An in-depth look at various ignition apparatuses and engine systems, including hot tube ignition, low-tension systems and flame ignition
Sectional disgram showing the construction of a valve-in-head engine.
Editor's Note: This is a reprint from the book Internal Combustion Engines and Tractors, Their Development, Design, Construction, Function and Maintenance. Notes of a series of lectures, delivered by Oliver B. Zimmerman, of the Engineering Staff, International Harvester Company, Chicago, copyrighted 1920, IHC. It was sent to us by Paul R. Bell, 247 Eldora do Avenue, Louisville, Kentucky 40218.
The function of the ignition system is to provide a controlled means of firing the charge of fuel and air mixture.
Various means have been used in the past for this purpose, yielding knowledge which, owing to its value in showing development and requirements, is here reviewed.
It is well to realize something of the exacting requirements of an ignition apparatus, especially the spark plugs or igniters. First, the parts within the cylinder are subjected to the intense heats produced during ignition and burning of the charge 2000 to 4000 degrees Fahrenheit, and to sudden changes during each cycle, ranging from this exceedingly high temperature down to the cool temperature of the incoming charge of air.
Next, consider the time required to complete this work. Take an engine running 600 rpm, and using spark plug ignition. It makes one revolution in 1-10th of a second. If we ignite the charge 30 degrees ahead of center, there being 360 degrees in each revolution, the time of ignition would be 30-360ths. This allows only 1-12th of the time required for one revolution or 1-120th of a second for the charge to burn and create its pressure within the cylinder a very short time even for an engine of moderate speed. In that time the spark must jump between the points, the flame start and spread throughout the cylinder to all its corners, if best results are to be obtained. If the cylinder has irregular pockets, the flame must follow into them requiring more time than if there were no pockets. This accounts for the fact that L-head and T-head engines act either slower or with less power than the valve-in-head types.
The first method of firing was by igniting the mixture with a naked flame. This flame was easily and often blown out, so the engine required constant attention from the operator. The uncertainty of action quickly put this system aside in favor of hot tube ignition. Hot tube ignition consists of a piece of pipe closed at one end and screwed into the cylinder so that the inside of the tube can receive a part of the burnable mixture during compression. This tube was heated by means of a flame from the outside, thus producing the heat necessary to ignite the charge, but without the danger of blowing out the flame except in windy weather. The control had, therefore, been much improved but permanence and close regulation were still lacking.
Further refinement resulted in the design of the hot bulb ignition, which operates much the same as the hot tube ignition. Its main weakness consisted in the time required to get the engine under way from six to fifteen minutes to get up to speed without load and longer to get to the point of taking full load.
Operation necessitating the open flame of a gasoline torch to start and get under way is a distinctly undesirable system when frequent starts and stops are made, where widely varying loads are frequent, where fires are made possible, or where close regulation is desirable.
Compression ignition was introduced with the Diesel and semi-Diesel types of engine and consists in a reduction of the compression space to a degree where sufficient pressure is produced and heat enough generated to cause complete burning of the fuel injected into the highly compressed air. Compression of from 250 lb. to 500 lb. is used, fuel being injected under pressures above those of compression. None of the types of ignition suit the requirements of tractors as well as electric ignition.
Two systems of electric ignition are used to deliver a spark to ignite the fuel mixture in the cylinder of an engine low tension or 'make-and-break' ignition and high tension or "jump spark" ignition. The source of electricity for either system can be a battery or a magneto. Low tension means low voltage; high tension means high voltage.
The low tension system requires an igniter which must have some mechanical means of opening or breaking apart the igniter points to produce a spark gap. To get a hot spark at this gap requires a large volume of current and a low pressure or a low tension. This kind of current requires only a primary winding in the spark coil or magneto.
The high tension or jump spark system always uses a spark plug which has a permanent spark gap that the spark must jump across.
The current produced by a low tension system while of large volume does not have enough pressure to jump across this spark gap. To produce the high pressure necessary we add a secondary winding to the primary winding of the spark coil or magneto and thus induce in this secondary winding and circuit a voltage high enough to bridge the spark gap of a spark plug. This secondary winding makes a high tension or high voltage system of a low tension system.
By a "good" spark we ordinarily mean a large, hot spark. Igniting the charge in a cylinder may be likened to kindling a fire. By using plenty of kindling and a hot blow torch flame all over it at once, the fire would burn much quicker than if it were lighted with an ordinary match. Just so with lighting a fuel mixture in the engine cylinder. A big, hot, fat spark that will make the mixture burn as fast as possible is most desirable.
