Question

What type of exhaust comes from a Steam Engine, Internal Combustion Engine, and a Hydrogen Fuel Cell?

Answer 1

see below

Explanation:

In a Steam engine the exausts depends by the fuel used for the phase of heating in the Watt cycle (or its modifications)
The worst one is the coal (the old train in far west) where you have a lot of `CO_2` but also a lot of particulate and black smoke.

More is little the molecula of fuel, the less are the exausts you have from a Internal Combustion engine, because it burnst better (with less particulate) an it need less air in excess. Moreover the smaller molecules produce less `CO_2` and more `H_2O` .

Air in excess lets to form, in the products, NOx because in whatever combustion are present both Oxygen and Nitrogen. Fortunately the reaction
`N_2 + O_2 = 2 NO` is very sfavourited and it can occur in the opposite direction with the catalytic muffler.

in an Hydrogen Fuel Cells burn only `H_2` and you obtain only `H_2O` with very littele amount of NOx, but `H_2` doesn't exist in nature and you must obtain it or obtain the enrgy yo produce it.
The exausts depend by the process to product `H_2`