Aviation Engines Part 13

CARE OF THE DIXIE MAGNETO

The bearings of the magneto are provided with oil cups and a few drops of light oil every 1,000 miles are sufficient. The breaker lever should be lubricated every 1,000 miles with a drop of light oil, applied with a tooth-pick. The proper distance between the platinum points when separated should not exceed .020 or one-fiftieth of an inch. A gauge of the proper size is attached to the screwdriver furnished with the magneto. The platinum contacts should be kept clean and properly adjusted. Should the contacts become pitted, a fine file should be used to smooth them in order to permit them to come into perfect contact. The distributor block should be removed occasionally and inspected for an acc.u.mulation of carbon dust. The inside of the distributor block should be cleaned with a cloth moistened with gasoline and then wiped dry with a clean cloth. When replacing the block, care must be exercised in pushing the carbon brush into the socket. Do not pull out the carbon brushes in the distributor because you think there is not enough tension on the small bra.s.s springs. In order to obtain the most efficient results, the normal setting of the spark-plug points should not exceed .025 of an inch, and it is advisable to have the gap just right before a spark-plug is inserted.

The spark-plug electrodes may be easily set by means of the gauge attached to the screwdriver. _The setting of the spark-plug points is an important function which is usually overlooked, with the result that the magneto is blamed when it is not at fault._

TIMING OF THE DIXIE MAGNETO

[Ill.u.s.tration: Fig. 69A.--Sectional Views Outlining Construction of Dixie Magneto with Compound Distributor for Eight-Cylinder Engine Ignition.]

In order to obtain the utmost efficiency from the engine, the magneto must be correctly timed to it. This operation is usually performed when the magneto is fitted to the engine at the factory. The correct setting may vary according to individuality of the engine, and some engines may require an earlier setting in order to obtain the best results.

However, should the occasion arise to retime the magneto, the procedure is as follows: Rotate the crank-shaft of the engine until one of the pistons, preferably that of cylinder No. 1, is 1/16 of an inch ahead of the end of the compression stroke. With the timing lever in full r.e.t.a.r.d position, the driving shaft of the magneto should be rotated in the direction in which it will be driven. The circuit breaker should be closely observed and when the platinum contact points are about to separate, the drive gear or coupling should be secured to the drive shaft of the magneto. Care should be taken not to alter the position of the magneto shaft when tightening the nut to secure the gear or coupling, after which the magneto should be secured to its base. Remove the distributor block and determine which terminal of the block is in contact with the carbon brush of the distributor finger and connect with plug wire leading to No. 1 cylinder to this terminal. Connect the remaining plug wires in turn according to the proper sequence of firing of the cylinders. (See the wiring diagram for a typical six-cylinder engine at Fig. 70.) A terminal on the end of the cover spring of the magneto is provided for the purpose of connecting the wire leading to a ground switch for stopping the engine.

A special model or type of magneto is made for V engines which use a compound distributor construction instead of the simple type on the model ill.u.s.trated and a different interior arrangement permits the production of four sparks per revolution of the rotors. This makes it possible to run the magneto slower than would be possible with the two-spark form. The application of two compound distributor magnetos of this type to a Thomas-Morse 135 horse-power motor of the eight-cylinder V pattern is clearly shown at Fig. 71.

[Ill.u.s.tration: Fig. 70.--Wiring Diagram of Dixie Magneto Installation on Hall-Scott Six-Cylinder 125 Horse-Power Aeronautic Motor.]

SPARK-PLUG DESIGN AND APPLICATION

[Ill.u.s.tration: Fig. 71.--How Magneto Ignition is Installed on Thomas-Morse 135 Horse-Power Motor.]

With the high-tension system of ignition the spark is produced by a current of high voltage jumping between two points which break the complete circuit, which would exist otherwise in the secondary coil and its external connections. The spark-plug is a simple device which consists of two terminal electrodes carried in a suitable sh.e.l.l member, which is screwed into the cylinder. Typical spark-plugs are shown in section at Fig. 72 and the construction can be easily understood. The secondary wire from the coil is attached to a terminal at the top of a central electrode member, which is supported in a bushing of some form of insulating material. The type shown at A employs a molded porcelain as an insulator, while that depicted at B uses a bushing of mica. The insulating bushing and electrode are housed in a steel body, which is provided with a screw thread at the bottom, by which means it is screwed into the combustion chamber.

