Oil vs. coolant temp & location of oil temp sensor
11-08-1999, 01:26 PM
#3
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Oil vs. coolant temp & location of oil temp sensor
A while back on this forum, people were discussing oil temperature versus coolant temperature. It seemed that the consensus was that people's coolant temperature increased much more rapidly than their oil temperature, and many asked if it was OK to drive their cars "hard" before the oil reached operating temperature, but after the coolant had.
This has led me to wonder: where exactly is the oil temperature sensor located on a 1.8T? I wonder because it seems improbable to me that coolant temperature could reach a toasty 190 deg. F before the oil. Oil has a much lower specific heat than water, and it runs through the hottest parts of the engine, being heated directly as it lubricates the bearings. Additionally, while I can't remember the capacity of the radiator off the top of my head, it's significantly greater than the 3.7 quart oil capacity, which means there's more water to heat than oil. The oil also passes through the oil-coolant heat exchanger after leaving the pan and before entering the engine, does it not? (It's right after the filter and the filter is upstream of the engine, right?) This would further serve to pre-heat the oil if it were cooler than coolant.
All of which leads me to speculate that the oil temperature sensor might be located in the oil pan or upstream of the engine, and that the rate of oil circulation through the engine might be low. This could explain why the oil temps always seem so low; if it took 20 minutes to circulate all 3.7 quarts of oil through the engine, then the oil in the pan would hardly heat up before half of it had been circulated after 10 mintues or so. The oil temperature reading could also stay low indefinitely if outside temperatures were low enough that the oil was cooling significantly in the pan before being returned to the engine. However, the oil in the engine probably reaches "operating temperature" at least as fast as the coolant does.
This would also help to explain the higher oil temperatures people experience at the track, even when their coolant temp stays dead at 190. The rotary gear pump circulates oil faster when the engine is held at high-rpm. Oil that normally has a chance to cool to in the pan, before being recirculated, is being recirculated faster, allowing the sensor reading to rise, even though the engine temp, measured by the coolant temp, isn't rising.
Any comments on the theory would be appreciated.
This has led me to wonder: where exactly is the oil temperature sensor located on a 1.8T? I wonder because it seems improbable to me that coolant temperature could reach a toasty 190 deg. F before the oil. Oil has a much lower specific heat than water, and it runs through the hottest parts of the engine, being heated directly as it lubricates the bearings. Additionally, while I can't remember the capacity of the radiator off the top of my head, it's significantly greater than the 3.7 quart oil capacity, which means there's more water to heat than oil. The oil also passes through the oil-coolant heat exchanger after leaving the pan and before entering the engine, does it not? (It's right after the filter and the filter is upstream of the engine, right?) This would further serve to pre-heat the oil if it were cooler than coolant.
All of which leads me to speculate that the oil temperature sensor might be located in the oil pan or upstream of the engine, and that the rate of oil circulation through the engine might be low. This could explain why the oil temps always seem so low; if it took 20 minutes to circulate all 3.7 quarts of oil through the engine, then the oil in the pan would hardly heat up before half of it had been circulated after 10 mintues or so. The oil temperature reading could also stay low indefinitely if outside temperatures were low enough that the oil was cooling significantly in the pan before being returned to the engine. However, the oil in the engine probably reaches "operating temperature" at least as fast as the coolant does.
This would also help to explain the higher oil temperatures people experience at the track, even when their coolant temp stays dead at 190. The rotary gear pump circulates oil faster when the engine is held at high-rpm. Oil that normally has a chance to cool to in the pan, before being recirculated, is being recirculated faster, allowing the sensor reading to rise, even though the engine temp, measured by the coolant temp, isn't rising.
Any comments on the theory would be appreciated.
11-08-1999, 01:48 PM
#4
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Coolant temp is thermostatically controlled, oil is not.
On cold days, oil temp in this type engine may never get above 80°C.
Location of temp sending unit is not very important.
The dynamic interaction between coolant and oil temp is quite complex and depends on many factors.
Location of temp sending unit is not very important.
The dynamic interaction between coolant and oil temp is quite complex and depends on many factors.
11-08-1999, 06:36 PM
#5
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Here's my theory...
You raise a lot of good points, but....
The water surrounds the heads where *combustion* occurrs. The heads are going to get hot the quickest. The bottom of the block has a higher specific gravity and stays relatively cool. This is where most of the oil resides. Some oil is pumped up to the valves and through the crank bearings, etc., but those are massive pieces of metal that are not going to get hot nearly as fast as the heads. And the relatively small quantities of oil returning to the oil pan will take some time to heat up the rest of the oil.
The water surrounds the heads where *combustion* occurrs. The heads are going to get hot the quickest. The bottom of the block has a higher specific gravity and stays relatively cool. This is where most of the oil resides. Some oil is pumped up to the valves and through the crank bearings, etc., but those are massive pieces of metal that are not going to get hot nearly as fast as the heads. And the relatively small quantities of oil returning to the oil pan will take some time to heat up the rest of the oil.
11-09-1999, 04:38 AM
#6
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Sounds reasonable...
If I had waited to post until after I went home to read the relevant sections in my Bosch Automotive Handbook, I probably wouldn't have posted quite what I did. I figured out that you are right last night around 7 pm.
Still, the oil temperature sensor must be located in the pan (or thereabouts), because there is no single point of return from the engine. It seems to me that we shouldn't really be worried about the oil in the pan getting hot, but just the little oil actually lubricating the engine. There being such a thin film doing the actual bearing lubrication, it must get hot pretty quickly, especially since cold oil's viscosity (internal friction) is so much higher than hot oil's. The lubricating oil might warm up even before the water jackets surrounding the heads. Waddaya think?
Still, the oil temperature sensor must be located in the pan (or thereabouts), because there is no single point of return from the engine. It seems to me that we shouldn't really be worried about the oil in the pan getting hot, but just the little oil actually lubricating the engine. There being such a thin film doing the actual bearing lubrication, it must get hot pretty quickly, especially since cold oil's viscosity (internal friction) is so much higher than hot oil's. The lubricating oil might warm up even before the water jackets surrounding the heads. Waddaya think?
11-09-1999, 04:41 AM
#7
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But the oil actually circulated through the engine must get hot fast, right?...
...Or else we'd all have problems with water condensing in the oil during winter, would we not? (OK, I know that statement only applies to those of us in northern climes.)
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11-09-1999, 06:16 AM
#8
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The short answer was in the message header: . . . . . . . . .
Coolant temp is thermostatically controlled, oil is not -- at least not directly.
There is a thermostat that regulates the coolant temperature. The coolant removes heat from the engine AND keeps the oil temp with a certain range. Oil temp is more of an indicator of how hard the engine is working; it will increase with engine load whereas coolant temp is quite constant.
Condensation is a different issue.
Sounds like you are interested in engine thermodynamics. Maybe you could study this and make a career of it.
There is a thermostat that regulates the coolant temperature. The coolant removes heat from the engine AND keeps the oil temp with a certain range. Oil temp is more of an indicator of how hard the engine is working; it will increase with engine load whereas coolant temp is quite constant.
Condensation is a different issue.
Sounds like you are interested in engine thermodynamics. Maybe you could study this and make a career of it.
11-09-1999, 07:38 AM
#9
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Logic...
I like your supposition, but as Tim alludes to...with the thermostat closed (I am assuming there are 2 or 3 minithermos in the actual theromostat) you are actually only heating a limited amount of water. Brearing temps do take a while to build up and ther is lots of metal mass and insulating air. The water is in direct contact the cyl. head and has nowhere to exchange heat until the thermo opens (does warm the oil). There is not really very much water in the head and interior heater portions.
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