When fossil fuels are burnt in the open air, then carbon dioxide (CO2) and water vapour (H2O) are the main combustion products that are created.
However, in an enclosed space which limits the amount of air available, the combustion process also generates carbon monoxide (CO) and hydrogen (H2>).
The presence of sulphur in the fuel, diesel fuel contains in excess of 2% sulphur, results in sulphur dioxide (SO2) being created, and this is known to cause acid rain.
Various forms of nitrogen oxide are generated, these are referred to as NOX, and are a cause of acid rain as well as causing respiratory problems. NOX can combine with sunshine to create smog.
The lean burn engine operates with a fuel/air ratio of 18:1. This optimises the power and minimises the gas emissions.
The three way catalytic converter works by completing the combustion process that took place in the petrol engine, passing the exhaust emissions over a series of catalyst which complete the combustion process started in the engine. Thus many dangerous gases are not released into the atmosphere.
Diesel engines can produce clouds of particulates which have potential health issues to the people breathing them in.