Зображення сторінки

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.


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


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.


Fig. 10.—Coupe Body, a Closed Type Suitable for Two or Three Pas

sengers 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.

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed]

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





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, while a gasoline engine can be started under these conditions only by means of some mechanical starting arrangement that will enable the crank shaft to be turned until a charge of gas has exploded in the cylinder. A steam engine starts as soon as steam is admitted to the cylinders. A gas engine as used in automobiles will not start unless the crank shaft is rotated by a starting handle or some other mechanical equivalent. . A steam engine is an external combustion motor while the gas engine is an internal combustion type.

Q. Define "external" and "internal" combustion.

A. External combustion means burning the fuel which produces power outside of the cylinder as is the case when water is heated in a steam boiler, which is separate from the steam engine. Internal combustion is what takes place when the gas that produces the heat from which power is obtained is burnt or exploded directly against the piston in the cylinder. An engine which depends upon the internal combustion principle is more efficient than an external combustion form because loss of heat is less.

Q. How is fuel supplied to gas engine cylinders?

A. The gas, which may be derived from the burning of coal in a gas producer, manufactured by the usual methods of making illuminating gas or taken from natural underground sources as natural gas, is supplied to the cy inders of a gas engine directly from the source of supply through the medium of piping which leads to the combustion chamber of the motor. When liquid fuel such as gasoline or kerosene is used, it is first transformed into a gas by atomizing or spraying a small quantity of the liquid into an entering charge of pure air which changes the liquid spray to a vapor that afterward mixes intimately with the air to form an inflammable gas.

Q. What is necessary before the gas in the cylinder can be exploded?

A. Before exploding the gas charge it is customary to compress it into from one-third to one-fifth of the cylinder volume. A charge of gas that is not compressed will burn comparatively slow and will not produce much power. The more the gas charge is compressed the greater the force of the explosions. There is a limit to the amount of compression, because if kerosene or gasoline vapor pressure is in

« НазадПродовжити »