Aircraft technical Basics: Introduction to Airplanes - Navy Training Courses Edition of 1944: Chapter 6 Engine Systems

CHAPTER 6 ENGINE SYSTEMS

WHAT THEY ARE

In any aircraft engine, old or new, there is a number of SYSTEMS that make it function. You have a MECHANICAL SYSTEM consisting of such parts as cylinders, pistons, connecting rods, and the crankshaft. You have a FUEL SYSTEM which feeds the mixture of gasoline and air into the cylinders.

You have an IGNITION SYSTEM which controls the electrical spark and ignites the fuel mixture in the cylinders at just the right time. You have a LUBRICATION SYSTEM which oils the moving parts and keeps them running smoothly. And you have a COOLING SYSTEM which carries off the terrific heat generated by the engine in operation and prevents it from burning itself up.

MECHANICAL SYSTEM

A CYLINDER consists of two main sections - the BARREL and the HEAD. Both of these parts are covered with numerous COOLING FINS in air-cooled radial engines (which are the engines you are going to read about in this book because most Navy airplanes use radial engines). The cooling fins provide a large radiating surface so that the air can carry away the heat rapidly.

The EXHAUST and INTAKE valves are installed in the cylinder head, as are the SPARK PLUGS. Usually there are two spark plugs for each cylinder. In-take and exhaust PORTS provide the entrance and exit for charges of fuel and burned exhaust gases. The intake port is connected to the fuel induction system by means of the intake manifold. Exhaust ports are connected with the exhaust manifold, which collects the burned gases and passes them into the exhaust pipes.

A PISTON is a sort of metal plunger which forms the movable "end" of the cylinder and transmits the force of the expanding gases of combustion to the crankshaft by means of the CONNECTING ROD. The connecting rod is hitched to the piston by means of a WRIST PIN, and to the crankshaft (or to the master connecting rod, as the case may be) by a KNUCKLE PIN or by a BEARING. Figure 18 illustrates the relationship of these parts.

Figure 18.--Pistons, connecting rods, and master rod.

Figure 19. - Crankshaft.

The CRANKSHAFT has one or more offset sections (depending upon the number of cylinder banks the engine may have) called THROWS. The round part of the throw to which the master connecting rod is attached is known as the CRANKPIN. To counter-act the weight of the throw and permit the crank-shaft to turn without jerking, a COUNTERBALANCE must be added to the opposite side of the crank-shaft, as you see in figure 19.

The cylinders are fastened to a metal shell in which the crankshaft is housed. This shell is called the CRANKCASE, and is fitted with bearings which hold the crankshaft in position. Crankcases are usually made in several sections which are fastened together with bolts or studs.

FUEL SYSTEM

The FUEL SYSTEM includes the FUEL TANKS, the FUEL LINES (with their necessary pumps), various VALVES, STRAINERS, the SUPERCHARGER, and the CARBURETOR. Its basic job is to provide fuel in proper quantities, and correctly mixed, to the engine cylinders for burning.

In the simplest type of fuel system, the fuel flows from the tanks to the carburetor by GRAVITY. In other words, it just flows "down hill." This system, diagrammed in figure 20, would be the berries if it didn't mean that the tanks have to be fairly high ABOVE the engine to provide gasoline to the carburetor under pressure. Tanks, however, are usually in the airplane's wings. Actually, you rarely find gravity fuel systems in anything but small trainers.

Figure 20.-Simple gravity fuel system diagram.

The majority of fuel systems include a MECHANICAL PUMP that supplies fuel from the tanks to the carburetor under pressure. With such a pump in the system, the tanks may actually be below engine level. Fuel comes from the tank through a SELECTOR VALVE and a STRAINER. Another item in this system is a hand pump - your old friend the WOBBLE PUMP. It supplies fuel under pressure to the carburetor for starting, since the mechanical pump doesn't function until the engine is running.

