轮机长
考试试题
[问答题]Supercharging and Scavenging
Air supply is very important to the perfect combustion of fuel oil in the diesel engine. As the air
density determines the maximum weight of fuel that can be burned per working stroke in the cylinder, it
also determines the maximum power that can be developed by the engine. If the charge-air density is
increased by a compressor before it enters the cylinder, a greater amount of fuel can be burned, resulting in
an increase of power output. This is the main principle of exhaust turbo-charging.
In today’s practice, the compressor, i. e. a blower, is driven by the heat energy obtained from the
engine exhaust gases. The turbo-blower comprises a gas-turbine, driven by the engine exhaust gases and
directly coupled to the blower, which draws fresh air from the engine-room and discharges it into the air
inlet manifold on the engine. The power produced in the turbine must be equal to that required by the
compressor. The air is pumped into the cylinder through the inlet ports. The turbo-blower is provided with a filter-silencer.
It is usually desirable to fit charge-air coolers to turbo-charged two-stroke engines. As the charge-air
density increases, the amount of air flowing into the cylinders is increased and as the charge-air
temperature lowers, the exhaust temperature and the engine thermal load are reduced.
It is very important that each cylinder should be well scavenged before the charged air is com-
pressed, otherwise this fresh air charge will be fouled by the residual exhaust gases from the last cycle. The
necessary scavenging is obtained by providing a satisfactory pressure difference between the air manifold and the exhaust manifold.
