By Rav. T Bohacz
photographer: Rav. T Bohacz
There are a few ways to tell what is happening inside a diesel engine without taking it apart. There are two that you can do in your driveway: looking in the tailpipe and having the engine oil scientifically analyzed.
Getting Scientific
Known as oil analysis, it's a procedure that's very common in the military and with large, expensive engines such as those used in aviation and farming applications. Many trucking companies employ oil analysis as a quick and inexpensive window into the engines put to work in their fleets.
Using microscopic and spectrum analysis, the engine oil can reveal the condition of bearings, fuel contamination, moisture content, and antifreeze dilution. The study also looks for silicon, iron, chromium, aluminum, copper, lead, tin, nickel, and silver. All these metals are used in some form as consumables in the engine such as in the piston rings and bearings, so high levels can tell you when an internal part needs to be replaced.
Understanding Oil
Keeping a diesel engine lubricated poses many different problems. The friction between surfaces is reduced by the presence of oil. The function of engine oil is to displace the two surfaces and reduce the mechanical interference. The oil should be able to adhere to each surface yet be amenable to displacement.
The viscosity of a lubricant helps to drag the fluid into the separating space between the bearings and then retards its escape when the squeezing action brings the two surfaces together. The term "viscosity index" is a measure of the viscosity/temperature behavior of an oil, based on an arbitrary scale of 0-100, with 0 being representative of an oil that thins down rapidly when heated and 100 being representative of an oil that thins less rapidly. Because of these effects, a pressure is created in the lubricant. This pressure can be induced and maintained by employing a wedge action within the bearing surface.
Oil at the Crankshaft
When an engine is not running, the crankshaft rests on the bearings. When the engine starts and oil pressure is created, the crankshaft moves up off the bearings and pulls some oil under it. As the crankshaft rotation continues, the oil is wedged between the shaft and bearing, keeping them separated.
As engine speed and loads change, the position of the crankshaft, with respect to the bearings, also changes. Under certain conditions, the mating surfaces may actually break through the oil wedge. There is still enough of an oil film to prevent scoring but not enough to keep the crankshaft and the bearing completely separated. Don't confuse the designed oil clearance with the space between the crankshaft and bearing made by the oil wedge. The oil film is approximately 0.0001-inch, while the machined clearance for a main or connecting rod bearing is usually between 0.004-0.006-inch.
The engine oil then needs to support the pressure load placed upon it as the flame in the cylinder expands and sends the piston down in the bore. The higher the viscosity index number (what we define as weight), the greater the load the oil wedge can support, because its escape from the clearance area is hindered by friction.