LOW-PRESSURE ENGINE CYLINDERS Cylinder cylinders used on two-cycle low-pres sure engines may well be separated into two classes—those having the crankcase enclosed to act as a scavenging air compressor, and those. which make use of the front end of the cylinder for the same purpose. However, the general designs of these two are quite similar, the chief difference being in the extra length of cylinder employed in the second class.
Figure 251 is a typical horizontal cylinder where crankcase compression is used. It consists of a simple casting with the necessary cored passages for the exhaust ports, air-intake, etc. This design is closely followed by a number of builders. The details in which they vary are usually the shape and extent of the air ports and the position of the exhaust ports.
To attain a good scavenging effect with the air charge, it is necessary to have the air passages enter the cylinder at an angle so as to direct the air toward the cylinder head, and then it is not necessary to rely exclusively on the deflection plate on the piston. It is about as harmful to have too much air-port area as too little. Some builders attempt to place the ports as far around the cylinder girth as the exhaust passage permits— planning, in this way, to obtain a better scavenging effect. How ever, as the air enters the many ports it tends to focus on the center of the cylinder head, causing eddy currents that do but little good. Furthermore, with a decreased port area the air velocity is higher with a consequent better cleansing of the cylinder.
The majority of engines are arranged to have the piston un cover the exhaust ports before the air ports are opened; in this way a greater part of the gases pass out of the cylinder before the air enters. This decreases the amount of air required, or the case is better stated by saying that the available air supply is used to better advantage. At the same time, it allows the air pressure to be less since the gases in the cylinder would be at practically atmospheric pressure when the air starts to blow through. In many installations the exhaust falls below atmos
pheric pressure, owing to the inertia of the column of exhaust gases forming a vacuum at the engine. To obtain this the exhaust pipe must be of a length that allows the gases to leave the open end at the moment the piston uncovers the exhaust port for the discharge of the succeeding charge.
Cylinder with Front-end Compression.—Figure 255 is a cross section of a cylinder wherein the front end is enclosed and acts as the scavenging air compressor, in place of the enclosed crank case. The layouts of the air and exhaust ports are quite like the cylinder shown in Fig. 251.
Cylinder with Removable Liner.—Figure 257 shows a design of cylinder that follows accepted gas and high-compression oil en gine practices. The cylinder jacket is cast integral with the engine frame while the cylinder itself is a removable cast-iron liner pressed into the jacket. At the rear or cylinder-head end a flange rests in a recess in the jacket, while the other end is sealed against water leakage by packing rings. The this method of anchorage, is free to expand lengthwise. The design has much to commend itself. It makes cylinder replace ments lower than in the case of the combined jacket and cylinder; and there is but little danger of leakage, even though many engineers are prejudiced because of this remote possibility.
Vertical Engine Cylinder.—Figure 258 is that of the cylinder used on several makes of vertical engines. It is a one-piece casting of simple design, having the air passage formed in the casting. An attractive feature is the hand plate at the side. Opening this allows the piston pin and brass to be inspected, through a like opening in the piston. It is also handy to see if the piston rings are fast in their grooves. This cylinder design is employed on the Fairbanks-Morse engines and is similar to that of the Bolinder and of the Mietz and Weiss engines.