Types of scavenging
Loop or cross scavenge
• Cross flow
• The main difference between the two
types is that uniflow requires an exhaust
valve or piston to operate. Loop or cross
flow relies on the piston to open and close
After ignition of the fuel the piston travels
down the liner uncovering firstly the
exhaust ports. The exhaust gas at a
pressure above atmospheric is expelled.
This is often referred to as blowdown and
its effect can be seen on the power card
for all the types of scavenging as a rapid
drop in cylinder pressure towards the end
of the cycle.
• The method of loop scavenging is similar
to the cross flow except the exhaust and
scavenge ports may be found on the same
• Some 2 stroke engines do not have
exhaust valves; As well as scavenge
ports in the cylinder liner, they are fitted
with exhaust ports located just above the
scavenge ports. As the piston uncovers
the exhaust ports on the power stroke,
the exhaust gas starts to leave the
cylinder. When the scavenge ports are
uncovered, scavenge air loops around
the cylinder and pushes the remaining
exhaust gas out of the cylinder. This type
of engine is known as a loop scavenged
engine. Note that the piston skirt is much
longer than that for a uniflow scavenged
engine. This is because the skirt has to
seal the scavenge and exhaust ports
when the piston is at TDC.
• Although simpler in construction with less
moving parts, these engines are not as
efficient or as powerful as uniflow
scavenged engines. The scavenging of
the cylinder is not 100%, and thus less fuel
can be burnt per stroke.
• All modern large 2 stroke crosshead
engines now being built are of the uniflow
• 2 stroke engines with
an exhaust valve
mounted in the
cylinder head are
known as uniflow
This is because the
flow of scavenging
air is in one (uni)
MAN B&W MC series uniflow
The scavenge air enters through the
scavenge ports in the lower part of the
cylinder liner, the exhaust gas is expelled
through the centrally mounted exhaust
valve in the cylinder cover. The scavenge
ports are angled to generating a rotational
movement of the rising column of air.
Air is forced out of the cylinder by the rising
piston leading to low flow resistance, the
effect is often compared to squeezing the
contents out of a tube.
Disadvantages of the loop/crossflow method of
The greatest disadvantage of this system, and
the one that has led to the abandonment of
its usage where once it was widespread is its
inefficiency in clearing the cylinder of all
Following the exhaust blowdown the scavenge
ports are opened.
The period available for scavenging is limited
to the recovering of the exhaust ports and is
only at its most effective until the closing of
the scavenge ports.
Therefore, high air velocities are designed in,
the air entering through steeply angled
The possibility exists for the scavenge air to
shortcut directly to the exhaust ports, a
situation which worsens with blockage of the
scavenge ports due to carbon build up.
Due to the inefficiencies above there is a high
volume of scavenge air requirement with this
This has led to the complicated underpiston
effect designs to augment the turboblower
output with some engine power being
absorbed dropping cycle efficiency.
A problem with having fixed ports is that
difficulty is encountered with port timing.
On the piston down stroke the exhaust port is
opened followed by the scavenge port to
make effective use of blowdown.
However, the same timing for closing the
ports means that the effective
compression stroke is reduced.
To try to remedy this differing means of
closing the exhaust ports before the
piston covered the ports was tried, One
such method was by engine driven
rotating valves which opened and
closed the exhaust ports.
All the designs increased complexity
and often proved unreliable due to the
arduous conditions they had to operate
To prevent exhaust gas entering the cylinder
under the piston as the piston moves up to
TDC, extended piston skirts are fitted.
This adds to the reciprocating mass and
increases load on the crosshead bearing.
The small amount of side thrust not
absorbed by the crosshead is spread over
the larger of the skirt reducing loading and
wear on the liner, however, problems of
increased lubrication requirements for the
increased surface area largely negate any
The requirement for both the exhaust and scavenge
ports being fitted into the liner makes for a more
complicated design with increased liner lubrication
difficulties especially in way of the exhaust ports.
This region suffering not only the washing away
effect of the gas flow but also contamination from
combustion products and increased temperature.
Cylinder lubricating oil volume demand is
therefore higher with this design.
In an attempt to improve the scavenge efficiency
shaped pistons have been used which produce a
combustion chamber shape not the best for
Asymmetrical piston designs can also lead to
excessive thermal loading and complicated
strengthening and cooling designs.
One advantage of the Loop method of
scavenging is that it does away with the
requirement for an exhaust valve or
This means that all the extra running gear
associated with this can be omitted.
That means, simpler cylinder cover design,
simpler and less stressed camshaft and
camshaft drive train.
Where an exhaust valve is fitted in the
cover, there is increase thermal stressing
especially in way of the valve where higher
temperatures are encountered.
Additional advantages of the Uniflow method
The fitting of an exhaust valve does give a major
advantage in that the timing of opening and
closing of the valve can be altered which is
used to its fullest with modern designs with
'Variable exhaust timing' control fitted.
This means that the effect of the scavenge air
inertia entering the cylinder, and optimising the
closing of the valve to increase the effective
compression (which starts when the exhaust
valve is closed) can all be taken into account
for the varying loads and engine speeds.
The increased scavenging efficiency with this
type of scavenging creates greater scope for
increased economy and so all modern designs
are based on this design.
The opposed piston design once in favour due to its
inherent dynamic load balancing has now largely
disappeared due to its increased mechanical
There is a minimum air demand as the ingressing air
pushes the combustion products ahead of it with
little requirement for scavenging by dilution.
As the air flow is symmetrical rising up the liner the
thermal influencing on the liner walls, cylinder
covers and piston crowns is also symmetrical.
this allows simpler oil cooling of the piston crown
The reduced number of ports (no exhaust ports), and
reduced size of the scavenge ports (due to reduced
air volume requirements), this reduces the problems
of liner lubrication allowing reduced oil consumption.