Sometimes, such as when the mechanical pump is out of commission, it may be necessary to pump all fuel by hand from tank to carburetor. It is hard to pump enough gasoline THROUGH the mechanism of the engine-driven pump by hand, so there is a BYPASS VALVE in the fuel line to carry hand-pumped fuel AROUND the engine pump in such cases.

Don't get the idea that mechanical fuel pumps are out of kilter half the time. Actually they're very reliable. Like any piece of machinery, however, they CAN go haywire, and need protection for such an emergency.

A mechanical fuel pump is operated by the engine. Generally it is mounted right on the crank-case and geared directly to the engine crankshaft. There are two kinds of engine-driven pumps GEAR PUMPS and VANE PUMPS. The vane pump is most commonly used at present in service air-planes.

Too much pressure built up by a fuel pump can easily damage the carburetor, so a PRESSURE RELIEF VALVE is placed in the fuel line between the engine-driven pump and the carburetor. When a given fuel pressure is exceeded, the valve opens and some of the fuel flows into a return line and back to the tank.

The FUEL PRESSURE GAGE On the instrument panel is connected to the carburetor and tells you at all times just how much fuel pressure is being delivered to the carburetor. If the gage reading is lower than it should be, somebody better get busy and man the wobble pump or the engine will probably stop. It might not matter when you're on the ground, but an engine that stops during flight can be very disturbing to your mental health.

Most airplanes have two or more fuel tanks. One reason for this is the balancing of the air-plane. As was pointed out before, fuel tanks are usually located in the wings. If only a single tank were used, the airplane would be "wing heavy" on one side. Also, it's WONDERFUL to have two or more tanks if one springs A LEAK.

Figure 21.-Fuel selector valve.

Turning the SELECTOR VALVE to "on" position is the first step in starting an engine. There are several types of selector valves made, but all of them work in fundamentally the same way. The basic kind used with two-tank fuel systems is shown in figure 21, and has four "on" positions, marked "both," "right, "left," and "reserve."

The RESERVE position does not lead to a separate tank, but taps a lower section of one of the regular tanks. The reserve system is installed to ensure a supply of gas during take-off and landing, as well as to take care of pilots who RUN OUT OF GAS.

Fuel tanks are usually provided with a SUMP, which is a depression at the bottom of the tank to collect dirt or water and keep it from circulating into the fuel lines. The sump should be drained at regular intervals to dispose of collected foreign matter. The STRAINER, mentioned previously, is another device which prevents impurities from reaching the carburetor. It is a cup-like container, fitted with a wire screen which acts like a sieve and filters out solid particles.

The PRIMER is nothing more than a small pump. When the primer handle on the instrument panel is pulled back, fuel is drawn into the pump through its inlet valve. When the handle is pushed forward, the fuel is forced through an outlet pipe to the cylinder or intake manifold.

In the CARBURETOR, gasoline and air are mixed in the proper proportions for the most efficient production of power. Gasoline alone won't even burn unless air is present to provide oxygen for the combustion process. However, if gasoline is broken up into extremely fine drops, as in a spray, and mixed with air, it will burn very rapidly. The purpose of the carburetor, then, is to convert liquid gasoline into the smallest particles possible, and to mix the particles with air.

There are three general types of carburetors.

The SIMPLE FLOAT TYPE is used on small engines, such as those in some trainer planes. The DIAPHRAGM TYPE, an anti-icing variety, is used on larger engines. The INJECTION TYPE, also anti-icing, is likewise used on larger engines. Don't be confused because there are different types of carburetors. They ALL perform the SAME FUNCTION. Fundamentally, "it ain't what they do, it's the way that they do it" that makes them at all different.

Operating conditions make necessary some variation in the proportion of air to gasoline mixed by the carburetor, but on the average it will be about 15 parts of air to 1 of gasoline. Changes in this proportion can be made by adjusting the MIXTURE CONTROL HANDLE. A RICH mixture contains a higher proportion of gasoline to air. A LEAN mixture increases the percentage of air being mixed with a given amount of gasoline.

