Oil warm up time

Messages
51
Location
Illinois
OK, Since I stirred up some good discussion about coolant and distilled water in the Mech. Forum, I thought I'd pour some oil on the fire this time. The temp gage on the dash of a car is, of course, the coolant temp gage. I have heard people make the claim here that the engine oil takes longer to reach operating temperature than coolant does. These folks have said you need to wait 10, even 15 minutes into engine operation to get full oil temp. This, frankly, mystifies me. A typical 4 cylinder engine has about 8 quarts of coolant and 4 of oil. OK, some of the coolant is isolated in the radiator until the thermostat opens, maybe half? (Based on the fact that opening my old Saturn's rad drain yielded under 1 gallon of the 2 total in the system) So why would a gallon of coolant circulated through the warming engine gain heat faster than the gallon of oil which has even closer contact with the fastest heating parts of the engine? Seems to me that, being about equivalent in mass, they would heat at the same rate. So why can't I assume that once my coolant reaches 190, my oil is at the same temp (and I can BOOST)? PS No AT fluid temp to worry about - see name.
 
Messages
40,164
Location
Great Lakes
I'm no expert here, but I believe it has to do with the fact that the whole engine cooling system is designed in a way that the oil is supposed to give its heat away to coolant. Having both a coolant temp gauge and an engine oil temp gauge in my 1.8T, I can say from experience that on a warm summer day, it takes about 5 minutes for the coolant temp gauge to get to 190F. At that time, the oil temp gauge doesn't even start moving (the scale begins at 150F). From that point, it takes approx. another 10 minutes before the oil temp gauge gets up to 190F.
 
Messages
1,432
Location
Virginia
quote:
I'm no expert here, but I believe it has to do with the fact that the whole engine cooling system is designed in a way that the oil is supposed to give its heat away to coolant.
I don't think this explains the faster warm-up, because the oil would have to be hotter than the coolant to transfer heat to it. The oil gives heat to the coolant only after everything is good and hot. If indeed the coolant heats up faster than the oil, then explanation might lie here: 1) When the engine is warming up, the thermostat is closed. Therefore, the quantity of coolant coming up to temp (before the stat opens) is only what is in the block and heater core/hoses (a lot less than the total system capacity). So there might actually be less coolant to warm up than oil, at least initially. 2) All of this coolant is flowing in a tight loop (while the stat is closed) right through the hottest part of the engine - the water jacket around the cylinders. Not all of the oil goes to the hottest parts as consistently, some goes to valve train / cam / crankshaft and then back down to the pan waiting to get pumped again etc. 3) I don't have the numbers, but I suspect that the specific heat of oil is greater, in other words it takes more heat energy to raise a given quantity of oil one degree than the same quantity of water. This would seem to be consistent with motor oil's other stable qualities... Maybe some of the chemist / engineer types can confirm or deny my hypothesis? Matt [ June 25, 2003, 11:07 AM: Message edited by: Matt89 ]
 
Messages
7,775
Location
Oklahoma
hmmmmmm..... I raised this question regarding warm ups a few ago. Didn't even think about it the way you are. That's good stuff to further my belief on engine warm ups. Thermodynamics comes into mind here........you know or remember from school days......heat flows from hot to cold....so if water is getting hot, heat will transfer to engine block, which would be picked up by the oil until a equilibrium is met. Interesting stuff. [Smile] [ June 25, 2003, 04:54 PM: Message edited by: Schmoe ]
 
Messages
23,591
The coolant in my Audi (2.8/V6) reaches operating temperature (195 degr F) within 3 minutes in ambient temps of 50 degr, F and up. The oil needs at least 5 miles before it reaches 200 degr F. Normal operating temperature is around 200 to 212 degr F. That means I won't drive over 3000 RPM, and I won't drive with WOT until the oil has reached 200 degr F.
 
Messages
40,164
Location
Great Lakes
quote:
Originally posted by pedaltothemetal: Get a oil pressure gage. As soon as the pressure drops to normal, it's fully warmed up. Leo
How do you know what a "normal" pressure is?
 
Messages
742
Location
Lake Anna, VA
I can usually tell when my oil comes up to operating temp in my 00 Wrangler. When first started it holds 55psi oil pressure at idle and a little higher under load. When the oil reaches operating temp in idles at 35 psi and under load 50 psi. Depending on temp it can reach operating temp in as little as 5 mins of driving in very hot weather.
 
Messages
4,805
Location
Lakeville, MN
Establish the "normal" pressure over time, if you have a real oil pressure gauge. Like Ryan, my Jeep Cherokee shows this behavior quite nicely: 60 psi on cold start, slightly higher when off idle cold. Fully warm is 25 psi idle, around 50 psi when under moderate throttle. In the summer this can happen in 5 minutes, in winter at -20 F it can take a half hour.
 
Messages
4,478
Location
Southern California
quote:
Originally posted by moribundman: The coolant in my Audi (2.8/V6) reaches operating temperature (195 degr F) within 3 minutes in ambient temps of 50 degr, F and up.
This may not be true. It could really be a case of the coolant temperature in the cylinder head reaching normal operating temperature while the thermostat is still in bypass mode. Might be interesting to measure the coolant temperature in the radiator just at the point when the temperature gauge first indicates normal operating temperature. I'd be willing to bet that the coolant in the radiator is no where close to 195 degrees F.
 
Messages
23,591
quote:
quote:Originally posted by moribundman: The coolant in my Audi (2.8/V6) reaches operating temperature (195 degr F) within 3 minutes in ambient temps of 50 degr, F and up. This may not be true. It could really be a case of the coolant temperature in the cylinder head reaching normal operating temperature while the thermostat is still in bypass mode. Might be interesting to measure the coolant temperature in the radiator just at the point when the temperature gauge first indicates normal operating temperature. I'd be willing to bet that the coolant in the radiator is no where close to 195 degrees F.
Ray, let me clarify: When I start the cold engine, I see the coolant gauge needle creep towards about 170 degr F. At that point the thermostat opens, and the needle moves within 30 seconds or so to 195 degr F. Nowhere did I say that the coolant in the radiator had an operating temperature of 195 degr F. It's good if the coolant sensor is NOT in the radiator, because if the sensor is closer to the head (as it is in my car), the first signs of overheating will occur earlier.
 
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