The MC series of the two-stroke MAN B&W Diesel were introduced in the early 80s of last century. Its main application is the propulsion of all types of vessels, medium and large sizes. These engines have also been used in terrestrial applications, mainly for production of electrical power.
An
advantage of the MAN B&W MC series is that they incorporate the uniflow
scavenging, much more efficient than the former competitors, such as Sulzer RND, MAN KSZ, among others,
who employ loop or cross scavenging.
Two-stroke
engines generally have a drawback which has a great influence on the
development of its operating cycle. This problem is motivated by the
neccesity of the four phases of the operating cycle
(expansion, exhaust, intake and compression ) in a single turn of the
crankshaft, therefore the periods required for each of the phases are
necessarily shorter than a four-stroke engine. Between them,
the most critical stages are exhaust-intake, when the charge in the
cylinder is renewed. For this reason, the design of the engine is extremely
important.
The
process of displacement the flue gases out of the cylinder, and
filling with fresh air charge, called "scavenging", has a decisive influence on fuel consumption, power
and pollution.
The following figure shows the airflow (red) and exhaust gas (blue) inside one of the cylinders.
As can be seen
in the previous figure, incoming air is used to blow out or exhaust gas
sweep and meanwhile filling the space with fresh air.
The following
figure was obtained through a CFD analysis. It shows the velocity distribution
of the flow in the cylinder during the renovation of the load.
The following figure shows the mass fractions exhaust (blue) and air (red) for a tour from 90° to 270° crank angle.
In this CFD analysis, the free software OpenFOAM was employed. The reason is that this is an open source and thus it allows a complete manipulation of the governing equations. This is essential to carry out this type of analysis. It is necessary to adjust several parameters and some of them cannot be modified with other commercial softwares.
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