8. The =BELT WHEELS= or =PULLEYS= are the means of transmitting the power of the engine to the work to be performed. The engine is generally connected to the driven machinery by a belt connecting the engine pulley with the pulley of the driven machine. 9. The =FLY WHEELS= by reason of their mass and their momentum, store up a portion of the energy expended during the working stroke, and return it to the engine in order to carry it through the idle strokes of compression, admission and expulsion. In some engines the fly wheels serve in double the capacity as pulleys. 10. The =BASE= or =FRAME= of the engine acts as a foundation for the various working parts, holding them in their proper positions. (42) Application of the Four Stroke Principle. While the five events of every commercial four stroke cycle engine are accomplished in exactly the same order, or routine as explained in paragraph (8), Chapter 3, the actual design and method of applying the cycle varies greatly in different makes of engines. This great difference in the details of construction often makes it difficult for the novice to identify the cycle of operations in that particular engine. The different forms of valve gears that are used to perform the same functions in the cycle are good examples of the variation in design, some makers using the poppet or disc type, some the sliding sleeve, and others the rotary type. [Illustration: Fig. 16. Ball Bearing Crank Shaft, Pistons and Connecting Rods of the “Maximotor,” in Their Relative Positions. ] Multiple cylinder engines vary in the cylinder grouping or arrangement, the arrangement and number of cylinders depending on the service for which the engine is intended, the amount of vibration permissible, or the weight. The question of speed also introduces modifications in the design, but no matter what valve arrangement is adopted or what grouping of cylinders is used, a four stroke cycle engine performs the five events of suction, compression, ignition, expansion and exhaust in four strokes, in each and every cylinder. With the exception of fuel injection (which in reality corresponds to the ignition event) in the four stroke Diesel engine, the indicator cards of all four stroke cycle engines passes the same characteristics as the diagram shown in Fig. 10.