Aviation Engines Part 23

Defective carburetion often causes misfiring or irregular operation. The common derangement of the components of the fuel system that are common enough to warrant suspicion and the best methods for their location follows: First, disconnect the feed pipe from the carburetor and see if the gasoline flows freely from the tank. If the stream coming out of the pipe is not the full size of the orifice it is an indication that the pipe is clogged with dirt or that there is an acc.u.mulation of rust, scale, or lint in the strainer screens of the filter. It is also possible that the fuel shut-off valve may be wholly or partly closed. If the gasoline flows by gravity the liquid may be air bound in the tank, while if a pressure-feed system is utilized the tank may leak so that it does not retain pressure; the check valve retaining the pressure may be defective or the pipe conveying the air or gas under pressure to the tank may be clogged.

If the gasoline flows from the pipe in a steady stream the carburetor demands examination. There may be dirt or water in the float chamber, which will constrict the pa.s.sage between the float chamber and the spray nozzle, or a particle of foreign matter may have entered the nozzle and stopped up the fine holes therein. The float may bind on its guide, the needle valve regulating the gasoline-inlet opening in bowl may stick to its seat. Any of the conditions mentioned would cut down the gasoline supply and the engine would not receive sufficient quant.i.ties of gas.

The air-valve spring may be weak or the air valve broken. The gasoline-adjusting needle may be loose and jar out of adjustment, or the air-valve spring-adjusting nuts may be such a poor fit on the stem that adjustments will not be retained. These instructions apply only to carburetors having air valves and mixture regulating means which are used only in rare instances in airplane work. Air may leak in through the manifold, due to a porous casting, or leaky joints in a built up form and dilute the mixture. The air-intake dust screen may be so clogged with dirt and lint that not enough air will pa.s.s through the mesh. Water or sediment in the gasoline will cause misfiring because the fuel feed varies when the water or dirt constricts the standpipe bore.

It is possible that the carburetor may be out of adjustment. If clouds of black smoke are emitted at the exhaust pipe it is positive indication that too much gasoline is being supplied the mixture and the supply should be cut down by s.c.r.e.w.i.n.g in the needle valve on types where this method of regulation is provided, and by making sure that the fuel level is at the proper height, or that the proper nozzle is used in those forms where the spray nozzle has no means of adjustment. If the mixture contains too much air there will be a p.r.o.nounced popping back in the carburetor. This may be overcome by s.c.r.e.w.i.n.g in the air-valve adjustment so the spring tension is increased or by slightly opening up the gasoline-supply regulation needle. When a carburetor is properly adjusted and the mixture delivered the cylinder burns properly, the exhaust gas will be clean and free from the objectionable odor present when gasoline is burned in excess.

The character of combustion may be judged by the color of the flame which issues from it when the engine is running with an open throttle after nightfall. If the flame is red, it indicates too much gasoline. If yellowish, it shows an excess of air, while a properly proportioned mixture will be evidenced by a p.r.o.nounced blue flame, such as given by a gas-stove burner.

The Duplex Model O. D. Zenith carburetor used upon most of the six- and eight-cylinder airplane engines consists of a single float chamber, and a single air intake, joined to two separate and distinct spray nozzles, venturi and idling adjustments. It is to be noted that as the carburetor barrels are arranged side by side, both valves are mounted on the same shaft, and work in unison through a single operating lever. It is not necessary to alter their position. In order to make the engine idle well, it is essential that the ignition, especially the spark-plugs, should be in good condition. The gaskets between carburetor and manifold, and between manifold and cylinders should be absolutely air-tight. The adjustment for low speed on the carburetor is made by turning in or out the two knurled screws, placed one on each side of the float chamber. After starting the engine and allowing it to become thoroughly warmed, one side of the carburetor should be adjusted so that the three cylinders it affects fire properly at low speed. The other side should be adjusted in the same manner until all six cylinders fire perfectly at low speed. As the adjustment is changed on the knurled screw a difference in the idling of the engine should be noticed. If the engine begins to run evenly or speeds up it shows that the mixture becomes right in its proportion.

