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Old 09-13-2021, 03:29 PM   #1
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EGT / Pyro for naturally aspirated diesel?

My Lehman 120's have EGT pyrometers in the exhaust elbows (ports are cast into the elbows), and analog gauges at the lower helm (not the upper).

I don't understand why one would need these on a non turbo engine, particularly one where the relationship between throttle, RPM and load is pretty much fixed (can't lug the engine towing up a long hill, etc.). Indeed, my gauges seem to correlate just to speed and nothing else. I struggle to come up with an engine failure/wear issue that I would pick up on these gauges before another more obvious sign somewhere else.

Anyone care to defend their inclusion as I re-do my helm? I'm always inclined to keep an instrument if it's already there, but only if there's some point to it.
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Old 09-13-2021, 05:10 PM   #2
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Losses of cooling by impeller wear, strainer clogging, tubesheet clogging, hose failure or exhaust riser corrosion will cause exhaust temps to go up. Make sure you have baseline temps recorded somewhere so you can compare later when you have questions. Don't rely on your memory.
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Old 09-13-2021, 05:19 PM   #3
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Losses of cooling by impeller wear, strainer clogging, tubesheet clogging, hose failure or exhaust riser corrosion will cause exhaust temps to go up. Make sure you have baseline temps recorded somewhere so you can compare later when you have questions. Don't rely on your memory.
Wouldn’t the EGT sensor be before the water is injected into the exhaust? I put Borel alarms on our last boat for this exact issue. They have a zip tie with a sensor in it that goes around the hose right after the water is injected to detect loss of cooling water flow.
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Old 09-13-2021, 06:10 PM   #4
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Wouldn’t the EGT sensor be before the water is injected into the exhaust? I put Borel alarms on our last boat for this exact issue. They have a zip tie with a sensor in it that goes around the hose right after the water is injected to detect loss of cooling water flow.
Yeah I did the same thing last season for the reasons High Wire states. The pyrometers measure EGT before the water gets mixed in - trying to figure out if those numbers are good for anything.
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Old 09-13-2021, 06:28 PM   #5
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Yeah I did the same thing last season for the reasons High Wire states. The pyrometers measure EGT before the water gets mixed in - trying to figure out if those numbers are good for anything.
An over propped boat would cause EG temps to drastically increase. EGT is the best way to gauge engine load. I know that Yanmar gives parameters in the shop manual.
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Old 09-13-2021, 09:40 PM   #6
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An over propped boat would cause EG temps to drastically increase. EGT is the best way to gauge engine load. I know that Yanmar gives parameters in the shop manual.
Yeah I understand that EGT will go up if load increases at a given RPM - but that never happens during normal operation unless you have a variable pitch prop. If I were to re-prop (not likely to ever happen) I could check temps with an IR gun.

Is there any reason to have a pyro gauge during normal operation?
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Old 09-14-2021, 06:11 AM   #7
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Yeah I understand that EGT will go up if load increases at a given RPM - but that never happens during normal operation unless you have a variable pitch prop. If I were to re-prop (not likely to ever happen) I could check temps with an IR gun.

Is there any reason to have a pyro gauge during normal operation?
Your Pyro gauge will show the effects of a dirty bottom, dragging a lobster trap, or the like. Its like any other gauge. The value lies not in daily operation but when it starts to change. Probably not important on a normally aspirated trawler. However I like to have alarms on the exhaust hose. That to me is extremely important.
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Old 09-14-2021, 06:40 AM   #8
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As previously mentioned an over propped boat can easily raise EGTs to uncomfortable levels. If your vessel can reach full rated RPM with full tanks and no overheats I can't see the EGTs as a daily metric. But, whether turbos or not knowing the EGT provides good initial data from which to gauge future load issues.

Think of an EGT reading like coolant temperature, volts, oil pressure, heading or wind speed - data points from which to assess your vessel's well being. Some of us even use an IR gun every 90 minutes or so during an ER check.
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Old 09-14-2021, 07:30 AM   #9
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In MY opinion if you monitor for black smoke you'll be good enough.
I ran a pyrometer on a turbocharged diesel truck (Dodge/Cummins) for years and the only time egt went high was when I was pushing black smoke.
But that's only my opinion.
I also don't like to monitor too many gages. I'd rather spend my time monitoring coolant temperature. and oil pressure. That's what is going to take out a Lehman.
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Old 09-14-2021, 07:44 AM   #10
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I think you are correct that it provides minimal information once you have done the initial verification that prop loading is correct. I could see it being useful if you regularly towed a dinghy, and perhaps useful as a diagnostic for a fouled bottom or entanglement. So it has some value, but is it enough to continue to dedicate dash space? If I were looking for something to ditch, it would be at or near the top of the list. But if I had space, I would probably keep it.


