The pump was invented by H. A. Humphrey and first presented in paper to the Institution of Mechanical Engineers on 19 November 1909.

The force of the expanding gas drives water downwards from the combustion chamber and accelerates it along the delivery pipe.

As the water begins to flow back down the U, this increases the chamber pressure and causes the exhaust valves to open.

There would still be a risk of inefficient four-stroking though, unless the scavenging is effective enough to sweep out the previous exhaust gases.

[2] A large amount of experimental work on the two-stroke version of the Humphrey engine was conducted by multiple companies in the hope of improving the type's efficiency and power density.

[2] The only 2-stroke Humphrey pump to enter service was a single example designed by A.P Steckel and installed at the Sun shipyard in Pennsylvania in 1925.

[5] The main limitation of the Humphrey pump is that it has no suction capacity, in fact it must be installed in a dry sump several meters below the supply level.

Given the pump's physical size and the need to protect the equipment against flood events, this is no small engineering exercise, requiring (in the Cobdogla example) several thousand tons of concrete.

[6] Its advantages however are: with no moving parts except for the dozens of spring-loaded inlet valves, it has low maintenance and high reliability.

The demonstration pump exhibited at the Brussels exhibition was tested by Prof W.C Unwin in 1909[5] and was found to require around 33% less fuel gas than would have been consumed had contemporary gas burning engines been used drive pumps moving the same amount of water to the same height.

[11][12] A 36" cylinder bore Humphrey pump was installed at Chester, Pennsylvania in 1927 by the Sun Shipbuilding and Dry Dock Company.

Final construction would have used ten pumps, five by Beardmore's and five by the Italian division of Brown Boveri.

This pump was thought to have some application for small-scale or portable tasks, where its convenience outweighed efficiency.