what are the substances emitted by diesel, gasoline and LPG?

Answer 1

They all emit carbon dioxide and water vapor (in addition to the 80% nitrogen that was largely unchanged).

For a given amount of work, the LPG will emit a bit less CO2 and a bit more H2O (because its hydrogen to carbon ratio is higher). The diesel will emit a bit less of both compared to gasoline because it is more efficient due to its higher compression ratio.

They can all emit oxides of nitrgoen (NOx) unless equipped with a reducing catalytic converter. Gas auto have had those for 25 years. LPG conversions often don't. Diesels don't, but will starting with the 2007 model year when "clean diesel" (low-sulphur) is fully available.

They all also emit some carbon monoxide, especially if running "rich" with too much fuel. That is greatly reduced in a gasoline engines catalytic converter.

They all emit unburned hydrocarbons which is a health and smog issue. Much, much less if equipped with a catalytic converter.

Diesels are much worse on particulate emissions and that "soot" has its own health problem because the particles are so small they get deep into our lungs.

Answer 2

carbon monoxide