Rationale for coolant system temps

[SBBlue]

What exactly is the rationale for a particular coolant system temperature range?

Granted, most IC engines are designed to run at 190-210 deg F. But why? I can certainly understand the problems if you tried to run an engine at 400 deg F, but why not 250 deg F? On the other hand, why not set the system to run at 160 deg F?

I'm very aware that diesel engine manufacturers are quite concerned about engine temps, to the point of derating engines when the temperature reaches 220 deg F and starting an automatic shutdown sequence for a little bit higher temps.

But what happens if you run the engine at 240 deg F? Is it abnormal wear? Or is the engine at risk for seizing up?

I would appreciate some insight into this!!!

 

[IRstuff]

One simple answer is that the temperature you're referring to is the coolant temperature and the most effective coolant is still water and water boils. While antifreeze and pressurization changes that somewhat, the upper limit must also allow for a certain degree of fault tolerance.

On the other hand, an engine that's too cool steals power from the combustion cycle.

TTFN

 

[TheBlacksmith]

Also bear in mind that the temperature you see is the bulk temperature, localized temperatures adjacent to the cylinder walls, etc. may be higher, cutting into the margin mentioned above; 50/50 ethylene glycol and water pressurized to 13 psi boils at like 249 degrees.

Blacksmith

 

[tbuelna]

SBBlue:

During WWII, some of the piston aircraft engine manufacturers investigated using very high coolant temperatures. The idea was to increase the coolant temperature and thus allow a reduction in heat exchanger size.

I also read recently that the F1 engine guys had tried doing the same thing for the same reason. The FIA now has a maximum cooling system pressure rule, apparently for safety reasons.

Regards,
Terry

 

[patdaly]

Unless I am wrong, they already are. The Corvette, I believe runs above 210 F, and I know on the LT1 used in the Caprice, the primary cooling fan does not turn on until the coolant temperature hits 225 F, secondary fan comes on at 231 F.

 

[patprimmer]

I agree the main driving force to limit coolantant temps are the economy and avaliability of water and safety

It is dangerous enough to remove the cap at 15 psi with water. Higher pressures are even more dangerous, and create more problems with hose life cost, water pump seals etc.

Hot oil is much more dangerous, and more expensive and a less efficient heat transfer medium.

Very hot oil or glycol also tends to rot hoses.

All these problems could be overcome, but at what cost for what gain.

The only gains are reduced radiator size, leading to improved aerodynamics, and improved thermal efficiency.

Another downside is increased under hood/bonnet temps, which can impact on volumetric efficiency and underbonnet component material choice and therefore costs for electrical insulation, hoses, belts, bottles, covers, manifolds, rockerboxes, etc.

Conversly, too cool decreases thermal efficiency and requires exceptionally large radiators.

The temperature has to be decided at design stage, as it effects many other decisions as indicated above, and also determines many clearances in the engine components.

Another limiting factor is that the engine must operate satisfactorily dureing it's warm up, so clearances wide enough to avoid seizing at the maximum temperature, still need to be tight enough to avoid excessive noise and damage at cold start

Regards
pat

 

[enginesrus]

One is like someone has said above. Other components with in, will be at more elevated temperatures if the coolant temp is allowed to go very high. And other than the water boiling issue. Is Oil. Especially before the synthetics. Oil film breakdown, additive package breakdown, and coking, to name a few.
But there have been IC Recip engines made to run at very high temps. Some years back Cummins was experimenting with an Adiabatic engine.

 

[BillyShope]

On a historical note, I recall reading that some early engines took advantage of the heat of vaporization. In other words, the coolant passages were large enough to carry water vapor.

 

[patprimmer]

Very old cars used thermocycle instead of a waterpump. This called for large passages and a continuous upward passage from the bottom tank, through the engine to the top tank

Regards
pat

 

[patprimmer]

I clicked to soon. Also meant to say that they were not pressurised and on steep slopes, the water boiled away happily, with steam being generated in the head, discharging out the vent just below the cap

Regards
pat

 

[overmod]

I know of some experiments with GM 6.5TD diesel engines where lower thermostat setpoints were used to counteract higher EGTs. There are marine versions of this engine that have 160-degree thermostats, and several people installed these thermostats in trucks.

I was told this was a bad idea because of engine oil. Apparently, the oil temp is substantially affected by the coolant temp (in several ways) and if it doesn't get hot enough, any condensate in the crankcase doesn't leave the oil. Water CAN get into oil, especially detergent oil, and can corrode all sorts of things...

Final rule: 180 degree stats instead of 210. Better at avoiding coking, etc. as previously discussed, and some reduction of the effect of high EGTs on things like oil sprayed on the undersides of the pistons to cool them. The decreased 'thermal efficiency' of the engine wasn't a major factor because exhaust temp was already too high, and that was limiting engine cycle efficiency...

RME

 

[evelrod]

To start, I believe that higher operating temps in street engines are for emission control reasons primarily and I am not into street engines. However, many race engines where thermal efficiency (as well as aerodynamic drag) is paramount, NASCAR in particular, run water temps ~240f and higher as a matter of course. This requires a very high pressure system as EG and PG coolants are either frowned on or outright banned (SCCA ,eg.) In order to keep other temps under control, additional heat exchangers are used (oil, trans, diff, etc.) and proper cool air ducting and heat insulation of the intake system are used.
On my Mini Cooper vintage race engine I shoot for ~220f max coolant (water and Red Line "Water Wetter&quot and ~240f oil temp. So far, so good!
I tried using 100% PG on my old Model A Ford a few years back and it seemed to work, it never boiled off, but coolant temps tended to go as high as 300f and I don't have an oil pump, much less an oil cooler. A note by the mfgr of the PG coolant is that it is NOT reccommended for a pressure system in 100% concentrations. I switched back to the same coolant as the race engines in the Ford before I melted the bearings or something else equally stupid!

Rod

 

[turboice]

The primary reason that I am aware of is the issue of vaporization at the cylinder wall. Coolant reaches its highest temperatures quite a distance from where the temperatures are usually measured. If the coolant vaporizes at the cylinder wall there is a spot that is not having heat absorbed since the heat capacity of the coolant at that spot has been sapped. This localized occurrence can be very small in size but the hot spot it leaves within the cylinder contributes to knock.

The different temperature ratings of different cars are likely based on the location of the sensor. I am sure they are all designed that given an average coolant mixture and the OEM system pressure that the coolant would be unlikely to reach over say 230*F at the cylinder wall.

There are waterless coolants that do not boil/vaporize even in a pressureless system. These have been successfully run in engines to 300*F coolant temps at the sensor - with no knock (i.e. no hotspots) and improved efficiency, especially on diesels.

Realistically if a theorectical coolant being used could still absorb heat at a sufficient rate at the cylinder wall this theorectical coolant could run at temps as high as 350*F.

It is the limitation of heat absorption if the coolant vaporizes at the cylinder wall that results in engines running coolant temperature below what would truly be ideal for the operation of the engine.

Ed.

 

[turboice]

The above was meant to expand on what theblacksmith was saying.