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are usually mounted on pneumatic tires, as air filled tires are called, while motor trucks which travel at slower speeds use solid rubber tires.

Motor trucks may be divided into two classes: those which utilize chassis patterned after pleasure types, but fitted with delivery bodies, as shown at Fig. 1, and those types of which a typical example is shown at Fig. 2, in which the motor is carried under the driver's seat and where the general design is such that the chassis can be used only for commercial purposes. There are classes of commercial cars, such as taxicabs, light delivery wagons, ambulances and vehicles of this nature, which are practically pleasure car chassis fitted with forms of bodies adapting it to a commercial use.

Q. What are the essential elements of the automobile?

A. The principal parts of automobiles are shown at Figs. 3, 4, 5, these outlining the chassis of pleasure cars and a typical truck. The foundation for the entire mechanism is the frame, which is attached to and supported by springs which are bolted to the axles on which the wheels are mounted. The power plant, which consists of the gasoline engine and a number of other parts, is usually carried at the front end of the frame, where it is housed under a hood, or in a special compartment under the body where it can be easily reached.

The clutch and gear set form part of the transmission system, as does the chain or propeller shaft used to transmit the engine power from the change speed gearing to the rear axle. A steering gear by which the direction of car is controlled and levers for shifting the change speed gears, for controlling the brake and for operating the spark and throttle of the engine, complete the chassis assembly. An automobile may be said to consist of the following main groups: the power plant; the clutch and change speed gearing; the power transmission system; the supporting frame and springs; the axle and wheels; the controlling elements; and the load carrying or body member. (Figs. 4 and 5.)

Q. What is the function of the power plant?

A. The purpose of the power plant is to generate enough power so that when this is transmitted to the rear wheels thru the medium of suitable reduction gearing, it will propel the vehicle.

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Fig. 4. Plan View of Typical Modern Pleasure Car Chassis With Important Components Indicated,

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Fig. 5.-Side Elevation Showing Location of Important Components on Typical Pleasure Car Chassis.

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Fig. 6.-Carburetor Side of Chalmers Unit Power Plant.

Q. What does the power plant consist of?

A. The usual gasoline automobile power plant consists of a gasoline engine, a carburetion or fuel supply group, an electrical ignition system, a lubrication system, and means for cooling the engine. (Figs. 6 and 7.)

Q. Name functions of carburetion group.

A. The purpose of the carburetion group is to supply the motor cylinders with an explosive gas.

Q. Name parts of carburetion group.

A. The principal parts of a carburetion group are: a suitable tank or container for the liquid fuel; piping by which this is transfered from the tank to the carburetor or vaporizer which changes the liquid fuel to a gas, and a suitable manifold by which the gas from the carburetor is supplied to the motor cylinder.

Q. Name functions of ignition system.

A. The purpose of the ignition system is to ignite the gas or vapor supplied to the engine cylinder by the carburetion group and to produce power by exploding this gas in the cylinder.

Q. Name parts of ignition system.

A. The parts of the usual electrical system are: some source of electrical current; wires for conveying it from the current producer to a switch or timer which regulates the time the electric spark occurs in the engine; an induction coil or similar apparatus for transforming current of low potential or voltage to current of higher power capable of producing a spark in the cylinder and some form of spark plug which is placed in the combustion chamber and between the points of which the spark that explodes the gas occurs. Some ignition systems consist merely of a magneto or mechanical generator which incorporates in its construction the necessary timing and current intensifying appliances and wires leading from this device to the spark plugs in the motor cylinder.

Q. Why is a lubrication system needed?

A. If the parts of the power plant were not properly oiled, they would wear out very quickly, and in fact it would not be possible to run a gasoline engine for any length of time without supplying oil to

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