Since the air is thinner at high levels and consequently is at a lower pressure, changes in altitude have quite an effect on the normal carburetor mixture. Most airplanes, therefore, have an AUTOMATIC MIXTURE CONTROL UNIT which regulates the fuel-air mixture so that it reaches the engine in the same relative proportion regardless of the altitude and pressure of the atmosphere.

Mixture control alone, unfortunately, isn't enough to provide the means of obtaining sea-level horsepower at higher altitudes. The quantity, or weight, of air-fuel mixture that enters a cylinder with the intake stroke of the piston varies according to the density of the air. The weight of a given volume of air taken in at high altitude is LESS than it would be at lower levels. So, for high-level flying, it is necessary to FORCE a constant weight of air-fuel mixture into the cylinders in order to maintain the horsepower. This is done by means of a SUPERCHARGER.

A supercharger is essentially just a big air pump. The types in use at present are driven by gears from the crankshaft, or by a turbine operated by the pressure of gases from the engine exhaust line. They can be put in operation by the pilot at will. A TWO-STAGE supercharger is standard equipment on many modern military air-planes. One blower forces air into the carburetor. The other delivers air-fuel mixture to the cylinders, under pressure.

Often it's necessary to provide heated air for the carburetor to prevent the formation of ice under certain weather conditions. One method you'll find in use is that which draws air for the carburetor from BEHIND THE HOT ENGINE CYLINDERS. Another system draws air around the hot EXHAUST MANIFOLD before it enters the carburetor intake.

As you undoubtedly know, aircraft engines operate at extremely high temperatures. Aviators used to find that cylinder heads got so hot that engines kept running, or perhaps kicked backward, after the ignition switch was shut off. To correct this bad habit, carburetors are often equipped with an IDLE CUT-OFF, which will stop the flow of fuel to the engine. Obviously an engine, no matter how hot, can't keep running without fuel.

IGNITION SYSTEM 

What makes the fuel burn, once it has been delivered to the cylinders? The answer is the IGNITION SYSTEM. Ignition, if you look in the dictionary, means "setting on fire." In airplane engines the "setting on fire" of the fuel is accomplished by the use of SPARK PLUGS and ELECTRIC CURRENT.

You remember from the description of the four-stroke cycle that at the start of the power stroke an electric spark from the spark plugs ignites the fuel, which then expands by combustion and forces the piston downward in its cylinder.

All service types of engines use two spark plugs for each cylinder. One purpose of this DUAL IGNITION is to provide better combustion, as a charge of fuel mixture will burn more evenly if ignited at two points at the same instant.

Some airplanes use a battery system of ignition, much like that found in an automobile. But most airplane engines obtain ignition current from MAGNETOS. A magneto is a special type of electric generator which produces pulses of high voltage current that will jump across the gaps between the arc points of spark plugs.

To make a spark plug "spark" at the proper instant, there is a device called a DISTRIBUTOR in the electrical circuit connecting the magneto to the spark plug. A revolving arm, or ROTOR, of the distributor brushes past CONTACT POINTS in the distributor's outer shell, making a series of electrical contacts which allow a surge of current to travel to the spark plugs in proper firing succession and at just the right instant.

Most dual ignition systems require TWO magnetos. One spark plug in each cylinder of the engine is fired in turn by the first magneto. The other spark plug in each cylinder is fired by the second magneto. The distributors, of course, are timed so that electrical impulses are delivered to both spark plugs in a single cylinder at the same moment.

Magnetos, obviously, cannot furnish any current unless the engine is turning, since they depend on gearing from the crankshaft to make them work. Many airplanes, therefore, have small BOOSTER MAGNETOS installed in their engine ignition systems. These are geared to the starters so as to provide a spark when the starter is engaged and the main engine-driven magnetos are not turning at all, or when they are turning too slowly to provide a good hot spark.

