Advance timing for fuel pump

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Pengze
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Advance timing for fuel pump

Postby Pengze » Sat Oct 20, 2012 1:50 am

Dear Sirs,
These are my queries:-
1. Is it correct and advisable to advance the fuel pump timing for a diesel engine if the fuel pump barrel & plunger is worn out ?
2. What is the maximum deviation or allowable range of degrees BTDC compared to the original fuel pump timing setting ?

Your advice on this matter is highly regarded. Thank you.

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JK
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Re: Advance timing for fuel pump

Postby JK » Sat Oct 20, 2012 10:40 am

The only time I have seen something like that done was on older Merlee diesels that had the lobes wearing down on the cam shafts. We would occasionally adjust the fuel pumps so they would be in correct timing. After a while there was a overhaul on the pair and the camshafts were pulled out of each engine and installed in the opposite engine. The engines were directly coupled to the shaft turning opposite directions so the astern (or ahead) lobe on the opposite engine was not worn..

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Big Pete
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Re: Advance timing for fuel pump

Postby Big Pete » Tue Oct 23, 2012 12:14 am

If the fuel pump barrel and plunger are worn out, there will be leakage between them, this will mean that after the spill Port is closed it will take longer for the pump to reach sufficient pressure to open the fuel injection valve, delaying injection.
It also means that the total quantity of fuel delivered will be less than it should be for any given fuel rack setting.

The consequences are that fuel injection will start late, even if the spill timing is correct, and the maximum amount of fuel that can be injected will be less.

The symptons will be higher exhaust temperatures in relation to the load, and reduced maximum power output. The severity of these symptoms will be dependant on the viscosity, and therefore the temperature of the fuel, becuase the viscosity & Density of the fuel, together with the clearance between the Barrel & Plunger will determine the mass of fuel "Lost" from the fuel pump.

The best solution would obviously be to change the Barrels and plungers as soon as possible. You do not say what grade of fuel the engine is running on, it is common for Engines that normally run on HFO to have problems after changing over to MDO because the reduced viscosity of the fuel causes such a large increase in leakage that the engine will no longer start.

The effective delay in commencement of injection could be corrected by advancing the timing, depending on the type of engine this can be done ny adjusting the "Bucket Tappet" below the fuel pump, removing shims from under the fuel pump, or adjusting the coupling on the pump drive shaft.
If the fuel cams are worn, these methods can also be used to correct the fuel timing, however, if the cams are worn the stroke of the fuel pump will be reduced so the maximum fuel delivery, and Power, will still be reduced.

If you are unable to replace the Barrels and plungers for any reason, I would suggest that you first make sure that the rest of the engine is in as good a condition as possible, especially the injectors and cylinder head valves.

Then advance the timing by small amounts, say 1 degree, take a full set of performance readings, especially power cards if possible, if not peak pressures. Compare these readings with the Test Bed and Sea Trials Data.
Sea Trials Data are more comparable with what you will see onboard. The test Bed trials always have significantly lower exhaust temperatures than will be achieved on the ship because the exhaust pipe on the test bed will be short and straight while the exhaust pipe on the ship wil cause a much higher back pressure.
If you can not find the Sea Trials Data, look for the oldest set of performance data that have been recorded and use them for comparison.
So long as the the rest of engine is in good condition the exhaust temperature after the Turbocharger and the Turbocharger RPM are good references to compare with the peak pressure. Late injection would increase the exhaust temperatures and energy in the exhaust hence the turbo charger RPM, in relation to the peak pressure.

Continue advancing the timing 1 degree at a time until the peak pressures correspond with the TC exhaust and RPM from the Sea Trials or old performance Data. If the engine is being used as an electrical generator it is of course easy to measure the actual power generated and this can be used as a reference rather that the T/C readings.

This will mask the symptom caused by the dealyed injection but maximum power will still be reduced.
To restore the maximum power output it will be neccessary to re adjust the mechanical stops on the fuel racks and any electrical/ electronic load control systems.

I hope this helps.

BP

This should concea
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Pengze
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Re: Advance timing for fuel pump

Postby Pengze » Tue Oct 23, 2012 6:15 pm

Dear Sir,
Very well written. Your advice is highly regarded.
Actually we are using MGO & HFO 180cst. for changeover.
Further to that, could you pls clarify for the following;-
1. if we advance the timing too much, would it cause heavy knocking?
2. if the barrel & plunger is worn, the fuel rack index will increase for the same given power output, is that right?
3. and the governor index will also increase correspondingly., right?
4. if let say we advance the timing to get the required peak pressure, would the exhaust temperature increase respectively?
5. how about if we have leaking fuel pump delivery valve, what would be the consequences? is it the same as having the same symptoms as worn barrel & plunger?
6. how to determine correctly if the fuel pump delivery valve is leaking?

Would appreciate yr valuable advice.
Once again ,thank you very much.