One thought would be to relocate it somewhere else. Perhaps you could find a spot in the ER for it, or otherwise in proximity to the engine? Then the info is still available, just not right in front of your nose all the time.
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Old 09-14-2021, 11:47 AM   #11
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One thought would be to relocate it somewhere else. Perhaps you could find a spot in the ER for it, or otherwise in proximity to the engine? Then the info is still available, just not right in front of your nose all the time.
I really like that idea. As an engineer I hate the idea of throwing away data, but agree that from a User Interface standpoint, having extra data can actually be a detriment if it's not providing useful insight.
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Old 09-14-2021, 11:57 AM   #12
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what if you hit a whale on the way to Catalina. a bent shaft or prop will send the EGT skyrocketing
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Old 09-14-2021, 12:07 PM   #13
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what if you hit a whale on the way to Catalina. a bent shaft or prop will send the EGT skyrocketing
There you go. As the boat shakes itself to pieces and sinks out from under me I'll benefit from a clear picture of my trusty Lehman's exhaust temps. "uh oh - better back off the throttles!"
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Old 09-14-2021, 02:08 PM   #14
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You don't need pyrometers on a natural, but since you have them, they can give indications of cylinder health. In theory all cylinders should run close to the same temps. High or low numbers can indicate fuel, valve, and ring problems. Most naturals would have trouble making exhaust temps high enough to damage the engine under normal setups.

On larger continuous duty engines like tugs doing a long tow, the engine speed is set to produce an exhaust temp below damaging temps. Some engines can make micro adjustments to the fuel in each cylinder and use temps to make each cylinder pull the load equally.
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Old 09-14-2021, 08:53 PM   #15
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Thanks Lepke.
Could you explain more about natural aspiration engines not needing EGT?

We have two Perkins M135's - big, heavy, NA's.
I was debating whether to go the band style EGT (Borel) or the probe/insertion to try and detect coolant loss AND something catastrophic happening meaning sudden high exhaust temps. It sounds you don't feel a NA engine will need that EGT information?
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Old 09-14-2021, 09:10 PM   #16
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If engine loses cooling water I suspect EGT will respond faster than thermistor stuck in the water flow and having redundant systems is usually beneficial but I agree, there is a limit to how many gauges you can monitor. I would rather have an overtemp signal go to an alarm relay.
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Old 09-14-2021, 09:12 PM   #17
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If you read Calder’s book on marine engines you’ll see where he states that it is much more useful to have an EGT gauge on each individual cylinder. That way you can monitor individual cylinder health like TwistedTree was stating. Only having one on the manifold gives limited info.

If your engine were a turbo then the manifold temp gauge at the helm would be useful, but keep the other six gauges down in the engine room in either case.
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Old 09-14-2021, 09:22 PM   #18
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If engine loses cooling water I suspect EGT will respond faster than thermistor stuck in the water flow and having redundant systems is usually beneficial but I agree, there is a limit to how many gauges you can monitor. I would rather have an overtemp signal go to an alarm relay.
There's some confusion on the thread on what EGT is measuring - here's a quick summary of the sensors we're talking about & their utility:

1. EGT via pyrometer measuring exhaust temp prior to raw water injection: This is what I have on my boat, and what this thread has confirmed to be pretty useless on a naturally aspirated engine

2. Pyrometers on each cylinder: Useful (but rare and probably not important to most) in monitoring differences between cylinders to detect premature engine wear over relatively long periods of time

3. Exhaust temp alarms (e.g. Borel style straps) measuring (usually a binary switch) exhaust temp downstream of raw water injection: Extremely useful safety feature - an impeller failure will cause engine coolant temps to increase (probably in minutes), but will elevate (in seconds) temps downstream of raw water injection to levels above what your exhaust system (rubber, fiberglass) can handle, leading to fires, sinking, and all of our worst nightmares. The difference between an exhaust temp alarm going off immediately versus coolant temp going off 3-5 minutes later could quite plausibly save your boat.
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Old 09-15-2021, 09:54 PM   #19
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Thanks socialrider - that's a lot clearer!

Now I just have to try and find a cheaper source of the Borel, or another strap-style, for #3 since the shipping cost is more than half the cost of the item
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Old 09-15-2021, 10:35 PM   #20
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Thanks socialrider - that's a lot clearer!

Now I just have to try and find a cheaper source of the Borel, or another strap-style, for #3 since the shipping cost is more than half the cost of the item
Don't know if it helps but I used these - the band itself is $49 but not sure about shipping to you:
https://aqualarm.net/exhaust-engine-...rssensors-c-5/

They're just a normally open temperature switch that closes at 200degF; you could find an appropriate switch at an industrial supply house and epoxy it to a big zip tie.
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