Зображення сторінки

useful purpose and which only acts to attract and retain dust and grit. The instructions given on the chart can be followed to advantage on practically all types of gasoline cars, though. of course, the different constructions will have to be treated as the peculiarities of design dictate.

How the Motor Should Be Started.—One of the most important points in the education of the novice motorist is the best method of starting the motor if a self starting device is not provided. Before the engine is set in motion certain precautions must be observed regardless of the make or type of car. The gasoline tank, radiator and lubricating oil container should be inspected to make sure there is enough fuel, water and lubricating oil. The shut-off valve in the pipe line leading from the gasoline tank to the carburetor is opened so the fuel will flow to the vaporizer. The carburetor should be primed by means of a small plunger usually carried in the floatbowl cover, and if a small resistance is felt to the downward movement of the primer or if gasoline escapes from the bottom of the mixing device, this may be con sidered a positive indication the fuel from the tank has reached the carburetor and that gasoline is present at the spray orifice. The next step is to see that the change-speed lever is in a neutral position or that the clutch pedal is disengaged. The spark control lever, which is usually carried on the steering wheel, should be set at full retard point. In some cars this may be at the back of the sector, while the retard position may be the other extreme in other motor cars.

If one attempts to set an engine in motion by means of a hand crank with the spark lever advanced so that an early spark is obtained, the motor may “kick back," and this reversal of motion, which is due to premature combustion, may sprain the wrist or break an arm. It will be well to open the throttle or gas lever a little to insure that a charge of combustible gas will be inspired into the motor. The engine should be turned over several times as briskly as possible, and then the switch which completes the electrical circuit between the battery and the ignition mechanism should be put into circuit and the switch plug inserted. The hand crank is pushed in until it engages a ratchet member on the front end of the crankshaft, and then the motor should be turned by pulling up on the starting handle with the left hand.

The hand crank should always be engaged so that an upward pull will be necessary to turn the crankshaft, and the point that cannot be too firmly impressed upon the embryo motorist's mind is that gasoline engines should always be started by pulling up on the handle of the starting crank, never by pushing down. If the starting handle has been properly placed and the engine has been turned over enough with the switch open so the cylin. ders hold a gas charge, and the switch circuit is closed when a decided resistance is felt as the crank is turned, indicating that the piston in the cylinder in which the gas charge is about to explode is nearing the compression point, a single quick, strong up pull on the crank should be sufficient to start any properly adjusted motor.

Multiple-cylinder engines, especially those of the four and six-cylinder types, are started much more easily than the one and two-cylinder forms. These can often be started by turning the starting handle over briskly so the motor will take in gas but without the switch closing the electric circuit. To start the motor the switch is closed and a spark will be produced in the cylinder about to fire (only in cars equipped with battery ignition) by moving the spark lever from one end of the sector to the other. As soon as the engine becomes started it should be kept from racing by shutting down the supply of gas to the point where the motor will turn freely and yet slowly.

Handling Spark and Throttle Levers.—Most hydrocarbon vehicle motors have a certain degree of flexibility, i. e., they may be run slow or fast, and the speed may be accelerated or cut down as desired within a range from 200 revolutions per minute to the maximum, which will vary with the type of motor, some running as high as 3,000 r. p. m. This is an important advantage,

. inasmuch as it permits one to regulate the vehicle speed on most occasions by a touch of the throttle alone. The engine speed of practically all automobiles is controlled by two ways, though usually these are employed in conjunction. One of these consists of varying the time of the spark in the cylinder, the other regulating the amount of gas supplied. The throttle, in some cases, may be controlled by three distinct means. One of these is a governor which shuts off the gas supply automatically if motor speeds exceed a certain predetermined point. Governors are used only on commercial vehicles. The governor may be temporarily dispensed with by pressing down on the accelerator pedal, which will open the throttle directly, or by means of the throttle lever carried on top of the steering column. The usual method of driving is to set the throttle lever at a point which will give the minimum speed desired and depend upon the accelerator to take care of other speed fluctuations. The function of the spark lever is to regulate the time of sparking to the point best suited to the needs of the engine.

The question of motor speed regulation seems to be a simple one, but many motorists learn proper methods of spark and throttle lever placing only after considerable driving experience has been obtained. Motor speed regulation depends upon two factors. First, advancing the time of sparking to the most efficient point after the engine has once been started; and, secondly, increasing the amount of mixture supplied the cylinders. The spark and throttle levers, while designed to be manipulated independent of each other, usually move with a certain definite relation. It would not be good practice to run an engine with the spark lever way advanced and gas supply throttle nearly closed; nor would good results be obtained if the spark lever was retarded and the throttle opened as it is desired to increase the motor speed. It is not difficult to understand the function of the throttle lever and how the admission of more gas to the cylinders would act in creating more power, just as augmenting the steam supply to a steam engine will increase its capacity.

Why Spark Is Advanced.—The rules for manipulation of the spark lever are not so well understood. In order to make clear the reason for intelligent manipulation of the spark handle there are certain points that must be considered. On most automobiles there is a position of the spark lever, usually at the center or intermediate point of the sector over which it moves,

[blocks in formation]

Fig. 26.-Showing Various Positions of Spark and Throttle Levers for

Obtaining Different Car Speeds. 3

which corresponds to the normal firing point. If the spark lever is not advanced beyond this position, and the motor is turning over slowly, the gas in the cylinders is being exploded when the pistons reach the end of their compression stroke. When the gas is fully compacted the explosion or power obtained from combustion is more powerful than if the spark fired gas which was not compressed properly. The electric spark is not produced at the exact time that the motor should be fired at all speeds, and if the spark was supplied the very instant of full compression, irrespective of the speed of rotation, there would be no need of moving the spark lever.

Not only is the current apt to lag, but it takes a certain definite amount of time to set fire to the gas. It requires nearly the same amount of time to ignite the gas, of given composition, regardless of the speed of the motor. If the motor is only turning at a few hundred revolutions per minute there is ample time to ignite all gas charges positively, but if the motor speed increases, and the explosions occur oftener, then one must compensate for the more rapidly occurring combustion periods by arranging to start igniting the gas earlier so the explosion will actually occur when the piston is at its highest point in the cylinder. The compensation for lag is made by advancing the spark. The spark lever on the steering wheel or column moves a timer distributor, if battery system is employed, or the magneto contact breaker box, if that form of current producer furnishes the ignition energy. The amount of spark advance needed depends on engine speed, and the greater the piston velocity the more the spark should be advanced.

It is possible to advance the spark lever too far, and when this occurs the gas is exploded before the piston reaches the top of its stroke and premature explosion takes place. As a result of this the upwardly moving piston is forced to overcome the resistance exerted by the expanding gas of the ignited charge in completing the remainder of the compression stroke, and before it will return on the power stroke. The injurious back pressure on the piston reduces the capacity of the motor and a pounding noise similar to that produced by loose motor parts

« НазадПродовжити »