[Ill.u.s.tration: Fig. 72.--Spark-Plug Types Showing Construction and Arrangement of Parts.]

When porcelain is used as an insulating material it is kept from direct contact with the metal portion by some form of yielding packing, usually asbestos. This is necessary because the steel and porcelain have different coefficients of expansion and some flexibility must be provided at the joints to permit the materials to expand differently when heated. The steel body of the plug which is screwed into the cylinder is in metallic contact with it and carries sparking points which form one of the terminals of the air gap over which the spark occurs. The current entering at the top of the plug cannot reach the ground, which is represented by the metal portion of the engine, until it has traversed the full length of the central electrode and overcome the resistance of the gap between it and the terminal point on the sh.e.l.l. The porcelain bushing is firmly seated against the asbestos packing by means of a bra.s.s screw gland which sets against a f.l.a.n.g.e formed on the porcelain, and which screws into a thread at the upper portion of the plug body.

The mica plug shown at B is somewhat simpler in construction than that shown at A. The mica core which keeps the central electrode separated from the steel body is composed of several layers of pure sheet mica wound around the steel rod longitudinally, and hundreds of stamped steel washers which are forced over this member and compacted under high pressure with some form of a binding material between them. Porcelain insulators are usually molded from high-grade clay and are approximately of the shapes desired by the designers of the plug. The central electrode may be held in place by mechanical means such as nuts, packings, and a shoulder on the rod, as shown at A. Another method sometimes used is to cement the electrode in place by means of some form of fire-clay cement. Whatever method of fastening is used, it is imperative that the joints be absolutely tight so that no gas can escape at the time of explosion. Porcelain is the material most widely used because it can be glazed so that it will not absorb oil, and it is subjected to such high temperature in baking that it is not liable to crack when heated.

The spark-plugs may be screwed into any convenient part of the combustion chamber, the general practice being to install them in the caps over the inlet valves, or in the side of the combustion chamber, so the points will be directly in the path of the entering fresh gases from the carburetor.

Other insulating materials sometimes used are gla.s.s, steat.i.te (which is a form of soapstone) and lava. Mica and porcelain are the two common materials used because they give the best results. Gla.s.s is liable to crack, while lava or the soapstone insulating bushings absorb oil. The spark gap of the average plug is equal to about 1/32 of an inch for coil ignition and 1/40 of an inch when used in magneto circuits. A simple gauge for determining the gap setting is the thickness of an ordinary visiting card for magneto plugs, or a s.p.a.ce equal to the thickness of a worn dime for a coil plug. The insulating bushings are made in a number of different ways, and while details of construction vary, spark-plugs do not differ essentially in design. The dimensions of the standardized plug recommended by the S. A. E. are shown at Fig. 73.

[Ill.u.s.tration: Fig. 73.--Standard Airplane Engine Plug Suggested by S.

A. E. Standards Committee.]

It is often desirable to have a water-tight joint between the high-tension cable and the terminal screw on top of the insulating bushing of the spark-plug, especially in marine applications. The plug shown at C, Fig. 72, is provided with an insulating member or hood of porcelain, which is secured by a clip in such a manner that it makes a water-tight connection. Should the porcelain of a conventional form of plug become covered with water or dirty oil, the high-tension current is apt to run down this conducting material on the porcelain and reach the ground without having to complete its circuit by jumping the air gap and producing a spark. It will be evident that wherever a plug is exposed to the elements, which is often the case in airplane service, that it should be protected by an insulating hood which will keep the insulator dry and prevent short circuiting of the spark. The same end can be attained by slipping an ordinary rubber nipple over the porcelain insulator of any conventional plug and bringing up one end over the cable.

TWO-SPARK IGNITION

On most aviation engines, especially those having large cylinders, it is sometimes difficult to secure complete combustion by using a single-spark plug. If the combustion is not rapid the efficiency of the engine will be reduced proportionately. The compressed charge in the cylinder does not ignite all at once or instantaneously, as many a.s.sume, but it is the strata of gas nearest the plug which is ignited first.

This in turn sets fire to consecutive layers of the charge until the entire ma.s.s is aflame. One may compare the combustion of gas in the gas-engine cylinder to the phenomenon which obtains when a heavy object is thrown into a pool of still water. First a small circle is seen at the point where the object has pa.s.sed into the water, this circle in turn inducing other and larger circles until the whole surface of the pool has been agitated from the one central point. The method of igniting the gas is very similar, as the spark ignites the circle of gas immediately adjacent to the sparking point, and this circle in turn ignites a little larger one concentric with it. The second circle of flame sets fire to more of the gas, and finally the entire contents of the combustion chamber are burning.

