Aviation Engines Part 15

This strainer should be removed after every five to eight hours running of the engine and cleaned thoroughly with gasoline. It is also advisable to squirt distillate up into the case through the opening where the strainer has been removed. Allow this distillate to drain out thoroughly before replacing the plug with strainer attached. Be sure gasket is in place on plug before replacing. Pour new oil in through either of the two breather pipes on exhaust side of motor. Be sure to replace strainer screens if removed. If, through oversight, the engine does not receive sufficient lubrication and begins to heat or pound, it should be stopped immediately. After allowing engine to cool pour at least three gallons of oil into oil sump. Fill radiator with water after engine has cooled.

Should there be apparent damage, the engine should be thoroughly inspected immediately without further running. If no obvious damage has been done, the engine should be given a careful examination at the earliest opportunity to see that the running without oil has not burned the bearings or caused other trouble.

Oils best adapted for Hall-Scott engines have the following properties: A flash test of not less than 400 F.; viscosity of not less than 75 to 85 taken at 21 F. with Saybolt's Universal Viscosimeter.

_Zeroline heavy duty oil_, manufactured by the Standard Oil Company of California; also,

_Gargoyle mobile B oil_, manufactured by the Vacuum Oil Company, both fulfill the above specifications. One or the other of these oils can be obtained all over the world.

Monogram extra heavy is also recommended.

OIL SUPPLY BY CONSTANT LEVEL SPLASH SYSTEM

The splash system of lubrication that depends on the connecting rod to distribute the lubricant is one of the most successful and simplest forms for simple four- and six-cylinder vertical automobile engines, but is not as well adapted to the oiling of airplane power plants for reasons previously stated. If too much oil is supplied the surplus will work past the piston rings and into the combustion chamber, where it will burn and cause carbon deposits. Too much oil will also cause an engine to smoke and an excess of lubricating oil is usually manifested by a bluish-white smoke issuing from the exhaust.

A good method of maintaining a constant level of oil for the successful application of the splash system is shown at Fig. 78. The engine base casting includes a separate chamber which serves as an oil container and which is below the level of oil in the crank-case. The lubricant is drawn from the sump or oil container by means of a positive oil pump which discharges directly into the engine case. The level is maintained by an overflow pipe which allows all excess lubricant to flow back into the oil container at the bottom of the cylinder. Before pa.s.sing into the pump again the oil is strained or filtered by a screen of wire gauze and all foreign matter removed. Owing to the rapid circulation of the oil it may be used over and over again for quite a period of time. The oil is introduced directly into the crank-case by a breather pipe and the level is indicated by a rod carried by a float which rises when the container is replenished and falls when the available supply diminishes. It will be noted that with such system the only apparatus required besides the oil tank which is cast integral with the bottom of the crank-case is a suitable pump to maintain circulation of oil. This member is always positively driven, either by means of shaft and universal coupling or direct gearing. As the system is entirely automatic in action, it will furnish a positive supply of oil at all desired points, and it cannot be tampered with by the inexpert because no adjustments are provided or needed.

DRY CRANK-CASE SYSTEM BEST FOR AIRPLANE ENGINES

[Ill.u.s.tration: Fig. 78.--Sectional View of Typical Motor Showing Parts Needing Lubrication and Method of Applying Oil by Constant Level Splash System. Note also Water Jacket and s.p.a.ces for Water Circulation.]

In most airplane power plants it is considered desirable to supply the oil directly to the parts needing it by suitable leads instead of depending solely upon the distributing action of scoops on the connecting rod big ends. A system of this nature is shown at Fig. 77.

The oil is carried in the crank-case, as is common practice, but the normal oil level is below the point where it will be reached by the connecting rod. It is drawn from the crank-case by a plunger pump which directs it to a manifold leading directly to conductors which supply the main journals. After the oil has been used on these points it drains back into the bottom of the crank-case. An excess is provided which is supplied to the connecting rod ends by pa.s.sages drilled into the webs of the crank-shaft and part way into the crank-pins as shown by the dotted lines. The oil which is present at the connecting rod crank-pins is thrown off by centrifugal force and lubricates the cylinder walls and other internal parts. Regulating screws are provided so that the amount of oil supplied the different points may be regulated at will. A relief check valve is installed to take care of excess lubricant and to allow any oil that does not pa.s.s back into the pipe line to overflow or bi-pa.s.s into the main container.

[Ill.u.s.tration: Fig. 79.--Pressure Feed Oil-Supply System of Airplane Power Plants has Many Good Features.]

