Twin diesels - unequal power output - help
Twin diesels - unequal power output - help
(OP)
I have a twin diesel set up on my boat.
They were factory-test engines that I bought for a much-reduced price, and I DIY-marinised them.
Although one was as a factory turbo-only version while the other was a factory turbo+aftercooler version, I standardised all parts to the spec of the turbo+aftercooler version, which is rated some 10HP higher.
To be more specific, the injectors, injection-pumps (and settings)and turbos are now all identical, as the INTERNAL ENGINE PARTS ALSO ALWAYS WERE IDENTICAL.
However, the engine that started as a turbo-only version is STILL lower-powered than the factory-issue turbo+after-cooler one.
I suspect that the compression ratio of the turbo-only engine is LOWER than the other turbo+after-cooler engine (?) . This is because when I installed some additional pulleys concentric with the crankshaft and tightened the nut on the central hub , the lower-powered engine offered much lower resistance to turning than the higher-powered one.
I have looked at the factory drawings on the Internet and they say that the (TDC) piston-top to head clearance on these engines can vary between 0,95 and 1,15 millimetres - quite a bit of variation there !
They also say NOT TO GRIND the piston head and/or the engine deck. Is there a special surface treatment dished out in the factory, once the compression ratio has been determined for a particular version of the engine, and which will be impaired if the surfaces are ground ?
What are my chances of success, if I decide to standardise the piston-to-head clearance on both engines, with a view to getting equal power output - or very nearly so - from them ? What precautions, if any ?
They were factory-test engines that I bought for a much-reduced price, and I DIY-marinised them.
Although one was as a factory turbo-only version while the other was a factory turbo+aftercooler version, I standardised all parts to the spec of the turbo+aftercooler version, which is rated some 10HP higher.
To be more specific, the injectors, injection-pumps (and settings)and turbos are now all identical, as the INTERNAL ENGINE PARTS ALSO ALWAYS WERE IDENTICAL.
However, the engine that started as a turbo-only version is STILL lower-powered than the factory-issue turbo+after-cooler one.
I suspect that the compression ratio of the turbo-only engine is LOWER than the other turbo+after-cooler engine (?) . This is because when I installed some additional pulleys concentric with the crankshaft and tightened the nut on the central hub , the lower-powered engine offered much lower resistance to turning than the higher-powered one.
I have looked at the factory drawings on the Internet and they say that the (TDC) piston-top to head clearance on these engines can vary between 0,95 and 1,15 millimetres - quite a bit of variation there !
They also say NOT TO GRIND the piston head and/or the engine deck. Is there a special surface treatment dished out in the factory, once the compression ratio has been determined for a particular version of the engine, and which will be impaired if the surfaces are ground ?
What are my chances of success, if I decide to standardise the piston-to-head clearance on both engines, with a view to getting equal power output - or very nearly so - from them ? What precautions, if any ?





RE: Twin diesels - unequal power output - help
RE: Twin diesels - unequal power output - help
I had a 3rd party give both the engines a top-end overhaul prior to marinising them, that is, both received new pistons, new wet-liners, new O-rings etc.
So they are identical in every respect, manufacturing tolerances notwithstanding.
As for the the lower-power engine.....this is obvious from the respective propellor wakes, as well as not making it above 2200-2300 rpm on load at full throttle, with the stronger engine on idle. Reversing the roles, the stronger one makes for 2600 rpm and an impressive wake. Props are identical , but counter-rotating.
If I had to unbutton the heads, would TDC piston -head (or engine deck) clearance show up with a good quality dial gauge ?
Would it be feasible to have the engine deck skimmed on the lower-powered engine to put the two on an equal footing ?
Would it solve the unequal power problem ?
RE: Twin diesels - unequal power output - help
An engine running backward from the design direction would have all the swirling disturbed messing everything up in the combustion process
Hydrae
RE: Twin diesels - unequal power output - help
Hydrae: Pistons go up and down, not around in a circle. They know little about what direction the crankshaft rotates. The only difference may be that the rod pins are offset in the opposite direction.
Paulista: As for the difference in engine performance, it could be a number of things. You already stated that they started life as different engines, and skimming the block is a no-no, only to bring block settling back to spec. Also, NEVER swap pistons around to even out the overall compression ratio, there are too many variables in power cycles, first and second order vibrations, and so on. A VERY sharp engine technician on site (Thats the secret there) should be able to pinpoint the differences in a reasonable amount of time. It could be cam profiles, compression pressure, injector pumps, injectors, turbo, etc. Lastly, I doubt that you would be able to see or feel a 10 bhp difference between the two. It appears from your description that there is a drastic difference.
