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In many years large low speed marine diesel engines have consumed heavy fuel oils with sulfur contents in the order of 2.5−4.5 wt %. Present legislation requires that the fuel sulfur is reduced, and in the near future the limit will be 0.5 wt % globally. During combustion most of the sulfur is oxidized to SO 2 from which a fraction is further oxidized to SO 3 . SO 3 may combine with H 2 O and condense as liquid sulfuric acid that promotes corrosive wear on e.g. cylinder liners. To extend engine lifetime and reduce costs for lubrication it is pivotal to identify formation of SO 3 with respect to operational conditions and sulfur feed. This work presents a computational model of a large low speed two-stroke diesel engine where a 0D multizone approach including a detailed reaction mechanism is employed in order to investigate in cylinder formation of gaseous SO 3 where fuel burn rates are based on experimental pressure traces. In contrast to NO the SO 3 does not really form at the highest combustion temperatures, but like NO the formation of SO 3 is very sensitive to the rate that fresh air mixes with hot combustion products. Consequently a simple mixing rate is proposed and calibrated in order to meet experimental results of NO. For a large low speed diesel engine the model shows that 3−5% of the injected sulfur is oxidized to SO 3 that is formed primarily in the temperature range from 2000 to 1300 K during cylinder expansion. In addition the model is used to reduce the full reaction mechanism from 96 to 7 elementary sulfur reactions without compromising the SO 3 to SO 2 ratio.

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