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The lower view is the valve side of the motor and shows clearly the governor case, the method of installing the magneto and carburetor, the design and method of securing inlet and exhaust manifolds and the location. of the valves in pockets at the side of the L-shaped individual cylinder castings.

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FIG. 38.-Single Cylinder Four-Cycle Motor With One Half of Cylinder and Crankcase Removed to Show All Important Parts.

CHAPTER IV.

GAS TRACTOR POWER PLANT COMPONENTS.

Parts of Tractor Engines and Their Functions-Cylinder Construction-The Valve System-Piston and Rings-Connecting Rod Types-Crankshaft Forms-Utility of FlywheelEngine Base and Bearings.

Parts of Tractor Engines and Their Functions.— The general design of tractor engines having been considered and the principles upon which their action is based outlined, it will be advisable to consider more in detail the various parts comprising the engine, the materials of which they are made, the various common forms they exist in and their functions. In the crosssectional view of the typical four-cycle engine depicted at Fig. 38 all the important parts of a single-cylinder power plant are shown. In a four-cylinder engine most of these parts pertaining to the cylinder and valve operation would be multiplied by four, though one set of cam shafts and cam shaft gears, one engine base and crank shaft and one fly-wheel would be all that would be needed whether the engine has one or six cylinders. The parts of a multiple cylinder engine are thus duplicates of each other to a large extent and when the parts of one cylinder and their functions are understood, it will not be difficult to understand any four-cycle gas engine.

The cylinder, which is a "T" head form, having valves at both sides is attached to a base of aluminum in which the cam shafts and crank shaft are housed. The piston

is the reciprocating member that moves back and forth in the cylinder and it is joined to the connecting rod by means of a wrist pin passing through the upper end of the connecting rod. The lower portion of the connecting rod is attached to the crank pin of the crank shaft. As the piston travels up and down in the cylinder it turns the crank pin just the same way as the reciprocating motion of a sewing-machine treadle is transformed to a turning movement of the large driving wheel. The cylinder is provided with a water jacket, which is a hollow space cored between the inner and outer walls through which water is circulated to keep the engine cool. The valves are carried in side pockets extending on each side of the cylinder and are held against their seats by means of coil springs.

The cam shafts, which turn the cams used to operate the valves are driven from a small gear on the crank shaft, the ratio of drive being two turns of the crank shaft for one turn of the cam shaft. In all four-cycle motors of conventional design regardless of the number of cylinders used the cam shafts always turn at one half crank shaft speed. The valves are raised from their seats by means of push rods, which transfer the motion imparted by the cam point raising the roll which bears against the cam. These members are steadied by guide bushings screwed into the engine crank case.

The starting crank is used to turn the crank shaft through the preliminary movements necessary for the piston to draw in a charge of explosive gas and compress it. When the gas is exploded by the electric spark which occurs between the points of the spark plug mounted in the cylinder head, the piston is driven down and a certain amount of the energy produced by the explosion is stored in the fly-wheel rim. As a general rule, after the

first explosion, the engine will continue to run without hand cranking.

The inlet valve is used to admit the fresh gas from the carburetor to the combustion chamber and is raised from its seat when the piston starts to go down on the induction stroke. It remains open until the piston has reached the bottom of this stroke and the cylinder is filled with gas. While the piston has been going down the cam shaft has been turning and when the piston reaches the bottom of its stroke the cam point has revolved until it is out of contact with the roller in the valve plunger and the valve spring, which is not indicated but similar to that shown on the exhaust valve, returns the inlet valve to its seat in the valve chamber.

The exhaust valve does not open until the piston has made two more strokes, one the compression the other the expansion, or power stroke. When the piston has covered approximately seven-eights of its movement on the power stroke the exhaust cam turns around, raises the push rod, which in turn raises the exhaust valve from its seat, and permits the burnt gases to escape through the open exhaust port. The valve remains open during the entire upward stroke of the piston, closing when the piston reaches the top. The inlet cam has now come into action again and another fresh charge of gas is drawn into the cylinder during the beginning, of the next cycle of operation. From the foregoing description it will not be difficult to understand the action of the various parts of the four-cycle engine and the relation these members bear to each other.

Cylinder Construction.-The cylinder is one of the most important parts of any form of heat engine and it is subjected to greater stress than any other member. As a rule cylinders are made of cast iron and unless of

the air-cooled form have water jackets cast integral. Tractor engines of the usual pattern use individual cylinder castings, though some of the forms which are built according to automobile practice have the cylinders cast in pairs. Tractor engines are generally of large bore and long stroke because power is obtained by large piston displacement rather than high crank-shaft speed.

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FIG. 39.-Cylinder With Both Valves on One Side, Four Ring Piston and Marine Type Connecting Rod.

The conventional form of gas-engine cylinder is outlined at Fig. 39 in connection with the piston and connecting rod. This cylinder is of the L form and has the water jacket cast integral. Both valves are carried at one side of the cylinder head which is a unit with the cylinder.

A cross-section through a cylinder of this form is shown at Fig. 40. The large water spaces surrounding the cylinder walls and combustion chamber are clearly indicated. The bore of the cylinder is finished very smoothly

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