Auto Racing Articles: Cylinder Head Air Flow

Air Flow Through Valves
Article courtesy of http://RacingSecrets.com

By way of preliminary analysis, the application of the law of geometrical similarity presents a strong case for valves in pairs. For example, at a given pressure drop and the same lift, one valve would require a diameter of 4 inches to provide an area of opening equal to that of a pair of valves each of 2 inches diameter. The superficial area of the one 4-inch valve is twice the combined area of the two 2-inch valves, and if opened against a pressure in the cylinder, this is a measure of the comparative forces involved. The 4-inch valve would weigh four times the combined weight of the 2-inch pair, and the necessary spring tension would differ in that proportion for the same lift and the same engine speed. It may be noted here that while the above is correct upon the assumption of geometric similarly, the effective valve areas differ from. the actual as the coefficient of efflux varies at different lifts; also that the weight of a welldesigned valve increases somewhat less than the third power of the diameter would indicate.

Briefly stated, he assumes that two valves of 2.83-inch diameter should be substituted for one of 4-inch diameter (equal cross-sectional port area which required that the smaller diameter be 0.707 of the larger diameter) and that the valves in each ease are lifted 31.85 per cent of their respective diameters. Es then computes the hydraulic mean radii for the two cases, applies the laws of friction, and reaches the conclusion that the two valves would have a frictional resistance 39 per cent greater than the single valve.

The contrast is sharp. The tentative conclusion geometrically derived is that two valves of onehalf the cross-sectional port area and equal opening area, as compared to the single valve, would afford the same flow. Mr. Pomeroy's tentative conclusion is that two valves having the same cross-sectional port area as the single valve, and the same opening area with a lift 0.707 that of the single valve, would have a frictional resistance 39 per cent greater, and therefore less capacity. This discrepancy seemed to afford ample ground for experimentally determining the relative flow in similar combinations of valves.

This work was carried on by the Clarke Thomson Research in connection with problems involving exhaust gas scavenging at the Bureau of Standards and under the general direction of the National Advisory Committee for Aeronautics. Appreciation of the many courtesies extended by the Bureau of Standards is gratefully acknowledged.

APPARATUS.

The apparatus consisted principally of a centrifugal blower, a model cylinder, and U-tubes for measurements of pressure.

The blower was one of special design with a balanced rotor 11.25 inches in diameter, composed of 10 forward curved blades. An electric motor furnished the power, rheostat control permitting speeds from 3,000 to 6,500 revolutions per minute, corresponding approximately to pressures of 9 to 32 inches of water. The number of impulses varied from 30,000 to 65,000 per minute, affording practically continuous flow. The blower was connected to the cylinder with rubber hose, care being taken to see that the alignment of the hose remained perpendicular to the face of the cylinder at point of entrance throughout the tests.

The cylinder is shown in longitudinal cross section in plate 1. The cylinder head was carved out of white pine by an excellent pattern maker, and carefully finished as to its interior in accordance with dimension drawings. At the entrance end, the passages leading to the valves were cylindrical in form with axis perpendicular to the cylinder axis and 2.5 inches in diameter, the passages then curved as shown to the ports. The approach to the large valve, which had a diameter of 2.5 inches, was circular in cross section at all points. The approach to the pair of valves on the opposite side of the cylinder became narrower in the plane of the cross section shown and widened laterally to smoothly divide about 1.5 inches from the ports into two passages of 1.75 inches diameter. The angle between the valve axis and the cylinder axis was 15 degrees. No valve guides or bushings extended into the passages.

The diameter of the counterbore was 5.75 inches and of the cylinder proper, 5 inches. The valves were seated with a bevel of 30 degrees in the two planes forming the cylinder head. The diffuser shown was constructed of thin brass soldered together and inserted so as to divide the whole area of the cylinder at that point into rectangular passages about-seven-eighths inch square and 2 inches long.

The jet at the opposite end of the cylinder was likewise carved out of white pine as shown, and was connected to the cylinder head by a length of 5-inch wrought-iron pipe, smoothly galvanized inside, used to obtain sufficient length for rectification of the air current. Gaskets and shellac were used at the joints and the assembly drawn together with four long bolts extending from end to end, outside the cylinder.

In addition to the single valve with a diameter of 2.5 inches and the pair of valves with diameters of 1.75 inches already mentioned, another pair with diameters of 1.25 inches was tested. False seats were used with this smaller pair, consisting of turned hardwood rings carefully fitted to the 1.75-inch seats and beveled to receive the smaller valves as shown in Plate 4, fig. 2. These false seats obviously left a circular shelf or projection 0.25 inch wide immediately above the ports. As a matter of interest, two readings were taken with these shelves projecting above the port, but before running off the main test on these 1.25-inch valves the lines of the passages were smoothed off by filling in above these projections with putty, giving the approximate stream lines shown.

