Operational Information

Lost Motion

 
 

On a two stroke engine, the fuel pumps must be retimed when the engine is required to reverse direction (i.e. run astern). This is done by moving the fuel pump cams or fuel pump cam follower positions relative to the crankshaft.

If one cylinder of the engine is considered (left), the piston is just before TDC with the engine running ahead and the crankshaft rotating clockwise. The piston is moving up towards TDC. The picture on the right shows the fuel cam at this point; where the cam follower is rising up the lift of the cam as it rotates clockwise. This point can be considered as the start of injection.

 

The fuel pump cam follower is moving up the rise of the cam on the delivery stroke. The cam is correctly in time with the engine

 

If, at this point the engine is stopped, and is started in the reverse direction (astern), the crankshaft now moves in an anticlockwise direction. Then the piston in this particular unit is now moving down the cylinder and is just after TDC. At this point fuel injection should have just finished. However, by studying the picture of the cam (right) it can be seen that the camshaft has reversed direction (because it is directly driven from the crankshaft), and is also rotating anticlockwise.

 In the picture the follower is moving down the cam which means the fuel pump plunger is just finishing the suction stroke;  i.e completely out of time with the engine.

 

Here the fuel pump cam is in the wrong position. When the piston is just after TDC, fuel delivery should have finished and the follower should be approaching the peak of the cam.

 

So that the Fuel Pump cam is timed correctly with the crankshaft when the engine is reversed, the fuel pump cams are rotated by a hydraulic servomotor which changes the position of the cams relative to the crankshaft. The angle through which the cams are turned is known as the Lost Motion angle.

Although this can be made to happen when the engine is still rotating, it is probably easier to think of the engine stopped as shown left and the camshaft moving as shown on the animation below. Once the fuel cams have moved, the engine can then start running in the reverse direction (anticlockwise).

Because the engine is started using compressed air admitted through the air start valves, the operating mechanism for these must also be retimed.

More details on the operation of the reversing servomotor as used on the Sulzer RTA engine can be found in the members section

The angle that the cams move through is the lost motion angle.

This is not the only method of reversing a two stroke engine. Other methods include moving the whole camshaft axially so that a different set of cams are used, and a rather clever method used by MAN-B&W which alters the position of the cam followers.

In case you are wondering, it is not necessary to retime the exhaust valve operating cams on a Two Stroke engine. Think about it!!

If you can't work out why, click here

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