Sequence of events (1)  Scavenging  (intake) (2)  Compression (3)  Injection/ignition and (4)  Combustion (5)  Expansion  (power) (6)  Exhaust Description of Events A  fresh  change  of  air  is forced  into  the  cylinder intake ports by the blower. Exhaust   gases   escape through  the  open  exhaust valves. As   the   piston   moves upward, the intake ports are covered and the exhaust valves  close.  The  air  is compressed  in  the  cylinder; the   piston   continues   to move towards TDC. When the piston nears the top  of  its  stroke,  fuel  is injected into the cylinder. The fuel ignites due to the heat of compression. The  rapid  expansion  of burning gases forces the piston  down. As the piston nears BDC, the exhaust valves open, starting   the   release   of exhaust. As shown earlier, a power stroke is produced every crankshaft  revolution  within  the  two-stroke-cycle engine,  whereas  the  four-stroke-cycle  engine  requires two revolutions for one power stroke. It might appear then  that  the  two-stroke-cycle  engine  can  produce  twice as much power as the four-stroke-cycle engine of the same size, operating at the same speed; however, this power increase is limited to approximately 70 to 80 percent because some of the power is used to drive a blower that forces the air charge into the cylinder under pressure.  Also,  the  burned  gases  are  not  completely cleared  from  the  cylinder,  reducing  combustion efficiency.  Additionally,  because  of  the  much  shorter period the intake port is open (compared to the period the intake valve in a four stroke is open), a relatively smaller amount of air is admitted. Hence, with less air, less power per stroke is produced in a two-stroke-cycle engine. You need to know the differences between a two- stroke and four-stroke engine. Study the following chart. TWO-STROKE FOUR-STROKE 1. One cycle equals one  1 .   One cycle equals two crankshaft  revolution crankshaft   revolu- a n d    t w o    p i s t o n tions  and  four  piston strokes. strokes. 2.    Requires a blower. 2.    Blower is optional. 3.    Requires  intake  and    3.    Requires  only  intake exhaust   ports   or and  exhaust  valves. intake   ports   and exhaust valves. Figure 2-8 shows a comparison of events that occur during the same length of time for both two-stroke- and four-stroke-cycle engines. Notice the shaded areas that represent the overlapping of events. Q1. For a vehicle to move, reciprocating motion must be changed to what type of motion? Q2.   On   what   three   things   must   an   internal combustion engine rely to operate ? Q3. A one-cylinder engine consists of what number of parts? Q4.   A two-stroke engine has approximately what percentage  of  power  increase  over  a  four-stroke engine? Q5. In a two-stroke diesel engine, what sequence of events  happens  during  the  intake  stroke? CLASSIFICATION OF ENGINES LEARNING  OBJECTIVE:  Recognize  the differences  in  the  types,  the  cylinder arrangements, and the valve arrangements of internal  combustion  engines. Engines  for  automotive  and  construction equipment may be classified in a number of ways:  type of fuel used, type of cooling used, or valve and cylinder arrangement.   They   all   operate   on   the   internal combustion principle, and the application of basic principles of construction to particular needs or systems of  manufacture  has  caused  certain  designs  to  be recognized as conventional. The most common method of classification is by the type of fuel used; that is, whether the engine burns gasoline or diesel fuel. ENGINE COMPARISON Mechanically and in overall appearance, gasoline and diesel engines resemble one another; however, in 2-8