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Marin 05-13-2013 08:24 PM

Engine temp question for RickB
Rick--- With all the talk that occurrs on this and other boating forums about the proper operating temperature range for a diesel I have been pondering a failry basic question for a bit now.

The short version of the question is this: is the relationship between coolant temperature and combustion chamber temperature such that if the coolant temperature is indicating properly on the gauge per the manual, does this always mean that the combustion chamber temperature is also in its correct range?

The background is this---- People often talk about their engine temperature in terms of what they see on the coolant gauge. They talk about the horrors that will occur if you run a diesel at less than it's optimal temperature range. The typical comment, which I myself have made from time to time is, "If you run your engine at too low an rpm it will run too cool and over time this could be detrimental to the engine."

When I have said this, I have meant the engine's combustion temperature being too cool.

Now I assume the engine manufacturers design the cooling systems of their engines, from coolant circulation volume and flow to thermostat characteristics to coolant temperature sensor and gauge, so that when the engine is operating in its proper temperature range (combustion) the temperature gauge on the panel will indicate the proper temperature (coolant).

So.....if on the typical diesel engine installed in the typical recreational boat the coolant temperature gauge is indicating the correct operating temperature per the manufacturer's manual, does that always mean that the combustion chamber temperature is also in the correct range?

The application of my question is this: At a cruising rpm of 1650 the coolant temperature gauges of the two FL120s in our boat indicate the proper temperature per the manual. If I pull the power back so the engines are operating at 1500 rpm (or even less) and the coolant temperatures remain at the same place on the gauges (kept there by the thermostats) can I assume that the combustion chamber temperatures are also remaining in the correct range?

So I would not have to worry about "too-cool" combustion chamber temperatures as long as the coolant temperature was at the right figure on the gauge per the operating manual.

RickB 05-13-2013 09:32 PM

Believe it or not, the temperature in the combustion chamber doesn't vary all that much. The heat of compression is about 500*C, the heat of combustion in the center of the "fireball" (if you don't mind me calling it that) is close to 2000*C. The temperature next to the cylinder wall is fairly close to that of the coolant and the temperature of the gas passing the exhaust valves will vary greatly with load but a good figure is around 1000*C at high load.

Now, getting back to your question. First we have to understand that there is a difference between temperature and heat. Your cooling system removes heat from the engine. The result of rejecting that heat is a lower temperature of the engine and fluids.

The amount of heat rejected to the coolant is directly related to the amount of fuel burned. At idle, the center of the "fireball" is just about what is is at max power, it is just a hell of a lot smaller and releases far less heat - fewer BTUs are available to heat the charge air, cylinder walls, pistons and all the other bits exposed to it. If you didn't control the jacket water temperature, it would go up and down proportionally to the amount of heat released by burning the fuel.

Throwing another log on the fire makes the room hotter but it doesn't change the temperature of the flame.

High power puts lots of weight of fuel in the combustion chamber, lots of weight of fuel means more heat is released. About 30 percent of that heat goes out the exhaust, another 20 percent is carried away by the coolant. When everything is right with the world, your engine can run at full power and the coolant will transfer enough heat to the ocean to maintain a constant coolant temperature that equates to a constant temperature at the cylinder wall and around the exhaust valves. The thermostat opening at full power should be enough to allow a coolant flow rate that transports just enough BTUs overboard to balance heat production with heat rejection and the coolant temperature remains constant at the thermostat setpoint.

If you reduce the heat input to the engine by reducing the fuel flow, the thermostat will reduce the coolant flow to maintain the setpoint. It will work that way until there is more heat produced than the coolant can carry away or not enough heat is produced to keep the coolant at the setpoint.

Either way, the temperature in the combustion chamber is, for all practical purposes, the same.

You may read about technologies that manage emissions by reducing combustion temperature. Some of them inject up to 50 percent by weight of water (in a fuel/water emulsion or a separate injection nozzle) to reduce the flame temperature in the fireball. But, to produce X amount of power, Y amount of fuel must be burned and the coolant doesn't even know that the flame temperature is lower, it just knows it has the same heat load to carry at the same power output as before.

You might look at it like a pot of boiling water. Water in an open pot stays at the same temperature regardless of the size of the fire under it. The only thing that changes is if you build a bigger fire under the pot, it will boil dry faster.

That is the long version, the short version is: When the cooling system is working correctly, the temperature differential across the heat exchanger is a measure of how much heat the coolant is carrying from the combustion chamber to the sea. Combustion temperature is fairly constant and is not altered by the cooling system.

New designs created to reduce fuel consumption and emissions will give us combustion chamber temperatures that are no higher than the compression temperature and hold that temperature longer by controlling the rate of heat release. The cooling system will never know the difference.

