Do I need a thermostat in Fiji?

[Muckle Flugga]

Originally Posted by Alan/Rhapsody

We have a Perkins 4-107 and the engine overheated. The local mechanic tested the thermostat and said it was no good. I have heard that in warm tropical water one can get away without a thermostat. Is this OK? . Water temp in the Marina is 81 F. Can't find a replacement locally, so can I do without at least temporarily ? I will be ordering a new one, but it will take awhile.

Do not worry about the thermo. At least temporarily you will not need it in Fiji. It is far from a critical issue though you will want to replace eventually if bad. Leftbrain's stopgap should work nicely. But you can easily test it yourself. The rating will be stamped on it or stated in the manual. Just put it in a pot of water, and slowly bring the temperature up, monitoring the water temp with an adequate thermometer, until you see it opening. Note the temp at opening and compare to the rated temp. This will let you know if the mechanic is full of it or not. Sometimes pulling a thermo can mask a more serious issue! Test it yourself.

[Alan/Rhapsody]

I tested the thermostat and it appears to be ok. So I will put it back and test it.
I am also ordering a new one from. New Zealand.
Thanks

[River Cruiser]

Originally Posted by FSMike

How would the lack of a thermostat cause an engine to overheat?

If the coolant moves thru the engine to quickly it doesn't remove as much heat so the engine will have hot spots. The temperature gauge will read the coolant temp and not indicate a problem but the iron in the area of the combustion chamber will be much hotter leading to a shorter life.


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[svlamorocha]

Originally Posted by a64pilot

The dirt track car racing guys use what looks like a washer where the thermostat goes, theory is the water in fact does not overheat, but the engine, specifically the heads do, based on the water moving too quickly through the engine if not obstructed.

That theory is BS.

If you increase the mass/volume flow of coolant (as when removing a thermostat) then the heat removed per mass/volume unit will go down a bit but not as much as the increase in mass/volume flow. In other words, if the water moves faster the engine will run cooler.

You can learn this in engineering school or just take the thermostat out and use a infrarred pyrometer to measure temperature in various places.

Another thing is that when an engine has been modified to increase HP without upgrading the cooling system then folks may find that removing the thermostat (to increase a lot the coolant flow) and installing a washer ( to limit that increase in order to prevent the engine from running too cold) is the only practical way to keep it at a reasonable temperature.

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[jimbunyard]

Unless you've somehow managed to upset the temporal order of the universe, this,

Originally Posted by svlamorocha

That theory is BS.

If you increase the mass/volume flow of coolant (as when removing a thermostat) then the heat removed per mass/volume unit will go down a bit but not as much as the increase in mass/volume flow. In other words, if the water moves faster the engine will run cooler.

You can learn this in engineering school or just take the thermostat out and use a infrarred pyrometer to measure temperature in various places.

Another thing is that when an engine has been modified to increase HP without upgrading the cooling system then folks may find that removing the thermostat (to increase a lot the coolant flow) and installing a washer ( to limit that increase in order to prevent the engine from running too cold) is the only practical way to keep it at a reasonable temperature.



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is BS.

Transfer of heat takes time, therefore the speed, or lack of speed, of a fluid through a system will have a direct, but not necessarily linear, effect on the rate of transfer in that system.

And of course that leaves out the hydrodynamic and mechanical variables involved in any complex, real world system...

[svlamorocha]

Originally Posted by jimbunyard

Unless you've somehow managed to upset the temporal order of the universe, this,




is BS.

Transfer of heat takes time, therefore the speed, or lack of speed, of a fluid through a system will have a direct, but not necessarily linear, effect on the rate of transfer in that system.

And of course that leaves out the hydrodynamic and mechanical variables involved in any complex, real world system...

Let me get your thoughts straight. Are you supporting that "theory" that says that if you insert a restriction on the water flow then the engine will cooler**?

I do not need to go back to my numerical modelling days, I can disprove that with a thermometer in the old Fiat I play around with...

[a64pilot]

I did say theory, because to be entirely truthful what I don't understand is since these guys are trying to extract every HP they can, why not underdrive the water pump, slow it down to decrease flow as opposed to restricting it, which would seem to waste power?

But a lot of what Billy Joe and Jim Bob do at the race track seems silly at first glance, but often when you dig into it, they have stumbled onto something that works, don't discount Redneck Engineering.
Look up Gurney flap as a perfect example of Redneck Engineering

https://en.wikipedia.org/wiki/Gurney_flap

[jimbunyard]

Originally Posted by svlamorocha

Let me get your thoughts straight. Are you supporting that "theory" that says that if you insert a restriction on the water flow then the engine will cooler**?

I do not need to go back to my numerical modelling days, I can disprove that with a thermometer in the old Fiat I play around with...

I am stating a fact;

"Transfer of heat takes time, therefore the speed, or lack of speed, of a fluid through a system will have a direct, but not necessarily linear, effect on the rate of transfer in that system."

And making a joke by asking (what I assume to be) a rhetorical question.

Have you managed to upset the temporal order of the universe?