Be sure the b.u.t.terfly throttle is closed as far as possible by s.c.r.e.w.i.n.g out the stop screw which regulates the closed position for idling. Care should be taken to have the b.u.t.terfly held firmly against this stop screw at all times while idling engine. If three cylinders seem to run irregularly after changing the position of the b.u.t.terfly, still another adjustment may have to be made with the knurled screw. Uns.c.r.e.w.i.n.g this makes the mixture leaner. s.c.r.e.w.i.n.g in closes off some of the air supply to the idling jet, making it richer. After one side has been made to idle satisfactorily repeat the same procedure with the opposite three cylinders. In other words, each side should be idled independently to about the same speed.

Remember that the main jet and compensating jet have no appreciable effect on the idling of the engine. The idling mixture is drawn directly through the opening determined by the knurled screw and enters the carburetor barrel through the small hole at the edge of each b.u.t.terfly.

This is called the priming hole and is only effective during idling.

Beyond that point the suction is transferred to the main jet and compensator, which controls the power of the engine beyond the idling position of the throttle.

DEFECTS IN OILING SYSTEMS

While troubles existing in the ignition or carburetion groups are usually denoted by imperfect operation of the motor, such as lost power, and misfiring, derangements of the lubrication or cooling systems are usually evident by overheating, diminution in engine capacity, or noisy operation. Overheating may be caused by poor carburetion as much as by deficient cooling or insufficient oiling. When the oiling group is not functioning as it should the friction between the motor parts produces heat. If the cooling system is in proper condition, as will be evidenced by the condition of the water in the radiator, and the carburetion group appears to be in good condition, the overheating is probably caused by some defect in the oiling system.

The conditions that most commonly result in poor lubrication are: Insufficient oil in the engine crank-case or sump, broken or clogged oil pipes, screen at filter filled with lint or dirt, broken oil pump, or defective oil-pump drive. The supply of oil may be reduced by a defective inlet or discharge-check valve at the mechanical oiler or worn pumps. A clogged oil pa.s.sage or pipe leading to an important bearing point will cause trouble because the oil cannot get between the working surfaces. It is well to remember that much of the trouble caused by defective oiling may be prevented by using only the best grades of lubricant, and even if all parts of the oil system are working properly, oils of poor quality will cause friction and overheating.

DEFECTS IN COOLING SYSTEMS OUTLINED

Cooling systems are very simple and are not liable to give trouble as a rule if the radiator is kept full of clean water and the circulation is not impeded. When overheating is due to defective cooling the most common troubles are those that impede water circulation. If the radiator is clogged or the piping of water jackets filled with rust or sediment the speed of water circulation will be slow, which will also be the case if the water pump or its driving means fail. Any scale or sediment in the water jackets or in the piping or radiator pa.s.sages will reduce the heat conductivity of the metal exposed to the air, and the water will not be cooled as quickly as though the scale was not present.

The rubber hose often used in making the flexible connections demanded between the radiator and water manifolds of the engine may deteriorate inside and particles of rubber hang down that will reduce the area of the pa.s.sage. The grease from the grease cups mounted on the pump-shaft bearing to lubricate that member often finds its way into the water system and rots the inner walls of the rubber hose, this resulting in strips of the partly decomposed rubber lining hanging down and restricting the pa.s.sage. The cooling system is p.r.o.ne to overheat after antifreezing solutions of which calcium chloride forms a part have been used. This is due to the formation of crystals of salt in the radiator pa.s.sages or water jackets, and these crystals can only be dissolved by suitable chemical means, or removed by sc.r.a.ping when the construction permits.

Overheating is often caused by some condition in the fuel system that produces too rich or too lean mixture. Excess gasoline may be supplied if any of the following conditions are present: Bore of spray nozzle or standpipe too large, auxiliary air-valve spring too tight, gasoline level too high, loose regulating valve, fuel-soaked cork float, punctured sheet-metal float, dirt under float control shut-off valve or insufficient air supply because of a clogged air screen. If pressure feed is utilized there may be too much pressure in the tank, or the float controlled mechanism operating the shut-off in the float bowl of the carburetor may not act quickly enough.

