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December, 1920 FIG. §--TELESCOPE TUBE PRINCIPLE UTILIZED IN PISTON COOLING the piston in its upward travel covers the exhaust ports, at about 20 per cent of the stroke, the cylinder is filled with air at only slightly above atmospheric pressure, this air being compressed in the remainder of the stroke. Thus the weight of air compressed does not ex- ceed 85 per cent of the weight of a cylinder full at atmospheric pressure. This led to the design of the scav- enging valve shown in Fig. 5. This is known as the Sulzer scavenging sys- tem and utilizes the principle of port scavenging but it employs twa tiers in place of one tier of ports. In Operation, the piston uncovers the upper tier of ports before and_ the. lower tier after it uncovers the ex- haust ports. At the same time, com- munication between the interior of the cylinder and the scavenging air sup- ply or receiver, through the upper ports, is controlled by a timed and mechanically operated: rotory valve with two ports situated well out of the reack of heat and dirt. It operates slowly. the period available for its opening and closing being equivalent to more than 134 strokes of the piston. Thi valve remains closed until the exhaust ports have been uncovered long enough to reduce the pressure of the gases in the cylinder nearly to that of the atmosphere. Then the valve is opened while the piston uncovers the lower scavenging ports which causes a rapid and thorough purging of the cylinder. This feature, it is pointed out, results in protection against a blowback. Blowbacks Are Prevented On its return stroke, the piston first covers the lower scavenging ports and then the exhaust ports, while the upper scavenging ports and their valve re- main open enabling the scavenging air to fill the cylinder at full scaveng- ing pressure before communication is shut off by the moving piston. By this system it is claimed that a blow- THE MARINE REVIEW 643 VIG. 8--TYPICAL 2-CYCLE DIESEL ENGINE INDICATOR CARD back of burned gases into the cylinder cannot occur and the double tier ar- rangement and the form of the scav- . enging ports insure a thorough purg- ing of the cylinder. As a matter of fact, tests conducted recently have: proved that the amount of burned gases remaining in the cylinder does not exceed 3 per cent. The weight ot air thus compressed is the weight of a evlinder full at atmospheric pressure permitting the consumption of the full -- quantity of fuel injected. The cylinder liner with its scavenging and exhaust ports is shown in Fig. 10, The operation of the scavenging and charging system is simple and inter- esting. It is shown in detail in Fig. 9. In A, the piston covers all ports while the scavenging valve is closed. The piston is on its down, or impulse stroke, the gases have been fired and are burn- ing and expanding. During the first part of this stroke, the valve opens and fuel injection and combustion Occur. The piston has uncovered the upper scavenging ports in B but the scavenging © i (a) Piston traveling down; all valves and ports closed; fuel injected and burnt during first part of stroke; gases expand. (b) Piston travel- ing down; piston uncovers exhaust _ports; burnt gases escape through ex- haust ports, reduc- ing their pressure to atmospheric. (c) Piston traveling down; exhaust ports still uncov- ered; scavenging valves open; air under light pressure enters through scavenging valves, and blows gases out of cylinder. (d) Piston traveling up; piston covers exhaust ports; scav- _ enging valve closed; air in cylinder is being compressed, FIG. 7--PRINCIPLE ON WHICH THE 2-CYCLE TYPE OF DIESEL ENGINE WORKS