All you have 'proved' in playing around with your Fiat is that your Fiat runs cooler without the t'stat, if that is in fact your implication.

Other heat transfer systems, such as the TAMD41 in my fishing boat, may be different.

Restrictions or lack of restrictions is a heat transfer system will have effects that can go either way; to make blanket assertions about what those effects will be is grade A prime BS.

[svlamorocha]

Originally Posted by jimbunyard

Restrictions or lack of restrictions is a heat transfer system will have effects that can go either way; to make blanket assertions about what those effects will be is grade A prime BS.

I am familiar with complex thermodynamics problems and I am yet to see a an example in which a restriction in the mass flow of a *single* stream of coolant reduces heat flow. One thing is that the the heat removed per unit of coolant mass (say delta T in coolant) goes down when you increase mass flow , but the reduction is smaller than the increase in mass flow. Therefore you have a nonlinear relationship but you know which way it goes. When you multiply mass flow (that increases a lot) by delta T (that decreases by a small percentage) you get higher heat flow for a given engine temperature, or lower engine temperature for the same heat flow.

The restrictor in the TAMD41 is not evidence against what I say above.
As far as I know the TAMD41 needs a restrictor in the turbo not to improve heat flow there, but to balance the mass flow in the two streams of coolant (oil cooler and turbo) that go through the same thermostat and coolant pump. Here you restrict cooling of the turbo to make the oil run cooler (at the expense of the turbo running a bit hotter)Did I get that wrong?

I believe that the Stewart Components folks are well placed to talk about engine cooling systems; here is what they have to say:

TECH TIP #3 - THERMOSTATS & RESTRICTORS

Thermostats & Restrictors
We strongly recommend NEVER using a restrictor: they decrease coolant flow and ultimately inhibit cooling.

See this link for detail on their explanation as to the origin of the myth that restricting coolant flow can make an engine run cooler.

[leftbrainstuff]

Originally Posted by jimbunyard

Theoretically, the water moves through the engine too fast to pick up all the heat required to cool it to the proper operating temperature. I've never seen it though...

+1. Very common in automotive and motorsport. Many modern powerplants will overheat in minutes. In many motorsport applications the coolant flow is often reversed to cool the heads first. Thermostats are removed and replaced by restrictors.

In older marine engines with partially fouled cooling passages you might not have any issues just by removing the thermostat. Watch your temp gauge. If it reaches steady state under load your fine. If it keeps climbing then you'll need a restrictor.

The thermostat when fully open provides a flow restriction that is designed to ensure coolant flow can extract the maximum heat. The heat exchanger is also sized to do the reverse.

Because we use raw water, which is more temperature consistent than air, a marine cooling system is more tolerant.



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[Dsanduril]

Flow restrictors don't work because they slow flow down, that is a myth (I could link lots of web sites, but let your Google-** go if you want to read more). Heat transfer is actually most efficient when the delta T between the two mediums is highest. However, as A64 noted, there is a reason and a method to redneck (or grandpa's) engineering. Flow restrictors are used to increase pressure in the cooling system. This has two positive effects:

1. Helps prevent cavitation of the water pump. Centrifugal pumps running at high RPM tend to cavitate at low pressures on the discharge side. In a racing engine that is frequently/usually run at high RPM the coolant pump is also running at high RPM, leading to a high cavitation potential.
2. Fluid under pressure boils at a higher temperature. Boiling coolant at hot spots in the engine has no ability to carry away heat. In fact, the steam generated from boiling turns to a gas, decreasing the available heat transfer surface area (the amount of your block in contact with liquid coolant becomes smaller). Increasing the pressure on water from 0 to 14 psi raises the boiling point from 212F to 245F (0 to 965hPa -> 100 to 118c). While your overall coolant temperature may be well below those numbers, the places where the heat transfer is actually occurring can be considerably hotter than the average coolant temperature as it comes through the thermostat.

How does that apply in a marine engine cooling system? In a raw water cooled engine it (for the most part) doesn't. Raw water cooled engines generally carry a flexible impeller positive displacement pump and run at almost no pressure. Cavitation is more an issue of inlet water restrictions than discharge pressure, and the engines are designed to run at a very low coolant pressure. Some restriction (equivalent to the fully open restriction of the thermostat) may be desirable to maintain the same pressure, but there isn't a lot of pressure there.

Engines cooled by heat exchanger, on the other hand, are much more like their dirt track brethren. Most of them run a secondary centrifugal pump for coolant circulation, and most of them run at some pressure.

Those are all big generalities. If you want generalities, then most raw-water cooled engines will probably have little heat problem (they will have problems from running too cool) running without a thermostat. Most heat-exchanger cooled engines will likely have some level of heat problem running without a thermostat because they will run at lower then design coolant pressure.

[donradcliffe]

The theoretical short answer is that reducing the restrictions and increasing the coolant flow rate will make the engine run cooler.

The long answer is that in the Dittius Boelter equation for enclosed turbulent heat transfer, the heat transfer coefficient is proportional to the the mass flow to the 0.8 power.

The Perkins 4107 is a simple engine design, and taking the thermostat out will make it run cooler, period.