The standard "dry air" (0% humidity) composition at 59 F is about 21% oxygen. At 100 F and 95% humidity, approximately 7% of the atmosphere will be water vapor. Furthermore, the density change of the remaining oxygen, due to the temperature change from 59 F to 100 F, will further reduce the oxygen content by another 6% from dry air.
This means that air at 100 F and 95% humidity will contain approximately 87% of the oxygen present in air at 59 F and 0% humidity. In other words, your gasoline engine would have to run using air containing 18.3% oxygen, as opposed to 21%.
This decrease in O2 will force the driver to open up the throttle a little more to keep the same cruising speed. This will reduce pumping losses through the throttle plate and intake manifold, placing the engine nearer its ideal BSFC. The result is a measurable increase in fuel economy.
|