Radiator Cap Question

[97windsor]

Originally Posted by spike45

Mike,

I know I come onto this forum and start rebutting a lot of statements I am reading and that may rub some the wrong way. Please bear with me as I have 40 years in the diesel service industry to draw on. I am only too happy to share my knowledge. I am currently a service engineer and cooling system specialist with Cummins (Cummins Filtration business unit). While formation of steam bubbles can happen, it does not generally cause cavitation pitting. Coolants have defoaming agents to suppress formation of steam or air bubbles.

Engine Cooling System 101: Engines are designed to cool in the high temperature regions by the process of nucleate boiling heat transfer. The steam bubble action you ascribe as the cause is a process called film boiling. Film boiling only occurs in engines when coolant flow is interrupted as in stuck thermostat or water pump drive is disabled causing the impeller to discontinue coolant flow.

Nucleate boiling occurs primarily on the coolant passage surfaces of the cylinder heads adjacent to the exhaust valves. There, the metal surface temperatures at maximum engine load are in the vicinity of 800 degrees F. While the coolant never actually sees that temp, it does have a contact surface boiling forming tiny bubbles that are removing the heat flux at the maximum rate per the glycol/water mix ratio and system pressure. Nucleate boiling is something most of us have seen. Put a pan of water on the stove and turn on the flame! When you see the tiny bubbles on the bottom of the pan form but the liquid volume above the pan bottom is not showing any rising bubbles the coolant (water) is removing the heat from the nucleate bubbles thus transferring the maximum heat from the pan surface. If you had a temp gauge on the pan surface, you would see that the temperature has leveled out. It will not begin to rise further until the water bulk volume temperature has reached the boiling point for that atmospheric pressure. Then the pan/water interface experiences the change from nucleate to 'film" boiling. The 'film' meaning the interface has a film of steam and no liquid in contact with the pan. The coolant bulk volume not having sufficient cooling by evaporation on the air surface then climbs to boil temps to achieve evaporation cooling. Meanwhile the pan temp keeps climbing.

So, for those who are able to follow my explanations, keeping the cap pressure at OEM specs and keeping a cap that can actually hold that pressure is paramount to engine protection against a "melt down".

What can you do to increase the boil point of the coolant safely? Do not run minimum glycol levels like 40% antifreeze/60% water. Do not exceed 60% glycol content as concentration above that takes away from convective heat transfer (cooler liquids on hot surfaces) that the engine needs when not operating at maximum heat rejection loads (engine full low). Do not remove the pressure cap to look into the system unnecessarily as it releases pressure build up. Coolant pressure builds by the expansion of the coolant volume due to normal heating. Coolant can only expand so much and further expansion is limited. Check the level when the coolant is cold or cool. If you have a plastic surge tank, coolant level is best observed through the translucent walls. If your surge tank is metal, a "level window" is very beneficial.

Thanks Spike for taking the time to explain this using YOUR 40 yrs of knowledge in the actual field this thread discusses, and not "a friend told me so that must be correct and taken as gospel.
I still think the OEM spec is the proper info to use when replacement parts are needed, weather it is a filter, rad cap, or any other part that someone is changing. Thanks again

[spike45]

I have read the rhetoric from Final Charge on this forum as well as the Powertrain, Cummins Forum.

Final Charge is an excellent OAT coolant. But to tell customers that story about better able to cool because there are no coatings like conventional coolants have is pure BALONEY. Our company (Fleetguard) used Final Charge as a base product for our ES Optimax OAT coolant with nitrite. I know that Final Charge has actually been tested under the CAT EC-1 coolant spec including the organized field test (big $$). It passed. Conventional chemistry (non-OAT) coolants leave a microscopic passivating film of a few microns thick. A micron is one millionth of a meter or 39 millions of an inch. No real film to impede heat transfer. Not like scale formation in engines from hard water where you are dealing with 1/32" or more that can cause great heat issues.

[spike45]

Originally Posted by Diplomat Don

If you're running OAT Coolant......you don't even need a cap!

Don,

Are you serious? Check your O&M Guide for the engine.

[spike45]

Originally Posted by 97windsor

"Turbocharged engines operating at full load as on hill climbing can achieve an exhaust temperature of 1050*F. Cruising should yield exhaust temps of around 850 -900F. " This is a post from spike, from another discussion.

Mike says the difference in the coolant boil point between a 7# cap and a 13# cap is approx 10-15 degrees F depending on the altitude.

My question is this. With the cyl temps in the 850-900F at cruise and increase approx 150-200 degrees F at full load. Will a 10-15 degree difference in the boiling point of the coolant really matter?

Each pound of of pressure raises the boiling point of coolant by 2.7*F, round it off to 3*F. Premix 50/50 coolants have a sea level boil point of 226*F. Add in the additional 35 degrees from system pressure cap and the coolant boiling temperature is 261F.

