Engine Oil Temperature and Pressure


Engine Oil Temperature and Pressure

Introduction

Many new pilots do not fully appreciate the relevance of the Oil temperature and pressure relationships. They will call T&P's green, but not really interperate them. This also translates to the starting of the engines, and deciding if what they see is normal or faulty. Respecting oil temperature/pressure limitations protects against cumulative engine damage and reduces failure risks. 

Cold Engines

It's normal to expect a high oil pressure when the engine is cold. The effectiveness of engine lubrication and cooling depends on oil temperature. Low oil temperature increases the viscosity and flow resistance through oil distribution channels. Reduced flow through the engine driven pump increases oil pressure.

A cold engine must be warmed appreciably before high RPM is demanded. Oil pressure increases with engine RPM, because the pump is directly driven by the engine. If you combine high RPM with a low temperature oil, you risk excessive oil pressure. Reduced oil flow additionally reduces heat transfer and lubrication. This is a bad combination and can cause ruptures in seals and engine hotspots.

In countries with freezing climates, external heater and power sources are recommended for very cold starts below freezing (‹32°F). Cold and warm starts are distinguished by body temperature (100°F). A cold started piston engine should be warmed before selecting power above 1500RPM.

Warm Engines

Hot engines will show high Oil Temp and a Low pressure. The pressure is very often in the lower caution range (yellow arc) during idle below 1200rpm. This is normal. 

What is important to notice is that the pressure rises if RPM is increased slightly. What you dont want to see is red. 

One way to increase oil pressure if you're concerned is to increase the oil quantity. If you normally run 6qts, you could elect to run 7qts in hotter weather. This increase the volume of oil, hence the heat is shared woth more quantity thus better cooling.

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