pulled back until the throttle is almost closed in order to prevent the engine "racing."

Q. What is the objection to engine racing?

A. If a motor runs at high speeds while the car is not in motion it will be subjected to unnecessary wear and the car will vibrate unduly.

Q. How is engine speed controlled?

A. The approved method of controlling engine speed is by spark

Fig. 250.-Motor and Car Control Group of Pope-Hartford Car.

and throttle levers, as shown at Figs. 247-C, 249, and 250. In most cars the spark and throttle levers are carried above the steering wheel where they may be easily reached, though sometimes they are mounted on the steering column, as shown at Fig. 249. As a rule, advancing the spark and giving the engine more gas by moving the throttle lever will accelerate motor speed, while retarding the spark and cutting down the supply of gas will slow the engine down. The various positions of the spark and throttle levers of the Pierce-Arrow car for starting, slow, medium and fast running are indicated at Fig. 249.

Q. Describe an automatic engine stop to prevent excessive speed.

A. Some forms of engines are provided with a governor connected with the throttle in such a way that when the engine speed exceeds a certain predetermined point the throttle will be closed by the governor and the engine speed will slacken automatically.

Q. Outline common method of cutting out governor action. A. In cars where a governor is fitted on the engine the gas lever or a foot accelerator may be used to open the throttle independently of the governor so that this member will only be operative when the throttle lever or accelerator pedal is set for low or medium speeds.

Q. Why should spark be advanced after engine is started? A. The spark lever should be moved from the retard position as soon as the engine is started in order to bring the ignition point so the spark takes place just when the piston reaches the end of its compression stroke instead of after it has started to go down on the power stroke. Continuous operation with lever in retarded position will mean overheating and loss of engine efficiency.

Q. What happens if spark lever is too far advanced? A. If the engine is run with the spark lever advanced too much, the charges will explode before the piston reaches the end of its compression stroke and this preignition, or early explosion, will impose stresses on the motor bearings and will produce a pronounced knocking sound because it tends to oppose piston movement.

Q. What is the best position for spark lever under average running conditions?

A. The spark lever can be placed to best advantage at that point on the segment which corresponds to ignition when the piston reaches the end of the compression stroke, because this condition is one favorable to efficient engine action and makes possible very flexible control by throttle lever alone. In some cars using magneto ignition no provision is made for advancing or retarding the time of the spark which is set so it occurs slightly in advance of top center or when the piston has just compacted a charge of gas and is ready to descend on its power stroke. In cars using this system of control the engine speed is regulated entirely by the throttle lever or accelerator pedal.

Q. What should be done if any unusual noises materialize after engine is started?

A. If any unusual sounds are heard after the engine is started the operator should immediately stop the motor and investigate the cause of the noise. Sometimes considerable damage may be done to the mechanism by a loose or broken part if the engine is kept running when a defective condition has made itself known.

NOTE. For further information on Engine Control see "The Modern Gasoline Automobile."-By Pagé

LESSON TWENTY-SEVEN

METHODS OF SPEED CHANGING OUTLINED

Q. Name the principal types of speed changing systems. A. Speed changes may be effected by means of friction, planetary, sliding gear, and individual clutch gearing.

Q. Which are the simplest types to control?

A. The friction and planetary gearing are the easiest operated by the novice, though the individual clutch and selective sliding gear are handled just as easily by the more experienced motorist. Q. What is the feature of the planetary gear set?

A. The important feature of an epicyclic or planetary gear set is that the speed reduction gears are always in mesh and that any speed may be obtained instantly by a simple movement which tightens a friction band or clutch acting only for the speed selected. As the gears are always in mesh and as the operation of obtaining a speed and applying a clutch is simultaneous, there is no danger of stripping gears or of doing other damage to the transmission. Q. Name principal planetary gear car and describe controlling system.

A. Practically the only planetary gear car now on the market, but one that is sold in very large quantities, is the Ford Model T. The control system of this is shown at Fig. 251. The spark and throttle control are on the steering post under the wheel which is located at the left side of the car. The lever at the outside is for the spark advance while that on the right is the throttle lever controlling the gas supply. A single hand lever, which combines the function of an emergency brake actuator and high speed clutch release, is provided on the left side of the car. Three pedals are used, the one at the extreme left engaging the low speed when pushed forward and allowing the direct drive clutch which is spring engaged

High And Low Speed

Fig. 251.-Control System of Ford Model T Car Which Uses Planetary Change Speed Cearset.

to come into action if allowed to drop to its rearmost position. When the emergency brake lever is applied the high and low speed pedal is in a neutral position. The middle pedal engages the reverse ratio when pressed forward, while that at the extreme right is a foot brake acting on the gearset.

Q. Describe simplest form of planetary control system.

A. The simplest form of planetary gear control system is that by which a single lever convenient to the operator's hand is used to engage all speeds. The lever has five positions, three of which