Twin diesels - unequal power output - help

[Paulista]

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 ?

 

[ivymike]

how big is the difference between the two? You could get a few percent difference just due to mfr variation in just about any combustion-related component (injectors, cam, pistons, etc). If you really think it's compression ratio variation that's causing the difference, why not swap half the pistons from each engine into the other? How about all the pistons from one into the other? I've heard that tolerances on PRL groups can result in as much as +/- 1 compression ratio (ie nominal 17:1, max 18:1, min 16:1).

 

[Paulista]

Ivymike,
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 ?

 

[hydrae]

How is the counter-rotating prop done? transmision? or is the engine running in the oposite direction via a replaced cams?
An engine running backward from the design direction would have all the swirling disturbed messing everything up in the combustion process
Hydrae

 

[franzh]

WHAT?
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

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.

 

[patprimmer]

I agree with Franz

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

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.

 

[rmw]

Were these opposite turning engines originally, or did you convert one to turn opposite direction as part of the marine set up?

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

 

[MikeHalloran]

Did you happen to record the boat speed during your experimental runs?

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

 

[ivymike]

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.

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!

 

[franzh]

The comment by yourself stated that if the pistons were different compressions, could they be swapped around to even it out. Obviously, if the pistons were the same part number and as you state "identical", there would be no problem.

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

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.

 

[ivymike]

The comment by yourself stated that if the pistons were different compressions, could they be swapped around to even it out. Obviously, if the pistons were the same part number and as you state "identical", there would be no problem.

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.

 

[franzh]

Unequal power production from pistons that are evenly different (even and odd firing order) can indeed produce shaking vibration in both the primary and secondary levels, more noticable in primary, or first order, as long as the engine is under power. The greater the power level the greater the vibration, or shake. Under no load, or decel, there is no perceived shake.

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

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.

 

[ivymike]

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.

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,

 

[GregLocock]

I've seen diesel pistons selected at the factory for:

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

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[franzh]

Ok Mike, we're pulling at semantics now.
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

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.

 

[patprimmer]

If I had 2 engines to do, I would measure the piston deck heights, bore clearance and weights, the rod centre to centre and weights, block deck height at each end and each crank stroke.

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

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.

 

[MikeHalloran]

I still wouldn't lay a wrench on the engines until I had swapped the props to see if the 'power' asymmetry follows them.

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

 

[strokersix]

I'm not sure swapping props is feasible. Aren't the props righty and lefty? Or can the drive system run full power in reverse?

 

[hydrae]

Lets see if I get smashed on again.

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

 

[SomptingGuy]

Excuse my simplistic quesion, but how do you reverse the rotational direction of a diesel?

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.