Engine cooling theory?

[pwilletts]

Lets say I have small diesel(1 cyl) using raw water cooling, that can be run with or without the thermostat...(using the thermostat there is a by pass to allow water to pass through until the thermostat opens)...will I get better cooling of the head with the relatively quick flow of water through the head( w/o the thermostat)...OR..will the water held 'captive ' by the smaller flow through the thermostat act as a heat sink and infact remove more heat?? yes I know diesels like hot..

[Eigenvector]

You can't really easily answer this question. It could be the same net effect, with or without, high flow / low flow.


The highest heat transfer rate (energy) occurs with the biggest timperature difference. That being said, it's nolinear and asymptotic as the two temperature approach equal.


Since the energy of the media (water) is a function of temperature and specific heat capacity, there is a solution where the two flow rates result in the same heat rejection rate (energy) but have different exit temperatures.


The thermostat is simply a flow regulator that seeks a desired temperature.

[a64pilot]

In racing the thermostat is a failure point. Of course you try to eliminate all the failure points possible, you don’t want to DNF over something simple and preventative, so of course they were left out.
Well once racing got sophisticated enough so that most things were monitored, it was discovered that even tough the coolant temp was lower, actual Cyl Head temps were higher, so a restrictor washer was fitted that matched the flow rate of the thermostat and cyl Head temps cooled down.
These same people realized that all US manufactured engines coolant was run through the engines backwards of what made sense, that it through the cylinder and out the head and they started running the coolant in reverse, letting the hottest part, the cylinder head get the coolest coolant.
GM at least adopted this in their V8 small block in the early 90’s I think.
So the Redneck racers are sometimes smarter than we give them credit for.

When you go modifying things from design, sometimes you get unintended results. Keep the thermostat

[jamhass]

Several separable issue here.

1. Is this an older engine -- one probably with restricted cooling passages, and or one with overheating issues? If so then, yes, remove the TStat, as it mainly adds another restriction into the system.

2. Newer engine -- almost certainly best to leave it in. The TStat keeps the temp roughly constant under varying loads, which keeps all the engine parts the same (design) temperature with correct clearances, etc etc.

3. Do you run the engine a LOT at lower power? If so, then see #2.

[Cheechako]

Any restriction to flow will reduce the ability to take heat out. So in theory the T-stat is a restriction... even when open. In reality, once open, it doesnt matter much unless you have a marginally designed system. Most auto fresh cooled engines will reach their temp eventually with or without. Marine engines... some will not reach temp.... and raw cooled ones often will not reach temp without a T stat..

[billknny]

Originally Posted by pwilletts

Lets say I have small diesel(1 cyl) using raw water cooling, that can be run with or without the thermostat...(using the thermostat there is a by pass to allow water to pass through until the thermostat opens)...will I get better cooling of the head with the relatively quick flow of water through the head( w/o the thermostat)...OR..will the water held 'captive ' by the smaller flow through the thermostat act as a heat sink and infact remove more heat?? yes I know diesels like hot..

The theory is simple: Assuming the passages are always full, slower water flow will NEVER give better cooling than faster water flow.

[svlamorocha]

Originally Posted by billknny

The theory is simple: Assuming the passages are always full, slower water flow will NEVER give better cooling than faster water flow.

+1 Thermodynamics 101

[a64pilot]

Originally Posted by billknny

The theory is simple: Assuming the passages are always full, slower water flow will NEVER give better cooling than faster water flow.

And yet there is the race cars experience?
Perhaps due to turbulence or maybe even cavitation at high RPM, who knows, but they did run hotter unrestricted.

[hellosailor]

Rashly assuming the engine was competently designed (because some marine installations are just clapped together and can be improved) then the cooling system has been designed to take raw seawater form outside the boat, in sufficient quantity and speed, that it will cool the engine sufficiently during all normal operation. From idling, up to full throttle against storm winds and seas.

Without a thermostat, you CANNOT run an engine at any fixed optimum temperature. In fact many old Peugeot cars and some commercial trucks often had a cardboard or zippered cloth cover placed in front of their radiators because otherwise, they would never warm up in the winter. The cooling system design was simply inadequate.

