Aircraft technical Basics: Aircraft Engines - RAF Flying Training Manual - Chapter VII.- Engines: First Principles

FIRST PRINCIPLES

The four-stroke cycle

1. Before going on to the constructional details of engines, it is desirable to examine briefly the basic principles on which they work. In all internal combustion engines the power is produced by the expansion of air in a cylinder when heated by the burning of the fuel in it. The heating causes a rise of pressure which drives the piston down* on its power stroke. There is no more gas in the cylinder after combustion than before and the expansion is solely due to the heat generated. Before it can be usefully burnt the charge of fuel and air must be brought into the cylinder (induction stroke) ; then it is compressed (compression stroke) ; in its compressed form it burns and as it expands drives the piston down (power stroke) ; and finally the waste products of combustion are disposed of in the exhaust stroke.

* Throughout this chapter, whatever the position of the cylinder in relation to crankshaft, "down" means towards the crankshaft, and " top " and " bottom " of a piston etc. are to be taken as the parts remote from and nearest to the crankshaft, respectively.

The induction stroke

3. On the induction stroke, the piston descends in the cylinder and thereby lowers the pressure inside, so that the combustible mixture is forced into it by the pressure outside. This does not happen instantaneously, because like all other matter the gas has inertia and does not start moving at once, and it also has to overcome the resistance of the induction passages through which it has to travel on its way to the cylinder. As the power output of the engine depends on the amount of gas which can get in during the short period (about 1/100 second at high speed) of the induction stroke, these passages must be kept as free as possible from any obstruction such as sharp bends, or constrictions at the valve ports, but when reduced power is required from the engine additional resistance is applied intentionally by throttling at the carburettor. Whatever the design of the induction system, it is not possible to fill the cylinder quite up to the same pressure as exists outside, and the ratio of the actual amount of gas in the cylinder at the end of the induction stroke to that which would be there if there were no resistance or inertia is called the " volumetric efficiency" of the engine.

The compression stroke

4. As the piston moves upwards, after the induction stroke, the gas is compressed. The ratio of its volume at bottom dead centre to that at top dead centre (for there is always some space left in the cylinder, even at T.D.C.) is called the " compression ratio ", which in aero engines is usually about 6 1/2 to 1 or 7 to 1. It should be noted that this is strictly speaking merely a ratio of volumes, and does not indicate the actual pressure in the cylinder at any time. As the gas is compressed, it gets hot (in the same way as a bicycle pump gets hot in use) and this increases the compression pressure to something more than that represented by the mere reduction of volume. Just before top dead centre (T.D.C.) the charge is fired by a spark, and the next part of the cycle is perhaps the most important of all because the exact behaviour of the charge during combustion has a great effect on the running of the engine.

Detonation

5. The burning of the gaseous charge in a cylinder is sometimes spoken of as an explosion, but this is misleading for it gives a false impression that it is a process so quick as to be virtually instantaneous. Under normal conditions the process of combustion occupies about 30° of crank movement and it is very important that it should be neither too quick nor to slow. The purpose of the burning is to heat up the gas and raise the pressure in the cylinder so that the piston is forced down and in order that full use should he made of the heat produced the combustion must be complete before the piston has gone too far on its stroke. If, on the other hand, the explosion is too quick it causes an excessive pressure rise and thereby heavy loading on the working parts of the engine, high temperatures causing excessive heat loss, and damage to the material of the pistons, valves, etc. Under certain conditions these effects are produced in an acute form by what is known as " detonation ". When this happens the burning begins normally but when part of the charge is burnt the rest explodes all at once and a very sharp rise of pressure and temperature is the result.

6. Fig. 95 shows actual photographs of a normal (non-detonating) explosion in a side-valve cylinder head of ordinary motor car design, obtained by means of a very high-speed cine camera. The figure under each picture is the position of the crank at which the picture was exposed—indicating before, and after, T.D.C. The spark occurs in the third picture, but the flame does not really get under way until about the 8th. After that, its steady progress across the combustion space and the subsequent cooling down of the hot gases as the piston descends can be clearly seen. In fig. 96, the spark occurs at the same point, and at first the progress of the flame is very similar to fig. 95, but in picture 13 a small globule of self-ignited flame can be seen at the right of the cylinder. In the very short time—only 1/2,000 second--before the next picture was taken almost the whole of the charge was burnt, showing the very sudden burning and accompanying pressure rise due to detonation. Such a sudden rise of pressure is akin to hitting the inside of the cylinder with a hammer and is audible as a " knock " outside the engine. Detonation is to be avoided as far as possible, but unfortunately some of the very features of design which tend to increase the power output also promote detonation if carried too far. These will be considered more fully later on, but it may be observed now that the search for more and more power involves a continual battle with detonation.

The exhaust stroke

7. After ignition, the burning charge expands, and as the piston descends the temperature and pressure fall, the heat energy generated by combustion being transmitted to the moving parts of the engine, and converted by them into useful work. The remainder of the cycle, the exhaust stroke, needs little comment except to point out that any obstruction to the exit of the exhaust gases not only absorbs energy in over-coming it but causes an excess of burnt gas to be left in the cylinder at the beginning of the next induction stroke, and thus contaminates the fresh charge. It is therefore important to see that a free outlet is provided, for the time available is no greater than for the new charge to come in on the induction stroke.