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The Malone engine was invented and patented by John Fox Jennens Malone (1880-1959) of Newcastle.
The engine used high temperature liquid as its working fluid, and was also referred to as the Hot Water Engine. A US patent, describing it with Mercury or an Mercury-Lead Alloy as working fluid, was granted in 1924.
Malone's first 50 hp prototype was completed in 1925, and used coal to heat high pressure water sealed inside a cylinder. Malone's second prototype, demonstrated in 1931, also produced 50 hp but in a much smaller design.
The engine worked on a modified version of the Stirling Cycle.
US Patent No. 1487664 here.
The above information is condensed from the Wikipedia entry.
See the Douglas Self website for more information and excellent drawings.