The bypass system (not the same as what cars use, they simply close off the flow until they are warm) allows for raw water to cool the engine AND the exhaust. A "wet exhaust" being necessary to the usual design on small craft. Otherwise they would have large runs of dangerously hot exhaust piping installed in them.

When salt water is used to cool an engine, salts crystallize out around 140F and then form deposits that block the cooling passages, killing the engine. But with fresh water, you can run the engine with a 170F thermostat, and the additional temperature rise allows the engine to run with more power and efficiency. Thermostats in cars typically are around 170-180F since they used closed cooling systems. Some cars have run up to 210F on stock engines for even higher power.

So for your boat engine? Yes, you NEED the thermostat, or else the engine will always be running cold and inefficient, unless there's some other problem. And you need the PROPER thermostat, 140 or 170 or whatever the maker specifies.

Thermostats are mechanically simple and ingenious things, but they can come DOA out of the box. So when you get a new one, drop it in a pot of water and look to make sure it actually does open as the water starts to bubble then boil. And that it closes when the water cools. That's much easier than replacing a thermostat and gasket if the new one is defective.

[FPNC]

The water flow restriction offered by the thermostat is not so much a benefit to picking up heat into the coolant as much as it is a benefit to removing heat from the coolant. The hot coolant needs to spend the correct amount of time in the heat exchanger/radiator in order to not have hot water rushing in and hot water rushing right back out. It takes time to remove heat.

Leave the thermostat in place. In an emergency situation a stuck thermostat can be "popped" to stay open and installed until the new one can be obtained. Back in the day, popping a stock thermostat was common for use in a race engine.

[hellosailor]

Let me throw one out there. In auxiliary transmission and oil coolers, which are still "radiators" when all is said and done, there is often talk of creating a "spin" or turbulence in the fluid flow, so that more fluid contacts the radiating surfaces and there is greater heat transmission, resulting in more cooling. If you remove a thermostat, I could see that might increase flow speed AND decrease the turbulence that the physical "interruption" of the thermostat causes. By creating a more laminar flow, you are keeping one layer of water at the outside of the cooling system, and then trapping the hotter inner "core" of the fluid, away from the radiating surfaces.
Yes, that stuff can be rocket science. It does not always conform to what apparent simple logic might lead you to expect, but there are rules for it, and the real engineers know them.

[leftbrainstuff]

Originally Posted by pwilletts

Lets say I have small diesel(1 cyl) using raw water cooling, that can be run with or without the thermostat...(using the thermostat there is a by pass to allow water to pass through until the thermostat opens)...will I get better cooling of the head with the relatively quick flow of water through the head( w/o the thermostat)...OR..will the water held 'captive ' by the smaller flow through the thermostat act as a heat sink and infact remove more heat?? yes I know diesels like hot..

Air to water and water to water heat exchangers are well understood in engineering.

Some turbulence is good as it maximizes heat transfer in most fluids including air and water.

But cavitation will radically decrease heat transfer. As you are now trying to transfer heat through vapor which has a much poorer thermal performance.

Flow rate is also key to maximizing heat transfer. Too slow and local boiling will occur. Too fast and heat won't be transferred quickly enough.

In most motorsport applications we reverse the flow of cooling water to cool the heads first. This usually require more complex bleeds to ensure air pockets don't form. We will also usually replace the thermostat with a fixed size orifice. This removes the potential of having to deal with thermostat failures.

By measuring water, oil and surface temperatures around the circuit it is trivial to evaluate changes and to troubleshoot.

Note that engines don't like to run hot or cold. The ideal running temperature, for any engine, is where the internal clearances are optimized for maximum sealing (cylinders, valves) and for minimum friction. Power at flywheel, crankcase blowby, oil temperature, ignition and burn rate and specific fuel consumption are key measures. The manufacturer determines this. Note that the boiling point of the coolant is an upper limit.

Note that the thermostat allows for fast warmup until the wax pellet melts and the thermostat opens. This minimizes engine wear. The thermostat acts as a variable orifice for flow control. It is not a heat sink.