An Investigation of the Airbox Method of Measuring the Air Consumption of Internal Combustion Engines

Abstract

The mass of air taken in per unit time by an internal combustion engine is a quantity of considerable importance in any analysis of engine performance, and a convenient means of measuring this quantity will often be required by the research worker or development engineer. Where reciprocating engines are concerned, the problem of measurement is not easy, since the flow is of a pulsating nature, for which “constant flow” types of measuring apparatus will, in general, be unreliable. A few suitable methods for measuring pulsating flows have, however, been devised, and what is generally known as the “airbox method”—consisting of the use of a measuring orifice in conjunction with a smoothing capacity—has often been employed, mainly because of its simplicity and the ease with which the necessary apparatus can be constructed.The work of Watson and Schofield (Proc. I.Mech.E., 1912, p. 517) has been of great value to users of the airbox method, but, though it has to some extent been supplemented by later investigations, several important questions still remain unanswered. The experiments described in the present paper represent an attempt to furnish additional information, in particular as regards the amplitude of the pressure fluctuations occurring in boxes of inadequate size and the errors in measurement produced thereby, a simple theory being given to account for these latter. The various factors governing the design of a reliable airbox meter are discussed, and it is shown that they can be related to the value of a dimensionless criterion which may be determined for any given engine.