IGNITION: Heartbeat of the Gasoline Engine
Spark plug fundamentals and their application to fire apparatus motors
JUST AS modem aircraft engines utilize dual ignition systems as a safety factor in flight, a large percentage of fire apparatus has had double ignition for over a decade as a precaution against engine failure. In aviation or in fire apparatus, the spark plug leads a double life in each cylinder. Should one ignition system falter, the other set of ignition components will furnish the vital spark until the job is done.
Fire apparatus dual-ignition systems usually fire 24 plugs in a 12-cylinder and 12 plugs in a 6-cylinder engine. In contrast to aircraft, fire apparatus dualignition systems may consist of a com District Service Engineer, Champion Spark Plug Company bination magneto and coil-battery system or a dual coil-battery system. In fact, very large aerial ladder units sometimes employ a four-coil, four-battery system. If you, as a fireman, have ever waited for your master mechanic to track down an elusive “bug” in 12 or 24 spark plugs, then you can imagine the time required in checking 144 to 224 spark plugs and the thousands of ignition parts on a fourengine plane.
The aircraft flight engineer has an instrument, a cathode-ray oscilloscope, which he uses in flight to trace impending ignition failure to any of the 144 to 224 spark plugs and to pinpoint with accuracy the particular cylinder and spark plug in question. This instrument is called an ignition analyzer and, like a doctor’s x-ray, can look inside the ignition system and show the exact performance of every plug and part of the intricate network. This oscilloscope is wired directly to the ignition system and acts as an electronic seeing-eye dog.
Over four years ago, the service department of the Champion Spark Plug Company, realizing the value of the ignition analyzer, equipped their service engineers with automotive-type models. As a service engineer, I had my first lesson in humility when I tried to hook up my oscilloscope on a pumper with four coils, 24 plugs, two distributors and two sets of points in each distributor. Having an extra ignition system on the same engine more than doubles the complications of tune-up and ignition suppression. But it didn’t take me long to learn that you need this kind of dependability when it takes some 200 hp to supply high volume and pressure to hose lines. Engine failure at such a critical time may cost many hundreds of thousands of dollars in damage—or even lives.
But even a dozen ignition systems are to no avail if the master mechanic is not completely familiar with some of the problems that can come up on ignition service. Manufacturers go to great lengths to make all the latest information available to the automotive trade on the subject of improving ignition performance.
Let’s take a look at some of the fine points of ignition tune-up on modern high compression engines. Remember that the minute details in ignition service can make the difference between dependability and failure.
Distributor assemblies, for example, must be checked carefully as a part of the regular tune-up. In addition the high resistance corrosion of the contact points inside the distributor cap, small cracks and carbon tracking between the inserts often cause missing under heavy load. These conditions are often overlooked during the course of a superficial examination of the distributor cap. The best way to do it is to look inside the cap with a concentrated light in order to find the trouble spot.
As compression ratios increase, ignition timing becomes more critical. Retarded timing will noticeably cut down on power on the newer engines, while excessive advance will create abnormally high temperatures in the cylinders. This can result in harmful detonation and preignition. This is why manufacturers recommend the use of timing lights for spark adjustments. The old trial-anderror methods of timing are no good on modern high-compression engines. Detonation usually occurs in an engine before you can hear it. so “timing by ear” is a poor guess at best.
It must also be remembered that automatic spark advance occurs within the distributor. As the engine speed increases, a timing light will show whether or not the advance mechanism is working properly or whether it is erratic due to excessive wear in the distributor bushing or the drive gears. Field test procedures call for accelerating the engine slowly to see if timing advances smoothly. Sometimes this excessive wear in distributor drive train will cause fluttering or “fanning” of the timing marks at certain rpm. If the flutter exceeds 3 degrees, distributor overhaul or other corrections is advised as the engine is obviously not firing at the proper time.
Keep in mind, too, that breaker point spacing will directly affect spark timing. A wider gap advances the timing while a closer spacing retards the spark. As a general rule .001 inch will change the timing about I degree. Therefore it’s a good practice to rechcck the distributor adjustment whenever the points are installed or respaced.
Another point in ignition tune-up is the question of polarity—the direction in which current “jumps” the spark plug gap. Battery’ ignition systems should provide negative current at the spark plug terminal, meaning that less voltage is required to fire the plugs. On older model engines, reversed polarity presented no particular problem. Ignition systems had surplus power for the lower voltages required. How’ever, in current high-compression engines, it’s another story.
According to the findings of research engineers 35 to 45 per cent less voltage is required from a typical ignition system when spark polarity is correct. This can be enough margin to eliminate hard starting, rough idle and missing under loads when usual corrections would fail to eliminate the trouble.
Polarity is changed by reversing the primary wire leads at the coil. A vehicle Will run with the wrong connections, so it’s something to check on a tune-up. This can be done with a standard coil tester or an ordinary high-tension volt meter. Simply ground the positive lead and touch the primary lead momentarily to the spark plugs. If the needle swings up, polarity is O.K.
Polarity can also be checked by holding the ignition cable about a quarter of an inch away from the spark plug terminal while the engine is running. Then insert the point of a soft pencil in the spark discharge. (I always use a wood pencil myself!) If the spark “feathers” and takes an orange tinge toward the plug, polarity is correct. If the feathering occurs towards the cable, reverse the coil primary leads.
An unsuspected source of trouble can be paint which is sprayed on the spark plug insulators, ignition wires and distributor. The lead in the paint is a good conductor of electricity, causing short circuits, and also increasing the capacitance of the ignition lead. This tends to load the system and encourage misfire. Also a new or reconditioned engine which has been spray-painted may have a clogged sent hole in the distributor housing, caused by oversprax. This in turn will cause high resistance corrosion inside the distributor, plus point burning, both of which can ruin an otherwise good tune-up job.
