Auto Racing Articles: Performance at High Altitudes

Engine Performance at High Altitudes
Article courtesy of http://RacingSecrets.com

INTRODUCTION.
This investigation was originally undertaken as a continuation of the work on carburetion reported on by Prof. Lucks in the second annual report of the committee. The. intention was to undertake a study of carburetion and carburetor performance in connection with several types of aeronautic engines under varying conditions of temperature and pressure.

While this program was under development members of the French War Mission and others urged upon the committee the importance of making a special study of the problem of engine performance at high altitudes. The original carburetion program was, therefore, modified and made a. part of a somewhat more comprehensive undertaking, viz, the design, construction, and equipment of a special laboratory for the study of engine performance in general, including carburetion, under conditions corresponding to the highest altitudes reached by aviators.

Such a laboratory has been designed, built, and fully equipped.

Preliminary runs have been made for developing teat procedure and the plant is in such condition that the following operations and measurements can now be made:

(a) A test chamber in which the engines are mounted can be reduced to any air pressure as low as one-third of an atmosphere. The air in this chamber can be maintained at a low temperature and circulated about the engine by means of fans at a high velocity. The engine is controlled and all measurements are made from outside this chamber.

(b) The temperature of the cylinder-jacket water is controlled by means of a thermostat, the amount circulated is metered, and its rise in temperature measured. Thus the heat taken away by the circulating water can be calculated.

(c) Provision is made to weigh the fuel supplied to the engine.

(d) The air supplied to the carburetor is metered and its temperature measured.

(e) The exhaust is water-cooled. This water is also metered and its rise in temperature measured so that the heat given out in the exhaust can be calculated.

(f) The pressures in the intake, the exhaust chambers, and the main test chamber can be measured, and there is provision for partial automatic control. Provision is also made for measuring such other pressures as may be desired.

(g) The temperature of the air in the test chamber and such other temperatures as may be of interest in various parts of the engine or elsewhere can be measured.

(h) Provision is also made for such other incidental measurements as may be necessary for special purposes.

Experimental work is now in progress to determine the effect of different fuels on the performance of typical aeronautic engines, particularly at very low air pressures and temperatures. this immediate program was laid out to assist in. the specification for aircraft engine fuels to be supplied to the Army for service in Europe.

DESCRIPTION OF LABORATORY AND EQUIPMENT.
In designing the laboratory it was decided that results could be best obtained in a test chamber in which the entire engine in operation could be surrounded by conditions of air pressure, air temperature, and air velocity substantially duplicating actual flying conditions. To accomplish this result required a practically air-tight chamber of sufficient strength to withstand an excess of air pressure of 10 pounds per square inch or more on the outside and large enough to contain an engine. Also, auxiliary chamber in which the exhaust gases can be cooled before reaching the blower tanks for supplying and weighing the gasoline, water supply controlled by a thermostat for cylinderjacket cooling, water supply for cooling the exhaust gases, a gravity vacuum drain pipe for removing the exhaust cooling water, and the experimental measuring equipment demanded by the various problems to be undertaken.

The equipment has all been designed for use with engines up to 300 horsepower and any attainable speed with liberal allowances so that considerably higher powers can probably be handled, particularly as the power available at low pressures will be correspondingly less than normal. The Sprague dynamometer is rated at 300 horsepower with considerable overload capacity. The Nash blower has a volumetric capacity about twice the displacement of a 300horsepower engme at a pressure of one-half atmosphere, and the Yorkrefrigeration machine is of 25-ton refrigeration capacity which should prove ample.

The refrigerating apparatus exhaust blower, and dynamometer Reunite are conventional units purchased for the purpose in hand. Hence the only portions of the equipment specially designed are the building, low-pressure chamber, the air-cooling System, the engine support, and the apparatus for measurements of air intake, fuel, temperatures, and pressures.

Of the above the building offers no unusual features, being a temporary structure of frame and stucco, designed to accommodate the vacuum chamber and auxiliary machinery.

The low-pressure chamber shown in flure 1 is of concrete, 1 foot in thickness, heavily reinforced with 3/4-inch steel bars to provide against pressure from without. The two door openings, the front one being shown in the figure, are on opposite sides of the test chamber andare4feet by%feet insize. The doors swing onhinges andare built up of 2 by 7 inch oak beams 41 feet long and spaced? inches between centers, the outside being covered with 1/2-inch soft wood loosely held with headless nails and covered over with air-proof roofing paper. This construction was adopted as a safeguard against possible explosion inside the chamber. The very light covering of the doors might be blown off without danger to the concrete walls. To guard against excessive air leaks the outside of the chamber is covered with a very heavy coating of asphalt paint and the doors close against heavy rubber gaskets.

