within the cylinder. The relationship between volume, pressure, and temperature within a cylinder of the engine is explained in the chart below and shown in figure 2-2. Note the changes within the cylinder while the temperature outside remains a constant 70°F. View Description A and B T h e    p i s t o n    m o v e s    u p w a r d , compressing the air in the cylinder. B and C As   volume   decreases,  pressure increases,  and  temperature  rises. These changing conditions continue, as the piston moves upward. D As the piston nears TDC, volume is still  decreasing.  Because  of compression   within   the   cylinder, both pressure and temperature of the air are now greater than at the beginning. This   up-and-down   motion   is   known   as RECIPROCATING  MOTION.  This  motion (straight-line  motion)  must  be changed into ROTARY Figure 2-2.—Volume, pressure, and temperature relationships. 2-2 MOTION (turning motion) to turn the wheels of a vehicle. A crankshaft and a connecting rod change their reciprocating motion to rotary motion. All internal combustion engines, whether gasoline or  diesel,  are  basically  the  same. We  can  best demonstrate  this  by  saying  they  all  rely  on  three things—FUEL, AIR, and IGNITION. FUEL contains potential energy for operating the engine;  AIR  contains  the  oxygen  necessary  for combustion;  and  IGNITION  starts combustion. Each one is fundamental, and an engine cannot operate without  them.  Any  discussion  of  engines  must  be  based on these three factors and the steps and mechanisms involved  in  delivering  them  to  the  combustion  chamber at the proper time. DEVELOPMENT  OF  POWER The power of an internal combustion engine comes from burning a mixture of fuel and air in a small, enclosed space. When this mixture bums, it expands greatly, and the push or pressure created is used to move the piston, thereby rotating the crankshaft. This motion is eventually sent to the wheels that move the vehicle. Since similar action occurs in each cylinder of an engine, let’s use one cylinder to describe the steps in the development of power.     The one-cylinder engine consists of four basic parts, as shown in figure 2-3. First, we must have a CYLINDER that is closed at one end; this cylinder is similar to a tall metal can that is stationary within the engine block. Inside  this  cylinder  is  the  PISTON—a  movable plug. It fits snugly into the cylinder but can still slide up and down easily. This piston movement is caused by fuel burning in the cylinder and results in production of reciprocating motion. You  have  already  learned  that  the  up-and-down movement of the piston is called reciprocating motion. This motion must be changed into rotary motion, so the wheels or tracks of a vehicle can rotate. This change is accomplished  by  a  throw  on  the  CRANKSHAFT  and the CONNECTING ROD which connects the piston and crankshaft throw. The throw is an offset section of the crankshaft that scribes a circle, as the shaft rotates. The top end of the connecting rod is connected to the piston and must therefore  go  up  and  down.  The  lower  end  of  the connecting rod is attached to the Crankshaft. The lower end of the connecting rod also, moves up and down but,