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Three impulses are given to the crankshaft of a six cylinder engine per revolution and that of an eight cylinder motor receives four impulses per crankshaft revolution. It will be apparent that the greater the number of cylinders the smoother the engine operation will be because the crankshaft receives more power strokes in a unit of time. A two-cycle motor crankshaft will receive twice as many impulses per revolution as that of a four-cycle engine having the same number of cylinders.

Q. Are five or seven cylinder engines practical?

A. Five cylinder engines of the conventional form having five throw crankshafts have been made and have operated satisfactorily though they do not run enough smoother than a four cylinder to warrant their replacing this type of standard engine and at the same time their torque is not equal to that of a six cylinder engine, so they have not been applied to automobile service. Any number of cylinders that will go into 360 evenly can be utilized in conjunction with a crankshaft having the same number of throws as there are cylinders, and these cranks may be arranged so that the explosions will follow each other evenly. A seven cylinder engine cannot be made of the usual form, but has been made with cylinders revolving around a fixed crank for aeroplane use. A five cylinder revolving engine was formerly utilized as a power plant on an automobile sold in limited numbers.

Q. What is the limit of engine weight per horse power?

A. The ordinary automobile power plant will weigh about ten pounds per horse power, but very light gas engines for use as aeroplane power plants have been made in the water cooled form that weighed less than five pounds per horse power complete and in the air cooled revolving cylinder forms that weighed but three pounds per horse power. Automobile power plants have been improved greatly in recent years, as 25 or 30 pounds per horse power was not considered excessive weight when the first gas engines were applied to automobile service.

LESSON THREE

PARTS OF GASOLINE MOTORS AND THEIR FUNCTIONS

Q. What are the parts common to all engines?

A. All internal combustion engines, regardless of type, must have the following parts: Cylinder, piston, connecting rod, crankshaft, and engine base.

Q. What additional parts are used only by four-cycle engines?

A. In addition to parts previously enumerated, four-cycle engines must have inlet and exhaust valves, valve operating push rods, valve springs for closing the valves, cams to open them and gearing of some form to drive the camshaft from the crankshaft. (See Fig. 21.)

Q. What is the cylinder and of what material is it made?

A. The cylinder is the portion of the engine in which the gases are confined prior to ignition and which serves as a guide for the piston member which transmits the power of the explosion to the crankshaft. Cylinders are very accurately machined, the bore being about .005" larger than the piston and the interior walls are made straight and true by boring and grinding. Cylinders are invariably made of cast iron of special mixture because this material withstands the heat better than any other and is easily poured into moulds in a molten condition to form very intricate shapes that would be difficult to produce commercially in any other way.

Q. Where is the combustion chamber?

A. The combustion chamber is at the upper end or closed portion of the cylinder. (Fig 21.)

Q. How are cylinders cast?

A. Cylinders may be cast individually, in pairs, or in blocks of three, four or six.

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Fig. 22.-Outlining Parts of Two-Cycle Motor and Also Showing the Members Necessary to Secure Action of a Four-Cycle Motor that Are Eliminated in the Two-Cycle Forms.

Q. What is the water jacket?

A. The water jacket is a wall surrounding the cylinder castings and separated from the cylinder wall by a space through which water is circulated around all portions of the cylinder that are liable to become unduly heated while the engine is in operation.

Q. How is it attached to the cylinder?

A. The water jacket is usually incorporated as part of the cylinder by casting it integral. The spaces through which the water circulates are formed by the use of cores made of sand which separrate the two walls until the molten metal which has been poured around and between the cores has had a chance to cool. When the casting is removed from the mould the sand is taken out of the water jackets and leaves a space through which water can circulate.

Q. Are cylinders ever made without jackets?

A. Some forms of water cooled cylinders are cast without the water jacket which is afterward applied, either by an electro-deposition process or by fastening a sheet metal water jacket to suitable flanges on the cylinder by screwing it in place, by brazing or hard soldering it or by any other suitable mechanical means.

Q. What is the valve chamber?

A. The valve chamber is a projecting portion of the cylinder in which the valves are placed that control the gas flow in or out of the combustion chamber and to which the inlet and exhaust manifolds are attached.

Q. Where is it placed on the cylinder?

A. The valve chamber may be placed at one side of the cylinder and of sufficient size for both valves or two valve chambers may be utilized, one placed at each side of the cylinder head and carrying only one valve. When cylinders are cast in pairs and a valve chamber is used on but one side there is room enough for four valves. But when two valve chambers are utilized each need only be large enough to provide accommodation for two valves. Valve chambers are sometimes made in the form of separate castings which are attached to the top of the cylinder or the valves may be housed in cages which are inserted directly into suitable openings made to receive them in the cylinder head. (See Figs. 21, 23, 25, 26.)

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Fig. 23.-End Sectional View of L Head Motor with Important Parts Clearly Outlined.

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