The engine is mounted at the right end of the test chamber, figure 1. The wails of the chamber are pierced by a number of holes appropriately placed, each being provided with a flange and gasket through which the various connections are made. The flexible coupling of the engine to the dynamometer is made through 9. The engine is controlled through 8 by means of a system. 61 pull rods and bell cranks. The gasoline is supplied from. overhead tanks through 17. These tanks rest on platform scales so that the weighing is done at the right of the front door. The pressure tubes are brought through 18 to appropriate manometers while an oil-gauge lead and necessary thermometer leads are brought through 13. All this apparatus, together with the manometers for the venturis of the two watercooling systems, is mounted near the engine controls. Likewise the dynamometer control board and the auxiliary switch board for the blower and refrigerating compressor motors is near by. Thus the chief operator not only-has complete control of the engine but also of all the auxiliary equipment. At the same time he can see at a glance everything that is happening. Electrical leads used in making the necessary beat measurements are brought through 5 and 6 to an instrument table. The cylinder jacket water enters through 15 and returns through 16 to an overhead miring tank supplied with a thermostat to regulate the temperature.

The air-cooling system consists of two parts. A series of direct expansion ammonia coils mounted on top of the chamber is provided for cooling the carburetor air. These coils consist of about 2,000 feet of 1 1/4-inch piping inclosed in a box insulated with 4 inches of sawdust. The air is made to pass through this coil box in a tortuous path and is then led through an insulated pipe provided with a regulating valve to the bottom of the test. chamber at 7. From this inlet the air passes through a box in which it-is metered directly to the carburetor, thus supplying cold air to the intake when desired.

The second part of the cooling system consists of about 800 feet of ammonia expansion coils mounted in the left end of the test chamber as shown in the sectional diagram, figure 2. Four fans are furnished to force air over the coils at high velocity. The object of these coils is to absorb the heat given out by the exposed surface of the engine. A fan is also provided for producing a high air velocity past the engine itself, if so desired.

The engine exhaust cooling system is shown diagrammatically in figure 2. The exhaust pipes connected to the engine are waterjacketed, the inner pipe extending about 8 feet from the engine, while the water jacket is continued from the exhaust elbow to the main exhaust manifold in the form of a flexible rubber hose. In this way the whole connection is made flexible and the water from the jacket enters the exhaust pipe at a considerable distance from the engine. The water enters the test chamber through 14 and is distributed to the exhaust pipes as shown in the figure.

The mixture of exhaust gases and water passes through a 5-inch pipe to the auxiliary exhaust chamber where the water is drained off througj the gravity drain pipe and the gases pass to the exhaust blower. Two exhaust manifolds and chambers are provided, one for each set of cylinders in a. V-type engine. Each side is provided with six inlets for pipes from the engine.

The engine support was designed for the purpose of duplicating as nearly as possible the flexibility and the inertia of the typical fuselage mounting. The design developed makes possible an accurate adjustment of stiffness as regards transverse and vertical vibration and rotation about each of the three principle axes of the engine, but as no data were at hand as to the corresponding characteristics of any fuselage mountings, the support was constructed on the basis of estimates of these constants, and appears to possess nearly the desired characteristics for the engines mounted on it up to the present time.

The design of this support is illustrated in figure 3. Two oak beams, A, in this case 2 by 6 inches by 6 feet 3 inches supported at the ends form the basis of the mounting. The engine is mounted directly on two supplementary beams, B, of 2 by 4 inch section and of the length required for the particular engine under test. These supplementary beams are free from the mean beams except at two points where they axe bolted together through a thin separating block, U. Two yokes, K, are provided to prevent torsion of the individual beams, but have no other effect as they are free from contact with any other part of the structure.

Selection of the dimensions of the main beams and adjustment of the spacing between the points of support of the secondary beams permits of adjustment of vertical and lateral stiffness and approximate adjustment of resistance about a vertical axis and a horizontal axis at right angles to the axis of the crank shaft. Stiffness as regards rotation about the latter axis can be adjusted by a third beam of proper dimensions rigidly connected at the ends and to the yoke rods, P.

The experimental program laid out in connection with the altitude laboratory, including a general study of the performance of aeronautic engines under high-altitude conditions: is in progress and much valuable information is in prospect. The subject of precompression engines and the development of precompression blowers or pumps is to be given special attention.

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