- The definition of a working substance is the liquid, air or gas that makes an engine run.
An example of a working substance is oil that helps to make the pistons move.
working substance definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
the air, gas, or liquid that works the pistons, vanes, etc. of an engine
working substance definition by American Heritage Dictionary
A substance, such as a fluid, used to effect a thermodynamic or other change in a system.