TDC BDC Piston Spark plug Intake valve Exhaust valve Connecting rod Stroke Top center Bottom center Crankshaft Cylinder flange Combustion chamber Intake stroke A Compression stroke B Power stroke C Exhaust stroke D Intake open Valves closed Valves closed Exhaust open 1-26 Figure 1-36. Four-stroke cycle. Figure 1-35. Components and terminology of engine operation. Operating Cycles 1. Four stroke  2. Two stroke  There are several operating cycles in use:  The bore of a cylinder is its inside diameter. The stroke is  the distance the piston moves from one end of the cylinder to  the other, specifically from top dead center (TDC) to bottom  dead center (BDC), or vice versa.  [Figure 1-35] top or head of the cylinder opens to allow the burned gases to  escape, and the momentum of the crankshaft and the propeller  forces the piston back up in the cylinder where it is ready for  the next event in the cycle. Another valve in the cylinder head  then opens to let in a fresh charge of the fuel/air mixture. The  valve allowing for the escape of the burning exhaust gases  is called the exhaust valve, and the valve which lets in the  fresh charge of the fuel/air mixture is called the intake valve.  These valves are opened and closed mechanically at the  proper times by the valve-operating mechanism. Therefore,  the order of the five events of a four stroke cycle engine are  intake, compression, ignition, power, and exhaust. In this type of engine, four strokes are required to complete the  required series of events or operating cycle of each cylinder.  [Figure 1-36]  Two complete revolutions of the crankshaft  (720°) are required for the four strokes; thus, each cylinder in  an engine of this type fires once in every two revolutions of  the crankshaft. In the following discussion of the four-stroke  cycle engine operation, note that the timing of the ignition and  the valve events vary considerably in different engines. Many  factors influence the timing of a specific engine, and it is most  important that the engine manufacturer’s recommendations  in this respect be followed in maintenance and overhaul. The  The vast majority of certified aircraft reciprocating engines  operate on the four-stroke cycle, sometimes called the Otto  cycle after its originator, a German physicist. The four-stroke  cycle engine has many advantages for use in aircraft. One  advantage is that it lends itself readily to high performance  through supercharging. Four-Stroke Cycle 3. Rotary  4. Diesel 