Understanding Your Truck: Lubrication
By Paul Abelson
A better way
Early engines used slippery vegetable oils and whale oil. Castor oil was a favorite in race cars and World War I aircraft for high temperature stability. In order to prevent those highly stressed engines from seizing from lack of lubrication, drivers and pilots put up with the side effects from several hours of inhaling castor oil fumes. But clearly, a better alternative was needed. Petroleum provided that alternative.
Chemists developed additives that let refined petroleum stand up to the stresses that engines create — shear, extreme pressure and high temperature — all of which break down oil molecules and destroy its lubricating properties. At high temperatures, oil tended to char, creating soot that would circulate in the oil abrading the engine from the inside. Engines would last only 50,000 miles or so before needing to be overhauled.
There are many reasons why we can expect more than a million miles today. Better materials, improved manufacturing techniques and better design are involved, but improved lubrication and lubricants make a major contribution.
Early engine lubrication relied on the crank churning through oil in a pan (the sump) and splashing it up onto the camshaft and cylinder walls. A major breakthrough occurred with the development of the oil pump. Oil under pressure could now be delivered to remote corners of the engine. Engine speed (rpm) could increase, since the crankshaft no longer had to splash oil, it no longer created foam that diluted the oil with air.
Modern oils need additives. Petroleum is a naturally occurring slippery substance. It needs to be fortified in order to reliably perform all its functions. Viscosity improvers let it flow when cold and prevent thinning when hot. Detergents and dispersants help oil keep the engine clean and suspend contaminants and impurities so they can be carried to filters without re-depositing on engine surfaces. Anti-foam chemicals prevent the oil from stiffening when churned, so the oil doesn’t wind up like a meringue.
Good premium oil can have between 30 and 35 percent of its volume as additives. They help meet specific properties demanded by ever more complex emissions- controlled engines. Synthetic oil is manufactured from hydrocarbon sources such as natural gas, coal and coal gas. It is uniform, free of impurities and stable. It flows at lower temperatures, maintains its properties at high temperatures, and lasts longer between changes.
Oil’s Other Functions
When oil circulates through the engine, it absorbs heat from every surface it contacts. That heat is carried to the sump, which transfers much of it to air flowing around the oil pan. Large diesel truck engines have oil coolers to dissipate heat. Oil from the sump is pumped through a heat exchanger similar to a radiator before entering the filters on its way to the engine. Oil gets hotter than engine coolant.
During the combustion process, soot is created when diesel is incompletely burned. That soot is captured in the engine oil, which carries it to filters that hold it and other contaminants, such as metals worn from engine parts. The main reason we change oil is to dump the contaminants.
Tremendous pressures build up inside cylinders during combustion. The gases produced force pistons down, converting heat energy to mechanical energy. The gases also try to escape between the cylinder walls and the piston rings, seeking the path of least resistance. The film of oil on the cylinder walls resists this blow-by, sealing the engine.
Lubrication is essential for the survival of almost all moving parts on a vehicle. It is based on the slippery properties of oils. The earliest recorded lubricants consisted of strips of animal fat wrapped around wood axles where wheel hubs were fitted. Friction was reduced so less effort was needed. Axles and hubs lasted longer, too.
In the thousands of years that followed, lubricants became more sophisticated, but the principle of separating moving parts with an oily film remained. Rendered fats, from bears to whales were used, as were oils pressed from seeds. Often, the oils needed thickening to stay in place. Mixed with soap, oil becomes grease, a semi-solid lubricant well suited for applications characterized by continuous motion. But even greases thinned when warmed, either from summer or friction. To prevent grease running out, leather discs were used as seals. While materials have improved tremendously, the principle of sealing grease is in use today.
Grease lubricates shafts well, but something more was needed for the complexity of internal combustion engines. They have rotating shafts and also other motion. Pistons reciprocate within cylinders. Gears mesh together. Cams rub against followers or tappets. Solid greases could not function inside an engine, but liquids could.