Twin diesels - unequal power

[tr7v8]

If both turbo's are identical and on identical size & condition of engines then boost should be identical. Turbo's are driven by the volume of exhaust gases hence unloaded engines no boost, heavily loaded engines max boost.
So if both at equal revs should give equal boost.
Another thought is how are your rev counters driven, from the alternators? The Perkins referred to earlier gave us heart failure at full throttle on the fly bridge because the revs on the port were 150-200 ish lower than the starboard, checked with the cabin rev counters which were indicating an identical 2400RPM, bugger lazy gauge.

Jim
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[Carioca]

Agree that turbo boost should be identical, marginal differences notwithstanding, which is not the case here.
Rev counters driven by engine alternators. Will swap instruments around and check FREQUENCY of port/stbd. alt. outputs for equal revs. with DVM.
Current headache : undoing nuts on port exhaust manifold in order to change gaskets.
Acid tests on port engine to follow. Would welcome offerings.

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[tr7v8]

Not sure what offerings you mean, prayers, WD40?
Is it not possible to lift head with manifold(s) attached saves issues of undoing bolts. otherwise plenty of soaking in WD40, plus gas or whatever the favourite brew is.
I also have started using flank drive 6 pointed sockets and find a lot more comes undone without the fight and rounded off hexes!

Jim
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[oldharry]

Hmmm - interesting one!. I suggest while you have it all stripped down - get your injectors checked. A faulty injector can cause some of the symptoms you describe. While the head is off have it checked for cracks in the casting, and to ensure it has not warped - if it has, the new gasket will fail quite soon.

Is there any sign of water in the sump oil? Slight leakage will cause the oil level to have risen after a run, while heavy leakage will make the oil go a dirty grey colour. This could be indicative of a cracked block or cylinder head and would need further investigation.

It is unlikely the injector pump is at fault here, although the timing should be checked. Over or under fuelling is usually characterised by excessive black or white smoke (NOT steam in this case - but like the white smoke given off by older diesel at cold start, being in fact atomised unburnt diesel fuel. The visible difference to steam being that in still conditions it hangs around much longer, and smells much more strongly)

Have the high pressure pipes to the injectors been changed? If so, check they have the same i/d bore as those fitted to on Stbd unit. Incorrect bore pipes have a very marked effect on power and smooth running with symptoms similar to those you describe.

Next isthedrive train OK? Power absorbed by a fault in the gearbox or prop shaft (over tight bearings for example) will rapidly reduce power output - and would cause the engine to warm up quicker with the extra drag. Parts of the gearbox are still rotating even in neutral!

Is there any blow by on the offending engine? This will show by increased crankcase pressure blowing fumes and as it get worse, engine oil, out through the crankcase breather system. At anything above tick over there should be little or no pressure in the breather system. If there is then you have ring or piston troubles - ranging from bore glazing (the bores have a golden varnished appearance - lovely on the woodwork but bad news in the engine!), to a sticking or broken ring(s). A quick test while the head is off - set the crank so the pistons are level with each other (the point of maximum wear in the bore) then pour a little engine oil into each bore enough to cover each piston. Ring troubles will how up as the oil drains down past the piston more quickly than on a good one. This is only a rough guide and does not show up all types of fault.

Finally check that the copper flame guard washer at the bottom of each injector is present - and not doubled. They are not easy to see, and its not uncommon to find a new one installed over the old one - which leads to some very tiresome rough running and power loss problems!

Good luck!

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[Carioca]

WD40 + brake fluid (yes !) + punch +hammer freed the nuts. With individual heads, it is necessary to dismantle intake / exhaust manifolds etc.
The port engine heads and piston tops had a great deal more carbon (soot) than Stbd engine - both were top-overhauled (new wet liners, pistons, rings, water and oil pump etc.) less than 100 hours back.
A MWM mechanic said today that with the added soot, it does smell like late ignition timing, something he has experienced in road transport vehicles.

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[Carioca]

Your rather handy tips will come of use as I reassemble the port engine to-morrow.
Like to make a wild guess now that excess carbon (soot) was found coating the inside of the heads and piston crowns ?

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[Carioca]

Further tests on port engine to resume on 13/03.

In the meantime, I was wondering if instead of a 2,0:1 ratio, the gear-box vendor sold me a 1,5:1 ratio by accident.

Could this factor be responsible for causing all (or some) of the low-power-output symptoms described earlier, viz. :

- port prop wash (wake) much weaker than Stbd
- top-end rpm and fuel consumption o port and Stbd are equal
- port + Stbd @ 2200 rpm give speed of 14-15 mph, while Stbd alone @ 2300 rpm gives 10,5 mph

Is there a simple way of checking the gear-ratio on the Borg Warner 5000 ?

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[tr7v8]

It's certainly possible but I'd expect your engine peak revs to be a lot lower than the small difference you have. eg 2200/2 = 1100 Prop RPM whereas 2200/1.5=1467Prop RPM a fair difference which I doubt the engine would achieve so given the absorption of power being the same I'd expect engine RPM to be 1100prop RPM X 1.5 =1650RPM peak is this the case?

As regards the gearbox I'd expect some form of plate on it which would show the gear ratio, or at least a part no. which could be given to BW and they could find out.


<hr width=100% size=1>Jim

Draco 2500

 

[Carioca]

Nameplates on gear-boxes affirm 2,0:1 ratio but I´ve known cases where the innards were found to be different.
Assuming that port and Stbd. gear ratios are 1,5 and 2,0 resp.:
- For identical engine rpm, port prop rpm some 30% higher than Stbd, which implies port prop load 1,3-squared X Stbd prop load (prop load square law);
- Consequently, port engine considerably overloaded, which does not seem to be the case, at least to the degree that the aforementioned numbers suggest; in fact, port engine under such duress would not attain 2200 rpm.
I think it is this that you said (in shorthand !) in your last post.
Port head-gasket job completed and trials rescheduled for this morning, complete with strobe tach etc.