The only reason internal combustion engines must be operated with an early spark is in order to give the mixture time to thoroughly burn. We have not yet developed a spark so hot and so big within the cylinder that burning will be practically instantaneous. With present ignition sparks, flame propagation is so slow that unless the mixture is ignited a long way ahead of dead center it will still be burning when the exhaust valve opens, and a great deal of fuel will be thrown out of the exhaust valve unburned. Excessive heat and loss of power also result.
Two types of magnetos for generating electrical energy to produce a spark are used, oscillating magnetos and rotary magnetos. Oscillating magnetos are low tension, rotary can be either high or low tension. As a general rule high tension or jump spark ignition is used on the higher speed engines, and the low tension or make-and-break ignition is used on the slow speed engines. Formerly in all cases batteries were used for starting, and when the engine reached proper speed the magneto was switched in and then the engine ran on the magneto. Now some means is provided in the magneto it self, so that practically the same spark is given when the engine is turned over slowly as when it is running at normal or full speed. In the high tension rotary type of magneto this is taken care of by an impulse starter. In the oscillating type of magneto this is taken care of by retarding the spark.
The impulse starter on a high tension rotary magneto is a mechanical contrivance which temporarily locks the armature of the magneto and winds up a spring, and is arranged so that the impulse starter will trip off about 3 degrees after dead center. When impulse starter is released or tripped, the spring is at such tension that it revolves the armature at practically the same speed as the-armature would be revolving were the engine running at its normal speed.
On the oscillating magneto the time of tripping is made later than when the engine is-running at normal speed, and by doing this the oscillator springs are at a greater tension when oscillator is tripped, so that the magneto actually produces a better spark when the engine is just turned over slowly for starting than when the engine is running at its normal speed with the spark advanced. Tripping of the oscillator for starting is about dead center of crankshaft.
It is necessary to use a late or retarded spark when starting, as the engine is moving very slowly, and if the mixture ignites before the piston comes to dead center, then the rotation of the engine will be reversed, as there is not enough momentum in the flywheel to carry it over dead center putting heavy strain on bearings and crankshaft. A retarded spark is also a hotter spark than spark in the advanced position. This hotter spark is necessary because, when starting the engine, the fuel mixture is not well balanced, as the speed is slow and the engine is cold.
The high tension rotary magneto with impulse starter furnished current for jump spark ignition on Titan 10-20 and International 15-30 tractors.
High tension magnetos may be tested for spark by removing cable and holding cable connection about 1/4" from end of binding post; or turning the flywheel, a spark will result if magneto is in good condition. A good fat spark should result at all times if impulse starter operates.
The magneto is driven clockwise (looking at it from impulse starter end) from the shaft through a heavy coil spring and runs as a rotary type.
(see image 7 in the image gallery) For starting the engine, the armature of the magneto is prevented from rotating by the pawl B engaging the notch A when the lever C is released while the shaft revolves through a part of a revolution, increasing the tension in the coil spring.
When the point D strikes the lug on the side of the pawl B lifting it out of the notch A, the tension of the coil spring causes the armature to spring forward quickly and produces a spark.
This process is repeated until speed of shaft reaches such a point that the force of point D striking the pawl B throws it up far enough to permit the lever C to lock it.
a. Align well with drive coupling and shaft.
b. Keep bracket and magneto tight on engine.
c. Time so impulse starter will trip when crank is about 3" past inner dead center of compression stroke.
d. Then adjust spark advance connection between armature control arm and advance lever so full retard and advance can be obtained.
e. Impulse starter should always hook up automatically when engine is about up to speed. If it does not, then spring controlling hook-up is too stiff and must be adjusted. Then again, if impulse starter hooks up too quickly or before engine is under way, spring is too weak and must be adjusted to act as stated above.
f. In starting trip impulse starter, put spark advance lever in retard position. This will prevent a kick back and reduce strains on engine that otherwise would occur with spark lever advanced. Engine speed is too slow at starting for advanced spark.
g. Distributor brush must be on and in good contact on distributor segment at both early and late spark positions.
h. Movable breaker arm action must be free, and stationary breaker arm must be well insulated and tight in housing so that the proper action is obtained to secure a good spark. The gap between breaker points should always be 1/64" or not to exceed .016" to .018".
i. Keep distributor brush and race free from carbon dust and dirt and keep breaker points clean, even and smooth.
j. Oil sparingly and keep clean to prevent short circuits. Use a light oil.
k. Don't allow tools or any hardened steel in contact with magnetos. Don't try to magnetize things from magnetothat will kill its life.