Franz
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RE: Twin diesels - unequal power output - help
I would like to add, if you decide to remove the heads, measure everything, including combustion chamber volume, cylinder head airflow, piston dome or dish volume, deck (piston to head) clearance. A depth mike is the best way to measure piston to head, but a straight edge and feeler gauge can do the job. Cylinder head volume takes a skill that is described in common cheap hot roding books and engine blueprinting articles.
My first checks would be to compare manifold pressure and temperature, followed by exhaust pressure and temperature, and airflow if possible.
Regards
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RE: Twin diesels - unequal power output - help
Are these two cycle or four cycle engines?
I concur with Pat's recommendation to measure some fairly easily measured performance variables before going too much farther.
I agree with Franz that the difference you describe is much more than 10 HP.
There is a rat in this woodpile somewhere.
rmw
RE: Twin diesels - unequal power output - help
I suspect the propeller on the 'low power' engine is a little overpitched. Maybe an inch, maybe two.
I'd bet if you pulled the props and measured them side by side on a bench, you'd be able to detect the difference.
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
RE: Twin diesels - unequal power output - help
Huh? If the pistons are the same part number & rev level ("identical," as stated), who cares which holes they go in? They certainly didn't pick the holes in any rigorous fashion the first time the pistons were installed!
RE: Twin diesels - unequal power output - help
If, however, they are indeed different, do NOT swap half of them in the same engine; I hold firm by my comments about harmonics and vibrations. Pistons that weigh the same may have different compression ratios, producing different amounts of power. Power production that is not consistent can indeed produce vibrations identified as both first and second order vibrations, noticable on power, not on decel.
Your last comment is correct, they are installed at random, with todays production tolerances, individual fitting on production engines is time intensive, read as expensive.
Franz
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RE: Twin diesels - unequal power output - help
Yes, the point was that if he'd received 12 pistons of the same P/N but with naturally occuring compression ratio variation (again, this natural variation can be +/- 1 CR per info I've been passed, but not due to the pistons alone), there would be no harm in shuffling the pistons to try to balance the power.
As far as the comments about engine vibrations, I assume you're either referring to torque recoil, engine shaking vibration, or crankshaft TVs? If you're referring to torque recoil or crank TVs, I don't know what you mean by primary and secondary vibrations. If you're talking about shaking vibration, then I'm not sure how uneven power production with equal-mass pistons will have any significant effect. I'm interested to hear the explanation, though. Please clarify.
RE: Twin diesels - unequal power output - help
About 15 years ago, I was trying to troubleshoot an engine with a strange vibration which had a piston mislabeled, with the wrong pin height. It was about 3.5 mm down the bore than the rest, with a resulting 2 points lower compression. Cranking compression showed the variance, while blowdown did not.
The question is not equal mass pistons, and if any manufacturer supplied the market with pistons with a +/- 1 compression variance as acceptable tolerances, they would quickly find themselves out of a market.
Back to the thread, I dont think that is the problem with this engine.
Franz
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RE: Twin diesels - unequal power output - help
I did say "not due to pistons alone," didn't I?
You might want to check the math sometime, though...
* A +/-0.5mm variation in piston-head clearance can easily account for +/-1 CR variation, for commonly available bowl and bore sizes. The tol stack for that clearance includes head gasket compression variation, conrod length, crank throw, block height, piston height, and maybe something-or-other that I'm forgetting at the moment. Piston height by itself accounts for 10% to 20% of that figure, typically +/-0.05 to +/-0.1 tols. Rod length would typ. be another +/-0.1 to +/-0.15 or so. (so the rod length and piston compression height alone can give around +/-0.2mm, or about +/- 0.5CR) Throw in a head gasket compression variation of +/-0.15mm, and a crank throw variation of +/-0.05mm, and a deck height variation of +/-0.1mm, and you're there.
* Looking at the pistons alone, bowl volume variations are not insignificant either. Bowl volume is not usually toleranced on a print, but the bowl diameter, depth, radii, and other tolerances can easily accomodate 10% or more bowl volume variation. Piston top land diameter variation (say +/-0.03mm) and top groove position variation (say +/-0.25mm) contribute to the CR variation as well.
* Cylinder bore diameter variation is not insignificant,
RE: Twin diesels - unequal power output - help
deck height/conrod length
bore
weight
And I've seen conrods selected for
length
weight
big end diameter
I'm gently wondering exactly how many of each are stocked as spares!
Cheers
Greg Locock
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RE: Twin diesels - unequal power output - help
Fact, you should never mix pistons with different CR's on an engine.
Fact, you should never attempt at correcting an engine deficiency by mixing and components of different values (certainly not outside of the manufacturing stack accumulates, which should never reach +/- 1 cr.) High compression ratio engines are critically aware of any cylinder to cylinder variances.
