Diesel engines for land and marine work . A, the exhaust valvecam by B, the air suction valve by C, and the starting valvecam by D. In a four-cycle engine each valve must be openonce in two revolutions, and the cam shaft must necessarilyrotate at half the speed of the crank shaft. In the diagrams,therefore, one revolution of the crank shaft is representedby a semicircle or 180°, while during one stroke of the F 66 DIESEL ENGINES FOR LAND AND MARINE WORK piston each cam makes a quarter of a revolution. The verti-cal and horizontal diameters in Fig. 20 therefore representtop and bottom dead cent
Diesel engines for land and marine work . A, the exhaust valvecam by B, the air suction valve by C, and the starting valvecam by D. In a four-cycle engine each valve must be openonce in two revolutions, and the cam shaft must necessarilyrotate at half the speed of the crank shaft. In the diagrams,therefore, one revolution of the crank shaft is representedby a semicircle or 180°, while during one stroke of the F 66 DIESEL ENGINES FOR LAND AND MARINE WORK piston each cam makes a quarter of a revolution. The verti-cal and horizontal diameters in Fig. 20 therefore representtop and bottom dead centres of the crank, the vertical linesbeing taken as top dead centres and the horizontal ones asthe bottom dead centres. The arrangement of the cam isnow easily understood. The fuel cam opens the fuel valvejust previous to the piston reaching the end of its up stroke,thus giving pre-admisson to the extent of perhaps 1 percent, of the stroke or less, depending on the speed of theengine. The valve is then held open for the required period,. Fuel Vslve Exhaust Valve Cam. Admission Valve Cam. Starting Valve CD Fig. 20.—Diagram showing Arrangement of Camswith Diesel Engine. the total amount of the opening being through an angle of8 or 10 per cent. The exhaust valve cam similarly opensslightly before the end of the working stroke, remains openduring the whole of the next or exhaust stroke, and closesjust after the top dead centre is reached. Air admissioncommences through the air suction valve just before theend of the exhaust stroke, and the valve is kept open duringthe next stroke, and closes immediately after the crankpasses the bottom dead centre. The starting valve cam isarranged to open the valve just before the top dead centreis reached, and to close it some considerable time before theend of the stroke. All the cams are arranged so that the CONSTRUCTION OF THE DIESEL ENGINE 67 valve opens very slightly during the first moment of contactof the cam with the lever,
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