Diesel engines for land and marine work . h point the exhaust opens andthe products of combustion begin to pass out. Air vinder apressure of about 4 to 8 lb. per sq. inch then enters the cyHn-d(r through a separate valve or port in the cylinder, from a so-called scavenge pump, which is quiteseparate from the air compressor for the provision of fuelignition and starting air supply, the necessity of which isapparent. All the exhaust gases are thus forced out through 38 DIESEL ENGINES FOR LAND AND MARINE WORK the exhaust ports, and at the end of the stroke the cylinderis left full
Diesel engines for land and marine work . h point the exhaust opens andthe products of combustion begin to pass out. Air vinder apressure of about 4 to 8 lb. per sq. inch then enters the cyHn-d(r through a separate valve or port in the cylinder, from a so-called scavenge pump, which is quiteseparate from the air compressor for the provision of fuelignition and starting air supply, the necessity of which isapparent. All the exhaust gases are thus forced out through 38 DIESEL ENGINES FOR LAND AND MARINE WORK the exhaust ports, and at the end of the stroke the cylinderis left full of pure air with all the valves closed, ready forthe first stroke of the next cycle. The diagram for this cycle does not differ mr/icrially fromthat for the four-stroke cycle as is seen in Fig. 13, whichillustrates the two-stroke cycle, cd represents the combus-tion of fuel, de the expansion until e, when the exhaustports begin to open, the rapid fall of pressure to/, along ef,being notable, and during the process of exhaust the cylinder. Fig. 13.—Two stroke Cycla Dia;5rarQ. is filled with air from the scavenge pump. There is no longera horizontal line representing the entrance of the air atatmospheric pressure as in the four-&troke diagram, since allthe air is admitted at a pressure above atmospheric. Theadmission of air continues along fg to the predeterminedpoint g, after which compression takes place. The scavengevalve or port opens slightly after the exhaust port, so thatthe pressure has already dropped to some extent before theadmission of the scavenge air. As far as constructional details go, the two-stroke cycle ACTION AND WORKING OF THE DIESEL ENGINE 39 engine differs from the four-stroke type in the arrange-ment of valves, and the provision of a scavenge pump in theformer case. Otherwise the engines are identical. In largoengines this pump is usually placed in line with the enginecyhnders, and its piston driven through a connecting rodoff an extension of the crank s
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