A simple system of this nature is shown graphically in a phantom view of the crank-case at Fig. 79, in which the oil pa.s.sages are made specially prominent. The oil is taken from a reservoir at the bottom of the engine base by the usual form of gear oil pump and is supplied to a main feed manifold which extends the length of the crank-case. Individual conductors lead to the five main bearings, which in turn supply the crank-pins by pa.s.sages drilled through the crank-shaft web. In this power plant the connecting rods are hollow section bronze castings and the pa.s.sage through the center of the connecting rod serves to convey the lubricant from the crank-pins to the wrist-pins. The cylinder walls are oiled by the spray of lubricant thrown off the revolving crank-shaft by centrifugal force. Oil projection by the dippers on the connecting rod ends from constant level troughs is unequal upon the cylinder walls of the two-cylinder blocks of an eight- or twelve-cylinder V engine.

This gives rise, on one side of the engine, to under-lubrication, and, on the other side, to over-lubrication, as shown at Fig. 80, A. This applies to all modifications of splash lubricating systems.

When a force-feed lubricating system is used, the oil, escaping past the cheeks of both ends of the crank-pin bearings, is thrown off at a tangent to the crank-pin circle in all directions, supplying the cylinders on both sides with an equal quant.i.ty of oil, as at Fig. 80, B.

WHY COOLING SYSTEMS ARE NECESSARY

The reader should understand from preceding chapters that the power of an internal-combustion motor is obtained by the rapid combustion and consequent expansion of some inflammable gas. The operation in brief is that when air or any other gas or vapor is heated, it will expand and that if this gas is confined in a s.p.a.ce which will not permit expansion, pressure will be exerted against all sides of the containing chamber.

The more a gas is heated, the more pressure it will exert upon the walls of the combustion chamber it confines. Pressure in a gas may be created by increasing its temperature and inversely heat may be created by pressure. When a gas is compressed its total volume is reduced and the temperature is augmented.

[Ill.u.s.tration: Fig. 80.--Why Pressure Feed System is Best for Eight-Cylinder Vee Airplane Engines.]

The efficiency of any form of heat engine is determined by the power obtained from a certain fuel consumption. A definite amount of energy will be liberated in the form of heat when a pound of any fuel is burned. The efficiency of any heat engine is proportional to the power developed from a definite quant.i.ty of fuel with the least loss of thermal units. If the greater proportion of the heat units derived by burning the explosive mixture could be utilized in doing useful work, the efficiency of the gasoline engine would be greater than that of any other form of energizing power. There is a great loss of heat from various causes, among which can be cited the reduction of pressure through cooling the motor and the loss of heat through the exhaust valves when the burned gases are expelled from the cylinder.

The loss through the water jacket of the average automobile power plant is over 50 per cent. of the total fuel efficiency. This means that more than half of the heat units available for power are absorbed and dissipated by the cooling water. Another 16 per cent. is lost through the exhaust valve, and but 33-1/3 per cent. of the heat units do useful work. The great loss of heat through the cooling systems cannot be avoided, as some method must be provided to keep the temperature of the engine within proper bounds. It is apparent that the rapid combustion and continued series of explosions would soon heat the metal portions of the engine to a red heat if some means were not taken to conduct much of this heat away. The high temperature of the parts would burn the lubricating oil, even that of the best quality, and the piston and rings would expand to such a degree, especially when deprived of oil, that they would seize in the cylinder. This would score the walls, and the friction which ensued would tend to bind the parts so tightly that the piston would stick, bearings would be burned out, the valves would warp, and the engine would soon become inoperative.

[Ill.u.s.tration: Fig. 81.--Operating Temperatures of Automobile Engine Parts Useful as a Guide to Understand Airplane Power Plant Heat.]

The best temperature to secure efficient operation is one on which considerable difference of opinion exists among engineers. The fact that the efficiency of an engine is dependent upon the ratio of heat converted into useful work compared to that generated by the explosion of the gas is an accepted fact. It is very important that the engine should not get too hot, and on the other hand it is equally vital that the cylinders be not robbed of too much heat. The object of cylinder cooling is to keep the temperature of the cylinder below the danger point, but at the same time to have it as high as possible to secure maximum power from the gas burned. The usual operating temperatures of an automobile engine are shown at Fig. 81, and this can be taken as an approximation of the temperatures apt to exist in an airplane engine of conventional design as well when at ground level or not very high in the air. The newer very high compression airplane engines in which compressions of eight or nine atmospheres are used, or about 125 pounds per square inch, will run considerably hotter than the temperatures indicated.