The bore and stroke size do indeed have a relationship with piston height, as you allude.
Lastly, we are way off track of this thread, we both have points which may be considered, and frankly, we are both correct. I do not entirely disagree with your last post, and when I worked at a major engine research and design firm, we were told to hold variances to less than the manufacturer’s minimum acceptable level.
Modern manufacturing techniques allow for precision castings, CNC machining of piston bowls, ring land grooves, pin locations, etc.
Thanks for a great discussion this Monday! I needed this to get my last remaining brain cell jump started!
Franz
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RE: Twin diesels - unequal power output - help
Some of these things can be done before disassembly.
I would then mix and match to get the 2 best sets, or the two sets that requires least work to get 2 reasonably equal engines. ie, heaviest set of rods in one engine, lightest in the other. Heaviest pistons with the lightest rods. Longest rods with the shortest pistons. longest rod piston combo in the tallest block, longest stroke in the tallest block etc etc.
Obviously there will be some conflicts and a decision on what is most important, and what is easiest to correct will influence the decisions.
Regards
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RE: Twin diesels - unequal power output - help
Before doing that, I'd run a WOT backpressure check. In days gone by, engines came in symmetrical pairs, but engine mfgrs. got tired of stocking symmetrical castings and such, so now a pair of engines is two identical engines. Which almost always means that the exhaust systems are _not_ symmetrical. An extra inch of water backpressure on one side, or a few tens of gpm seawater flow asymmetry, could explain a difference of 300+ rpm on the prop curve. Ten HP couldn't.
Use either a Dwyer Magnehelic 0..50"wc gage with switching valves, one gage for both engines, or a pair of water manometers. Also measure the seawater pressure at the heat exchanger discharge. If the pumps are centrifugal, also measure the seawater pressure at the pump discharge.
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
RE: Twin diesels - unequal power output - help
RE: Twin diesels - unequal power output - help
My comment earlier about the combustion process being mixed up due to reversing the rotation of the engine still stands, with the following assumption: is the firing order now reversed? because the intake and exhaust orders are now reversed, if so, then the effect of what cylinder received a charge just prior to the cylinder in question will be affected by changing the amount of charge, same goes to the exhaust which would change the backpressure, the easy way around this issue is to use tuned headers which I imagine is not easy with a turbo or in a boat, I also assume the turbos are impulse not constant pressure so the different firing order may influence turbo performance.
Is the turbo direction also reversed? I imagine not. One other item to look at how close are the turbos to each other? Is it easy to trade the turbos? Do you have a way to measure turbo speed?
Hydrae
RE: Twin diesels - unequal power output - help
My guess is that you must install new camshafts that operate the valves in the right, new sequence. And the FIE will need to be fiddled with (if nothing else, the advance should become retard). I can't believe that a 4-stroke diesel engine can be run backwards rhat easily.
RE: Twin diesels - unequal power output - help
But, are you sure you have equal gear ratio's in the transmissions? And equal prop pitch?
You could get a rough check of comp ratio balance by checking both engines compression pressure. Actually using pressure transducers would be a good check. There is mention of that here on this site. That would determine if true balance between the two.
RE: Twin diesels - unequal power output - help
Thank you all for the multi-faceted approach to addressing the problem at hand.
Do bear in mind that the two engines in question were "factory-test", and the low-powered one always performed as such (below par), both BEFORE and AFTER they were given a top-end overhaul. They are 4-cylinder, 4-stroke, 4,3 litre displacement, turbo-diesels, anti-clockwise rotation when viewed from the rear fly-wheel.
The silly mechanic who performed the overhaul job, did say something to the effect as :"Yes, the turbo-only model did have different pistons from the turbo+after-cooler one, but now after the overhaul, they are both going to be great, specially after receiving IDENTICAL pistons, wet-liners, head gaskets and O-rings - the engine manufacturer´s STANDARD overhaul kit !"
That said and taking into consideration that the culprit engine is also of OLDER factory-test manufacture, I doubt very much that engineering tolerances have singled out this particular engine for a shameful debut in my boat ! To add to the mystery, the factory does not have any records of the serial numbers pertaining to these test-engines.
Aside from the wake, the engine note of the stronger one is like a growl while the weaker one sounds more like a sewing-machine ! The exhaust of the former(starboard) has a bend (starboard) while the latter is straight out (port).
By the way, the props are IDENTICAL - I have closely accompanied an expert who checked them, with all the tools of the trade. The starboard one is right-hand while the port one is left-hand rotation. The gear boxes are BW 5000, fully-reversing.
