Suck Squish Bang Blow part 2- The Four Stroke Cycle
By Mike Kojima
Now that you know what all of the basic parts of an engine are and what they do, it’s time to understand how they work together in a system. Almost all cars except for the rotary powered RX-8 and nearly all motorcycles use 4 stroke engines and what is called by engineers, the Otto Cycle to turn the chemical energy of gasoline into power. Manipulation of the 4 stroke cycle is essential for obtaining more power from a motor so it is important to know what the different parts of the cycle are and how they affect your power output. Let’s jump into and explain the 4-Stroke Cycle. Sorry if this is all a bit basic for you advanced people but in order to get into more engine theory later, we have to start with the most basic of basics.
|This is Otto, a German dude who was one of the pioneers of the four stroke cycle around the turn of the century. Otto was probably more of the manager and ideas guy of the group consisting of Otto, Daimler and Maybach who invented the four stroke. I don't think that Otto was the engineering brains behind the concept but the four stroke cycle bears his name.|
The Intake Stroke
|The intake or suck stroke.|
This is where the suck part of our title comes from because that’s what the engine is doing. Let’s start with the piston at TDC or Top Dead Center. The intake valve is starting to open by the camshaft as the exhaust valve is closing. As the crankshaft turns, the connecting rod starts to pull the piston down away from TDC. The turning crank is linked to the camshaft by a chain or belt so as the crank turns, the intake valve is opened more and more until it is fully open and the exhaust valve is fully closed. The downward traveling piston creates suction in the cylinder so air and injected gasoline from the intake manifold are drawn into the cylinder by this suction. This continues until the piston is all the way to the bottom of the cranks stroke or BDC also known as Bottom Dead Center. Because of the shape of the cam, the intake valve is almost totally closed by the time the piston is at BDC. At the end of the intake stroke we are left with a cylinder full of fresh fuel air mixture with a closed exhaust valve and a rapidly closing intake valve.
The Compression Stroke
|The compression or squish stroke.|
This is where we got the squish in our title. Now the piston starts its trip upward being pushed up by the crankshaft and the connecting rod. Now the intake and exhaust valves are fully closed and as the piston is forced upward, the fuel air mix is compressed. This compression forces the fuel and air molecules closer and closer together until they become a highly reactive explosive mixture, the closer the proximity of the molecules; the easier it is to initiate an explosion. When the piston nears TDC the ignition system fires the sparkplug which triggers an explosion in the cylinder.
The Power Stroke
|The Power or Bang stroke.|
You guessed it, this is the big bang. By the time the piston is at TDC the explosion of fuel and air in the tightly contained cylinder is well under way. The heat and pressure of the explosion of gasoline and air rise rapidly and the piston is pushed strongly back down the cylinder with great force. This is the driving power that spins your wheels and propels you down the track. As the piston is pushed down the bore the cylinder pressure starts to decrease as the volume of the cylinder increases. As the piston nears the bottom of the bore the camshaft starts to open the exhaust valve.