The (Typical) 4-Stroke Piston Engine


     So here we’ll talk about the basics of the engine.  Its essential function is to convert the energy stored in gasoline into motion to move your car.  And to do that, we burn it; or make it combust.  This is why these engines are called “internal combustion” engines; the burning of the fuel takes place inside the engine, not outside where it can damage everything around it.  Specifically, we’ll talk about the most common internal combustion engine; the 4-stroke engine.  There are other varieties, such as diesel and rotarys, but I’ll save that for another post.

     Put simply, we ignite the gasonline by creating a spark.  And to aid in the combustion, we need air since we can’t burn anything in a vacuum.  So we mix air, fuel, and a small spark to create a “boom”, and release the energy from the gasoline and harness it to move our car.  How does it happen exactly?  I’ll break it down into the 4 different strokes, so take a look:


     Ok.  This is step one.  Please disregard most of the letters for now.  This is basically how an engine looks from the “front”, or “side”, depending on the configuration.  But basically this shows you one cylinder inside the engine.  Understand how one cylinder works, and you understand how all of your cylinders work.  What I would like you to pay attention to are 4 things: the piston in the middle (which the “N” is pointing at), the two orange sticks at the top (which my top arrow is pointing at), what the letter “P” is pointing at, and what the “L” is pointing at.

     The piston is what slides up and down within the cylinder block.  The two orange things, called “valves” open and close the two openings leading into the cylinder block.  The “P” is the crankshaft, which is what the piston(s) turns to channel our power.  And the letter “L” is the spark plug, which is what creates the spark.

     This photo shows the first step in the 4 stroke cycle; the “Intake Cycle“.  This is where one of the valves (the intake valve) opens up (where my top arrow is pointing) to let the air and a drip of gasoline into the chamber.  The piston itself is moving downwards, away from the top of the cylinder and turning the crankshaft to create the horsepower/torque.  When the piston reaches its lowest point, it then moves back upwards, and into the:


     The “Compression Cycle“.  Here, the valve that was last opened to let the air and fuel in has closed up, and the piston has reached its lowest point and continues upwards (my arrow).  The blue mist shows the air/fuel being compressed towards the top of the cylinder.

     The end of this cycle completes exactly half of the process, and this is when the combustion happens.  When the piston has compressed the air/fuel enough, the spark plug (letter “L“) will ignite the air/fuel and create a small explosion.  This is what converts the energy inside the fuel into kinetic energy which we can harness.


     So after the compression cycle, and after the piston reaches the top and the spark plug creates our explosion, we get the “Combustion Cycle“.  This picture is already past the mid point where the spark plug does its thing and the piston is moving back downwards.  Why is the piston moving back down?  Because that combustion will push the piston down.  Imagine standing next to an explosion.  You will obviously be pushed backwards, away from the source.  So, here the combustion is what creates our horsepower/torque and thus turns our wheels.


     When the piston has reached its lowest point, and the combustion is over, we enter the “Exhaust Cycle“, where the piston pushes the leftovers of the combustion (the chemical reaction of the combustion changes our air/fuel into something else) out of the cylinder and out of our car.  My top arrow here points to the other valve (the exhaust valve), which then opens up to let the exhaust gases out and into our exhaust pipes.  Then when the piston finishes pushing all of the exhaust out and reaches the top of its cycle, it will come back down again while the intake valve opens up again and starts the cycle all over again.  (If you’d like to see what all those letters point to, you can click here.)

     Here it is in action (click the image if it doesn’t animate):


     And there we have it.  I didn’t explain every little bit and part and what every part does, since I’ll leave that to another section.  I’ll be talking about the rotary next, and then the diesel engine.  Stay tuned.


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March 2009
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© 2009 Rusi Li

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