Interpreting Yanmar EGT

[CG Weinert]

I am struggling to draw the right conclusions from our RPM/EGT data and hoping you can help steer me in the right direction. I am pretty nerdy so please don’t hold back on the technical depth (eg math) of a reply. BACKGROUND: Our prop has a small defect, so I will be pulling the prop off for pre-season service this week. The question at hand is: are we propped right? My latest conclusion is YES, but I am confused by light soot. Was hoping you might take a minute to help me sort it out; my running assumption was that soot always was an indicator of being overpropped, but perhaps I am just cruising a little harder than I should be?. SETUP INFO: we have a Yanmar 6LYA-STP (370) in a Duffy 35 downeast cruiser (eng plate photo attached)

• ~500 engine hrs; factory (Walker) airsep (super clean)
• verified throttle lever drives fuel injection pump to the WOT hard stop.
• RPM are collected with laser tach.
• last 2 season clean bottom WOT was 3350 rpm at launch (EGT 950F post turbine)

BASIC QUESTIONS:

1. Can you offer insight into what should target WOT RPM should be? Would that be the ‘fuel stop’ RPM +50-100 (3300 on our engine plate) or some other number?
2. Does soot, at any level, always indicate being overpropped? We cruise at 2650-2700 (actual) RPM (post turbine EGT of 925) and have light soot on the transom after an hour ride. If one looks really hard off the transom, on a bright day, a fine haze can be seen dragging out behind us, but you have to be looking for it – normal people do not notice this. This is confusing to me as we consistently get 3350 WOT with a clean bottom on launch day.  I have never been smart enough to look for the haze on launch day, so it may only be a mid season phenomenon resulting from a slightly slimy bottom.
3. Attempting to translate what you have written elsewhere on your awesome site, is the right way to tackle engine propping and operation for longevity:

• Prop engine to peak RPM+50-100
• observe EGT at WOT
• establish cruise RPM at ~80% of the observed WOT temp?
  • e.g.- with WOT EGT 950, I need to slow my cruise down to something that gives me ~750F, correct?
• Do you have a preference to measure EGT at turbine inlet or outlet? Is this most commonly done at the outlet?

Apologies I am far from the boat right now or I would have thrown in a few thermocouple pics too. Many thanks,cgboston  

Topic

[David Marchand]

With the white smoke going away shortly after startup, you can be pretty sure that your injectors are fine.

While I agree with Tony’s recent post, I still wonder if there might be some reason your engine isn’t making its 370 rated hp and that is why it is smoking. First confirm governor performance by opening the throttle to wot in neutral. You should hit 3,700 rpm, the high idle spec. If not then investigate your governor/fuel injection pump. You previously said that the throttle hits the stops, but check again.

Then look at what might be causing the engine to produce less than 370 hp at wot. If the after cooler is fouled on the air side, the engine won’t get enough air at wot but it will get maximum fuel and that causes black smoke.

Another cause is that the turbo housing has corroded, the blade tip clearance is high and the turbo isn’t putting out full air at wot and the same thing happens as with a fouled air cooler.

Here is what I would do: Read Tony’s Tips on this site, first the Cummins one about 6BTA after coolers as it has text and then the Yanmar one for pictures. Then service your after cooler accordingly. If you see the fins on the air side fouled with soot and oil, then that may be your problem.

Then pull off the elbow at the exhaust exit from the turbo charger and look inside. You should see a smooth surface with a light coating of soot on the turbo throat. Any roughness is a sure sign that sea water has backed up from the exhaust. You will have to replace your turbo and rework your exhaust if this is the case.

Once you have done these things you can be pretty confident that your engine is putting out its rated hp. If you are still getting black smoke at cruise, then reduce the prop pitch an inch or so which should let the engine breath better at cruising rpms. You can cruise at 2,800 or a little more if the engine revs to 3,450 at wot to get back your cruising speed with the lower pitch.

David

[CG Weinert]

Thanks all. 

I’ll shoot for 3425-3525 and not fret about the soot.

Best!

CW

[Clark Leighs]

Sorry guys,   I did not look up the specs for the engine but read the plate in the photo.   Guess I got it wrong.

I’ll bow out 

[Tony Athens]

Yanmar Propping Specs

Study this for a bit.. Yanmar engines like to spin up well above their minimum rated RPM..    If you are asking, my suggestion would be to completely IGNORE the word “continuous duty”, as that term used by Yanmar on their pleasure duty engines means absolutely as to the universal term of continuous duty used by manufacturers like CAT,  Detroit, MTU, Cummins, Scania, etc etc..  I would also not read anything into this spec sheet other that what is says,— “target rpm” –This is what you shoot for.. This sheet came from my Yanmar schooling manuals in the early 2000’s and I have followed it since.. Zero issues, even on the 6LYA2’s  that I have installed & watch over. Your interpretation from what ever you read is wrong as to the governor. That did not come from me. Your engine will make FULL rated power to the upper range of the target RPM as will all the rest. Also, add this to the equation: Spinning up easy to well ABOVE the minimum rated RPM will guarantee you and much faster and easier path to getting over the HUMP, AND running at a higher RPM to get your “Happy Vessel Cruise Speed’ means you are pumping more seawater, more coolant, more oil, plus your cylinder  pressures (BMEP)  are lower to make the same HP.. These are all GOOD things to these types on high performance engines………………….. Tony  

[CG Weinert]

Thanks for the replies Clark and David.

