On 13 Nov, 10:27, The Natural Philosopher
wrote:

efficiency of a heat engine is pretty much down to the ratio of
combustion temperature to exhaust temperature.

Only if you ignore phase changes (and pressure changes too). Assuming
that temperature is the _only_ significant factor will mislead you.

Admittedly it is the case for a condensing boiler where you draw the
system boundary in the right place, but it won't help you to
understand how it's working internally.