The paper presents in reduced form the combustion process in diesel engine cylinder, formation of nitric oxides and soot, as it is known in engine research at the moment. Experimentally measured values for the nitric oxides and the exhaust gas opacity are presented for the water cooled diesel engine along with explanation of the temperature and air excess ratio influence. Results of numerical simulations applying the two-zone model for the nitric oxides formation are presented for the air cooled engines. The numerical model is validated using the measured values on water cooled diesel engine. Results of numerical simulations are comented and corelated to the in-cylinder process temperature and pressure.