Зображення сторінки
PDF
ePub

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.)

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed]

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

[graphic][subsumed][subsumed][subsumed][subsumed][merged small][subsumed][ocr errors][subsumed][merged small][subsumed][merged small][subsumed][subsumed][merged small][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][merged small]

Fig. 24.–Sectional View Through Cylinder and Crankcase of

Pierce-Arrow T Head Motor.

a free

matic valves cannot be used except for letting in a charge of fresh gas as they will only open during the suction stroke of the piston.

Q. Why must exhaust valves be mechanically operated?

A. Exhaust valves must be mechanically operated because they are forced to open against the pressure of the burnt gas in the cylinder which may be three or four times that of the atmosphere.

Q. Where are valves placed as a rule?

A. Conventional practice is to place the valves in pockets or valve chambers at one or both sides of the cylinder castings. They are commonly placed so that when opened the entering fresh gas stream will flow directly into the combustion chamber or so the exhaust

gas

will find passage to the outer air. Q. Describe the three common methods of valve placing.

A. The three common methods of valve placing are at the top of the cylinder opening inward, all valves at one side, or half on one side and the remainder on the other. In some engines one valve may be placed at the top of the cylinder while the other is carried in a side pocket.

Q. How are valves operated ?

A. Valves are usually operated by means of push rods which are actuated by revolving cams. The method of valve operation depends upon its placing in the cylinder. If placed in a pocket at the side it may be operated directly by means of a movable p unger interposed between the cam and the valve stems. If placed in the cylinder head it is usually actuated by a push rod and centrally fulcrumed lever which changes the upward movement of the valve plunger to a downward movement of the valve stem. (See Fig. 29.)

Q. What is the advantage of L head construction?

A. It is claimed that greater compactness is obtained by placing all valves on one side of the engine and that the piping is more easily installed. (Figs. 23 and 27.)

Q. What is the advantage of T head construction?

A. Carrying the valves on both sides of the cylinder makes it possible to use larger valve members and ports of greater area for the passage of the gases. (Fig. 24.)

« НазадПродовжити »