When a diesel engine is put under a load there is greater resistance to the expansion of combustion gases in the cylinder. (en.wikipedia.org)
You can also test for combustion gas in the coolant by over filling the reservoir, get the engine hot and check for bubbling in the reservoir. (mechanics.stackexchange.com)
Inside your car's engine, combustion gases expand as gasoline is burned, creating force that drives a piston. (discovermagazine.com)