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Big Pete
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Re: Advance timing for fuel pump

Postby Big Pete » Wed Oct 24, 2012 2:32 am

img001.jpg


Thank you for your compliments.

1) If the fuel injection is advanced too far, the Peak Pressures will be excessive at full power, this could start leaking around the cylinder head, mechanically overload the cylinder head bolts, pistons, connecting rods, crankshaft and bearings causing severe damage to the engine, however, what you are trying to do is correct the slight injection delay caused by the worn fuel pumps. This is why it would be best to check the fuel injection timing by studying the power/Draw card for the engine from when the engine was new with ones taken before and after any adjustment to the timing. If the start of injection was adjusted to be earlier than designed it would cause knocking. The old check to see if the barrels and plungers are worn was to strip down the pump, make an airtight seal over the end of the barrel /Plunger unit with the palm of the hand and turn it upside down, the plunger should be held in place by the vacuum between the hand and the plunger, if the plunger drops out, they are worn out.

2) & 3) The Governor will try to maintain the engine at constant speed, if the fuel pumps are worn (and therefore delivering less fuel than they should in relation to the fuel rack position) the governor will compensate by increasing the fuel rack setting until the engine produces the required amount of power to maintain the set speed. However, the fuel pump leakage will reduce the rate at which fuel is injected into the cylinder, increasing the fuel rack position means that fuel will be injected over a longer time/crank angle period, so the efficiency of the engine may be reduced. The peak pressure will be lower but the pressure will be sustained further down the expansion stroke, so that the area under the curve (power developed) will be the same as it should be.

4) I would not try to advance the injection timing to give a specific peak pressure, the "notch" on the pressure curve of the Draw card indicates the point of injection, I would only advance the timing sufficiently to correct the injection delay caused by the wear. Correcting the timing should, if everything else on the engine is correct, slightly reduce the Exhaust temperatures.

5& 6) Most modern fuel pumps are fitted with 2 spring loaded non return valves, the larger one, with the greater spring force opens to allow fuel to flow to the injector and closes at the end of the delivery stroke, the other opens at the end of the delivery stroke and releases the fuel pressure in the H.P. fuel pipe and injector, to prevent " dribbling" from the injector. On some engines the smaller valve is actually mounted inside the larger one. It should be fairly easy to test if these are opening at the correct pressure, and closing tightly, unfortunatly they are often sealed units (especially on Medium speed engines) that can not be dismantled and lapped in. If the Non Return valves are leaking this can reduce the volume of fuel delivered IF air is drawn in from leaking discharge pipe connections, or combustion gas leaks back through the injector during the pump suction stroke.


See the attachment from my ancient (1972) copy of C C Pounder's Marine Diesel Engines.
It is always better to ask a stupid question than to do a stupid thing.

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JK
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Re: Advance timing for fuel pump

Postby JK » Thu Oct 25, 2012 3:01 am

If you are unable to replace the Barrels and plungers for any reason, I would suggest that you first make sure that the rest of the engine is in as good a condition as possible, especially the injectors and cylinder head valves.

Somehow, if they are going to screw around with the pumps like this, I can't see the rest of the engine being any better. But that's just me.

Thanks for the very complete answer Big Pete. I know for such a complete response you have taken quite a bit of time and effort for research..

Pengze
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Re: Advance timing for fuel pump

Postby Pengze » Fri Oct 26, 2012 4:46 am

Dear Sir,
For a medium speed diesel engine like ours, we cannot take the Power/Draw Card to check the injection timing. We are using dial indicator gauge to only measure peak pressure.

Just to clarify,
1. If the barrel & plunger is leaking, can we determine from the leak-off pipe attached at the fuel pump body ?
2. If the delivery valve is leaking, can we also determine from this same leak-off pipe ?
3. Is it correct to say that, if we change 1 unit of defective fuel pump only (for a 8 cyl engine), we have to re-adjust the fuel rack index of the existing units on the engine in order to have sharing power balancing so as to achieve almost similar peak pressures for all cylinders ?

Thank you very much for yr guidance.

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Big Pete
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Re: Advance timing for fuel pump

Postby Big Pete » Mon Oct 29, 2012 8:39 am

Hi again Pengze,

Ref. Your questions:-

1) If you have drain pipes from the base of the fuel pumps down to a fuel oil drain or leakage tank that should be filling up and going into alarm and being pumped out if the barrels and Plungers are worn. (Especially when running on MDO)
By disconnecting the individual pipes you should be able to determine which pumps are the most worn.

