As the sibling comments have pointed out, power-to-weight is the main reason, but it doesn't make diesel engines completely unfeasible for light aircraft. Diam

As the sibling comments have pointed out, power-to-weight is the main reason, but it doesn't make diesel engines completely unfeasible for light aircraft. Diamond Aircraft have had success selling planes with diesel engines recently, and even started a subsidiary company to manufacture them: https://en.wikipedia.org/wiki/Austro_Engine https://en.wikipedia.org/wiki/Austro_Engine