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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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4.-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.

Q. What is the cylinder head?

A. The cylinder head is the name generally given to the upper or closed end of the cylinder.

Q. Name two methods of constructing cylinder heads.

A. Cylinder heads may be a separate casting attached to the cylinder member by bolts or they may be formed integrally with the cylinder during the casting process. The latter is the conventional method.

Q. What are the advantages of individual cylinders?

A. It is claimed that individual cylinders are simpler to form at the foundry, easier to handle in the machine shop and that they are more cheaply replaced in event of damage than where cylinders are cast in a block. With some form of engines it is possible to use only individual cylinder castings. This is true of the two cylinder opposed form, engines having mechanically applied sheet metal water jackets and air cooled engines. (Fig. 26 )

Q. What are the advantages of block castings?

A. Block castings, in which three or more cylinders are cast together, not only make a shorter and more compact cylinder assembly possible, but produce a stronger engine because the cylinder block actually reinforces the engine base. With unit castings of the individual type the engine is longer and the crankcase must be made heavier in order to secure the proper degree of strength. (Fig. 27.)

Q. What are the valves?

A. The valves are members that control the ports through which the gases enter and leave the combustion chamber.

Q. Name two types of valves.

A. Valves are either automatically or mechanically operated and may be of the conventional mushroom or poppet valve type or of the sleeve, slide, piston, ring or rotary disc forms.

Q. Can automatic valves be used for exhaust?

A. Automatic valves cannot be used for exhaust because they can only open when there is a partial vacuum in the cylinder or when the pressure in the cylinder is less than that of the outer air. Auto

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