SOME CAUSES OF NOISY OPERATION

There are a number of power-plant derangements which give positive indication because of noisy operation. Any knocking or rattling sounds are usually produced by wear in connecting rods or main bearings of the engine, though sometimes a sharp metallic knock, which is very much the same as that produced by a loose bearing, is due to carbon deposits in the cylinder heads, or premature ignition due to advanced spark-time lever. Squeaking sounds invariably indicate dry bearings, and whenever such a sound is heard it should be immediately located and oil applied to the parts thus denoting their dry condition. Whistling or blowing sounds are produced by leaks, either in the engine itself or in the gas manifolds. A sharp whistle denotes the escape of gas under pressure and is usually caused by a defective packing or gasket that seals a portion of the combustion chamber or that is used for a joint as the exhaust manifold. A blowing sound indicates a leaky packing in crank-case.

Grinding noises in the motor are usually caused by the timing gears and will obtain if these gears are dry or if they have become worn. Whenever a loud knocking sound is heard careful inspection should be made to locate the cause of the trouble. Much harm may be done in a few minutes if the engine is run with loose connecting rod or bearings that would be prevented by taking up the wear or looseness between the parts by some means of adjustment.

BRIEF SUMMARY OF HINTS FOR STARTING ENGINE

First make sure that all cylinders have compression. To ascertain this, open pet c.o.c.ks of all cylinders except the one to be tested, crank over motor and see that a strong opposition to cranking is met with once in two revolutions. If motor has no pet c.o.c.ks, crank and notice that oppositions are met at equal distances, two to every revolution of the starting crank in a four-cylinder motor. If compression is lacking, examine the parts of the cylinder or cylinders at fault in the following order, trying to start the motor whenever any one fault is found and remedied. See that the valve push rods or rocker arms do not touch valve stems for more than approximately 1/2 revolution in every 2 revolutions, and that there is not more than .010 to .020 inch clearance between them depending on the make of the motor. Make sure that the exhaust valve seats. To determine this examine the spring and see that it is connected to the valve stem properly. Take out valve and see that there is no obstruction, such as carbon, on its seat. See that valve works freely in its guide. Examine inlet valve in same manner. Listen for hissing sound while cranking motor for leaks at other places.

Make sure that a spark occurs in each cylinder as follows: If magneto or magneto and battery with non-vibrating coil is used: Disconnect wire from spark-plug, hold end about 1/8 inch from cylinder or terminal of spark-plug. Have motor cranked briskly and see if spark occurs. Examine adjustment of interrupter points. See that wires are placed correctly and not short circuited. Take out spark-plug and lay it on the cylinder, being careful that base of plug only touches the cylinder and that ignition wire is connected. Have motor cranked briskly and see if spark occurs. Check timing of magneto and see that all brushes are making contact.

See if there is gasoline in the carburetor. See that there is gasoline in the tank. Examine valve at tank. Prime carburetor and see that spray nozzle pa.s.sage is clear. Be sure throttle is open. Prime cylinders by putting about a teaspoonful of gasoline in through pet c.o.c.k or spark-plug opening. Adjust carburetor if necessary.

LOCATION OF ENGINE TROUBLES MADE EASY

The following tabulation has been prepared and originated by the writer to outline in a simple manner the various troubles and derangements that interfere with efficient internal-combustion engine action. The parts and their functions are practically the same in all gas or gasoline engines of the four-cycle type, and the general instructions given apply just as well to all hydro-carbon engines, even if the parts differ in form materially. The essential components are clearly indicated in the many part sectional drawings in this book so they may be easily recognized. The various defects that may materialize are tabulated in a manner that makes for ready reference, and the various defective conditions are found opposite the part affected, and under a heading that denotes the main trouble to which the others are contributing causes. The various symptoms denoting the individual troubles outlined are given to facilitate their recognition in a positive manner.

Brief note is also made of the remedies for the restoration of the defective part or condition. It is apparent that a table of this character is intended merely as a guide, and it is a compilation of practically all the known troubles that may materialize in gas-engine operation. While most of the defects outlined are common enough to warrant suspicion, they will never exist in an engine all at the same time, and it will be necessary to make a systematic search for such of those as exist.

