Performance of the 'Gas Engine' invented by Mark Isamabard Brunei (MIB), who was a one of the most famous British engineers and also a father of Isamabard Kingdom Brunei (IKB), is discussed by researching the document of specifications of the patent, which was applied for in 1825. After the outline and working principle of the engine are shown from the information of the document of specifications, the reason why the engine could not work in practice is shown as the result of the discussion with today's knowledge under some assumptions. It is concluded that the main reason was that the temperature difference between the high temperature heat source (hot water) and the low temperature heat source (cold water) was too small to drive such a heat engine, from thermodynamics and thermal engineering points of view.