Evaluation of the Results of Initial Engine Tests with STID Equipment
The report provides an evaluation of selected cylinder pressure records of a Wärtsilä 320 engine operating according to the STID principle with early, low pressure steam injection, and a discussion of the effects of the steam injection on emissions. It also contains an evaluation of the application potential of low pressure steam injection on engines.
Due to pressure limitations of the available boiler, only tests with steam pressures up to 50 bar and steam temperatures up to 320°C could be performed. The steam pressure limitation caused that only injection during the intake stroke and early during the compression stroke was feasible which strongly restricted the scope of the study. However, the research has contributed considerably to the understanding of the STID process and in particular to evaluation of the steam effects on emissions and combustion. The low pressure steam injection is also one of the possible STID application versions to the normal, not thermally upgraded (hot), engines which requires only minor modifications of the steam generating facilities used in current ship installations.
It is shown that the ignition and combustion processes in the engine are practically unaffected by injection of the required mass of steam, which typically shall not be larger than 3.5 times the mass of fuel, at which soot, unburned hydrocarbon and CO emissions start to grow. Within this range the NOx emissions can be described by a simple formula NOx =NOx,o(1-0.2m), where m is the steam to fuel mass ratio and NOx,o the emissions of the engine with out steam injection.
The optimal way of using low pressure steam is to inject the steam into the inlet manifold of the engine which eliminates the complications of direct steam injection apparatus and eliminates the growth of maximum pressure. The steam temperature shall be as low as possible to minimize the reduction of volumetric efficiency of the engine and to reduce heat losses. The surplus steam enthalpy shall be used to additionally supercharge the engine using an ejector or tangential injection into the impeller of the compressor. It is recommended to utilize the low pressure steam injection on efficiency optimized engines by which both fuel consumption, NOx and soot emission reduction can be achieved