The manifold pressure gage indicates the pressure existing inside the intake manifold and is essential to the safe and efficient operation of supercharged engines.
Primarily, the supercharger enables an engine to deliver its designed horsepower during flight at high altitudes. For full power output, the explosive charge that operates the engine must be maintained at a specified weight. This explosive charge is comprised of gasoline and air. Air decreases in density as elevation increases, and therefore the weight of the explosive charge, incorporating air, will drop correspondingly unless steps are taken to add to the weight of the air. The supercharger performs this essential task, by compressing the total charge to the required pressure.Supercharger compresses the explosive mixture of air and gasoline, thus maintaining the charge at a specified weight for full power output at high altitudes.
At all elevations, but especially at the relatively lower altitudes, the danger of over-compression is ever present. The manifold pressure gage is the sentinel that guards against too high a boost for the air strata through which the plane is flying. For example, a certain type of supercharged engine will develop its designed hp with wide-open throttle at 10,000 feet. If this engine is given full throttle at sea level, where the air is much denser, the resulting over-compression in the cylinders will cause serious damage to the engine.
To permit the pilot to conveniently and assuredly limit the amount of power fed to the engine, manifold pressure gages are usually equipped with adjustable warning sectors. These signaling devices consist of colored bands which are visible through the dial of the instrument and which underscore the various operating-pressure ranges.
A dial of the manifold pressure gage, manufactured by the Eclipse-Pioneer Division of Bendix Aviation Corporation, is illustrated showing the typical warning sector arrangement. Note that the dial is graduated in inches of mercury absolute. Absolute means that the zero of the scale is mathematical zero, not the sea-level atmosphere pressure of 29.92 inches of mercury, which is the zero of many pressure-measuring instruments. A green band runs from 30 to 36, representing the pressure range for efficient cruising. A yellow band, beginning at 36 and ending at 42, denotes the high-boost operating range. Starting at 42 is a red band; it underscores the dangerous pressures. The warning sector arrangement described is merely suggestive and, in actual use, the bands are set in accordance with manufacturer's specifications for a particular engine.
An example will serve to explain the use of the gage. An airplane is wheeled from its hangar and the pointer of the instrument is at the barometric pressure of the airport say 30 inches Hg. The instrument functions somewhat like a barometer when the aircraft is not operating The engine is started; while it is idling, the pointer swings toward 1, because pressure, inside the manifold, is being exhausted during this low-speed operation at a higher rate than atmospheric pressure is entering through the partially closed throttle. The pilot wishes to climb at a rate which will utilize the full output of the engine's designed hp. He opens the throttle at the take-off until the pointer of the manifold pressure gage stops at the top of the yellow sector.
Shortly after take-off, the pilot brings the throttle back to the maximum efficient manifold pressure for climbing. As altitude is gained, the pointer of the instrument drops, indicating pressure loss. The pilot brings pressure back to whatever figure the manufacturer has specified as high-boost maximum for the climb, by applying more throttle. The throttle is so manipulated until it is brought to the full open position as the airplane reaches the designed altitude of its engine. Thus controlled by manifold pressure gage indications, the throttle is safely applied to enable the engine to deliver its high-power output at take-off and during the climb at all altitudes up to the elevation for which the supercharger is designed. Above the altitude at which the supercharger loses effectiveness, danger of over-compression no longer exists and power output falls off.
The manifold pressure gage consists, basically, of a two-celled, evacuated diaphragm plus a simple linkage system. The latter transmits movements of the diaphragm to the indicating pointer amplifying the minute diaphragm motions in the transmission. The linkage system starts with the flexible strap which connects the diaphragm to the bellows seal assembly. A link ties the bellows to the rocking shaft. Next come a sector, a pinion and the handstaff which is an integral part of the pinion. A pointer is attached to the handstaff. The hairspring performs the task of keeping all units in the linkage taut and in proper relation to each other.
In operation, the diaphragm, which is evacuated, and therefore pressureless, collapses as the pressure of the intake manifold is introduced through the damper tube to the airtight chamber surrounding the diaphragm. As the diaphragm deflects, the strap exerts a pull on the bellows causing it to expand on one side and contract on the other as shown in the diagram. The pivot arm rotates as per the arrow. A link is connected to both the pivot arm and the rocking shaft and it transmits motion to the rocking shaft, causing it to turn as shown by arrow b. The rocking shaft conveys the motion through the sector and pinion to the pointer. The latter indicates on the dial of the gage pressure, in inches of mercury. When pressure drops, the diaphragm expands, reversing the motions previously listed.
This article was originally published in the August, 1944, issue of Air Tech magazine, vol 5, no 2, pp 27-28.
A PDF of this article is available.
Photos credited to Eclipse-Pioneer Div of Bendix Aviation Corp.