COOLING SYSTEMS GENERALLY APPLIED

There are two general systems of engine cooling in common use, that in which water is heated by the absorption of heat from the engine and then cooled by air, and the other method in which the air is directed onto the cylinder and absorbs the heat directly instead of through the medium of water. When the liquid is employed in cooling it is circulated through jackets which surround the cylinder casting and the water may be kept in motion by two methods. The one generally favored is to use a positive circulating pump of some form which is driven by the engine to keep the water in motion. The other system is to utilize a natural principle that heated water is lighter than cold liquid and that it will tend to rise to the top of the cylinder when it becomes heated to the proper temperature and cooled water takes its place at the bottom of the water jacket.

Air-cooling methods may be by radiation or convection. In the former case the effective outer surface of the cylinder is increased by the addition of f.l.a.n.g.es machined or cast thereon, and the air is depended on to rise from the cylinder as heated and be replaced by cooler air. This, of course, is found only on stationary engines. When a positive air draught is directed against the cylinder by means of the propeller slip stream in an airplane, cooling is by convection and radiation both.

Sometimes the air draught may be directed against the cylinder walls by some form of jacket which confines it to the heated portions of the cylinder.

COOLING BY POSITIVE WATER CIRCULATION

[Ill.u.s.tration: Fig. 82.--Water Cooling of Salmson Seven-Cylinder Radial Airplane Engine.]

A typical water-cooling system in which a pump is depended upon to promote circulation of the cooling liquid is shown at Figs. 82 and 83.

The radiator is carried at the front end of the fuselage in most cases, and serves as a combined water tank and cooler, but in some cases it is carried at the side of the engine, as in Fig. 84, or attached to the central portion of the aerofoil or wing structure. It is composed of an upper and lower portion joined together by a series of pipes which may be round and provided with a series of fins to radiate the heat, or which may be flat in order to have the water pa.s.s through in thin sheets and cool it more easily. Cellular or honeycomb coolers are composed of a large number of bent tubes which will expose a large area of surface to the cooling influence of the air draught forced through the radiator either by the forward movement of the vehicle or by some type of fan.

The cellular and flat tube types have almost entirely displaced the f.l.a.n.g.e tube radiators which were formerly popular because they cool the water more effectively, and may be made lighter than the tubular radiator could be for engines of the same capacity.

[Ill.u.s.tration: Fig. 83.--How Water Cooling System of Thomas Airplane Engine is Installed in Fuselage.]

The water is drawn from the lower header of the radiator by the pump and is forced through a manifold to the lower portion of the water jackets of the cylinder. It becomes heated as it pa.s.ses around the cylinder walls and combustion chambers and the hot water pa.s.ses out of the top of the water jacket to the upper portion of the radiator. Here it is divided in thin streams and directed against comparatively cool metal which abstracts the heat from the water. As it becomes cooler it falls to the bottom of the radiator because its weight increases as the temperature becomes lower. By the time it reaches the lower tank of the radiator it has been cooled sufficiently so that it may be again pa.s.sed around the cylinders of the motor. The popular form of circulating pump is known as the "centrifugal type" because a rotary impeller of paddle-wheel form throws water which it receives at a central point toward the outside and thus causes it to maintain a definite rate of circulation. The pump is always a separate appliance attached to the engine and driven by positive gearing or direct-shaft connection. The centrifugal pump is not as positive as the gear form, and some manufacturers prefer the latter because of the positive pumping features. They are very simple in form, consisting of a suitable cast body in which a pair of spur pinions having large teeth are carried. One of these gears is driven by suitable means, and as it turns the other member they maintain a flow of water around the pump body. The pump should always be installed in series with the water pipe which conveys the cool liquid from the lower compartment of the radiator to the coolest portion of the water jacket.

[Ill.u.s.tration: Fig. 84.--Finned Tube Radiators at the Side of Hall-Scott Airplane Power Plant Installed in Standard Fuselage.]

WATER CIRCULATION BY NATURAL SYSTEM

Some automobile engineers contend that the rapid water circulation obtained by using a pump may cool the cylinders too much, and that the temperature of the engine may be reduced so much that the efficiency will be lessened. For this reason there is a growing tendency to use the natural method of water circulation as the cooling liquid is supplied to the cylinder jackets just below the boiling point and the water issues from the jacket at the top of the cylinder after it has absorbed sufficient heat to raise it just about to the boiling point.

As the water becomes heated by contact with the hot cylinder and combustion-chamber walls it rises to the top of the water jacket, flows to the cooler, where enough of the heat is absorbed to cause it to become sensibly greater in weight. As the water becomes cooler, it falls to the bottom of the radiator and it is again supplied to the water jacket. The circulation is entirely automatic and continues as long as there is a difference in temperature between the liquid in the water s.p.a.ces of the engine and that in the cooler. The circulation becomes brisker as the engine becomes hotter and thus the temperature of the cylinders is kept more nearly to a fixed point. With the thermosyphon system the cooling liquid is nearly always at its boiling point, whereas if the circulation is maintained by a pump the engine will become cooler at high speed and will heat up more at low speed.