I. Keep cables from distributor terminals to spark plugs well insulated from water, oils and metals.
m. Keep gap of spark plug points from .025" to .30" apart and keep points and core spacing free from water, dirt and carbon at all times for satisfactory starting and running ignition conditions.
n. Magneto bars or armature should not be removed. If they ever are, be sure to place soft iron or steel bars between ends of magneto bars before removing.
o. If magneto bars are removed be sure to put them back right so all north poles come together on same side and all south come together on the other side. If reversed they will neutralize one another and prevent operation.
p. The firing rotation of International 15-30 is 1, 2, 4, 3. Cylinder No. 1 is the left hand looking from rear to front of tractor. The firing rotation of the International 8-16 (four-cylinder) is 1, 3, 4, 2. Cylinder No. 1 is the first cylinder looking to the rear of the tractor.
q. It is good practice to put some cup grease on impulse starter springs when putting together after cleaning. This will improve workings and reduce wear.
To test a spark plug, remove plug from cylinder head and place in position on cylinder with a piece of paper the thickness of an ordinary business card between the points. Set trip on magneto for starting and crank engine over slowly. If the plug is in good condition a spark will occur between the points and a small hole will be burned through the paper. Spark plug points should be set about .025" or .030" apart.
Caution. Do not screw plugs in too tight or they will burn in so it will be impossible to re move them without ruining them.
The gap should be from .025" to .030" or the thickness of gauge on screw driver sent with tools.
There are so many International Harvester internal combustion engines and tractors in use everywhere that, as a matter of information, we give herewith suggestions for the care and operation of types of ignition systems other than used at present on International Harvester tractors.
a. Attach and time so magneto will trip at starting (magneto tripping retarded) when crank is on or close to inner dead center of compression stroke.
b. Oscillating magnetos, when tripping, do not produce spark in engine until trip finger returns after being released and strikes arm on movable electrode, opening igniter points, so the time elapsing between tripping and the parting of igniter points as on Webster-Horse Shoe and Accurate magnetos should be about 8 degrees earlier, considering crank position, than the time desired to have spark occur in that particular engine, and with an oscillating Bosch magneto tripping must occur about 12 degrees earlier, because springs on Bosch are longer, consequently slower acting.
c. Although magneto trip timing may be adjusted right, the opening of igniter points may not occur at right time, as indicated above, unless igniter trip adjusting screw, rod or hook rod, as the case may be, are adjusted to open igniter points.
d. To use batteries with oscillating magnetos, set igniter trip adjusting screw, rod or hook rod to hold igniter points apart not less than 1/16", then when trip rod moves trip finger away from trip adjusting screw or rods, igniter points will come together, setting up the circuit through battery and coil, filling the coil, making ready for spark, producing ignition conditions as satisfactory as with regular igniter.
e. For a good, hot spark, trip rod must come in contact with magneto trip finger when crank has traveled in on compression stroke about 45 degrees or 50 degrees.
f. Keep tight on bracket; keep clean; don't over-oil; keep contact and timing correct; one wire only from magneto terminal to stationary electrode; keep in good alignment; don't abuse by careless handling.
(a) Select dry cells of good strength.
(b) Keep dry and pack well for long life of cells.
(c) Keep coils insulated from one another. Never let zinc covering one cell touch zinc covering of another cellit will kill batteries.
(d) Cells are more active when warm than cold.
(a) Always have wires tight at all terminals.
(b) Always wire from positive, carbon or center terminal of dry cells to spark coil, and from coil direct to igniter for satisfactory results.
(c) Always wire from negative, zinc or outer terminal of dry cells to ground point or circuit breaker on engine.
(d) Always keep spark wires well insulated against water, oil and metals and from one another to insure satisfactory current and increased life of batteries.
3. Spark Coil
(a) Always keep spark coil dry. If wet, will rust and destroy its efficiency.
(b) Keep free from vibration to prevent wires from breaking.
(c) Always keep wires tight at coil terminals for in creased battery life.
(a) Stationary electrode must be well insulated against heat, pressures, fuel, oil and water penetration effects so that current can be carried to igniter points when needed for spark.
