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Fig. 20.-Sectional View of Six-Cylinder Motor Used on Pierce Arrow Cars. Note Seven Bearing Crank

shaft and General Simplicity and Strength of Design.

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Fig. 21.-End Sectional View of Motor With T Head Cylinder

Showing Important parts.

A. The crankshaft of a single cylinder four-cycle engine will receive one impulse every two revolutions. That of a two cylinder form will receive an impulse every revolution. A three cylinder crankshaft will receive three impulses in two revolutions or one every two thirds crankshaft revolution. Four explosions are obtained for every two revolutions of a four cylinder crankshaft or two each revolution. 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 Mqtor and Also Showing the

Members Necessary to Secure Action of a Four-Cycle Motor that Are Eliminated in the Two-Cycle Forms.

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