Experimental and Numerical Study of Swirling ... - Solid Mechanics

Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke
Marine Diesel Engines
Chapter 1
Figure 1.4 gives a comparison of scavenging performance of different
aforementioned scavenging types based on two parameters (Heywood, 1988):
Delivery Ratio
Mass of delivered air or mixture per cycle
Reference (exhuast gas) mass
Mass of air in trapped cylinder charge
Purity
mass of trapped cylinder charge
(Schweitzer P. H., 1949) describes the ideal engine scavenging process as:
‘‘The ideal engine scavenges the cylinder of all residual products of combustion, fills it with
uncontaminated fresh air, and in so doing wastes no fresh air through the exhaust.’’
This is also defined as ‘perfect displacement scavenging process’ by
Hopkinson (1914). In the perfect displacement scavenging process all fresh
air charge entering the cylinder is retained and perfectly displaces the exhaust
gases (Blair, 1990). In actually engine scavenging processes there always
occurs mixing of fresh air charge with the exhaust gases. However, the level
of mixing depends on the type of scavenging. In ideal scavenging, the
scavenge air acts like a piston and pushes the exhaust gases out of cylinder
without mixing with them (Schweitzer, 1949). From figure 1.4 it can be seen
that the uniflow scavenging type is most efficient one. Uniflow flow
scavenging is the most efficient scavenging system but require added cost of
exhaust valve system (Pulkrabek, 2003). Also it accounts for higher engine
thermal efficiency due to better air/ gas exchange (Pevzner, 1998). Today
almost all of the modern large marine diesel engines use uniflow scavenging
(Raunek, 2009). Earlier measurements conducted on some marine diesel
engines, manufactured by MAN Diesel A/S, resulted in 98% of purity at a
delivery ratio of 1.5 (MAN Diesel, 2010). This shows that the uniflow
scavenging in that engine has better performance than the one given in
figure 1.4. However, optimization of the amount of air used during the
scavenging process is still a challenging and potential task in order to
improve the overall engine system efficiency and also to reduce cost due to
scavenging system delivering excess amount of air than is actually needed.
1.5 Swirling Flow during Uniflow Scavenging
Process
As discussed in section 1.2, in addition to exhaust gas scavenging as the main
criteria, the design of uniflow scavenging process also considers to provide a
‘mixing favorable’ flow field to the injected diesel fuel at the end of
compression stroke. Introducing swirl in the flow has a wide spread use in
many engineering applications for enhancement of mixing process. Swirl in
the in-cylinder flow is introduced by making the intake ports at an angle to
7
Introduction