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FUNDAMENTALS OF THE DIESEL CYCLE
Compression - h1018v1_43

Mechanical Science Volume 1 of 2
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DOE-HDBK-1018/1-93 FUNDAMENTALS OF THE DIESEL CYCLE Diesel Engine Fundamentals To convert the chemical energy of the fuel into useful mechanical energy all internal combustion engines  must  go  through  four  events:  intake,  compression,  power,  and  exhaust.    How  these events are timed and how they occur differentiates the various types of engines. All diesel engines fall into one of two categories, two-stroke or four-stroke cycle engines.   The word cycle refers to any operation or series of events that repeats itself.   In the case of a four- stroke cycle engine, the engine requires four strokes of the piston (intake, compression, power, and exhaust) to complete one full cycle.   Therefore, it requires two rotations of the crankshaft, or 720 of crankshaft rotation (360 x 2) to complete one cycle.   In a two-stroke cycle engine the events (intake, compression, power, and exhaust) occur in only one rotation of the crankshaft, or 360. Timing In the following discussion of the diesel cycle it is important to keep in mind the time frame in which each of the actions is required to occur.  Time is required to move exhaust gas out of the cylinder and fresh air in to the cylinders, to compress the air, to inject fuel, and  to  burn  the  fuel.    If  a  four-stroke  diesel  engine  is  running  at  a  constant  2100 revolutions  per  minute  (rpm),  the  crankshaft  would  be  rotating  at  35  revolutions,  or 12,600 degrees, per second.   One stroke is completed in about 0.01429 seconds. The  Four-Stoke  Cycle In a four-stroke engine the camshaft is geared so that it rotates at half the speed of the crankshaft Figure 16   Scavenging and Intake (1:2).   This means that the crankshaft must make two complete revolutions before the camshaft will  complete  one  revolution.    The  following  section  will  describe  a  four-stroke,  normally aspirated, diesel  engine having  both intake  and exhaust  valves with a 3.5-inch bore and 4-inch stroke with a 16:1 compression ratio, as it passes through one complete cycle.  We will start on the intake stroke.   All the timing marks  given are  generic and will vary from engine to engine.  Refer to Figures 10, 16, and 17 during the following discussion. Intake As the piston moves upward and approaches 28 before top  dead  center  (BTDC),  as  measured  by  crankshaft rotation, the camshaft lobe starts to lift the cam follower. This causes the pushrod to move upward and pivots the rocker arm on the rocker arm shaft.  As the valve lash is taken   up,   the   rocker   arm   pushes   the   intake   valve downward  and  the  valve  starts  to  open.     The  intake stroke  now  starts  while  the  exhaust  valve  is  still  open. The flow of the exhaust gasses will have created a low ME-01 Rev. 0 Page 22







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