Internal combustion engine procedure:
A basic part of the cycle of an internal combustion engine is the supply
of fresh air and removal of exhaust gases. This is the gas exchange
process. Scavenging is the removal of exhaust gases by blowing in fresh
air. Charging is the filling of the engine cylinder with a supply or charge
of fresh air ready for compression. With supercharging a large mass of air
is supplied to the cylinder by blowing it in under pressure.
Efficient scavenging is essential to ensure a sufficient supply of fresh air
for combustion. In the four-stroke cycle engine there is an adequate
overlap between the air inlet valve opening and the exhaust valve
closing. With two-stroke cycle engines this overlap is limited and some
slight mixing of exhaust gases and incoming air does occur.
A number of different scavenging methods are in use in slow-speed
two-stroke engines. In each the fresh air enters as the inlet port is
opened by the downward movement of the piston and continues until
the port is closed by the upward moving piston. The flow path of the
scavenge air is decided by the engine port shape and design and the
Three basic systems are in use: the cross flow, the
loop and the uniflow. All modern slow-speed diesel engines now use the
uniflow scavenging system with a cylinder-head exhaust valve.
In cross scavenging the incoming air is directed upwards, pushing the
exhaust gases before it. The exhaust gases then travel down and out of
the exhaust ports. Figure above illustrates the process.
In loop scavenging the incoming air passes over the piston crown then
rises towards the cylinder head. The exhaust gases are forced before the
air passing down and out of exhaust ports located just above the inlet
ports. The process is shown in Figure below.
With uniflow scavenging the incoming air enters at the lower end of
the cylinder and leaves at the top. The outlet at the top of the cylinder
may be ports or a large valve. The process is shown here.
Fig: Uniflow scavenging
Each of the systems has various advantages and disadvantages. Cross
scavenging requires the fitting of a piston skirt to prevent air or exhaust
gas escape when the piston is at the top of the stroke. Loop scavenge
arrangements have low temperature air and high temperature exhaust
gas passing through adjacent ports, causing temperature differential
problems for the liner material. Uniflow is the most efficient scavenging
system but requires either an opposed piston arrangement or an exhaust
valve in the cylinder head. All three systems have the ports angled to
swirl the incoming air and direct it in the appropriate path.
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