From the conversation I am struggling with what target WOT to use for the propping conversation. Can somebody help – “3100 continuous” or “3300 fuel stop” from the engine plate?

You both mention “high idle,” which brings up an additional question.  I have attached an excerpt from the Yanmar manual which shows high idle at 3750. 

I have been presuming from my web recon, that 3300 + XX is the right WOT target for propping.

Reading Tony’s Tips: I’ve come to translate the terms thus, does the community concur?

·         Continuous RPM – in this case (Yanmar) this is the RPM where the mechanical governor starts to kick in and taper fuel delivery to moderate RPM under load (read Tony’s Tips on continuous duty and Yanmar for more)

·         Fuel Stop RPM – the point at which the engine will no longer allow speed to increase under load, for Yanmar coincides with peak power

·         High Idle – peak, no load RPM

I have also seen that Yanmar propping sheet, as well as hearing from my local Yanmar dealer that “Yanmar likes to prop that engine to 3350”; which may not be quite as conservative as that worksheet.  The worksheet does have a certain vintage look, so I have wondered if the worksheet is ‘old news?’

This local Yanmar recommendation to prop to 3350 , if 3300 is the right baseline WOT, is more or less aligned with the Tony approach of “propping the vessel at full working weight to rated RPM plus 100 to 200 rpm” (http://www.sbmar.com/articles/engine-life-vs-engine-loading/). 

Tony / Corey – if you can make a minute to weigh in, could you be sure to help me understand which number (3100/3300) is the right starting point from your view of the world, please?  (I suspect “3300”).  Also thoughts on how to select max cruise speed (80% of WOT EGT?) would be wonderful while I have your attention.

I certainly want to make sure I am on the conservative side of the line.

Answering David’s other questions:  we see less than 30 seconds of almost-not-there white haze outside the exhaust port at startup, and then never any more.

 

[David Marchand]

Clarke: 3,300 is not high idle for that engine. 3,300 is where the maximum hp is produced. See the attached bulletin from Yanmar.

OP: I have the same engine on my Mainship Pilot 34 but no EGT gauge and run her the same way- cruise fast at 2,700 rpm. I am a little over propped- 3,250 at wot down from 3,350 when newer and lighter, all phototached rpms. But I have no soot, so it would seem that your soot is not related to propping.

What is the engine like at idle when cold. Mine puts out noticeable white smoke that diminishes when warm but not entirely. I attribute that to injectors, but I am mindful of Tony’s advice that injectors usually last for thousands of hours and I only have 1,000 on mine. It may be just the way that engine is.

Those props specs in Yanmar’s bulletin are really conservative. I think you are where you should be at 3,350 IF THE ENGINE IS REALLY PUTTING OUT 370 HP. I agree with Clarke’s list of things that can cause black soot, but they all also cut down on maximum power. So maybe you are under propped (relative to where you should be if the engine can put out 370 hp) which lets you hit that rpm but you are really putting out less than 370 hp for one of the reasons Clarke indicates. Also repitching the prop to 3,450 might be worth trying.

David

[Clark Leighs]

#1   —  yes, 3300 is also called High Idle.    That is what the governor is set to as the absolute max. rpm the engine will be allowed to attain,   +/- a wee bit. 

You want to get the engine propped so FULLY LOADED the engine will attain between the Rated rpm of 3100 and the High Idle [3300]

 

#2 —soot can be a result of a dirty air filter, clogging aftercooler, charge air leaks, dirty bottom, damaged  or fouled propeller,   so although sooting is indicative  of overpropping it is not the only cause.   I’m sure I’ve missed something.

 

#3— basically yes.   A couple notes:

                    3a—I’m being picky but  what I think you are calling peak is normally called RATED [3100 ].   Peak could mean 3300 to some folk, not what you want.     Terminology becomes quite important especially on the internet as we cannot go huh or raise an eyebrow as a question.  So prop to 3150 to 3200.  A bit higher is also fine as it will unload the engine more and give more cushion as fouling develops or an extra guest and gear are brought aboard.

                    —3b    Yes, you want to know the EGT at WOT.      

                   —3c     roughly sure.    But keep in mind that you may find that at some lower rpms that the egt actually rises.  The turbo boost often falls off not linearly so the quantity of cooling air may drop to the point that egt rises.    You must watch for this and then avoid those revs.    Better pick revs that are several hundred lower and then see how the egt does and how the boat sounds/responds.

                   -3d  either thermocouple location will work.   Ahead of turbo is more accurate but if the probe breaks it will damage the turbo.    For that reason many probes are installed after the turbo.    Just note that the temps will be different.   Ahead will be 200-300 oF higher than when mounted after the turbo.

When testing all info pertaining to performance should be recorded.    Boost, EGT at WOT, boat speed max., rpm reached at WOT,  even fuel use if available and then recorded.    Then pick what you think  will be a cruise and do it again.        That way you can look back if you find egt seems higher or boost down or boat speed off.     Troubleshooting often needs that kind of info to minimize guessing and if asking for help, in person or internet, that eases the process a lot.     Record all of that data, both WOT and cruise even if you change the cruise later.

And of course get that prop dealt with.  Testing with fouling or damage is counter productive.

To be clear I am no expert but this is what I have learned over the years.

 

Have fun.   Sounds like you are on your way and have a decent understanding to start.

[Author]

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