River Cruiser 05-13-2013 09:35 PM

The thermostat controls the engine temp via the coolant if all other parts in the system are working as designed. The coolant removes the excess heat in the system by circulating faster at higher loads & slows down with a lighter load. The way I understand a engine cooling system is that the removal of the thermostat will allow the coolant to move thru the engine to fast and not remove enough of the excess heat, leaving hot spots in the engine while the gauge will not indicate a problem. I believe cruising at lower RPMs will not have a significant effect on engine life or dependability if the cooling system is operating properly, the thermostat will help maintain the desired temps by not allowing heat to be removed as fast.

Marin 05-13-2013 10:19 PM

Rick--- Good explanation, thank you, and I think I followed it correctly.

So it would seem, then, that if there is some truth to the notion that lower combustion temperatures (heat) at the outer edges of the combustion chamber can be detrimental to an engine over time, running our engines at 1500 or even a bit less will not be detrimental because even at that lower power setting the temperatures across the chamber will still be close enough to what they are at 1650 or 1700 that there will be no detrimental effect on the engine itself. And the fact the coolant temperature on the gauge remains consistent with the factory-specified figure means the cooling system via the thermostat is keeping things on an even keel with regards to the temperature of the engine's components like valves, cylinder walls, etc.

So as long as we're on this subject, when you have time perhaps you can explain to me what ARE the detrimental effects everyone likes to talk about from running a diesel "too cool?" Everyone talks about them but I do not recall ever seeing anyone describe what they actually are. Thanks.

Baker 05-14-2013 12:23 AM

You had me at...................


Originally Posted by Marin (Post 156232)

The short version of the question is this:

Baker 05-14-2013 12:28 AM

Riddle me this Batman.....

I always had a bard time evacuating all of the engine oil out of the Yanmar 4LHA in my previous boat. The reason being....even though I could idle my boat at the dock with a little bit of a load on it and it would get up to a solid 180 degrees coolant temp.... The oil temp would be lukewarm at best and would still be difficult to drain due to the fact that it was a little cool. I had fantasies of running the boat hard and then stopping at anchor and changing the oil immediately but of course the reality of the "contingencies" reared their ugly head and didn't want to be at anchor with an oilless engine. Anyway, it was a solid 5 minute no wake ride to my slip and by the time I got there, the oil was damn near cold even though the coolant temp was 180!!!!!!!!!!!!

Marin 05-14-2013 12:36 AM

Don't know about your setup but on our engines the raw water cooling of the engine oil is not thermostat controlled. The oil just goes through the lube oil heat exchanger which has a high volume of cold raw water going through it even at idle and back to the engine. So if the engine slows down and the heat production from the movement of the crank, cam, connecting rods, etc, drops, the oil temp is probably going to drop pretty fast because it's getting less heat transferred into it plus it's getting unregulated cooling from the raw water system.

When we change oil we run each engine (separately, we do them one at a time) in gear in the slip at about 1200 rpm until the oil filter is hot, but not real hot, to the touch. This usually takes about 20 minutes. Then we shut the engine down, punch holes in the top (bottom) of the oil filter, pump out the three gallons of oil and then change the filter.

Baker 05-14-2013 01:17 AM

My point here is that just because the coolant temp is 180 does not mean the engine is at operating temp. I would have a tendency to believe that oil temp is a better indication of "operating temp" than coolant temp. I have discussed this with other folks and they were having a hard time believing that the coolant temp could be 180 while the oil temp is not above 100....

THD 05-14-2013 02:19 AM

When I really think about it, RickB's explanation does make a lost of sense. If a 4 stroke is running at 1,800 RPMs (I picked 1,800 to make the math easier!), each cylinder fires 900 times a minute or 15 times a second. At 1,500 RPM, each cylinder fires 750 times a minute or 12.5 times a second. Even though that is a 16.7% decrease in the # of firings, it is only over a one second period. That does not seem to be enough to have any effect whatsoever on the firing chamber temperature. Perhaps that is why you don't see an actual "Engine Cylinder Temperature Gauge". In any event, that temperature seems somewhat irrelevant to me as that is indeed a constant temperature. The important trick is to maintain the metal temperatures around the firing chambers, i.e. the block and head, at temperatures that allow everything to function correctly, no (or controlled) heat expansion, etc. Thus the calculation by manufacturers that at 180 degrees, coolant is removing enough heat from the combustion that it does not adversely affect the engine operation. The thermostat increases or decreases coolant flow to maintain that temperature.

FF 05-14-2013 06:54 AM

The problem of underloading is not combustion chamber temperature.

