Doxford Opposed Piston Oil Engine
The Nem-Stop used on some Doxford engines
Authored by: Alfons Verheijden
Pictures by: Various Sources
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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
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.
A diagram of the NEM-STOP is shown, operation
of it, is as follows:
1) Engine running and normal manoeuvring.
The Nem-Stop lever should be in in position "1", when all relief valves are shut and function normally.
2) Using Single-stage Nem-stop for stopping the engine.
The fuel is first shut off, then the Nemstop lever placed in position "2", whereupon all relief valves are held open and, due to the continuous escape of air on both compression and expansion strokes, a net braking effect is obtained.
This quickly stops the engine even though the compression pressure which resists rotation of the engine is reduced.
3) Using Two-stage Nem-Stop for stopping the engine.
This is used when the engine has a tendency to crawl with the gear, as before in position "2".
When this occurs the Nem-Stop lever is moved to positon "3", when full compression is restored in cylinders 3 and 5, the immediate increase in resistance during the compression being sufficient to stop the engine.
4) Use of the Nem-Stop gear for scavenging combustion spaces after long "stand by" periods.
The usual procedure is to open the indicator cocks and give the engine a turn on starting air.
Nem-Stop gear can be used for this purpose by placing the lever in position "2" and applying starting air.
The engine will then turn very slowly and a good scavenging action is obtained through the relief valves.
5) An additional advantage claimed for the gear is that the engine can be quickly stopped to prevent consequent damage should any defect develop in the running gear and services when running at full speed.
On engines with "Timing Valves" only the Nem-Stop handle!
With Reversing handle + Nem-Stop handle, on LB and LBD Doxford"s, on which the forward Camshaft has to be changed during manoeuvring from Ahead / Astern v.v.
|The Special RELIEF VALVE used with a NEM-STOP|
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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