To use the list advantageously, it is necessary to know one main trouble easily recognized. For example, if the power plant is noisy, look for the possible troubles under the head of Noisy Operation; if it lacks capacity, the derangement will undoubtedly be found under the head of Lost Power. It is a.s.sumed in all cases that the trouble exists in the power plant or its components, and not in the auxiliary members of the ignition, carburetion, lubrication, or cooling systems. The novice and student will readily recognize the parts of the average aviation engine by referring to the very complete and clearly lettered ill.u.s.trations of mechanism given in many parts of this treatise.

LOST POWER AND OVERHEATING

------------------+------------------+------------------+-------------------- PART AFFECTED NATURE OF TROUBLE SYMPTOMS AND REMEDY EFFECTS ------------------+------------------+------------------+-------------------- Water Pipe Joint. Loose. Loss of water, Tighten bolts, heating. replace gaskets.

Spark Plug. Leakage in Loss of power. Replace insulation threads, Hissing caused by if defective, screw insulation, escaping gas. down tighter.

packing. Compression Leak in threads. Loss of power. Tighten if loose.

Release c.o.c.k. Leak in fitting. Whistling or Grind fitting to hissing. new seating in body.

Combustion Crack or blowhole. Loss of compres- Fill by welding.

Chamber. Roughness. Carbon sion. Preignition. Smooth out deposits. Sharp roughness. Sc.r.a.pe edges. out or dissolve carbon.

Valve Chamber Cap. Leak in threads. Loss of compres- Remove. Apply pipe Defective gasket. sion. Hissing. compound to threads and replace. Use new gasket or packing.

Valve Head. Warped. Scored or Loss of compres- True up in lathe.

pitted. Carbon- sion. Grind to seat.

ized. Covered with Sc.r.a.pe off. Smooth scale. Loose on with emery cloth.

stem (two-piece Tighten by valves only). riveting.

Valve Seat. Warped or pitted. Loss of compres- Use reseating Covered with car- sion. reamer. Clean off bon. Foreign mat- and grind valve to ter between valve seat.

and seat. Valve Stem. Covered with Valve does not Clean with emery scale. Bent. Bind- close. Loss of cloth; straighten.

ing in guide. compression. True up and smooth Stuck in guide. off. free with kerosene.

Valve Stem Guide. Burnt or rough. Valve may stick. Clean out hole.

Loose in valve Action irregular. Screw in tighter.

chamber. Valve Spring. Weak or broken. Valve does not close. Valve Operating Loose in guide. Valve action poor. Replace with new.

Plunger. Too much clearance Lift insufficient. Adjust screw closer.

between valve stem. Valve Lift Ad- Threads stripped. Poor valve action. Replace with new.

justing Screw. Too near valve. Adjust with proper Too far from reference to valve valve. stem.

Valve Lift Cam. Worn cam contour. Not enough valve Replace with new.

Loose on shaft. lift. Will not Replace pins or Out of time. lift valve. Valve keys. Set to open opens at wrong properly.

time. Cam-shaft. Sprung or twisted. Valves out of Straighten.

time. Cam-shaft Bushing. Worn. Not enough valve Replace.

lift. Cam-shaft Drive Loose on shaft. Irregular valve Fasten securely.

Gear. Out of time. Worn action. Time properly.

or broken teeth. Replace with new.

Cam Fastenings. Worn or broken. Valves out of Replace with new.

time. Cylinder Wall. Scored, gas leaks. Poor compression. Grind out bore.

Poor lubrication Overheating. Repair oiling causes friction. system.

Piston. Binds in cylinder. Overheating. Poor Lap off excess Walls scored. Worn compression. metal. Replace with out of round. new.

Piston Rings. Loss of spring. Loss of compres- Peen ring or Loose in grooves. sion. Gas blows replace. Fit new Scored. Worn or by. rings. Grind smooth.

broken. Slots in Replace. Turn slots line. apart.

Carbon in grooves. Overheating be- Remove deposits.

Insufficient open- cause of friction. File slot. Grind or ing. Binding on lap to fit cylinder cylinder. bore.

Wristpin. Loose, scores Loss of compres- Fasten securely.

cylinder. sion. Replace cylinder if groove is deep.

Crank-shaft. Scored or rough on Overheating be- Smooth up.

journals. Sprung. cause of friction. Straighten.