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 ?

 

[icrman]

I didn't have time to read all the posts here.
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.

 

[Paulista]

Gentlemen,
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.

 

[rdd48856]

Try to exchange turbos...

 

[Paulista]

The turbos are also IDENTICAL, both brand new units wih less than 20 hours of use.

 

[franzh]

One tried and true test, measure turbo boost at the inlet at WOT, on one engine at a time. Coast one engine and power the other for identical periods and sea conditions.
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

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.

 

[Paulista]

franzh,
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 ?

 

[MikeHalloran]

Do you have many hours on these engines? Is it possible that the weak engine's rings are not seated yet? Any indication of asymmetrical blowby or crankcase pressure?

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

 

[patprimmer]

If you doubt the compression ratio, simply do a compression test and a leak down test. In this case also test camshafts with a dial indicator and a degree wheel

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]

I remember learning an insignificant little factoid when I owned 18 wheelers (tractor trailers) some 25 years ago or so.

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

 

[xnicke]

Have you checked the propellers yet?

 

[Paulista]

Mike Halloran, Thanks for your input.

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:

http://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.