Operational Information

The Medium Speed 4 Stroke Trunk Piston Engine

The Fuel Pump

 

SEE ALSO THE TWO STROKE CROSSHEAD ENGINE FUEL PUMP

Medium speed four stroke engines are equipped with jerk type fuel pumps, one for each cylinder. A plunger operated by a cam reciprocates in a barrel.

The plunger has a helix machined into it which also forms a vertical groove and an annular groove at the base of the helix. The  barrel is located in the pump body which has spill ports, connected to the suction side of the pump, drilled so that they are above the top of the plunger when the cam is on the base circle. The plunger is keyed to a sleeve which has a gearwheel (pinion) machined into it. The pinion meshes with a rack which can rotate the plunger relative to the barrel. The rack is connected to the engine governor.

As the plunger moves upwards in the barrel, injection will commence once the plunger has closed off the spill ports and the pressure builds up. As soon as the helix or scroll passes the spill ports the pressure above the plunger will immediately drop, even though the plunger is still moving upwards. It should therefore be evident that the amount of fuel injected into the cylinder is dependent on the position of the helix relative to the spill port. When the vertical groove is lined up with the spill port, then no injection will take place and the engine will stop.

In the example shown above the plunger has a single helix machined into it. More common are pumps with two helices (and thus two no load grooves) diametrically opposite each other. This gives a balanced plunger. (shown left)

 

The plunger is machined to very fine tolerances, as is the matched barrel in which it reciprocates. Wear due to abrasive particles in the fuel will mean that the pump will take longer to build up the injection pressure required. Wear due to erosion also takes place on the top edge of the plunger and the edge of the helices and spill ports. This, together with the wear in the plunger and barrel, will lead to the injection timing becoming retarded, for which adjustment may have to be made.

Plunger and barrel from a  Sulzer ZA40

On the scroll or helical fuel pump, although the end of injection can be varied, the start of injection (i.e. when the top of the plunger covers the spill ports) is fixed. Fuels of different qualities may require advancing or retarding the start of injection, in addition to which if the injection timing is advanced when the engine is running at loads below the maximum continuous rating, then a saving in fuel can be achieved.

Different engine manufacturers achieve this Variable Injection Timing (VIT) using different methods.

The Wartsila 64 engine uses a fuel pump with two plungers and two barrels with common suction and discharge. The plunger for controlling the start of injection (timing) has a helix in the top of the plunger, while the plunger for controlling the end of injection (metering) is a conventional scroll type fuel pump plunger.

Both plungers are operated by the same cam. As they move upwards in their respective barrels injection will not start until the helix on the timing plunger has covered the spill port. This point is controlled by rotating the plunger in the barrel by means of a rack and pinion.

End of injection is controlled in the normal way, when the helix on the metering plunger uncovers the spill ports.

 

The MAN B&W 32/40 engine has a separate camshaft for the fuel pumps that can be advanced or retarded as the engine is running. The final drive gear on the timing gear train has an internal helically toothed sleeve bolted to it. The gear wheel and sleeve can be moved axially by means of a hydraulic piston. The toothed sleeve meshes with a matched helical gear fixed to the camshaft.

The camshaft is fixed so that it cannot move axially. Therefore as the sleeve moves up and down controlled by the hydraulic piston, so the  fuel pump timing is advanced or retarded.

The camshaft for the inlet and exhaust valves utilises two different profiles for economy and full power operation. The camshaft can move axially from one set of cams to the other whilst the engine is running similar to the method used for reversing the engine direction as shown on the camshaft page.

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