LUBRICATION SYSTEM

If you want to see a veteran railroad trainman stew in his own juices, simply suggest to him that there's a "hot box" on one of his cars. Likely as not he'll jump to the emergency cord and bring the train to a lurching, grinding stop. A trainman knows only too well that a "hot box" can cause serious trouble, and no fooling. It means the axle of a wheel isn't properly LUBRICATED, and something has to be done about it - DOUBLE QUICK - or there may be a bad break-down.

Lack of lubrication between moving metal parts means trouble in almost any kind of mechanism, and an aircraft engine is no exception to the rule. Metal rubbing directly against metal results in excessive FRICTION. In turn, friction generates heat, which has a tendency to expand metal parts until they swell out of shape. Add together the grind of constant high-speed rubbing and the distortion caused by heat and the result, in a precision-made engine, is WRECKAGE.

By using a lubricant - OIL, if you prefer that word - metal surfaces that move in contact with each other are both coated with a thin ANTIFRICTION FILM. Between the two films, other layers of oil slide in microscopic drops like tiny ball bearings, protecting the metal surfaces from harm.

It is quite a problem to keep the rubbing metal surfaces in an aircraft engine supplied with all the oil they need. That's the job of the LUBRICATION SYSTEM. Some engine parts need only be kept COATED with oil, while other parts must constantly receive oil UNDER PRESSURE. Adequate lubrication can't be accomplished, therefore, merely by running around with an oil can and squirting a few drops here and there. Proper equipment to supply oil where it's needed during engine operation must be BUILT-IN as an integral part of the engine.

Automobile engines have WET-SUMP lubricating systems, which carry oil directly in the crankcase and have no need for external tanks. This arrangement isn't satisfactory for most aircraft engines, however. You can't, for example, carry very much oil in the crankcase of a radial engine without running the danger of flooding the cylinders. There's the same problem, too, in an inverted in-line engine where there are cylinders BELOW the crankcase. Diving and climbing would, in many airplanes, bring up similar problems - not to mention what would happen in case such an airplane was flying upside down.

You'll find that practically all aircraft engines are of the DRY-SUMP type. The principles on which both wet- and dry-sump engines function are the same except for the matter of oil storage, however. As you are most interested in modern airplane engines, this book will be limited to discussion of the dry sump.

You can divide the lubrication equipment of an airplane into the EXTERNAL SYSTEM and the INTERNAL SYSTEM.

The purpose of the external system shown in figure 22, is primarily to get the oil from the tank to the engine and back again. The internal system, which is entirely inside the engine, conveys oil to the specific parts that must be lubricated.

The OIL TANK of a dry-sump system must be large enough to hold all the oil necessary for normal engine operation. In addition, it must have extra space to allow for foaming and expansion of oil when heated. At the top of the tank you'll find a VENT PIPE which leads back to the engine crankcase so that any oil forced out of the tank by excessive foaming will be saved. As the crankcase is ventilated to the outside air through the CRANKCASE BREATHER, air is allowed to enter the oil tank through the vent pipe. Without such a means of letting in air, the oil wouldn't run out of the tank.

Figure 22.-External lubricating system.

Oil passes through a pipe from the engine to an automatic oil TEMPERATURE-CONTROL UNIT and from there into the tank. The temperature-control unit "decides," by means of a thermostatic valve or a viscosity valve, whether the oil returning from the engine is warm enough or cool enough for normal operation. If it is too warm, the oil is automatically passed through an OIL COOLER, which is much like a small automobile radiator. If, on the other hand, the oil is too cool (as when you're starting the engine), the temperature-control unit causes the oil to return to the tank through an inlet, very close to the tank outlet leading back to the engine. The engine heat thus keeps warming the same oil over again until it is at a proper temperature.

A powerful OIL SUPPLY PUMP, mounted on the crankcase, pumps the oil from the tank through the engine. Another kind of pump, called the SCAVENGER PUMP, is used to remove the oil from the engine and pump it back to the tank.