2) If the delivery valve is leaking, when the pump is on its suction stroke, it will be trying to draw fuel out of the High Pressure fuel pipe and injector as well as drawing fuel from the fuel supply to the pumps. However, so long as the fuel in the pipe and injector does not "boil off" or air leak into the system through bad joints, personally I do not think this will have much effect. However, if the fuel turns to vapour in the pipes then the amount of fuel delivered on each stroke would be reduced and I would expect to hear a similar "Banging" or "knocking" to what you would hear if the engine was starved off fuel. You would notice an improvement in performance when running on cold diesel fuel.
While worn barrels and plungers would work better when running on more viscous HFO. I would not expect fuel in the injectors to leak back through the discharge valve, and all down the length of the clearance between the barrel & plungers and out of the drain, because it would have to come past the connection to the fuel oil supply rail. It would only come out of the drains if the barrels and plungers were badly worn and it would be impossible to tell whether it was leaking directly from the supply rail or back from the injector.

3) If you have one good pump on the engine it will be impossible to increase the peak pressures on the other cylinders to match it, the worn pumps would only produce the same peak pressure if you advanced the timing to a point where combustion was occurring during the compression stroke, which would be mechanically damaging to the engine, because the pressure would rise too quickly, and it would also reduce the power output, because the force of the explosion would be trying to push the piston down while it was still coming up on the compression stroke.

4) If you only have 1 good fuel pump it might make more sense to reduce the rack setting on the good pump rather than increase the rack setting on the other 7 pumps! If you increase the fuel rack settings on the worn pumps they will continue injecting fuel over alonger period off time (crank Angle) than the good pump so it will be impossible to truly balance the engine. The worn pumps will be burning fuel less efficiently, because it will not be expanded so much before the exhaust valves open, and have higher exhaust temperatures, there will also be more energy in the exhaust gas than there should be so the Scavenge Pressure and Turbo RPM will probably be higher than they should be for the power output, same as they would be if you had a dirty Turbo charger.

In general I would say that the cost of the additional fuel consumption caused by using worn fuel injection pumps and Fuel injectors is vastly greater than the cost of replacing the worn parts, and I would be trying to convince the Superintendant to supply new Barrels, plungers and discharge valves.

Good Luck.

BP
It is always better to ask a stupid question than to do a stupid thing.

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JK
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Re: Advance timing for fuel pump

Postby JK » Tue Oct 30, 2012 3:42 am

It is somewhat painful imagining what he must be dealing with on the engine...keep the lights on fellows!

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Mikkelsborg
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Re: Advance timing for fuel pump

Postby Mikkelsborg » Wed Nov 07, 2012 12:51 pm

The cost of plunger & barrels and nozzles have a fast ROI, when you take in consideration the extra fuel used due to poor performance.
Dependent of engine type we can recommend to have plunger & barrels reconditioned with new plunger, if cost is an issue..... If you advise the engine type we can give you a rough pricing on reconditioning....
For timing and adjustment we recommend to use an electronic indicator, where you can see the complete ignition and combustion for adjustments. By correct timing alone many operators reduce fuel consumption with 3-5 % dependent of engine condition...

http://www.scandic-diesel.com/product-l ... ndicators/
&
http://www.scandic-diesel.com/product-l ... -software/

Rgds Mikkel Elsborg
Mikkel Elsborg
PJ Diesel Group
(http://www.pjdiesel.com)

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Big Pete
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Re: Advance timing for fuel pump

Postby Big Pete » Thu Nov 08, 2012 12:23 am

Mikkelsborg wrote:The cost of plunger & barrels and nozzles have a fast ROI, when you take in consideration the extra fuel used due to poor performance.


For those who didn't understand ROI, it stands for "Return On Investment", that means you can regard the saving on the cost of fuel used as a return on the cost of the investment in new fuel injection parts. Just like the the return (Interest) you gain on the your "Investment" (Savings) in a savings account
If the engine is running a lot you should get your initial cost back in months rather than years.

As JK says, " keep the lights on". Good luck.

BP
It is always better to ask a stupid question than to do a stupid thing.

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Re: Advance timing for fuel pump

Postby jimmys » Thu Nov 08, 2012 4:58 am

The re-timing of an engine is not a push button job.
The last re-time I was involved in the angle of injection was 20 degrees before TDC and the commencement of injection was at 2 mm of fuel pump lift. Angle measured by vernier protractor and fuel pump lift measured by dummy fuel pump fitted with dial gauge. That involved 16 fuel pumps off the engine, 16 injectors of the engine all to be tested.
Due to wrong timing from manufacturer pistons were burned, valves and seats a mess. She was a two engined ship and both were wrongly timed at manufacture.
If the pumps are worn you cannot set the commencement of injection you do not have a clue what to set to. The injectors and pumps need to be in first class condition for injection at 2mm lift. Pump lift is set by individual shims under fuel pump.
I asked the makers for a tolerance, their reply, no tolerance, the engine is on the edge of safe working. A lot of engines are on the line. Be very careful if becoming involved in ad lib re-timing.

regards

PS engine running checked using dive time computer system, dont know what is used now. It was electronic engine sensors and PC computer


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