NACA Report No. 107

NACA Report 107 - A High-speed Engine Pressure Indicator of the Balanced Diaphragm Type was issued by the United States National Advisory Committee for Aeronautics in 1921.

Summary
NACA Report 107 describes a pressure-measuring device that was adapted for use in mapping indicator diagrams of high-speed internal combustion engines. The cards are obtained by a point-to-point method giving the average of a large number of engine cycles. The principle involved is the balancing of the engine cylinder pressure against a measured pressure on opposite sides of a metal diaphragm of negligible stiffness. In its application as an engine indicator the phase of the engine cycle to which a pressure measurement corresponds is selected by a filming device. The report discusses errors to be avoided in the development of an indicator for light high-speed engines, where vibration is serious, and outlines the principles underlying the design of this instrument in order to be free of such errors. A description of the instrument and accessories follows, together with operating directions. Specimen indicator diagrams are appended. The indicator has been used successfully at speeds up to 2,600 rpm, the highest speed available for trial. Its sensitivity is approximately that of a standard 6-inch dial gauge of the Bourdon tube type.

Prior to 1917 there were available several types of instruments for measuring and recording pressures internal combustion engine cylinders. Some of these were refinements of the conventional pressure indicator designed for low-speed steam engines while others were designed primarily for high-speed work with a view of minimizing the effects of inertia. These instruments were found to be useful for various classes of work, depending on their design and characteristics, although none of them had found more than very limited application in a comparatively few laboratories. Some of the inherent difficulties which have prevented the development of a wholly convenient and successful high-speed indicator are inertia, friction, and backlash in moving parts where mechanical means of recording are adopted; inertia and vibration of the system when a photographic method of magnification is applied to an instrument mounted on the engine; time lag of the gases in the connecting tube where a photographic apparatus is mounted independently of the engine and connects to it by a flexible tube; as well as the usual mechanical difficulties in the construction and operation of instruments of this class. It is noted that these difficulties are increased in the case of the aircraft engine, which usually must be mounted on a more or less flexible stand and in which at best the mechanical vibrations of the parts are excessive, due to their light weight and lack of rigidity. An important mechanical consequence of this excessive flexibility of the engine structure seems to have been overlooked, since it has not been discussed in the literature or taken account of other than accidentally in design of any indicator. To illustrate this effect, assume an indicator whose moving parts are mounted on the head of an engine cyIinder. In order to reduce the effect of inertia, the range of motion of the piston or diaphragm is reduced as much as practicable and the motion magnified either mechanically or optically so as to give a readable scale. It should be noted that the motion actually recorded is always the relative motion between the movable part of the indicator and the (supposeclIy) fixed part mounted rigidly on the cylinder head. But if the cylinder head itself flexes under pressure or mechanical vibration, this motion of the fixed support relative to the movable piston is recorded and magnified as well as the motion of the piston relative to its support. Hence it may happen that thus limiting the range of motion of the piston or heavy diaphragm and greatly magnifying the record, increases the bad effects of inertia. on a light flexible engine cylinder since the motions of the cylinder head itself, relative to the moving member, are subject to the same degree of magnification.

The rapid increase in volume and scope of experimental work on gasoline engines, particularly of aircraft engines, due to the impetus given by war conditions, intensified the need for suitable indicators and several laboratories undertook their development. The Bureau of Standards was particularly interested in securing an indicator suited to use in the altitude chambers where aircraft engines are operated for purpose of test and analysis of their performance under reduced pressure and temperature simulating conditions of flight. The altitude chambers enclose only the engine, all controls and measuring apparatus being outside whence in addition to all other requirements, it was essential that any indicator adopted should possess the feature of remote controI and reading. For the purpose in hand, for general analysis of engine performance, accurate indicator cards are of more importance than are individual records of single cylinder cycles; therefore a point-to-point method can be employed.

Conclusions
A successful instrument embodying the foregoing requirements has been developed and a half dozen of them have proved satisfactory for use under conditions of actual practice from 200 to 2,600 rpm (the highest engine speed available for test), and from 10 pounds/in² below atmospheric pressure to 1,000 psi above. The instrument has proved convenient in use and of high accuracy, being capabIe of measuring pressures to an accuracy comparable with that of the standard 6-inch pressure gauges used for recording these pressures. It is suited to measuring the pressures in internal combustion engines, as well as in any engine, compressor, or other machine in which gas pressures occur in successive cycles of the same form. For instance, the pressures occurring in the intake or exhaust manifold of a gasoline engine may be measured with the same instrument.