oil temperature

oil has a very different viscosity to water, thus takes longer to warm.

Besides, the oil is used for lubrication, the coolant is for keeping the engine ( i.e. the metal ) at an acceptable temperature.

True, but..

No.

Viscosity is the internal friction of a fluid; the coefficient of viscosity is defined as (tangential stress) / (velocity gradient). It has nothing to do with heating times, although it is temperature-dependent.

The time a substance takes to heat depends on its heat capacity.

R.

Viscosity has nothing to do with how long something takes to warm up. The Specific Heat Capacity of oil is about half of that of water, so if anything it's easier!

So, following this further, after the engine oil does its lubricating, engine oil behaves like any other fluid doing work - it gets hot. How else, if not to the cooling system, does the oil lose its heat?

I suspect my oil temperature guage is either broken, or is just very slow to respond to changes in oil temperature.

Actually, thinking about how a modern car's thermostat work, the probable answer is that the coolant sensor is in the cylinder head, where the coolant is hottest, and the oil sensor is in the sump, where the oil is coolest.

Also, immediately after a cold start, parts of the coolant system are cut off by the thermostat, so there's less coolant to actually heat up. Whereas the oil system has no thermostat, hence it takes longer for all of it heat up.

I'm surprised my oil takes so long to warm up though. Is this generally true for diesel engines?

I see where you're coming from, but coolant and oil in an engine do different things. So, yes, oil keeps the engine cooler than water would, but fortunately, water is not expected to cling to bearings!

What I was refering to in my OP, is that in an engine one would initially expect an equilibrium between all components (metal, oil and coolant) such that they would all heat up approximately evenly. Thinking again about how an engine actually works, this is not necessarily true (see other post).

The coolant circulates around the cylinders where considerable combustion is taking place and where a very great deal of heat is produced.

Oil, on the other hand circulates mainly around the crank, with some going up into the valve gear and some being squirted into the cylinders, but below the pistons. None of these places get as hot so quickly as the cylinders so the oil takes longer to warm up than the coolant which is very close to a raging furnace. The oil that is squirted up into the cylinders is only a relatively small amount, so does not heat the rest of it up when it is dragged back down into the sump by the descending piston/rings.

The heat that the oil picks up is dissipated mainly through the sump - unless there's an oil cooler as well, in which case this also helps here.

As you've said oil has a lower Specific Heat than water, thus making it easier to heat, but this also makes it easier to cool, i.e. oil 'holds' less heat than water. I had a friend years ago whose engine had a cracked block and his remedy was to fill the cooling system with oil rather than water, so that any intermingling was not harmful. Ran it for years with no problems.

Rob Graham

keeping

lubricating,

The water system has a thermostat so that a small quantity of water in the engine and heater is circulated so that the water heats up fast in the engine, the rest of the water gets added in as the water temp. in the engine rises, untill eventually all the water is in use. While the engine oil has no thermostatic control at all, the only extra cooling for the oil being from the surface of the sump and heat transfer through the block, so all the oil is in use all the time, therefore the oil takes much longer to get up to a high temp.

There is the possibility that your car has an oil cooler, these are not usually thermostatically controlled so all the oil goes round the oil cooling radiator no matter whether it is hot or cold, all the time.

Oil always takes a long time to warm through in an engine. On a previous car of mine it took at least ten minutes to warm through properly - and that's being on the m'way in five after leaving work.

"Mark Craft" mumbled:

Cor, there's a non sequitur if I ever saw one!

Its viscosity has nothing to do with how long it takes to warm up. Thermal mass of the engine and specific heat capacity of the oil affects how long it takes to warm up. The viscosity will change with temperature and by how much determines the grade.

I've been wondering about the oil temperature. My Omega 2.5v6 has an oil cooler in a coolant tank between the two cylinder banks so presumably this will help the oil get up to temperature from cold. Is the oil likely to be up to temperature when the coolant is or would it still be someway behind?

If you're saying that the oil is immersed in coolant (OK, within its own container) then the temperature must be very similar to that of the coolant I would have thought, albeit a bit behind. I'm assuming that the relevant coolant tank receives its coolant flow direct from the engine, not after it has been through the main radiator.

However, I'm speculating. I'm not familiar with this layout.

Rob

snip

Yes yes you have it, also generally where the heat energy goes in an engine varies with RPM, at very slow crankshaft speeds a lot of waste heat energy from combustion goes into the coolant, at higher crank speeds more of it goes out with exhaust gases. The bearing friction which generates the heat energy in the oil also increases with crank speed and throttle opennong but balancing this is the fact that oil system dosen't have thermostat to control the temperature and the sump acts as a very effective cooler when ever the vehicle is moving through the air so the oil tends to be over cooled particularly on motorways. at a typical 70 mph.

It all underlines the importance of not using an oil which is too thick at low temperatures and that oil coolers should only be fitted complete with an oil thermostat. Ideally the oil temperature should be slightly above the normal water temperature.

As does pretty much every other fluid. Water's a bugger to get hot. :-)

R.

Water has the greatest specific heat of any substance. It equals 1.

Rob Graham

Water is about 4.2 joules per gram per degree C.

Helium is 5.2, Hydrogen 14.3, which both appear to be greater than water.

cheers, clive

Thanks Rob. I think the coolant around the oil cooler gets it's water directly from the engine, at least the pipe from the thermostat goes to the radiator. Having just typed that I've realised that I don't have any idea where the return goes :-)

Not so much spare/wasted heat to go around, but then again, there's the heat absorbed by the oil in the turbo bearings, and on turbocharged Peugeot XUD9 engines there are jets spraying oil onto the underside of the pistons to keep them cool.

/john

I've also just discovered that, apparently, some VW diesels have glow plugs in the engine coolant as well as in the cylinder head. If that's the case, then it's no wonder the coolant heats up first!