The valves were all designed on similar lines with the exception that the smallest pair had stems five-sixteenths inch in diameter, to fit the guides used for the larger pair, this dimension being 40 per cent larger than true proportion dictated, equivalent to a reduction of 0.022 square inch or 1.8 per cent of the port area of the smaller pair.

The Pitot tube shown in the jet in Plate 1 was clamped in position at the axis of the-jet throughout the tests, velocity readings being taken as later described. The dimensions were three-sixteenths inch outside diameter and about 2.5 inches in length. The impact end was gradually rounded and the static holes were four in number, about 0.02 inch diameter, smoothly perforating the outer wall.

A static tube of one-eighth inch diameter penetrated the central portion of the cylinder, reading static pressure of the air column after passing the valves and the diffuser. This is for convenience termed the "lower static."

Static tubes of one-eighth inch diameter also tapped the flow where the air column entered the passage leading to the valves. These are for convenience termed "upper static," only one being used at a time, as indicated by its position with respect to the valves. All statics were slightlyrounded on the inner periphery and the end kept flush with the inner surface of the cylinder or passage and so located as to be perpendicular to the direction of air flow.

The upper and lower statics were connected to the two legs of a U-tube to read directly the pressure drop through the valve, and also connected to other U-tubes to read the upper static and lower static head separately.

All U tubes had an inside diameter of about 0.25 inch and were vertical with the exception of one, which was inclined at a slope of 10 to 1 to read with greater accuracy velocity pressures of 3 inches or less. -

A centigrade thermometer was damped with its bare bulb in the air jet at a point about 1.5 inches outside the apparatus. A similar thermometer was hung on the wall for readings of room temperature.

The moisture content recorded is the average for the period indicated, as taken from a recording hygrometer, the variations being but slight, as were those of the barometer. All readings were completed within a period of seven and one-half hours.

MEASUREMENT OF AIR FLOW.

The method used for measuring the velocity and quantity of air is based upon the principles of the impact tube and the jet.'

Briefly, the impact tube, when held in and parallel to the air stream, registers a pressure corresponding to the total energy in the air at that point. In case of continuous flow through a pipe of varying cross section, if the impact tube is moved up the axis of the air stream, the pressure registered is constant at all points, except for friction losses. The velocity pressure and static pressure vary with every change of cross section, but the sum of the two, which the impact tube reads, is constant at all points, as the law of conservation of energy indicates. This is similar to Bemouilli's theorem in hydraulics.

Where the section of the pipe is smaller, the velocity of the air must be higher, as the quantity passing all sections of the channel in a given time is constant under conditions of continuous flow. Higher velocity means greater kinetic energy in the moving air particle, and this increment can only arise out of a corresponding diminution of the static pressure?

The jet here used for flow measurement carried this case further, contracting the air column to about one-sixth of its area and discharging into atmosphere at a static pressure equal to atmospheric pressure, or zero U-tube reading, all energy in the air being kinetic, read as velocity pressure by the impact tube. This requires that the theoretical orifice be wholly convergent, i. e., that the radio of absolute pressure of the region into which the jet discharges to the absolute pressure of the region from which the jet discharges be greater than the critical value, 0.5272, for air.

After verifying the fact that throughout the range of velocities used, the impact side of the Pitot tube at any given velocity showed constant readings for various positions in the jet, the Pitot was clamped in position, and readings from the impact side only recorded as velocity pressures. At frequent intervals during the runs the static side of the Pitot was tested, but invariably showed zero reading.

The velocity in the jet roughly equaled the velocity through the average valve opening, being about six times the mean velocity in the cylinder proper. In actual magnitude, the velocities ranged from 1,500 to 19,000 feet per minute, or 25 to 320 feet per second, covering about the extreme range of mean inlet velocities encountered in practice.

CONCLUSIONS.

I. The coefficient of efflux is practically constant, for all pressure drops (at least below 1 pound per square inch) where the lower pressure is approximately atmospheric, and the theoretical flow is computed upon air at atmospheric density.

2. Under conditions of general similarity, the coefficient of efflux is very nearly the same for valves of different sizes, at equal lifts expressed in per cent of their respective diameters.

3. Lifting a valve one-quarter of its diameter may develop an area of opening geometrically equal to its port area, but affords a capacity less than 87 per cent of that of the unobstructed port, at the same pressure drop; a lift equal to one-half diameter develops SO to 90 per cent of this maximum capacity.

4. At the same pressure drop, one valve of diameter D and lift h is equal in. capacity to:

First. A pair of valves of diameter 0.707 D (equal port area) and lift 0.707 li.

Second. A pair of valves of diameter 0.6 D and lift h for values of h not exceeding about 0.25 V.

Send this article to a friend

Submit an Article

Webmasters: You may freely reprint this article and/or ebook on your website provided the following caption remains intact, along with a working link.

This article courtesy of http://RacingSecrets.com , your racing technology resource.