What makes a hot plug hot
The subject of the heat range is of particular importance to anyone maintaining a dual-ignition system. Quite often different heat ranges will be specified for spark plugs within the same cylinder, because of the difference of location of the spark plug. When the plug is situated right in the flame path or the exhaust flow, it is probably going to require a colder heat range than one located in the path of the incoming fuel.
A “hot” plug does not necessarily fire a “hotter” spark. It simply runs at higher temperature longer than a “cold’ plug. For example, in an application which requires extended periods of idling, with a tendency for the plugs to accumulate soft carbon deposits, a higher heat range may very likely be recommended in order to burn off this carbon. In highspeed operation, a colder heat range may be recommended. In this case it is necessary to dissipate the heat quickly, and carbon deposits are less of a problem.
Operating conditions for fire apparatus are such that a “happy medium” is most important. For example, many fire departments abide by the code that engines must be started up once or twice a day and idled to bring up oil to the cylinder top. This also serves the purpose on later model equipment of keeping pressure in the air brake reservoir. These periods of idle, of course, are conducive to spark plug fouling. At the same time, these engines may have extended periods of high speed, heavy load operation while pumping at the scene of a fire. While the idling conditions would be best controlled by a hotter plug, the high-speed operation requires a colder heat range. For this reason, a great deal of attention must be paid to selection of the best heat range.
A happy medium must also be established with regard to proper gap setting. If the gap is sot too narrow you may have a rough idle condition, while too wide a gap may cause misfiring under heavy load. That’s why it is important to give special attention to frequent spark plug check in order to keep electrode gap as close as possible to correct specifications.
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One development which has been extremely valuable to the fire apparatus engineer is the “U gap” or “auxiliary gap” spark plug. This plug, which has an air gap in the center electrode, is especially well suited to low speed and idling conditions, yet it performs well at high rpm. It has been approved, and is now being used, by a number of original equipment manufacturers.
Too many accessories?
A point well worth mentioning is the tendency of some mechanics to overload the primary wiring by installing sirens, flashers, blinkers, electric fuel pumps, and other accessories which cause a voltage drain. When ail this load is tapped into the primary ignition system, it lowers the secondary output to the plugs. If this condition exists in your equipment, you may be hindering your apparatus’ performance because of weak secondary voltage to the spark plugs. Service engineers will be glad to offer their assistance in checking this out for you.
Periodic servicing of spark plugs is, of course, essential to efficient operation. Certainly, on a tune-up, they should be serviced or replaced. Plugs should be cleaned and regapped every 3,000 to 5,000 miles in order to keep them clear of deposit which accumulates from the products of combustion. Such deposits may form rapidly at constant throttle during prolonged periods of pumping.
A wet, sludgy deposit is usually caused by oil which gets past worn piston rings or valve stem guides. This can be helped temporarily with the use of a hotter plug but eventually requires an engine overhaul.
A dry carbon deposit generally indicates that the fuel-air mixture is too rich. This may be the result of an improper adjustment within the carburetor, a stuck choke valve or a clogged air cleaner. Using a hotter plug will not permanently correct this difficulty.
When the electrodes are badly worn away, but the insulator is clean and dead gray in color, the probability is excessive heat. The next colder plug may correct this condition, generally caused by severe operating conditions. But spark timing should be checked first, since an overadvanced spark will cause plug burning.
If you’re having trouble with vehicle radio noise, you may be blaming the ignition system. Some critical radio installations have complete shielding of the ignition system to fully eliminate noise. But there are many other reasons for radio noise besides the ignition system, so check these major sources of interference before assuming that additional suppression is needed in the secondary circuit. It may save extra work!
Ignition noise is usually a popping sound varying in frequency with engine speed. It disappears when the ignition switch is turned off. This interference can usually be confined to the engine compartment by bonding (or grounding) of the hood, steering column, etc., to present its radiation to the antenna or radio. (Hood grounding may be especially poor on cars and apparatus two to three years old.) Many vehicles use suppressors in the rotor, distributor center lead or in the cables; these should be in good condition.
Generator noise is a high-pitched whine in the radio and varies with engine speed. It will persist, however, if ignition is cut off with the engine running at moderate speed. This can be corrected by the use of a by-pass capacitor between the armature terminal and ground. (Don’t connect it to the field terminal!)
Generator regulator noise is an irregular, rasping sound traceable to arcing of contacts. It will not persist below the charging cut-out point of the regulator and can be eliminated by a by-pass capacitor at the battery terminal.
Front wheel noise generally occurs only in dry weather and at speeds ox’er 15 mph. The irregular popping or “rushing” sound usually disappears when brakes are lightly applied. Static collectors in the hubs or special graphite antistatic powder in the tires handle this problem.
Shield grounds important
In all cases, while the antenna itself must not be grounded, the lead-in shield should not be broken and must be properly grounded at both the antenna and radio chassis. If noise disappears when the antenna is disconnected at the radio, it indicates that the antenna is picking up interference. If noise still persists, it is probably being transmitted to the radio via the vehicle power lines; this necessitates installation of a by-pass condenser from the “A” lead to ground.
If additional supression at the spark plugs is required for good reception on certain critical wave bands, the use of resistor plugs may be necessary.
The information set forth here merely highlights the main points of ignition tune-up service. Maintenance mechanics should also become entirely familiar with the service manuals accompanying their apparatus and should keep up to date with the service bulletins issued by the manufacturers. This literature, plus pertinent trade publication articles, will keep any mechanic abreast of latest procedures.