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Fig. 56.-Bottom View of Engine Base of Six-Cylinder Motor, Showing Four-Bearing Crankshaft.

Q. Why must flywheels be securely fastened?

A. It is imperative that flywheels be attached firmly to the crankshaft, because any looseness between the flywheel and the shaft will produce a pronounced knocking sound, and the methods of fastening, such as keys or bolts, may be entirely sheared off.

Q. What is the meaning of marks on flywheel rims?

A. The marks placed on flywheel rims, as indicated at Fig. 39, are placed thereon to assist in timing the valves and ignition spark. The abbreviation E. 0. means “Exhaust Opens," while E. C. means “Exhaust Closes.” The letters I. O. mean “Inlet Opens,” while I. C. means “Inlet Closed." The letter S. indicates the sparking point, while U. C. means “Upper Center,” which means that the piston is at the top of its stroke, and L. C. means “Lower Center,” which corresponds to the crank shaft position when the piston in the cylinder being timed is at the bottom of its stroke.

Q. What influence does flywheel diameter have on spacing of timing marks?

A. For a given number of degrees crank travel the marks are farther apart on large diameter flywheels than they are on those of lesser radius.

Q. What are the manifolds?

A. The manifolds are the built-up members or pipes that convey the fresh gas from the carbureting device to the valve chambers or which convey the inert products of combustion from the exhaust valve chambers to the muffling device.

Q. How are they secured to cylinders?

A. Manifolds are usually attached to cylinders by means of flange couplings bolted to the cylinders or by stirrups or retention bars which hold them securely in place.

Q. What precautions must be observed when designing manifolds?

A. Inlet or fresh gas manifolds should always be proportioned so that the gas from the carburetor does not have to turn any sharp corners and so the distance between each valve chamber of a multiple cylinder engine and the carburetor will be approximately the

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Fig. 57.—Bottom View of Four-Cylinder Motor Showing Five-Bearing Crank Shaft with Flywheel and

Timing Gear Attached.

same. Exhaust manifolds should cffer a free passage to the escaping gas and should be large enough in bore so that there will be no friction between the gas stream and the manifold wall.

Q. How is the engine base attached to vehicle frame?

A. The common method of construction is to cast arms integrally with the engine bed which extend to the main frame sides or rest on an inner frame or sub-frame member that is in turn supported by the main frame.

Q. What is the three point support”?

A. If the power plant is supported by three points placed in relation to each other so that the two on the same line form the base of a triangle while the third forms the apex, the engine is said to have a three point support. The advantages of this construction may be readily understood if one compares the action of a three legged stool and one having four legs on uneven ground. The stool with the three point support is not so much affected by irregularities in the surface on which it rests as is the four legged member. A power plant supported on three points is not affected by frame distortion or springing out of alinement as much as a four point support power, plant.

Q. What is "four point support”?

A. When a power plant is held in place in the frame by means of four arms so that the point of support forms a rectangular figure, the engine is said to have four point support. The four point system necessitates the use of a more rigid frame than does the three point method of suspension.

Q. What is a unit power plant?

A. The unit power plant construction is that where the engine, clutch and gearset are attached together so these three members may be said to be supported by the same common carrier or case. A unit power plant with the three point support is commonly used in motor car construction.

Q. What are the advantages of the unit power plant?

A. One of the methods of installing the motor and gearset of an automobile is to attach these to a frame as separate members and interpose some form of flexible coupling between the clutch shaft and the gear box to compensate for any disalinement of the two members due to frame distortion on rough roads. With a unit power plant, crankshaft, clutch and transmission shafts are held in the absolute alinement present when first assembled, and no amount of frame deflection can throw the three members out of the correctly alined position that insures maximum efficiency and minimum depreciation. (See Frontispiece.)

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