WE should be designing superchargers for better overall efficiency, according to Mr. Pigott, because the effect on engine capacity is pronounced, and gets to be more so at higher boost pressures. Mr. Pigott shows also that supercharger performance can be rather well predicted without trying out a variety of blowers in expensive engine setups. He sees no reason to suppose that the newer adiabatic rotaries can be considerably improved as they are further developed. He adds that the Roots type has seen considerable development and will be at the end of its string when boost pressures exceed 7 or 8 psi, which they are certain to do in the future.
WHILE the centrifugal supercharger is excellently suited to aviation service and to relatively constant-speed, diesel-engine service, Mr. Pigott states, it is not of value for cases of considerable variation in speed where full boost is required at all speeds. The Roots-type blower, the author says further, has had a justified development for low-pressure boost, but is not valuable for compression ratios much in excess of 1.6 because it has no adiabatic compression. The vane-type supercharger, while it has an adiabatic compression, as so far developed appears to the author to be somewhat complex in structure, requires internal lubrication, and has not yet shown satisfactory efficiency compared to other types. The Elliott-Lysholm screw-type and the new P. L. internal-gear type appear to him, however, to give great promise for satisfactory supercharging at the higher pressures beyond the range of the Roots type, and appear to be the best present line of development.
AN extensive study of oil foaming has brought Mr. Pigott to the following conclusions: 1. All suction-side resistance of either pressure or sump pumps must be reduced as much as possible. 2. Velocities in suction piping should be kept below 5 fps at all points. Bends that cannot be eliminated should be long radius. Valves and other restrictions must be avoided. 3. Spur-gear pumps can be improved by optimum exposure of teeth on the suction side, use of side pockets, and reduction of clearance. Without help, these pumps still won't be good enough for the highest altitudes. 4. Oil tank pressurizing will provide boost for the pressure pump. 5. Centrifugal boosters are useful, except with cold oil or with much aeration. For the pressure pump, the booster should be located directly at the oil tank, and for the scavenge pump, at the sump. Both must be drowned, no lift. 6.
A QUICK survey of the hydraulic aspects of the vapor lock problem is given here by Mr. Pigott. The hydraulic design of the supply system from the tank to the fuel pump is discussed by the author. The cause for carburetor vapor lock beyond the fuel pump becomes very largely a question of the ability of control orifices of any type to measure mixtures. In general, control orifices can only measure a homogeneous fluid correctly. The general methods of calculating a system for obtaining the limiting tank temperature for take-off and the limiting altitude for any system, are discussed, and the requirements summarized.
DISCUSSION is given of the increase in severity of conditions for lubrication in the last few years, during which the engine designer has been calling upon the oil chemist for special oils to correct difficulties in operation. The 51% increase in horsepower in 10 years is proportioned between increase of compression ratio, intake system, displacement, and speed. Increase in severity of mechanical and thermal loading is due both to increase of rotative speed and brake mean effective pressure. Cooling done by the oil has been increased, and pistons are hotter than formerly. Crankcase temperatures have risen. Compounded oils do not eliminate the basic rate of change of deterioration of oil with temperature. Compounding delays the start of deterioration and lowers the absolute rate. Too many engineering problems recently have been left to the chemist to solve.