While ordinarily combustion is sufficiently rapid with a single plug so that the proper explosion is obtained at moderate engine speeds, if the engine is working fast and the cylinders are of large capacity more power may be obtained by setting fire to the mixture at two different points instead of but one. This may be accomplished by using two sparking-plugs in the cylinder instead of one, and experiments have shown that it is possible to gain from twenty-five to thirty per cent.

in motor power at high speed with two-spark plugs, because the combustion of gas is accelerated by igniting the gas simultaneously in two places. The double-plug system on airplane engines is also a safeguard, as in event of failure of one plug in the cylinder the other would continue to fire the gas, and the engine will continue to function properly.

In using magneto ignition some precautions are necessary relating to wiring and also the character of the spark-plugs employed. The conductor should be of good quality, have ample insulation, and be well protected from acc.u.mulations of oil, which would tend to decompose rubber insulation. It is customary to protect the wiring by running it through the conduits of fiber or metal tubing lined with insulating material.

Multiple strand cables should be used for both primary and secondary wiring, and the insulation should be of rubber at least 3/16 inch thick.

The spark-plugs commonly used for battery and coil ignition cannot always be employed when a magneto is fitted. The current produced by the mechanical generator has a greater amperage and more heat value than that obtained from transformer coils excited by battery current. The greater heat may burn or fuse the slender points used on some battery plugs and heavier electrodes are needed to resist the heating effect of the more intense arc. While the current has greater amperage it is not of as high potential or voltage as that commonly produced by the secondary winding of an induction coil, and it cannot overcome as much of a gap. Manufacturers of magneto plugs usually set the spark points about 1/64 of an inch apart. The most efficient magneto plug has a plurality of points so that when the distance between one set becomes too great the spark will take place between one of the other pairs of electrodes which are not separated by so great an air s.p.a.ce.

[Ill.u.s.tration: Fig. 74.--Special Mica Plug for Aviation Engines.]

SPECIAL PLUGS FOR AIRPLANE WORK

Airplane work calls for special construction of spark-plugs, owing to the high compression used in the engines and the fact that they are operated on open throttle practically all the time, thus causing a great deal of heat to be developed. The plug shown at Fig. 74 was recently described in "The Automobile," and has been devised especially for airplane engines and automobile racing power plants. The core C is built up of mica washers, and has square shoulders. As mica washers of different sizes may be used, and accurate machining, such as is necessary with conical clamping surfaces, is not required, the plug can be produced economically. The square shoulders of the core afford two gasket seats, and when the core is clamped in the sh.e.l.l by means of check nut E, it is accurately centered and a tight joint is formed. This construction also makes a shorter plug than where conical fits are used, thus improving the heat radiation through the stem. The lower end of the sh.e.l.l is provided with a baffle plate O, which tends to keep the oil away from the mica. There are perforations L in this baffle plate to prevent burnt gases being pocketed behind the baffle plate and pre-igniting the new charge. This construction also brings the firing point out into the firing chamber of the engine, and has all the other advantages of a closed-end plug. The stem P is made of bra.s.s or copper, on account of their superior heat conductivity, and the electrode J is swedged into the bottom of the stem, as shown at K, in a secure manner.

The sh.e.l.l is finned, as shown at G, to provide greater heat radiating surface. There is also a fin F at the top of the stem, to increase the radiation of heat from the stem and electrode. The top of this finned portion is slightly countersunk, and the stem is riveted into same, thereby reducing the possibility of leakage past the threads on the stem. This finned portion is necked at A to take a slip terminal.

In building up the core a small section of washers, I, is built up before the mica insulating tube D is placed on. This construction gives a better support to section I. Baffle plate O is bored out to allow the electrode J to pa.s.s through, and the clearance between baffle plate and electrode is made larger than the width of the gap between the firing points, so that there is no danger of the spark jumping from the electrode to the baffle plate.

This plug will be furnished either with or without the finned portion, to meet individual requirements. The manufacturers lay special stress upon the simplicity of construction and upon the method of clamping, which is claimed to make the plug absolutely gas-tight.

