Many times the heat transferred to the lubrication system from combustion, and the kinetic components of an engine, are overlooked until a problem arises during dynamometer or  track testing.

A study was undertaken to develop a heat flow and heat transfer model of an engine's lubrication system to predict primarily sump oil temperatures, and secondarily, coolant temperatures. The temperature relationships between the oil and coolant were found to be most interesting. This technical brief is a summary of the referenced paper.

A model was developed to account for numerous items in the engine /lubrication system:
1. Engine Speed and Load,
2. Head and Block Geometry
3. Bearing Clearances
4. Piston Design
5. Coolant and Ambient Temperatures
6. Material Properties

(If the engine had an oil squirter system, this was included as well.)

Energy balance equations and heat input sources were incorporated into the model, with heat sources and heat sinks within the engine accounted for. In each test, a measured quantity of oil was used.

To verify the model, the model was plotted against full-up V8 engine data from a dynamometer. The model correlated quite well with the dynamometer data, with a slight under-estimation of oil sump temperature at high rpm.

It was found that the piston undercrown contributed about 70 to 80% of the heat into the oil with bearings contributing 10 to 20% of the heat energy to the oil.

Here is a breakdown of heat sources for engine oil 2,000 RPM:

Energy to Oil from

Piston Undercrown - 76%
Main and Big End Bearings - 13%
Camshaft Bearings - 1%
Cylinder Wall - 0%
Oil Deck in Head - 2%
Oil Pump Energy - 8%

At 4,000 RPM, the only increase in heat energy came from the Main and Big End bearings at 19%, a 6% increase from 2,000 RPM. The Piston Undercrown's energy contribution to oil temp was 4% less at 72% for the 4,000 RPM case.

The predicted sump (bulk) oil temperature versus the coolant temperature was shown to be about 18 to 20 degrees C higher for the oil than for the coolant at 2,000 RPM. At 4,000 RPM WOT, the oil sump temperature was about 50 degrees C higher than the coolant temperature. The slope for both RPM ranges was about 0.7 C/C, which means that on the average, the Oil Sump temperature is always 1.2 to 1.43 times higher than the coolant temp. Engine oil temperatures varied with engine output and coolant temperatures varied with engine output, airflow and radiator capacity. In a properly functioning engine, the resultant effect is they will be larely unrelated.

An interesting side-bars of the study was that the bulk oil temperature increased "only"  0.3 C for every 1.0 C increase in ambient (outside) temperatures.