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car is desired, having a capacity for but two or three passengers, is shown at Fig. 10. Truck bodies vary very widely according to the nature of the work they are intended for. The simplest form is a platform without sides, then stakes may be added to this platform body or various forms of closed wire panel delivery bodies or express wagon types fitted to the chassis.

Q. What are automobile bodies made of?

A. Pleasure car bodies may be made of wood or metal. Sometimes the panels are of sheet steel or aluminum attached to a wooden frame, or the body may be made entirely of metal. Motor truck bodies are usually made of wood and well ironed and strengthened by braces and angle pieces of steel or iron. The floors of motor trucks are often reinforced with strips of bar steel and the corners are provided with corner pieces of metal where they are liable to come in contact with a loading platform.

Q. What are "torpedo and stream line" bodies?

A. It is now common practice to make bodies of pleasure cars designed for high speed of such form that air resistance is reduced to a minimum. This is accomplished by the use of gradual curves and unbroken sides. The name torpedo has been applied to this form of body because in the exaggerated forms they resemble a projectile of this nature on wheels.

Q.

Under what conditions is air resistance noticed?

A. Air resistance is of little moment on car speeds lower than 30 miles per hour, but as this rate is exceeded, it is a very important factor which must be recognized, and the amount of exposed area of the car which will offer resistance should be reduced to as low a point as possible. The amount of power needed to overcome air res stance varies inversely as the cube of the speed. This means that if the speed of the car is doubled the power needed to overcome air resistance at half that speed is multiplied by eight. For instance, if one horsepower is absorbed at 20 miles per hour, 8 horsepower will be required to overcome the air resistance present at 40 miles per hour. It is for this reason that the torpedo body has been evolved and in most modern cars the frontal area, which means the number

of square feet in the radiator, dash, wind shield, mudguards, and other parts that would offer resistance to motion, is kept as low as possible.

Q. What is traction resistance?

A It takes a certain amount of power to overcome the friction existing between the wheels and the ground due to the car weight, and to produce movement of the vehicle. This is known as traction resistance and depends largely upon the nature of the road surfaces, the size of the driving wheels and the type of tire with which they are fitted.

[graphic]

Fig. 9.-Typical Two-Passenger Roadster Body on Pleasure Car

Chassis.

Q. What is the resistance of grades?

A. When an automobile is climbing a hill, it must lift its weight in opposition to the force of gravity, which exerts no retarding influence when the automobile is operated on the level, and which really accelerates car motion when the automobile is descending an incline. It requires an effective effort or push at the point of contact between the rear wheels and the ground equal to one per cent of the car weight for each one per cent increase in gradient.

Q. How much power does an automobile need?

A. The amount of power required to propel an automobile de

pends upon the nature of service for which it is intended. A high speed car of comparatively light weight may require more power than a heavily loaded, slow moving truck. An automobile that is to be used in hilly country or where road conditions are not good, needs more power than a machine of the same type operated under more favorable conditions. This is a point that is hard to decide, and no general rules can be given. If an automobile has too much power, it will use an excess of fuel over the amount actually needed to propel the vehicle and will rack itself to pieces sooner than one that is lacking in power. The under-powered car, however, usually performs very well on the level and under favorable conditions, but it is very unsatisfactory when used on hills or bad roads. Much depends upon the general design of the automobile, and the efficiency of the power transmission and driving elements.

[graphic]

Fig. 10.-Coupe Body, a Closed Type Suitable for Two or Three Passengers Fitted to Roadster Chassis.

Q. What is the usual proportion of power to weight?

A. Pleasure cars that are intended for touring or high speed

work, usually have one horsepower for each 50 pounds car weight. Medium speed touring cars and runabouts usually have one horsepower for each 75 pounds car weight. Delivery wagons designed for speedy service usually employ a power plant of such size that one horsepower is provided for each 100 pounds, while designers of heavy motor trucks that operate at very low speeds find that one horsepower to two or three hundred pounds weight is adequate.

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Fig. 11.-Sectional View of Overland Engine Showing Important

Parts.

LESSON TWO

ACTION OF THE TWO AND FOUR-STROKE CYCLE

MOTORS

Q. Describe the operating principle of the gas engine.

A. Power is obtained in a gas engine by exploding some combustible vapor or gas in a closed cylinder which is provided with an internal yielding member moved by the force of the explosion. The moving member converts the expansive power of the rapidly burning gases to mechanical energy by suitable leverage. The gas engine is a form of heat engine just as the steam or hot air motor.

Q. How does the gas engine differ from the steam engine? A. The gas engine, while it embodies some of the main elements used in a steam engine, differs greatly in general design. The cylinder is provided with a water jacket and most of the gas engines used as automobile power plants have the connecting rod attached directly to the reciprocating member or piston in the cylinder instead of to a crosshead as used in a steam engine Most forms of steam engines are double acting. That is to say, the steam is admitted first to one end of the cylinder, and then to the other, and both ends of the piston take a power impulse. All gas engines used in automobiles or similar vehicles are single acting, which means that the expansion of gas that produces the power takes place in the upper end of the cylinder and against the top of the piston only.

The power of a steam engine can be increased within certain limits by augmenting the pressure and quantity of steam supplied the cylinder. The effective force obtained by the explosion of gas in a gasoline engine attains a certain maximum volume which cannot. be increased. It is possible to start a steam engine under load,

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