It is combustion chamber pressure . With no load the piston does not keep the pressure up long enough most of the fuel to burn.

The only fuel that does burn is the fuel VAPOR , the excess fuel (that might have been vaporized and burned if held longer ) is blown out the exhaust , or as blowby to dilute the lube oil.

The unburned fuel in the exhaust is the cause of wet stacking (dry stack) or a fuel slick in the wet exhaust discharge.

On OTR trucks where the stack temperature is higher from more frequent high loads ,you will see black smoke (Bucky Balls) on acceleration , another over fuel condition.

AS the piston rings seal by pressure behind the rings , lo load allows the combustion gasses into the oil, and the light pressure burnishes the cylinder walls, removing the honing that controls oil on the cylinder walls.

More modern engines will use 3 or 4 shots from the injection system , in the hope of vaporizing more of the fuel, making a more efficient operating engine.

Warm , cool , not much difference , look at all the old Euro diesels that run at 140F or below.
BUT no load is a poor place to operate or even cruise.

This is why "start her up once a month" in storage is a rotten idea , and why some folks will chose to match the engine RPM and CRUISING loads to the propeller .

RickB 05-14-2013 10:33 AM


Originally Posted by FF (Post 156295)
The problem of underloading is not combustion chamber temperature.

You are right - when that term is used to describe the operation of propulsion engines it is almost universally a mythical conditon.


It is combustion chamber pressure . With no load the piston does not keep the pressure up long enough most of the fuel to burn.
Temperature, or more accurately, the amount of heat available to raise the temperature of the fuel (along with fuel quality itself) is the major factor that determines combustion quality.

Even during the phase of combustion called "afterburn" which occurs after injection is finished, and occurs only over a very small crank angle, the pressure is still well above the pressure at which injection began and the charge temperature is very much higher than at that point. The pressure is for all intents and purposes, irrelevant.


The only fuel that does burn is the fuel VAPOR , the excess fuel (that might have been vaporized and burned if held longer) is blown out the exhaust , or as blowby to dilute the lube oil.
That is a truism that applies at all times and all loads. However, if your connection to time was accurate, an engine driving a fixed pitch propeller at low rpm or even idling at minimum rpm provides the longest period possible for all the fuel to burn. It can't be "held" much longer


The unburned fuel in the exhaust is the cause of wet stacking (dry stack) or a fuel slick in the wet exhaust discharge.
Wet Stacking is an incredibly rare condition. How many readers have ever seen this phenomenon at all much less on their boats?

Wet stacking, when it does occur, is almost universally associated with generator engines running at a high constant speed with virtually no load, or unloaded (idling) stationary diesels in cold atmospheric conditions. Or clapped out Detroit 2-stokes. A cold stack will allow moisture in the exhaust (about a pound of water for every pound of fuel burned) to condense and wash soot and condensed hydrocarbons from the walls of the stack and eventually leak out to frighten the uninformed engine operator with visions of impending catastrophe.


AS the piston rings seal by pressure behind the rings , lo load allows the combustion gasses into the oil, and the light pressure burnishes the cylinder walls, removing the honing that controls oil on the cylinder walls.
This is another exaggeration promoted to mythical status. Unless you idle your engine for 8 hours a day the likelihood of this condition occuring is hardly worth thinking about.

Low load is a (no pun intended) loaded concept. The published operating load range for virtually any diesel engine shows a turn-down ratio of about 5 to 1 or better. That means the manufacturer allows continuous operation (below a certain maximum power output) down to about 20 percent of maximum rated power.

If you look at the propeller curve you will see that operating outside the low end of the rating curve would mean your boat is probably moving through the water at less than walking speed. Other than approaching your dock or mooring, how often and how long do you operate that low range?


This is why "start her up once a month" in storage is a rotten idea.
Idling a propulsion engine at the dock is not a good idea. Especially on those engines that use raw water cooled oil coolers. The only reason recreational boat engines use them is because they are cheaper to buy and install. A jacket water cooled oil cooler is far better for keeping the oil at the temperature is should be for viscosity and removal of contaminants.


...and why some folks will chose to match the engine RPM and CRUISING loads to the propeller .
I have no idea of what that means. The cruising load matches the rpm of a fixed pitch propeller all by itself ... it doesn't need any help from us.

Capthead 05-14-2013 12:41 PM

It seems then that to completely burn everything in the combustion chamber you need a larger fireball. If I understand this correctly. That means idling with a smaller fireball leaves carbon behind and running under a load which produces a larger fireball cleans out the carbon.

If that is correct, temperature isn't the issue as all combustion burns at the same temperature.

