Answer EKM Question 56

Q56. (a) Describe the procedure for checking and adjusting the timing of the main engine fuel injection pump. (b) Explain how diesel engine power balance is achieved, stating why it is essential.

Answer: For more details on this topic you can follow:

Fuel Pump and valve

Fuel Injection system

Checking of the timing of the Main engine fuel injection pump: 

When checking the timing of a fuel pump, the effective stroke of the plunger for a particular fuel setting is also sometimes measured. The effective stroke is the distance moved by the plunger between the start of delivery (when the plunger top edge covers the spill port) until the end of delivery (when the edge of the helical scroll uncovers the spill port). The distance moved and the crank angles at the start and end of delivery can then be compared to the engine test readings. 

    The distance moved (and therefore the volume displaced) by the plunger is a guide to the quantity of fuel being delivered for that fuel setting although it must be emphasised that the volume displaced is not equal to the volume of fuel being delivered because of the build-up of pressure and expansion in the HP pipes. 

    In plunger controlled fuel pumps, the beginning of injection takes place when the plunger covers the spill port. Turn the crank of the particular cylinder about 20° before TDC and make sure that the cam follower is on the base circle of the cam. The plunger will be at the bottom dead centre in this position and will partly cover the spill port. 

    One method to find when the plunger just covers the spill port is to open the suction and spill connection of the fuel pump. Hold a light through the opening and watch the same from the other end. Turn the crank of the cylinder slowly towards TDC from the above-mentioned position. Stop turning the engine moment the light disappears. Record the crank angle on the flywheel. This is the beginning of the injection. 

On some pumps it is possible, after isolating the fuel and draining the pump, to remove erosion plugs and sight through the spill ports while turning, the engine. At the point where the edge of the plunger is seen to cover the top of the spill port, the crank angle for that particular unit is noted and compared with the manufacturer's recommendation. 

    Small pumps mounted in a block can be set using spill timing, where the HP delivery pipe is exchanged for a U pipe led to a receptacle. With the rack turned to maximum and the fuel open to the pump (delivery valve removed) the engine is turned until the flow of fuel into the receptacle is cut off by the plunger (start of injection). 

    Larger pumps can use compressed air to establish when the plunger has cut off the spill ports. On other pumps, it may be necessary to remove the top cover of the pump and the delivery valve and measure the distance from the top of the barrel to the top of the plunger. The manufacturer will give the correct distance at which the plunger has covered the spill ports. Alternatively, the distance to the top of the plunger when the unit is on TDC may be given. 

A hand taken Out of Phase (draw) card or computer derived crank angle diagram can show the cylinder conditions at injection. Although the start of injection is normally a few degrees before TDC, the maximum pressure should be reached at about 12° ATDC so that there is a turning moment on the crankshaft. By inspection of the diagram, it can be seen whether the pressure rise caused by combustion of the fuel is too early or too late.

Adjusting the timing of the Main engine fuel injection pump: 

    On some pumps, the timing can only be adjusted by altering, the position of the fuel cam on the camshaft. This is done by hydraulically expanding the cam and turning it with a tool winch locates in holes in the side of the cam. 

    On other pumps, small adjustments can be made by adjusting, the relative heights of the plunger and barrel by placing shims under the foot of the plunger or under the fuel pump body in which the plunger is located. 

    On pumps fitted with VIT, the start of injection can be adjusted using the VIT rack for the individual pump. (MAN B&W MC engine). 

Power balancing achieving method:

To achieve a good cylinder power balance, firstly the compression pressures must be compared to establish that they are the same. Any deviation pointing to worn rings and liners or valve blow by must be investigated and corrected. Assuming the compression pressures are within an allowable range then the peak pressures should be examined to ensure they are approximately the same, as deviation can point to incorrectly timed fuel pumps. Next, the MIP for each cylinder is examined, taking into account the relative fuel rack settings and exhaust temperatures. If there are any large differences then an attempt can be made to balance the power output by adjusting the fuel rack settings. Each adjustment must be small, noted, and time is given for the engine to adjust to the change. In the easiest case, if a MIP, exhaust temperature and fuel rack setting are all low compared to the rest of the engine, then the fuel setting for that cylinder can be increased. 

    It must be appreciated that if the fuel on one cylinder is increased, then the fuel on the other cylinders will decrease for the same load setting. Care must be taken not to keep increasing the rack settings, or the pumps may not return to the no-load position when the engine is stopped. Sometimes it is necessary to reduce the rack settings on cylinders that are at a higher load. When adjustments have been completed, a further set of power cards are taken. All readings plus any adjustments made should be recorded in the logbook/pm sheets.

Requirement of Power balancing:

Engine power balance is essential for maximum power output with minimum fuel consumption without overloading the engine, either as a whole or on individual units. If the cylinder power outputs are varying by large amounts then the crankshaft will be subject to excessive stress, and overloaded cylinders will put excessive loads on the bearings leading to early failure.