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Two-stroke diesel engine
COMPRESSION  IGNITION  SYSTEM

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Figure 7-6.—A two-stroke diesel engine cylinder with the piston at the bottom of the power stroke. the  intake  and  exhaust  strokes  of  the  four- stroke  cycle. You  might  expect  a  two-stroke  engine  to develop  twice  as  much  power  as  a  four-stroke engine of the same size and to operate at the same speed. However, this is not true. With two-stroke diesel engines, some of the power is used to drive a blower (fig. 7-6) 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 ports  are  open  (as  compared  to  the  period  the intake  valve  in  a  four-stroke  cycle  is  open),  a smaller  amount  of  air  is  admitted.  Therefore,  with less  air  being  mixed  with  the  fuel,  less  power-per- power   stroke   is   produced.   Nevertheless,   two- stroke diesel engines give excellent service. VALVE   MECHANISM The  valve  mechanism  of  a  two-stroke  diesel cylinder  head  is  shown  in  figure  7-7.  This  cylinder head has two exhaust valves that are opened at the same time by the action of a single cam. They make a tight fit in the exhaust openings (ports) in  the  cylinder  head  and  are  held  in  the  closed position by the compression of the valve springs. The  rocker  arm  and  bridge  transmit  the reciprocating  motions  of  the  cam  roller  to  the valves. Figure 7-7.—A two-stroke diesel cylinder head, showing the valve-operating   mechanism. In figure 7-7, view A, the cam roller is riding on the base circle of the cam, and the valves are closed. As the camshaft rotates, the cam lobe or nose  rides  under  the  roller  and  raises  it  to  the position shown in view B. When the roller is lifted, the arm rotates around the rocker shaft; and the opposite  end  of  the  arm  is  lowered.  This  action pushes  the  bridge  and  valves  down  against  the pressure of the valve springs and opens the valve passages. On  some  types  of  engines,  the  camshaft  is located near the crankshaft. In these designs, the action  of  the  cam  roller  is  transmitted  to  the rocker  arm  by  a  pushrod. The camshaft must be timed with the crank- shaft so that the lobes will open the valves in each cylinder  at  the  correct  instant  in  the  operating cycle.  In  the  two-stroke  engine,  the  camshaft rotates at the same speed as the crankshaft. The four-stroke engine has an intake valve and an exhaust valve in every cylinder. Each valve is 7-7






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