An OIL FILTER is built into many external oil systems in airplanes. As you would assume from its name, it is a device for cleaning the oil. There are several types of oil filters, but they all serve the same purpose - GETTING RID OF DIRT.

Figure 23.-Internal lubrication system (diagram) for radial engine.

The internal oil system is, for the most part, made up of a series of small channels and holes leading to the various parts of the engine and its accessories which need lubrication. If you study figure 23 for awhile, you'll see how the internal system works. The crankshaft, for example, is made hollow so it can carry oil under pressure to the various bearings on its surface. Holes drilled from its bearing surfaces to the hollow interior keep the bearings supplied with oil.

If oil is delivered to the engine under too much pressure, excess oil will squirt out into the crank-case and probably work up into the cylinders where it can foul the spark plugs or form an undesirable coat of carbon. The oil pressure, therefore, must be kept at a constant figure, and should not be permitted to get too high. An oil-pressure safeguard is provided by the RELIEF VALVE, which bypasses excess oil back to the tank inlet pipe and doesn't let it reach the engine.

COOLING SYSTEMS

When in operation, an aircraft engine is really "hot stuff." So hot, in fact, that it would burn itself up in no time if there weren't some sort of COOLING SYSTEM built into it to carry off most of the heat generated by the combustion of fuel in the cylinders.

You recall that aircraft engines are of two types - air-cooled and liquid-cooled - and that both types have points in their favor. Air-cooling an engine is a lot like using an electric fan to cool yourself off on a hot summer afternoon. Liquid-cooling an engine can be compared to hopping into a cool shower bath. Either way - you HELP TO BEAT THE HEAT.

An airplane carries its own fan - the PROPELLER - around with it. Cool air from the propeller stream - plus the air blast created by the motion of the airplane in flight - if steered directly into contact with the hot cylinders, provide an effective means of getting rid of engine heat. The more cylinder surface you have exposed to the blast of air, the better the cooling process works. That is one of the main reasons for having the cylinders of an AIR-COOLED engine project out from the crankcase - so the air can get at them. And that is the reason for putting COOLING FINS on air-cooled engine cylinders and cylinder heads - to increase their radiating surface.

Several other devices are usually provided to increase the efficiency of air-cooling systems. If there is more than one row of cylinders in a radial-type air-cooled engine, AIR DEFLECTORS, or "baffles," are often installed to direct a sufficient flow of air to the rear row of cylinders. The collar-like COWLING around the engine is also a big help, as are the COWL FLAPS, which provide airflow control. Figure 24 shows you these two devices.

Figure 24.-Engine cowling and flaps.

You can check on the operating temperature of an air-cooled engine by readings from a dial on the instrument panel in the cockpit. The apparatus which operates the dial is called a THERMOCOUPLE, and is connected directly to one or more of the engine cylinders.

A liquid-cooled engine also depends on the air-stream from the propeller to keep its temperature down to normal. Instead of flowing over the cylinders, however, the air passes through a LIQUID-FILLED RADIATOR. The individual engine cylinders are surrounded with WATER JACKETS in which the coolant (the cooling fluid) circulates. The water jackets are connected to the radiator by a system of plumbing (including pipes, flexible hose connections, and drain plugs).

The radiator itself is made up of a network of small tubes through which the coolant is made to flow by means of a pump. An auxiliary EXPANSION TANK, controllable RADIATOR SHUTTERS and other attachments are frequently included in liquid-cooled systems.

Some progress has been made recently in the development of AIR-COOLED IN-LINE ENGINES for airplanes. They combine the light weight of the air-cooled engine with the smaller frontal area of an in-line engine, and are now installed in some Naval airplanes. Their efficiency depends on the principle of PRESSURE COOLING. An air chamber is provided adjacent to the cylinders, with its open end facing forward. The forward velocity of the airplane, as well as the fan action of the propeller, build up air pressure in this chamber. The air escapes between the cylinders, and its flow is controlled by BAFFLES, an AIR SCOOP, and COWL FLAPS.