Horsepower Correction for Atmospheric Humidity ¹

SEVERAL series of tests made on two multi-cylinder engines to determine the effect of humidity on engine performance are described and the results discussed. The basis for these tests was the so-called oxygen-content hypothesis that the presence of any given volume of water vapor in the cylinder, by lessening the oxygen present, reduces the quantity of fuel that can be burned efficiently per cycle and correspondingly decreases the power output. The results obtained closely verified this hypothesis.

As double interpolation is necessary in humidity tables for water-vapor pressure, the process is both laborious and conducive to errors, contour charts reducing both troubles only to a certain extent. Nomograms enabling the humidity correction to be obtained from thermometer and barometer readings were employed and are included in the paper. Instructions for using the nomograms are given and the method of their computation is reviewed briefly in an Appendix.