Answer MEP Question 14
Q. With reference to Main Engine Fuel Pumps;
(a) Explain how the setting of a variable injection timing fuel pump is checked and adjusted.
(b) State why it may be necessary to adjust the setting of a variable injection timing fuel pump.
Describe how a main engine fuel pump would be set and checked for:
1. Timing;
2. Quantity.
The primary method of checking the fuel pump timing would be using the manufacturer's instructions of setting the fuel pump with the engine stopped.
For the Bosch type fuel pumps, a small window is present at the base of the fuel pump. Within this window an engraved line can be seen on the spring holder of the fuel pump.
When the cam follower is displaced by the cam, it pushes the spring holder up. When the engraved line coincides with the line on the fuel pump body, then the fuel pump plunger is at the position Where it is just covering the spill ports.
Hence at this moment the fuel pump is starting to pressurise the trapped fuel volume, and hence fuel delivery will commence. Thus, the engine is rotated to the point where the two lines coincide, and this indicates the commencement of fuel delivery.
- The actual crank angle would be read off the flywheel.
- Should the fuel pump timing be required to be changed, then the shims fitted underneath the fuel pump would be changed. If shims are added the fuel pump will inject later, and vice versa for removing the shims.
- The fuel pump is attached to the fuel rack, and the fuel control lever set to the same readings as the other fuel pumps. This should deliver the same fuel quantity. The fuel pump rack should always read zero when the governor is in the no-fuel position, to ensure the engine will stop when required.
- To enable the actual fuel quantity delivered from each fuel pump to be measured, indicator cards would be required. Indicator cards would be taken from each cylinder and the power developed
would be found, either directly from the software for the electronic cylinder units, or using a planimeter to determine the area of the power card for the mechanically taken indicator cards.
(a) Explain how the setting of a variable injection timing fuel pump Is checked and adjusted.
- A variable injection fuel pump can also be adjusted for fuel timing with the engine running. Thus, whilst the engine is running the fuel pump timing can be checked by the draw card type of indicator card.
- On this type of card the position when the fuel has ignited can be seen by either the point at which the pressure-volume curve moves away from the normal compression curve, or the point on the pressure derivative curve on the electronic indicator when the rate of pressure rise suddenly increases. The derivative curve makes the point of fuel ignition much easier to determine.
(b) State why it may be necessary to adjust the setting of a variable injection timing fuel pump. Once the point of fuel timing is determined, then the fuel pump can be adjusted either
- Collectively for the whole engine by adjusting the sensor which determines the quantity of fue timing advancement by measuring the position of the fuel rack position.
part ( B ) answare is incompleted condition along with part ( C) also, kindly do need full as an earliest with placing an appropiate answars as I shall be obliged.
ReplyDelete( B ) ; The CCAI- The Calculated carbon aromaticity index.
ReplyDeletethe high CCAI and low ignition quality responsible for loss of engine power.
Beside that low CCAI and high ignition quality also responsible for loss of engine power.
To over come the above mentioned problems the VIT system is provided.
The VIT achieves 85% MCR maximum continuous ratingalong with a higher fuel economy by advancing the fuel pump injection timing.
At about 40% MCR low load there is obtained no alternation of injection timing.
But above 40% MCR the VIT mechanism starts to advance the fuel pump injection timing.
When the engine load reaches 85% MCR approx, then as per engine designe appears same the combustion pressure Pmax.
So the servo retards the injection timing and the Pmax being kept constant between 85% to 100% MCR. And at 90% MCR fuel can save 4 to 5 gm/h.p.-hour. Simultaneously wear of fuel pump can also be compensated ,
Beside that due to the change of cam shaft timing , timing chain elongation being up to 2 degrees.