ORIGINAL: wcmorrison
They put out the same stuff all combustion engines do; carbon monoxide, carbon dioxide, water, unburnt oil and so on.
But you will spew out more pollutants from your car than you will ever from your model airplane engine. Even your lawn mower will produce more pollution.
Chip
Not engines gives pollution only, your farthing are pollution too
NOx are nitrogen oxides created in combustion.
To preventing NOx out of engine are the exhaust recirculation used to lower combustion temperature. About 10-15% exhaust are entered into the intake manifold , then mixed with fuel mixture before combustion start. The EGR valve are controlled by ECU.