(b) Be sure to have movable electrode free enough so igniter points can part quickly, thereby producing a hot fat spark so necessary for good ignition. Igniters should have a break gap from 1/32" to 3/32", depending on size of engine.
(c) Be sure of good clean tight contact between spark wire and stationary electrode at all times.
(d) Cleanliness in any ignition system aids in producing good results.
(e) Always keep ignition timing correct, from 5 degrees to 7 degrees of inner center on compression stroke for every 100 rpm of engines of low or medium speed.
(f) In order to fill coil completely with current, be sure to have igniter points come in good contact when crank has traveled half way between outer and inner center of compression stroke.
(g) Ignition advance is directly related to engine speeds, kind of fuels used (quick or slow burning), engine design and altitude. High altitude gives rare air a slow burning mixture, consequently earlier ignition will be necessary.
(a) In addition to general points mentioned for make-and-break, and as this system carries a very much higher voltage current than in the make-and-break system, it becomes quite apparent that insulation in this system should be of the best in order to keep current in the right path. The tendency of an electric current is to seek the line of least resistance, so with poor insulation current would be lost on the way, and none or poor ignition will result.
(b) Wire from cells as per above to primary terminal of coil from the other primary coil terminal to commutator or contact terminal on engine, and from negative side of cell to engine ground, and high tension cable from induction terminal of coil to spark plugs.
2. Spark Plug
(a) Keep spark plugs clean.
(b) Spark plug gaps should be from .025 to .030 of an inch apart. No. 24 gauge sheet steel will be right for measuring gap space in plugs. If gap space is too small 6r too great, it tends to destroy points and unsatisfactory ignition results.
(c) Indications point to better results with single than with multiple gap plugs.
3. Spark Coil
(a) Adjust induction coil vibrator to produce silvery toned, evenly spaced vibrations.
To save batteries, don't have vibrator points too close together.
(a) Battery used in starting, auto sparker for running, then wire up as follows: From battery as above stated to one side of three-point switch, then from any one of armature terminals to other side of switch, then from primary coil to lower point of switch, then from coil terminal to stationary electrode of igniter, then join wires from negative side of cells and wire from second armature terminal of auto sparker, then the spliced ground wire to ground point or circuit breaker on engine frame.
(b) Adjust auto sparker governor so speed does not exceed 1,000 r.p.m., and see that auto sparker is properly lined up on bracket.
(c) Keep armature, brushes and contact points clean.
(d) Never wire battery and auto sparker on same circuit.
(e) By changing field wires from one terminal to the other the rotation of armature must be changed correspondingly to deliver current to brushes.
(a) Time engine correct for spark.
(b) Time magneto according to timing marks shown on armature shaft and hub so mark for either r. h. or l. h. rotation will be in line with one another when igniter is tripping in correct time, as described above.
(c) Wire up as described in Sec. I positive wire from coil to one side of three-point switch and from magneto only to terminal to the other side of switch, then wire from lower point of switch to igniter, and wire from negative terminal of cells to engine ground or circuit breaker.
(d) Take good care of your magneto. Keep tight on bracket, keep race and brushes clean. Don't over-lubricate, don't give it more than a few drops of oil once or twice a week.
(e) Keep tools away from magnetos and wiring.
(f) Don't hammer on bars because their effectiveness will then be lost.
(g) Don't try to magnetize everything possible by magneto bars. That will lower strength and shorten useful life of magneto.
(h) Don't wire coil nor battery and coil in magneto circuit, because it will kill battery and lower magneto life quickly and will also tend to prevent satisfactory starting and operation.
(i) Don't remove magneto bars nor armature, or they will lose about fifteen percent of their strength.
(j) If magneto bars or armature have to be removed, be sure to put a soft iron bar across from north to south poles of bars to prevent loss of magnetism.
(a) Keep in time with engine, Sec. IV, a and b.
(b) Don't wire up in circuit with battery and coil.
(c) No coil wanted or needed.
(d) Keep clean as described in Sec. III, c.
(e) Care for your magneto Sec. IV, e, f, g, h, i, j.
(f) Don't over-oil Sec. IV, d.
(g) Wire direct from magneto terminal to igniter.
(h) In starting with rotary magnetos only, two revolutions fast is better than 100 revolutions slow.
(i) Care of igniter Sec. IV, a, b, c.