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some types of cars where double-cylinder motors of the hori. zontal type are used the motor is placed under the seat or body. This type of construction is nearly obsolete at this time, and is found only on early forms of vehicles and one or two commercial cars. The power plant is nearly always combined with the clutch and change speed gearing in such a way to form a unit construction as shown at Fig. 4. This method of joining the parts is widely used at the present time, and is superior to the other common method where the motor and change speed gears are independent units. Each method has advantages. When the gearset and motor are separate the transmission may be removed from the chassis frame without disturbing the power plant and vice versa. At the other hand, when the unit construction is employed, it is sometimes difficult to remove one member without taking the entire unit from the frame. The unit construction has the advantage of retaining positive alignment of the gearset with the engine indefinitely. This relation between the parts is obtained when they are first assembled and the alignment cannot be changed by any condi. tion of operation after the unit is installed in the frame. This method of mounting also permits the three-point suspension, which is very desirable, as the power plant is not stressed by frame deformation when going over rough roads.

If one raises the hood at the front of a motor car, one will find a complete engine assembly very much the same as that depicted at Fig. 6, which outlines a popular engine with the various auxiliary parts lettered so that one can obtain an idea of their location relative to each other. Of the external parts shown the carburetor is employed to mix the gasoline used as fuel with a certain amount of air in order to form a gas that can be ignited in the engine cylinders. This explosive mixture is supplied to the cylinders by a conductor known as the inlet pipe. The spark plugs and magneto form part of the ignition outfit. The engine shown is a four-cylinder form and operates on the four-cycle principle. .

Automobile Engine Parts.-All internal combustion engines, regardless of type, must have the following parts: Cylinder,

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Fig. 6.—The Maxwell Unit Power Plant, Showing Location of Ignition Magneto and Carburetor, as well

as Change Speed Gearing Combined With Engine.'

piston, connecting rod, crankshaft, and engine base. 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. 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 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. The combustion chamber is at the upper end or closed portion of the cylinder. Cylinders may be cast individually, in pairs, or in blocks of three, four or six.

The piston is a reciprocating cylindrical member that moves in the cylinder and which transforms the power of the explosion to mechanical energy. Pistons are usually made of close grained gray iron of approximately the same mixture as the cylinder iron, though where great lightness is desired, as on aeroplane and high speed automobile motors, steel may be employed or aluminum alloy. A piston is usually provided with a series of grooves in which rings of cast iron are mounted to form a packing. The piston must be a free fit in the cylinder in order that it will not expand unduly when heated and bind. For this reason the packing rings are depended upon to keep the exploding gases from leaking by, and as they have considerable elasticity they conform to the cylinder bore and fit it very closely. As they are narrow they do not have much bearing surface on the cylinder and do not offer undue friction if properly lubricated. The piston rings are usually placed at the top of the piston.

The connecting rod is the member that forms the connecting link between the reciprocating piston and the rotary crankshaft. It describes a rotary movement at its lower end and oscillates at its upper end. Connecting rods are invariably made of steel drop forgings. The crankshaft is the part of the motor which converts the reciprocating motion of the piston to a continuous

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Fig. 7.—Part Sectional View of the Buick Six Cylinder Overhead Valve Motor, Depicting Important

Parts.

rotary movement suitable for turning the wheels of the automobile. The crank case is utilized to support the crankshaft and to act as a bed for the engine cylinders. It keeps the working part of the cylinder in perfect alignment with the crankshaft and camshaft carried and protected by the crank case, and at the same time it serves as a carrying or supporting member by which the power plant is attached to the chassis.

Automobile engine crank cases may be made of cast aluminum, cast iron, or bronze castings. The first named material is most generally used on account of its lightness. It has about the same strength as cast iron and weighs but one-third as much. On engines that are manufactured in large quantities, stamped sheet metal, such as steel and aluminum, have been utilized as the lower portion of the crank case. The small auxiliary shaft that carries the valve lifting cams and usually runs parallel with the crankshaft and which is driven by that member is called the camshaft. Some engines have but one camshaft, which carries the cams utilized in operating both inlet and exhaust valves. The “L” type cylinder engine needs but one camshaft, while the power plant provided with “T” head cylinders needs two camshafts, one at each side of the motor. The camshaft of a four-cycle engine is always driven at half the engine speed and always by positive gearing. A cam is a cylinder of metal having a raised portion at one point on its periphery. The difference between inlet and exhaust cams is in the cam profile, as the member intended to lift the exhaust valve has a longer dwell or larger and longer raised portion because the exhaust valve is kept open longer than the inlet member. 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. 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.

The flywheel is a heavy member attached to the crankshaft which has energy stored in its rim as the member revolves, and the momentum of this revolving mass tends to equalize the inter

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