In this matter of choice, I avoid jeneral terms when I can. For example, I rarely write "algae", because that could refer to moss that has a solid substrate or dissolved phytoplankton. The practical difference is that plankton can grow (and consume oxygen in decomposition) a lot faster than moss. Other writers figure that they want to, and can safely get rid of, both, so they lump it altogether in "algae", a word that I avoid.
Is no combusion external to an enjin? Or, external combusion is incidental to light enjinz like rockets and afterburning turbines. Does having the fire enclosed in solids for the vast majority of the burn make it internal combustion? I think that is a popular understanding that excludes rockets and turbines.