RE: Twin diesels - unequal power output - help
RE: Twin diesels - unequal power output - help
RE: Twin diesels - unequal power output - help
If you are getting 15lbs boost from one, and 12 lbs boost from the other, that would explain the problem, but to find out why the difference comes next.
Exhaust thermocouple temps may help here.
Mike Halloran has a good point about measuring outlet pressure, past the turbo.
Reading your original post raised an eyebrow, you say the lower powered engine has less rotating friction/apparent compression, that sounds like an arrow to follow.
Compression tests are in order.
Franz
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RE: Twin diesels - unequal power output - help
I knew someone would latch-on to the tell-tale symptoms I described earlier on, and which I myself forgot to reiterate in subsequent posts.
Yes, Mike Halloran´s suggestions for measuring turbine exhaust pressure - temp would be nice too, but for the measuring gear - plus your suggestion for measuring boost are pertinent to the case at hand.
In fact, I have run your test and there was no perceptible difference in boost levels between the two engines for similar RPM and loading conditions, notwithstanding the weaker one giving up at around 2300 RPM.
Now, the weaker one offered MUCH LOWER RRESISTANCE to turning while I tightened the nut on the central pulley hub, concentric with the crankshaft.
If it is, indeed, a lower-compression engine, as I am nearly sure it is, can it be fixed by skimming heads and/or decks, to bolster compression ratio, or will I be firing a shot in the unknown, only to have the ricochet catching up with me instants later ?
RE: Twin diesels - unequal power output - help
The parts to make two water manometer test kits will cost you ~$20 at Home Depot. A couple of 1/4"NPT brass pipe nipples, 6" long, adapters to get to 1/4" copper tube, a couple feet of copper tube, some flexible pvc tubing, a couple of yardsticks, and some blue masking tape should about do it. The plastic tubing will of course melt if it gets too close to the turbos.
You can measure seawater pressures with plastic tubes run up the flagpole. Just mark the water level at known rpms, and measure the elevation later.
The caution about not grinding the deck suggests that the head gaskets will ooze out and fail if the surface is too smooth. So, don't mess with the deck.
I >>think<< the cylinder liners are flanged at the top, and seated in counterbores in the block. I don't know if they are matched or shimmed or -what- to get the upper faces all even, but if the liners are shimmed too high, or not driven deep enough for whatever reason, that could affect your compression ratio with 'identical' parts. If you can find a place where you can get a feeler gage between the head face and the block face, the gap should be ~the same on both engines.
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
RE: Twin diesels - unequal power output - help
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Twin diesels - unequal power output - help
The first tractor I bought had a Cummins "350" in it, while a more popular version of the same engine was a "400". My drivers always wanted more power, so I looked into uprating it to make it a "400 HP" engine.
I already knew that the displacement was the same at 855 cu. in. and it was the same engine platform.
What I found out was that the only difference in the two versions of the same engine that produced the additional 50 HP was (1) timing, (2) compression ratio, and (3) turbo, with, of course, the appropriate fuel pump flow rate setting for the 400.
In your case, Paulista, you have stated that the turbo is the same, but I would question the timing and the compression ratio. As I remember it, the pistons and rods were the same in the Cummins 350 and 400, so the compression ratio must have been in the head configuration.
The engines had different torque curves, too. Have you compared the torque curves for the two engines you purchased?
I could have just had the fuel pump run for the specs of the "400", but that would not have produced the desired result. (various drivers did monkey with the pump, and it cost me a burnt up engine once.)
I asked if you reversed the rotation your self, of if the engines came opposite rotation. If you reversed the engines, could you have mis-timed it?
rmw
RE: Twin diesels - unequal power output - help
RE: Twin diesels - unequal power output - help
I looked again at the factory drawings on the internet. The piston top-to-deck height ("B") is the dimension that allows for considerable variation, which in turn (I suppose !) provides the mechanism for building engines of differing CR. This seems to fit in with what Greg Locock had to say earlier about his work experience at an engine builder.
Here is the Web Site:
www.mwm.com.br/site/downloads/posters/BLOCO.pdf
You may also look at the other drawings of piston and head (cabecote) assemblies on this site.
The boat has been going through a major overhaul since last September. Both engines haven´t been fired for some time, so it is natural that there is loss of seal between pistons and liners - but then again, the weaker engine has been singled out to show this up in rather dramatic fashion !
Coming to think about the exhaust back-presure test..... in as much as this is what provides the energy for the turbo, and thereby, the boost developed by the compressor, both engines show remarkably equal boost.
RMW and Pat Primmer - thank you both, and also the others who are too numerous to thank individually, for your generous input.
Will get back when I do some testing, like compression and blow-by.