Aircraft technical Basics: Aircraft Engines - RAF Flying Training Manual - Chapter VII.- Engines: Constructional Details

CONSTRUCTIONAL DETAILS

Cylinders

12. One of the most noticeable differences between aero engines and motor car engines is the number of cylinders. This is because it is impracticable for various reasons to get more than a limited power from one cylinder--about 100 h.p.—and consequently to get the high power needed from an aero engine a comparatively large number are required.

13. A large number of cylinders also gives smooth running and, in certain types of engine, a smaller frontal area. As the arrangement of the cylinders has a profound influence on the general design of the engine this will be considered first.

14. The two main types of engine are the in-line and the radial. In the former the cylinders are arranged side by side in a row along the crankcase, each piston working on a separate crank. Both for compactness and because it is difficult to make a long crankshaft stiff enough to avoid vibration, it is not usual to have more than six cylinders in a line. If a larger number is required two or more rows are mounted on the same crankcase at an angle to each other. A typical example of this is the MERLIN engine shown in Fig. 98 (a).

15. For very high powers four rows may be used, either in " X " formation, all pistons working on the same crankshaft, or in two groups of twelve set " H " fashion and working on two crankshafts alongside one another and geared together. The DAGGER engine is an example of this and is shewn in Fig. 98 (b). The " H " construction gives a small frontal area, but is apt to increase the weight.  Liquid cooled engines of the in-line type usually have all the cylinders of a row in one casting (cylinder block) for stiffness and lightness.

16. Radial engines have their cylinders arranged around the crankcase. This arrangement lends itself particularly to air cooling as it is easy to get a good flow of air round the cylinders and all modern radials are air cooled. The usual number of cylinders on one crank is seven or nine, but in large engines two or even three sets or banks of cylinders are used, with a two- or three-throw crankshaft. A single and a double bank radial are shewn in figs. 98 (c) and 98 (d). The radial arrangement tends to give a shorter, lighter and more accessible engine, one which is more easily air cooled, whereas in-line engines have the great advantage of a smaller frontal area and therefore less air resistance, they are, however, less easy to maintain.

17. As regards the cylinders themselves, air cooled cylinders closely resemble an ordinary overhead valve (o.h.v.) motor cycle cylinder, except that the head, for the sake of lightness and cooling, is made of light alloy, and is screwed, shrunk or bolted on to the steel barrel. Liquid cooled cylinder blocks, unlike motor car blocks, have separate steel cylinder liners surrounded by a cast light alloy jacket. The liners are open at both ends and clamped between the cylinder head casting and the crankcase, by means of long bolts.

Pistons and piston rings

18. Pistons are made of light alloy, usually forged, for the sake of lightness and heat conduction. Two or three piston rings are fitted at the top to keep compression and one or more special rings lower down to act as scraper rings and prevent too much oil getting past the pistons. The gudgeon pin, which connects the piston to the connecting rod, is made of case-hardened steel, and is hollow for lightness. It is usually " floating ", that is free to rotate in both piston and connecting-rod small end.

Valves and valve gear

19. The familiar mushroom-shaped " poppet " valve used on nearly all motor cars and motor cycles is also to be found on most aero engines, but the higher working temperature and the necessity for long life and reliability demand special materials. Valves are commonly made of chrome-silicon or tungsten steel, and exhaust valves are usually of the sodium cooled type. The part of the valve which makes contact with the seating in the cylinder head is usually faced with " stellite ", a material particularly resistant to burning and corrosion. The valve seats are screwed and shrunk into the cylinder head, and are made of nickel-chrome-manganese steel or aluminium bronze. Poppet valves are worked, in radial engines, by push rods and rockers operated by a single cam ring in front of the crankcase, and in " in-line " engines by camshafts running in bearings on the top of the cylinder block. The camshafts are driven by gearing from the crank-shaft and operate the valves through rockers.

20. On some engines an entirely different type of valve known as a sleeve valve, is used. This consists of a steel sleeve between the piston and cylinder, the ports being in the barrel, near the top, and in the sleeve itself. By combining rotary and reciprocating movement this single sleeve can be made to operate both inlet and exhaust ports. The advantages of this system are simplicity, durability and freedom from hot spots. A sleeve and cylinder assembly are shewn in fig. 100 which also shews the corresponding parts of a conventional poppet valve cylinder.

Crankcases

21. Crankcases of " in-line " engines consist of two light alloy castings joined in the horizontal plane, the top portion carrying the main bearings and the cylinders. In "X" and " H " engines, the bottom half also carries some of the cylinders ; otherwise it is merely a cover serving to keep oil inside and dirt outside. Crankcases of radial engines, owing to their shape, can usually be forged, thus obtaining lightness and strength. They may be either in two portions (three for a two-bank engine) split in the plane of the cylinders, or a hollow shell carrying the cylinders, with covers at the ends for the bearings.

