Page 29 - Introduction to Marine Engineering

Diesel engines 17

The gas exchange process

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. Older engines
were 'naturally aspirated'—taking fresh air only at atmospheric
pressure. Modern engines make use of exhaust gas driven turbo-
chargers to supply pressurised fresh air for scavenging and supercharg-
ing. Both four-stroke and two-stroke cycle engines may be pressure
charged.
On two-stroke diesels an electrically driven auxiliary blower is usually
provided because the exhaust gas driven turboblower cannot provide
enough air at low engine speeds, and the pressurised air is usually cooled
to increase the charge air density. An exhaust gas driven turbochargmg
arrangement for a slow-speed two-stroke cycle diesel is shown in Figure
2.9(a).
A turboblower or turbocharger is an air compressor driven by exhaust
gas (Figure 2.9(b)). The single shaft has an exhaust gas turbine on one
end and the air compressor on the other. Suitable casing design and
shaft seals ensure that the two gases do not mix. Air is drawn from the
machinery space through a filter and then compressed before passing to
the scavenge space. The exhaust gas may enter the turbine directly from
the engine or from a constant-pressure chamber. Each of the shaft
bearings has its own independent lubrication system, and the exhaust
gas end of the casing is usually water-cooled.

Scavenging
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
exhaust arrangements. 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.