Bisbee daily review., 1921-05-01
TIMING
 PROBLEMS ARE
 EXPLAINED HERE

the necessity of having the spark tim
 sd correctly. and the importance it
 has in the welfare of the engine. con-
 sidering power, overhesting, and gas
 oline consumption, and all around sat
 isfactory operation" says F. Thayer i of Thayer's Garage in Lowell.
 Thayer. looking over his mechani-
 Cal library on automobile construc-
 tion. supplys the following from "Au-
 tomobile Engineering:
 Like steam engine. an internal
 combustion motor depends for its power output on the mean effective
 pressure developed in the cylinder,
 usually referred to as it's m.e.p. This
 is effected directly by three factors:
 first, the initial compression of the
 charge. that is, the pressure to which
 the piston congresses the gaseous mixture on it's upward or compres-
 sion stroke just before firing; second. the time at which the charge Is ig
 nited; and third, the length of the stroke. It is with the second factor alone that this phase of the ignition l problem Is concerned In contrast ] with the steam engine in which the ! steam as admitted Is at compara- lively low pressure, and expands gradually, throughout tho stroke, the pressure develops in tho internal combustion motor at the moment of ignition Is tremendous, but it falls of very rapidly. The impulse given the piston is more in the form of a sharp blow that steady push, as with steam. The mean effective pres sure developed depends very largely upon the pressure reached at the mo- meut of explosion and this in turn depends upon the time ignition oc- CUTS without relation to the stroke. As the speed of an automobile motor varies over & wide range. it will bs apparent that means must be employ ed for the varying time of the ex- piosion. To be most efficient 1t must occur at the point of maximum com precision, i G., when the piston is ex- actly at the upper dead center on tho compression stroke As both a mechanical and electrical lag. or de- lay. must Do compensated for. the set ting which will give maximum eff ciency at 500 rpm. will be too slow at 15o0 rpm. aud the spark would then not take place until after the piston had started down again and the pressure had dropped consider. ably, causing great loss in power. On ths other hand, an attempt to run the motor slowly with a spark tim ing that would give the best results | at high speed would often result in causing the explosion to take place against tho rising piston. This is evi- denced by a hanamering sound and
 great falling off in the power
 Advance and Retard
 Means are accordingly provided in the majority Of ignition systems for causing the spark to occur earlier or later in the cylinders. This Is term | ed advancing and retarding the spark, the nomenclature being taken from
 the French with whom 1t originated.
 The explanation given in tho preced-
 ing paragraph for the necessity of this will make plain the car mak- . CTS often repeated injunction. to the
 novfce-uever drive with the spark retarded. Another and equally impor- tant reason Is that when operated this way, the combustion is incom- [ plete, the gas continues to burn throughout the stroke. and a greatly i increased percentage of its heat has to bo absorbed by the water jackets. causing the motor tn overheat badly. Adjusting for time factor coil: Ky cry induction coil has a certain time constant, which represents the period ! necessary to completely charge the coll. that is, tho time required for the current in tho primary winding to attain It's maximum value. This time constant depends very largely upon tho amount of magnetic energy which can be stored up In the coil. There must be added to this the tlme required to overcome the inertia of moving parts, such as the timer and vibrators Of q high tension battery system or the contact breaker and the distributor in megueto high ten sion system. As these parts are vrry small and light this would be practi- cally ineligible for any other purpose, but when figuring in .hundredths Of a second, as i'll tue case of the ignition timing of the high speed multi-cyiiu- der motors. it becomes of importance The object sought, ss already men toned ie to have the spark always occur at tho point of maximum com precision To accomplish this with the motor running at high speed, the ignition dsyises must act while the piston is still an spPrcciable distance below upper dead center. Tho timer in the case of battery system, or the contact breaker of a magneto, is ac- cordingly mounted so that 1t can be turned through part of revolution with relation to It's driving shaft or more particularly the cam carried by .f-N N N Nl.. ,e F.