The article discusses the results of actions performed by marine engine manufacturers towards the reduction of emission of harmful substances in the exhaust gas from these engines. The discussion is limited to main engines produced by MAN B&W. A model is proposed to describe the process of emission of harmful substances from Diesel engines based on the semi-Markov process, discrete in states and continuous in the operating time. The states of this process are: engine ability state (s1) and engine inability state (s2). A method to calculate probabilities P1 and P2 of the appearance of states s1 and s2 is proposed. State s1 is interpreted as corresponding to the inequality ekek(dop), where ek represents the mass of harmful substances emitted by the engine during its operation and ek(dop) is the maximum mass of emission of these substances permitted by relevant regulations.