Assessment of methods for quantification of exhaust emissions from marine dieselengines

Abstract

The focus on exhaust emissions has increased the last decades. In the coming years emission restrictions on nitrogen oxides (NOx) will be intensified while restrictions on PM are still pending consensus. With this, it becomes increasingly important that the emission measurements achieved during engine certification reflects the emissions formed during real engine operation. This study presents results of an assessment of measurement procedures for quantifying NOx and PM emissions from marine diesel engines for certification purposes. The present methods for quantifying NOx are questioned and the feasibility of applying the same measurement procedures for determining PM emissions is investigated. The study is based on the currently in use measurement procedures with focus on the application of steady state test cycles.

For NOx emission concern, the current test cycle was compared to cycles that were constructed based on specific vessel operation of a platform supply vessel, an anchor handling vessel and an oil tanker. Comparing the vessel specific cycles and the test cycle showed considerable differences in the chosen loads and weighting of them. Calculations where done on total weighted NOx emissions based on the test cycle and the constructed cycles. The results, giving difference in total weighted emissions comparing vessel specific cycles and test cycle, were not as distinctive as first thought. The difference found showed an increase or decrease in emissions of approximately 10 %. There were uncertainties to the calculations due to the specific NOx emissions used. The data had been collected from two engines, which might not be representative for a general average of specific emissions. New evaluations should be done with more extensive emission data.

An evaluation was done concerning application of the steady state test cycle for PM emission measurements. The focus was on evaluating if there are operational conditions inducing PM emissions that are not reflected in the use of test cycle. To evaluate this concern laboratory measurement of PM distribution during transient operation was conducted. The laboratory results gave inconclusive results, thus previous studies on the matter was reviewed. It was found from a study conducted on a ferry, that there was 6.5 times the number of particles formed during manoeuvring than during the sailing period. To determine the extent of manoeuvring and thereby transient operation, an assessment was made on operation of selected car ferries along the Norwegian coastline. It was found that manoeuvring constituted 5.5 minutes of a complete voyage, whereas an average of the total voyage duration was found to be 19.5 minutes. For the vessel from the external study, manoeuvring amounted to a total of 60 minutes, while sailing constituted 150 minutes. By taking the duration of the manoeuvring and sailing into account, a new relationship of PM number was found to be 2.6 times the amount of particles during manoeuvring than during sailing. For the given vessels, this constitutes that 70% of the particles formed for a voyage can be attributed to the manoeuvring phase. The findings give that application of a steady state test cycle for these vessels would give a poor reflection of the total emitted particles, and thus a generally applied test cycle should include transient loads in addition to the steady state. Current test cycles used for NOx quantification is with basis from my results unsuited for PM measurements.