With the thermosyphon, or natural system of cooling, more water must be carried than with the pump-maintained circulation methods. The water s.p.a.ces around the cylinders should be larger, the inlet and discharge water manifolds should have greater capacity, and be free from sharp corners which might impede the flow. The radiator must also carry more water than the form used in connection with the pump because of the brisker pump circulation which maintains the engine temperature at a lower point. Consideration of the above will show why the pump system is almost universally used in connection with airplane power plant cooling.

DIRECT AIR-COOLING METHODS

The earliest known method of cooling the cylinder of gas-engines was by means of a current of air pa.s.sed through a jacket which confined it close to the cylinder walls and was used by Daimler on his first gas-engine. The gasoline engine of that time was not as efficient as the later form, and other conditions which materialized made it desirable to cool the engine by water. Even as gasoline engines became more and more perfected there has always existed a prejudice against air cooling, though many forms of engines have been used, both in automobile and aircraft applications where the air-cooling method has proven to be very practical.

The simplest system of air cooling is that in which the cylinders are provided with a series of f.l.a.n.g.es which increase the effective radiating surface of the cylinder and directing an air-current from a fan against the f.l.a.n.g.es to absorb the heat. This increase in the available radiating surface of an air-cooled cylinder is necessary because air does not absorb heat as readily as water and therefore more surface must be provided that the excess heat be absorbed sufficiently fast to prevent distortion of the cylinders. Air-cooling systems are based on a law formulated by Newton, which is: "The rate for cooling for a body in a uniform current of air is directly proportional to the speed of the air current and the amount of radiating surface exposed to the cooling effect."

AIR-COOLED ENGINE DESIGN CONSIDERATIONS

[Ill.u.s.tration: Fig. 85.--Anzani Testing His Five-Cylinder Air Cooled Aviation Motor Installed in Bleriot Monoplane. Note Exposure of f.l.a.n.g.ed Cylinders to Propeller Slip Stream.]

There are certain considerations which must be taken into account in designing an air-cooled engine, which are often overlooked in those forms cooled by water. Large valves must be provided to insure rapid expulsion of the flaming exhaust gas and also to admit promptly the fresh cool mixture from the carburetor. The valves of air-cooled engines are usually placed in the cylinder-head, in order to eliminate any pockets or sharp pa.s.sages which would impede the flow of gas or retain some of the products of combustion and their heat. When high power is desired multiple-cylinder engines should be used, as there is a certain limit to the size of a successful air-cooled cylinder. Much better results are secured from those having small cubical contents because the heat from small quant.i.ties of gas will be more quickly carried off than from greater amounts. All successful engines of the aviation type which have been air-cooled have been of the multiple-cylinder type.

An air-cooled engine must be placed in the fuselage, as at Fig. 85, in such a way that there will be a positive circulation of air around it all the time that it is in operation. The air current may be produced by the tractor screw at the front end of the motor, or by a suction or blower fan attached to the crank-shaft as in the Renault engine or by rotating the cylinders as in the Le Rhone and Gnome motors. Greater care is required in lubrication of the air-cooled cylinders and only the best quality of oil should be used to insure satisfactory oiling.

The combustion chambers must be proportioned so that distribution of metal is as uniform as possible in order to prevent uneven expansion during increase in temperature and uneven contraction when the cylinder is cooled. It is essential that the inside walls of the combustion chamber be as smooth as possible because any sharp angle or projection may absorb sufficient heat to remain incandescent and cause trouble by igniting the mixture before the proper time. The best grades of cast iron or steel should be used in the cylinder and piston and the machine work must be done very accurately so the piston will operate with minimum friction in the cylinder. The cylinder bore should not exceed 4-1/2 or 5 inches and the compression pressure should never exceed 75 pounds absolute, or about five atmospheres, or serious overheating will result.

As an example of the care taken in disposing of the exhaust gases in order to obtain practical air-cooling, some cylinders are provided with a series of auxiliary exhaust ports uncovered by the piston when it reaches the end of its power stroke. The auxiliary exhaust ports open just as soon as the full force of the explosion has been spent and a portion of the flaming gases is discharged through the ports in the bottom of the cylinder. Less of the exhaust gases remains to be discharged through the regular exhaust member in the cylinder-head and this will not heat the walls of the cylinder nearly as much as the larger quant.i.ty of hot gas would. That the auxiliary exhaust port is of considerable value is conceded by many designers of fixed and fan-shaped air-cooled motors for airplanes.