Doxford Opposed Piston Oil Engine
The Nem-Stop used on some Doxford engines
Authored by: Alfons Verheijden
Pictures by: Various SourcesBrought to you by www.dieselduck.net, comments to [email protected]
When a large motor ship has to be stopped as soon as
possible, it is desirable to get the engine running in the astern direction
quickly, but owing to the heavy "drag" on the propeller, this operation presents
difficulties.
One method of assisting this operation is to apply some
form of braking to the propeller shaft so that its rotation is stopped;
the
engine can then be started in the astern direction, which is usually a much
simpler proposition than starting in the astern direction from the condition of
ahead rotation.
The NEM-STOP arrangement, developed by the "North Eastern
Marine Engineering Co", and for wich patents have been applied, provides a
simple and efficient method of braking such engines.
The method adopted
is the use of partial decompression and comprises shutting off the fuel supply
to the engine and allowing
part of the air charge in the cylinder to escape
through a special relief valve during the compression and expansion strokes.
By this method, a net braking effect is obtained during each cycle of engine
operation, during the expansion stroke
there will be less air in the
cylinders to expand and counteract the braking effect of the compressin stroke.
The control valve consists of a steel block having three connections, i.e.,
compressed air from the starting air system,
compressed air to the special
cylinder relief valves and a connection giving release to atmosphere.
Two valves are fitted in the block. One valve, when open,
supplies compressed air to the relief valves, the other,
when open, allows
the air to escape from the relief valves to atmosphere; only one of the valves
is open at a time.
The control valve is operated by the engine Reversing
Lever, and when the lever is in the ahead or the astern position, the cylinder
relief valves are closed. When the lever is in "mid position", however, the
relief valves are lifted and allow partial decompression in the manner
described.
1954: "BRITISH VISION", 16000 Tons Tanker for the British Tanker Co, with North Eastern Marine DOXFORD, type 67LB6, Stroke 2,320 mm, 6,800 b.h.p. at 119 r.p.m.
During Sea trials of the "BRITISH VISION" comparative stopping test were carried out, and it was shown that the engine under normal conditions, without braking, was stopped in 4 min. 45 sec., wheras with Nem -Stop braking as described, it was stopped in 50 sec.; this test also demonstrated that the use of Nem-Stop equipment reduced the time of stopping the vessel by over than 2 minutes.
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