RickB 05-14-2013 12:56 PM

Yes, that is a pretty good way to look at it. You need enough heat (a large mass of material such as charge air) at a high enough temperature to heat the cold fuel to vaporize it and raise it to its autoignition temperature. Insufficient heat will only char some droplets to make soot and some might go out the stack as unburned fuel (white smoke) or be condensed in a wet exhasut system to form a sheen on the water.

One reason a mechanical injection engine smokes white and makes a sheen is because the engine starts at (or even beyond) maximum fuel delivery. More fuel is injected than it takes to make full power. There is no way on Earth all that ice cold fuel can be heated to ignition temperature in an equally cold engine so a lot of it goes out as white smoke and much is condensed on the cold exhaust pipe.

Since the engine doesn't come up to speed quickly, the governor keeps injecting a high volume of fuel into the cylinder to try to speed it up and that just adds more smoke and sheen to the situation. Once the engine reaches governed speed and warms up the problem goes away.

Idling with a tiny little fireball also means there is unburned fuel scattered around the edges where the mixture is too cool to vaporize or too lean to burn. A mechanical injection system doesn't atomize the fuel all that well at low rpm anyway so that means there are large droplets flying around that turn to carbon before they vaporize completely or are ignited.

Marin 05-14-2013 03:25 PM

Great explanations, Rick, thanks much.

Capthead 05-14-2013 03:35 PM


Originally Posted by Marin (Post 156390)
Great explanations, Rick, thanks much.

Ditto!! Thanks Rick

moreton99 05-14-2013 04:48 PM

What a great forum. I have been churning over this question for a while. I have a 300 hp turbo older cummins that I run at low revs sometimes for a more economical cruise a 7.5 knots. After I do this I get a soot buildup that I clear but running her hard for 5 mins every 10 hours. From what I know that's ok but I was was still concerned about glazing/ soot in the oil. Is the fact that my oil is staying clean a indicator that all is ok?

On another note I have heard that they preheat the intake air on engines on oil rig support ships that do a lot of iding to help with the combustion. Could this reduce soot and get a better burn at lower revs?

RickB 05-14-2013 05:10 PM


Originally Posted by moreton99 (Post 156420)
Is the fact that my oil is staying clean a indicator that all is ok?

That is an excellent indicator of cylinder health.


On another note I have heard that they preheat the intake air on engines on oil rig support ships that do a lot of iding to help with the combustion. Could this reduce soot and get a better burn at lower revs?
The only application I am familiar with is using preheated jacket water in the charge air cooler so it acts as a heater during cold weather startup. Once the engine is running the turbos will heat the charge air very effectively. This is sometimes done on large medium speed engines that burn heavy oil operating in arctic conditions.

Giggitoni 05-14-2013 06:28 PM

Rick, thanks for taking time to explain the above. I appreciate it!

Peter B 05-15-2013 12:06 AM

Yes, thanks for enlightening us on that Rick, but you have sort of walked into this one, so as it's on everyone's lips, I'll ask it. Because I don't think, after all the times it has been discussed, we ever arrived at a definitive answer, even though we know even the definitive answer will still be a compromise of sorts, because clearly using the boat is the best way to keep it all good. However, accepting the reality many of us cannot get out for a proper run, what is the best thing to do when we go down to the boat to check it out, and feel that huge urge to do something because instinct says leaving the engine just sitting for maybe weeks or even months unused is not good. Do we....
1. Leave the damn thing alone...?
2. Turn it over without firing up for several revolutions to change position of parts and open and close different valves, & circulate a wee bit of oil...?
3. Run it under no load several minutes..?
4. Run it under light load for a few minutes..?
5. Run it under light load, (idle in gear, sort of thing), to operating temp..?
5a. Peter, you left out a choice. And that is to run the engines in gear under moderate load (1,200 rpm) until the coolant temperatures get to their normal reading at which point the engines continue to be run at that power and load for at least 30 minutes. This is what we do if we are unable to take the boat out for four to six weeks due to weather, my travel schedule, etc.
6. If none of the above...WTFESWD...translate that yourselves...

Hey, THD, don't distract him until he answers my questions first - get into line...and ok, Marin, I thought I had your technique covered in #5, but it's not quite the same, so I make that 5a...

THD 05-15-2013 01:15 AM

Rick-this all leads me to another question-on fuel usage. Does the injection system inject a set amount of fuel on each firing cycle? Thus, the difference in fuel usage derives from the increased firing cycles at higher RPMs. Or, does the injection system, also in some manner I can't figure out, sense "load" and inject increased amounts of fuel on each firing cycle if the engine is "working" harder? If so, how does the injection system sense the engine load?

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