Aircraft technical Basics: Aircraft Engines - RAF Flying Training Manual - Chapter VII.- Engines: Supercharging
SUPERCHARGING
Engine Power
48. The power output of any particular engine depends on the rate of revolution and the average pressure in the cylinder during the effective part of the power stroke. The permissible r.p.m. are limited by inertia forces, and in any case, after a point, pumping losses in the induction system restrict the gas flow so much that no more power is obtained. The gas pressure in the cylinder depends mainly on the compression ratio and the weight of gas burnt at each firing stroke. High compression ratios give increased power and increased efficiency, which is all to the good, but they also promote detonation, so here again there is a practical limit to power output. If, however, the amount of gas in each charge can be increased beyond that which the air pressure outside will force in, then an improved power output can be obtained.
Forced Induction
49. In the engines hitherto considered the combustible mixture is forced into the cylinders by atmospheric pressure only. These are called "normally aspirated" engines. In most Service engines, however, the power is increased by pumping the mixture in at a higher pressure. This is known as forced induction ; it is achieved in all engines used in the service by a high-speed fan, gear driven at the rear of the engine. This is called a blower, or supercharger, and is illustrated in fig. 116. It is fitted between the carburettor, which in this case is a single unit, and the induction pipe.
50. Forced induction may be used either to increase the power of an engine low down (boosting) ; to compensate for the lower atmospheric pressure at high altitudes, which would otherwise seriously reduce the available power output ; or to maintain sea-level power up to some considerable height (supercharging). This height depends on the size of the blower, and is usually about 12,000 to 15,000 ft. An engine may be both boosted and supercharged.
Boost
51. The pressure in the induction pipe is known as boost or boost pressure, and is measured in lbs. per sq. in. above (+) or below (—) that of the standard sea-level atmosphere. The boost pressure is indicated to the pilot by a gauge in the cockpit. It might be thought that with a large enough blower the power of an engine could be almost indefinitely increased, but in practice it is limited by the strength of the working parts of the engine, by the heat developed inside, and by detonation. Thus it will be seen that, as pointed out in para. 6, we are all the time up against detonation in the search for power. By reducing the compression ratio, a little boosting can, however, be made a definite advantage, and detonation can be minimised by using suitable fuels and rich mixtures at full power. The greatest boost that can be used in ordinary level flight is called " maximum level flight boost " and is equivalent to full throttle in a normally aspirated engine. It must only be used for short periods, and for continuous running a much lower figure (maximum continuous cruising boost) must not be exceeded.
52. For taking off a specially high power output is required and may be obtained by using more boost still, but for a very short period only, and then only with a very rich mixture (see para. 37) to avoid detonation. On the other hand, when cruising at low power with a weak mixture, the " maximum boost for economical cruising—weak mixture" is the limit, and is much less than the normal cruising boost.
Boost control
53. A blower capable of maintaining sea-level power up to a high altitude will naturally give still more power low down, and the boost must be kept within the stated limits by throttling. On certain early supercharged engines this was left entirely to the pilot, but in some circumstances, e.g. rapid climbing, dive bombing, etc., it is impossible for him to give that close attention to the boost pressure necessary to get the best out of the engine and at the same time avoid over-boosting. To get over this difficulty and protect the engine from damage, engines are now fitted with an automatic boost control, which is interposed between the pilot's throttle lever and the carburettor, and is arranged so that even if the pilot's lever is left wide open, the boost cannot rise above the maximum for level flight. Most boost controls now are of the "variable datum type", which is arranged so that any particular position of the pilot's lever corresponds to some definite boost pressure, and this boost will be kept automatically constant, independent of height, until the aeroplane rises above that at which the blower is just able to keep up with the falling atmospheric pressure. Above this, the boost falls off as with a normally aspirated engine. The details of the boost controls may be found in the engine handbooks. It is connected to the throttle linkage in such a way that, although the cockpit lever may be advanced, the carburettor throttle valve is not allowed to open further than is necessary to maintain the pre-determined boost pressure. By altering the setting, take-off boost can be given, but this is done by a separate control which automatically brings in the rich mixture from the carburettor. Some boost controls are provided with an emergency cut-out, which over-rides the automatic boost control. This must only be used in emergency and not, in any circumstances, for ordinary flying, because, even if the correct boost is not exceeded, the mixture enrichment is also put out of action and, as has been previously explained, high boost is only allowed with rich mixture, and without it may cause serious damage.