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[tr7v8]

"I think it is this that you said (in shorthand !) in your last post."
Err yes you even made it more complicated by going into prop loadings etc. but yes that is what I meant.

Good luck for the trials! Any chance of finding a handheld optical tacho to measure shaft revs as well?

What state were the head gaskets in? any signs of obvious blowing?


<hr width=100% size=1>Jim

Draco 2500

 

[david_bagshaw]

re BW box

do your props turn in opposite directions when going fwd?

If so the ratios will be definately different, 1.9 :1 & 2.01:1 if I remember correctly, as the take off power is from the inside or outside of a planetary set.
(72 series, the type we have fitted) dont know if the same with other versions


Is the boat still in displacement mode at the speeds published, if so... it looks likely to be a ratio miss match

Only method of determining the ratio without dissambly I can think of is to hire a strobe rev counter mark the shaft & front pulley, run engine at fixed rpm & measure shaft rpm for both sets.

(the sort of thing machine mart do)
good luck

<hr width=100% size=1>David
<A target="_blank" HREF=http://www.euroboating.net>http://www.euroboating.net</A>

 

[Carioca]

Yes, port engine head-gaskets with solid signs of blow-by.
I promise praise (and recognition !) to whosoever´s Gods, if refurbishing the port head-gaskets will see port engine making same power as Stbd engine !
Exhaust-manifold flanges being remachined to solve leakage problem on cylinder n°4, detected as port engine went on-line this morning.
Tests to resume on Tuesday / Wednesday next week.


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[Carioca]

Thanks for your comments on the fine-details of BW gear-ratios.
Boat test called-off this morning as port engine exhaust-manifold refitted after head-gasket replacement, started leaking at cylinder n°4.
Manifold is being remachined and tests rescheduled for Tuesday / Wednesday next week.

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[tr7v8]

Do you know if it was leaking at the exhaust manifold before, this hugely reduces the boost fom the Turbo?



<hr width=100% size=1>Jim

Draco 2500

 

[Carioca]

No leaks prior to refurbishing head gaskets...... at least as far as my senses were able to perceive.
Yes, leaks, like the one on start-up yesterday, have a devastating effect on turbo-boost.


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[oldharry]

Before you re-assemble, check the head for warping - or have it checked by an engineer. If it has blown once it will almost certainly blow again if the head is not skimmed. This should cost a lot less than the price of a new gasket set - and will save the cost and work of a further rebuild when (not 'if'!) the gasket fails again.

OBTW, to be 'picky' - 'blow by' in a diesel is a piston fault, not a head gasket leak. A gasket has been 'blowing' when there has been a compression leak past the gasket but has not damaged the gasket, while a 'blown' gasket is actually damaged either by a hole burnt in it, or a section having been blown out of it - frequently on the very narrow bit between one cylinder and the next.

Both conditions create a hot spot in the head which will have distorted it so that a new gasket is not properly supported and fails almost immediately. If the engine has remained in service any lenght of time after the gasket fails the combustion flame can start to burn away the head (or block) material. Ally heads MUST be checked before replacement - usually its worth skimming anyway to be on the safe side

'Blow by' happens when the crank case pressure rises due to compression leaking past worn or broken rings. The escaping gas blows engine oil out of the breather system eventually emptying the sump, and the pressure can blow crankcase oil seals out of their mountings - with terminal and very messy results! A quick check on an older diesel is to close off the breather pipe when the engine is warm for around 10 seconds, then see if there has been a build up of prssure when you release it. If so, the bores or rings are worn or damaged.

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[Carioca]

Port engine heads checked with a straight-edge and found to be in satisfactory condition.
Two head gaskets had signs of hot-spots by the water / oil passages (at lateral extremity of engine block). At these locations, gaskets clung on firmly to heads and when eased off, gasket surface material was laid bare.
Admittedly, the term "blow-by" was used loosely.
Wet-liners, pistons and rings in excellent condition as these have seen less than 100 hours of service since the "top-overhaul".


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[Piers]

Just in case - do your turbos have a waste gate? If so, is one of them stuck, or the sensor leaking, which would incorrect boost and low power

<hr width=100% size=1>Piers du Pré
http://www.dupre.co.uk

 

[Carioca]

Turbos have waste-gates factory-set at 1,3 bar (gauge).
1/4" rubber tubing betwen waste-gate actuator and compressor output replaced on both units - port tubing was frayed at the ends.
Presumably, if waste-gate stuck in "open", compressor pressure will fall and if
stuck "closed", over-pressure could arise ?
In case a leak is found in say the actuator, can whole assembly including actuating rod and valve be replaced successfully by competent turbo shop ?
These parts not available in the (general) engine rebuild market. Turbo manufacturer claims specialised know-how required to refit, aside from dissuading tinkerers who could blow up an engine, reset actuator to spec and make a warranty claim.
Thanks.

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[tr7v8]

Most decent Turbo specialists should be able to check them and rebuild or provide replacements. It's only a poppet valve, seat with housing and a big spring. Normally they are set and locked with locking wire on the nut and small spring. I'd think that a boost guage and knowing the correct figure would allow you to reset it yourself. 1.3 bar is 20PSI in english or as near as dammit. I've only ever replaced them not reset them and that's on petrols. Do you have boost guages as part of the instrumentation?

<hr width=100% size=1>Jim

Draco 2500