Any base stock formed by reacting an acid with an alcohol: diesters, polyol esters, complex esters and natural vegetable triglycerides. Esters are polar, so they cling to metal, dissolve additives and deposits, and swell seals slightly, which is why a few percent go into many PAO blends. Polyol esters dominate jet engine and refrigeration compressor oils, and readily biodegradable esters fill environmentally sensitive hydraulics. Their weak spot is hydrolysis: water and heat split them back into acid and alcohol.
Acid number's creeping up on the ester hydraulic, check for water before it hydrolyzes any further.
Also heard as
- synthetic ester
- polyol ester
- POE
Related terms
PAO
Polyalphaolefin, the synthetic hydrocarbon base oil built from alpha-olefins and classed as Group IV.
Base oil group
Five-group system that sorts base stocks by saturates, sulfur, viscosity index and chemical type.
TAN
Total acid number: milligrams of potassium hydroxide needed to neutralize the acids in one gram of oil.
PAG
Polyalkylene glycol, a synthetic base oil prized for low friction and clean running but hostile to most other oils.