Crankshafts and connecting rods

22. The crankshaft is machined from one or more forgings and in modern engines carries balance portions opposite to the crankpins. The shaft is hollow throughout, partly for lightness and also to enable oil to be distributed to the bearings. In some radial engines, the shaft is made in two or three portions clamped together, to enable a solid big end to be used. (Examples of radial and in-line crankshafts are shewn in figs. 101 (a) and (b).) Except for the main bearings of some radial engines, both main and crankpin bearings are of the plain type ; the exceptions being roller bearings. Connecting rods are made from steel forgings and have bearings at each end. The top one (small end) is usually a floating bronze bush but the big end bearing varies somewhat according to the type of engine. In-line engines have a plain and a forked rod working on the same crankpin ; only the forked rod. bears direct on the pin, the plain rod having a bearing on the forked rod bearing shell between the forked portions. This construction is shewn in fig. 102. Single-bank radial engines have one master big end bearing direct on the crankpin and the remaining rods are attached by means of wrist pins around the main big end. This may be either split, as in the in-line type of engine, or solid with a floating bush as shewn in fig. 102. The solid big end can of course only be used with a built up crankshaft, but it gives a much stiffer big-end construction, and by virtue of the floating bush thus made possible, reduces the effective bearing load.

Airscrew reduction gears

23. Owing to the inefficiency of a large airscrew at the high rate of revolution which is necessary for an engine to develop its full power, some gearing must be introduced between the crankshaft and the airscrew in order to reduce the speed of the latter to approximately half that of the former. In most engines the gear assembly, which is of course in front of the engine, is arranged so that it can be detached as a complete unit.

Auxiliary drives

24. In addition to its main duty of driving the airscrew, the crankshaft has also to provide power to drive various auxiliary services. Some of these, such as the oil pump and magnetos are for the engine itself, and others such as dynamos and air compressors, for various purposes on the aircraft. They are all usually grouped on the back of the engine, to avoid increasing the frontal area, and are driven by plain or bevel gears from the crankshaft. A typical group of auxiliaries is shewn in Fig. 103.

Recently, in order to simplify engine design and to facilitate interchangeabiity, there has been a tendency to remove most of the accessories from the engine itself to an auxiliary gearbox which is separately mounted. They are then driven by a single flexible shaft projecting from the back of the engine.

Valve timing

25. If we try to make the valves open and close too quickly, high stresses will be set up in the operating mechanism during opening and the springs will not be strong enough to make the valves follow the movement of the cams on closing. An appreciable time, reckoned in degrees of crank movement, must therefore be allowed for opening and closing of the valves. Moreover, while the valve is nearly closed not much gas can get through, and therefore, in order not to waste any of the induction and exhaust strokes, the valves begin to open earlier, and finally close later, than might at first be expected. The closing point of the inlet valve is further delayed because of the momentum of the incoming gas which causes it to continue entering after bottom dead centre (B.D.C.), and this " lag " in valve closing allows a bigger charge to enter. The opening of the exhaust valve is considerably advanced, because any pressure remaining in the cylinder after B.D.C. opposes the motion of the piston and the valve must be opened in time to allow the pressure to drop nearly to atmospheric before the piston begins to rise. At the end of the exhaust stroke, the momentum of the exhaust gas helps to evacuate the combustion space and thus clear it of burnt gas, an excess of which will interfere with the burning of the new charge. If both valves are open together for a short time, this partial vacuum encourages the fresh charge to enter, so that a considerable " overlap " is found, especially in high-speed engines. A typical valve timing diagram is given in Fig. 104.

Inertia forces

26. As the crankshaft goes round, the big ends tend to fly outwards but are restrained by a centripetal force at the crankpin. At each end of the stroke the piston has to be stopped and started in the reverse direction, and this needs a very large force at high speeds. Forces of this kind are known as " inertia forces."

27. Although they can be balanced as far as the whole engine is concerned, very large forces will nevertheless be set up in individual components, and these increase rapidly as revolution increases. The inertia forces are balanced to some extent by the forces due to gas pressure on the piston, but at high speeds they may be greater than these and so, on account of the heavy bearing loads, they limit the speed at which the engine may be safely run. For this reason very high r.p.m. must never be allowed unless there is at least some gas pressure to balance the intertia forces. When diving, therefore, the throttle should always be partially open, especially in radial engines where the big end loads are usually greater. As an example, the load on the crankpin of a radial engine when diving may be over 20 tons. (See Chap. II, para. 68.)