four stroke, where it has four. The two cycles in the two stroke engine are, intake/compression and power/exhaust stroke. Okay, during the intake/compression stroke, the fuel/air mixture is injected into the crankcase. When the piston moves down, it opens a port in which the fuel can enter the cylinder. As the piston moves up, it closes the port and compresses the fuel. The final cycle is power/exhaust cycle. When the piston reaches TDC, the spark plug ignites the fuel. When the piston is thrown down, it opens a different port, causing the exhaust fumes to be propelled out of the cylinder. At the same time but the other port, the fuel is injected once again. These two ports are located 180 degrees away; and because they are both open at the same time, some of the fuel does escape through the exhaust port. Although this happens, it is a minuet amount. Because the two stroke engine only has two cycles, it runs a lot faster than the four stroke engine.



So all in all, the four stoke engine is the more powerful engine of the two; which is why it is in trucks, cars and heavy equipment. The four stroke engine is meant to get things done. Different than this but not contrary, the two stroke engine is meant to get things done quickly. A dirt bike is a great example of this. All of this finally explains that the four stroke and two stroke engines are two different beasts with the same job, to get things done right.


Four stroke engine parts with description:




This is the piston. The top of the piston is where the explosion hits it. Those top two rings are called compression rings; they provide an air tight seal. The last ring is called the oil ring; it holds oil to keep the cylinder and itself lubricated, and somewhat cool. That rod that you can see is called the connecting rod. It connects the piston to the crankshaft.






This is the crankshaft. This is what the piston rotates around. The shinny part in between the two big discs is the actual part where the piston goes. The two big discs are the “flywheel” they keep the engine is constant rotation. Usually the flywheel is separate from the crankshaft.




This is the cylinder were the explosion happens. At the bottom of the picture, you can see the piston head.


These are the valves. These allow fuel to enter, and exhaust fumes to exit the cylinder. Usually there is one for each, but in high performance engines, there are two valves for each.




This little guy is called the spark plug. It creates a small spark which ignites the fuel, driving the piston down.


This is oil. It helps keep your engine cool and lubricated so that it won’t blow up.







This is the engine block; it holds all the main components in place. This particular block, is a V8. This means that it has eight pistons configured in a “V” formation.


This is an animation of the four stroke engine. The “induction” cycle show in the animation can also be called intake.















Now For two stroke engine parts:


This is a two stroke engine piston. Notice that it’s longer than the four stroke piston, and it also has a hole in it. This allows fuel/exhaust fumes in or out of the cylinder.



This is the cylinder of a two stroke engine. Notice how there are holes all around inside of it. This means that this is a high performance two stroke cylinder. All the holes allow for better fuel/exhaust flow.




This is an animation of a two cycle engine. The glow plug listed, is a version of a sparkplug. It doesn’t give of such a hot spark as a normal spark plug.





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