CHAPTER VII

Why Lubrication Is Necessary--Friction Defined--Theory of Lubrication--Derivation of Lubricants--Properties of Cylinder Oils--Factors Influencing Lubrication System Selection--Gnome Type Engines Use Castor Oil--Hall-Scott Lubrication System--Oil Supply by Constant Level Splash System--Dry Crank-Case System Best for Airplane Engines--Why Cooling Systems Are Necessary-- Cooling Systems Generally Applied--Cooling by Positive Pump Circulation--Thermo-Syphon System--Direct Air-Cooling Methods-- Air-Cooled Engine Design Considerations.

WHY LUBRICATION IS NECESSARY

The importance of minimizing friction at the various bearing surfaces of machines to secure mechanical efficiency is fully recognized by all mechanics, and proper lubricity of all parts of the mechanism is a very essential factor upon which the durability and successful operation of the motor car power plant depends. All of the moving members of the engine which are in contact with other portions, whether the motion is continuous or intermittent, of high or low velocity, or of rectilinear or continued rotary nature, should be provided with an adequate supply of oil. No other a.s.semblage of mechanism is operated under conditions which are so much to its disadvantage as the motor car, and the tendency is toward a simplification of oiling methods so that the supply will be ample and automatically applied to the points needing it.

In all machinery in motion the members which are in contact have a tendency to stick to each other, and the very minute projections which exist on even the smoothest of surfaces would have a tendency to cling or adhere to each other if the surfaces were not kept apart by some elastic and unctuous substance. This will flow or spread out over the surfaces and smooth out the inequalities existing which tend to produce heat and r.e.t.a.r.d motion of the pieces relative to each other.

A general impression which obtains is that well machined surfaces are smooth, but while they are apparently free from roughness, and no projections are visible to the naked eye, any smooth bearing surface, even if very carefully ground, will have a rough appearance if examined with a magnifying gla.s.s. An exaggerated condition to ill.u.s.trate this point is shown at Fig. 75. The amount of friction will vary in proportion to the pressure on the surfaces in contact and will augment as the loads increase; the rougher surfaces will have more friction than smoother ones and soft bodies will produce more friction than hard substances.

FRICTION DEFINED

Friction is always present in any mechanism as a resisting force that tends to r.e.t.a.r.d motion and bring all moving parts to a state of rest.

The absorption of power by friction may be gauged by the amount of heat which exists at the bearing points. Friction of solids may be divided into two cla.s.ses: sliding friction, such as exists between the piston and cylinder, or the bearings of a gas-engine, and rolling friction, which is that present when the load is supported by ball or roller bearings, or that which exists between the tires or the driving wheels and the road. Engineers endeavor to keep friction losses as low as possible, and much care is taken in all modern airplane engines to provide adequate methods of lubrication, or anti-friction bearings at all points where considerable friction exists.

THEORY OF LUBRICATION

The reason a lubricant is supplied to bearing points will be easily understood if one considers that these elastic substances flow between the close fitting surfaces, and by filling up the minute depressions in the surfaces and covering the high spots act as a cushion which absorbs the heat generated and takes the wear instead of the metallic bearing surface. The closer the parts fit together the more fluid the lubricant must be to pa.s.s between their surfaces, and at the same time it must possess sufficient body so that it will not be entirely forced out by the pressure existing between the parts.

[Ill.u.s.tration: Fig. 75.--Showing Use of Magnifying Gla.s.s to Demonstrate that Apparently Smooth Metal Surfaces May Have Minute Irregularities which Produce Friction.]

Oils should have good adhesive, as well as cohesive, qualities. The former are necessary so that the oil film will cling well to the surfaces of the bearings; the latter, so the oil particles will cling together and resist the tendency to separation which exists all the time the bearings are in operation. When used for gas-engine lubrication the oil should be capable of withstanding considerable heat in order that it will not be vaporized by the hot portions of the cylinder. It should have sufficient cold test so that it will remain fluid and flow readily at low temperature. Lubricants should be free from acid, or alkalies, which tend to produce a chemical action with metals and result in corrosion of the parts to which they are applied. It is imperative that the oil be exactly the proper quality and nature for the purpose intended and that it be applied in a positive manner. The requirements may be briefly summarized as follows:

First--It must have sufficient body to prevent seizing of the parts to which it is applied and between which it is depended upon to maintain an elastic film, and yet it must not have too much viscosity, in order to minimize the internal or fluid friction which exists between the particles of the lubricant itself.