The average coolant outlet temperature of an engine with a 192F thermostat and pulling a hill with a Toad, will be in the vicinity of 210 - 215F for a Cummins ISL and similar. While exhaust gas temps are much higher, the coolant does not experience that directly, not if it is flowing, has the right glycol/water ratio and the correct cap. No coolant temp gauge reads that temp but if you had a temp gauge embedded in the iron adjacent to the exhaust valve coolant passage, you would see those temps. No matter though. The engine is operating normally and that temp is not significant as a bit of knowledge you need. The coolant temp gauge is a better measure of the overall condition of the cooling system operation modes.

[Mike Canter]

Thanks Gary. The big question I have is why does everyone want to change their radiator cap pressure from the specified OEM pressure to a lower pressure like 7 lbs. I cannot understand the logic behind doing it. There has to be a reason why people are doing it.

[spike45]

Pitting failures on Detroit 71, 92 series hardly existed. Those engines operated at a lower BMEP than more current engines. Use of SCA in those was more to as a corrosion protection that antifreeze products of that era did not have.

[Dutch Star Don]

spike45.....No...I was not serious....Before you entered the thread, it had gone sideways and a dead horse was beaten until all there was was dust .....nothing to do with your comments.

[bullheaded]

Think about why all the NASCAR engine builders are using HIGH PRESSURE cooling systems now. More tape, more downforce, less air...no problem.

[spike45]

97 Windsor,

Steam does NOT cause cavitation pitting failures. A little knowledge is dangerous. If steam caused cavitation failures then any diesel engine regardless of SCA level and a non functioning pressure cap would have a pitting failure.

Only the upper two inches of liner have very elevated temperatures. Coolant circulation by design moves a lot of coolant past the liners. The coolant comes into the base of the liners or bottom area of a parent bore design. The cool coolant moves upward and into the cylinder head. That coolant does not come close to boiling unless the flow is restricted somewhere like a stuck thermostat, water pump impeller loose on the shaft.

If steam caused pitting failures why call the process 'cavitation'?

[walt2137]

Gary thanks for the informative explanation.

[spike45]

Originally Posted by Madrona

I've read most of the comments in this thread, understanding about half of them. Just to clarify for this senior mind, if you use the correct cap, the one designated by Cummins or whoever your engine manufacturer is, you'll be in good shape, right?

Using the correct cap will raise boil point, improve heat transfer due to boiling heat transfer design of the cooling system, suppress air and steam bubble growth. Steam bubbles formed during boiling heat transfer in the cylinder heads of the engine are quickly suppressed by the aid of a functioning pressure cap and the coolant flow volume.

So, you are in good shape for one aspect of engine cooling system maintenance.

The total coolant maintenance calls for
* a system filled with a coolant that meets your engine manufacturer's requirements
* maintaining the chemical additive level of your cooling system if it calls for the addition of chemical liquids or coolant filters with chemicals for the prevention of pitting
* always refilling low coolant levels with the correct specification coolant when the system is low.
* never refilling low coolant levels with water or mixed water/antifreeze without a supplemental coolant additive, SCA such as DCA4, Pencool 2000 to name a couple
* testing the coolant SCA level with test strips or lab testing yearly

[Mike Canter]

Now you are confusing me because I have talked to a diesel engineer from a major diesel manufacturing company and he will disagree with you and you have also said it both ways in your posts. If the motor is under extreme load such as climbing a steep grade for a long time then the coolant around the liner gets very hot and with the correct coolant mix and the correct pressure cap the coolant will not boil around the hot part of the liners but if the pressure cap rating is reduced to below the OEM requirement then you will have Nucleate boiling especially if the motorhome is being operated at higher altitudes. As you have stated this Nucleate boiling will cause steam bubbles to be generated against the liners and according to the diesel design engineer the sudden collapse of the bubble can cause pitting on the liners.

[spike45]

Originally Posted by Mike Canter

Thanks Gary. The big question I have is why does everyone want to change their radiator cap pressure from the specified OEM pressure to a lower pressure like 7 lbs. I cannot understand the logic behind doing it. There has to be a reason why people are doing it.

Mike,

I think this got started because the plastic (actually it is Nylon 6) coolant surge tank fittings are 'spitting' a little coolant pressure. Lowering cap pressure would reduce that effect. I have noted that some have said they have changed that tank more than once. Nylon 6 has a drawback in that it hydrolyzes, absorbs water, and becomes brittle, cracking. On other forums along this line that topic has come up on the surge tank failures.

There is a company called Evans Cooling. They promote non-aqueours (waterless) PG as a coolant. The idea is that by running a super high boiling point coolant, the engine is more fuel efficient and removes the need for a pressure cap to raise the boil point. No engine manufacturer endorses that off the wall thinking. Still, they have their following.

Lowering cap pressure lowers coolant boil temp thus allowing film boiling to more easily occur thus major heat transfer would be stopped. There is no heat transfer from the engine under film boiling conditions.

[Mike Canter]

So is Nucleate boiling or film boiling because they are both caused when the liner is hotter than the coolant?

We used Evans coolant is a race car but didn't like it.