A History and Overview of the Trucking Industry Part 6
Modern trucks have several different systems which all may require a specific type of lubrication with different properties. These systems include the engine crankcase, the transmissions, drive axle differentials, power steering system, steering gearbox and any hydraulic systems. The best source of information in which type of lubricant should be used for each application should come from the manufacturer. The manufacturer knows how it should be checked and when it should be changed. Lubricant marketers are also good sources of application information.
The Lubrication Process
Oil is considered the life-blood of an engine because it is the most important element for maintaining long engine life. A diesel engine is a very demanding machine. It operates in a range of temperatures and requires metal to rub against metal at incredible speeds. Lubrication systems in engines allow the engine to operate by reducing friction between the moving parts. The system also carries away by-products of combustion and cooling the hot surfaces. In modern-day diesel engines, the oil removes about 1/3 of the total heat generated.
Figure 00-00 shows a schematic for a lubrication system. The engine crankshaft gear drives the oil pump, which produces oil flow. Oil flow will decrease or increase directly proportional to the crankshaft/engine’s operating speed and within the range regulated by the oil pressure regulator because of this. The oil pressure regulator maintains appropriate pressure under all operating conditions.
Engines are built with internal oil passageways (galleys) that allow the oil to move to all of the many moving engine components. The oil filter prevents the passageways from becoming clogged with dirt, sludge or other types of contaminants which could possibly restrict oil flow. Extreme care should be taken when changing oil and the oil filter. It should be done in a clean and dirt-free environment. The engine oil fill cap and filler neck should be wiped down prior to adding oil to the engine, or else dirt on the outside may find its way inside.
When an engine is cold, it will produce a higher oil pressure because the oil is thick. As the engine’s temperature rises, the oil will thin and the pressure will come down to a steady state. This will be displayed on the oil gauge. Oil pressures vary depending on the engine, but normal pressure for most diesels under load should be 30 – 40 psi. While idling, the pressure should read about 10 psi.
After the water and oil gauges have stabilized, the engine can start to work. Any time a drop in the oil pressure is observed, it should be investigated immediately. The oil pump volume will be 40-100 gallons of oil through the system every minute depending on the engine. It could cause serious damage if there were to be a deficiency in oil flow if the engine is not shut off immediately.
The large rectangular-shaped container on the bottom of the engine is the oil pan. It is the collecting reservoir of the system oil. It has a suction screen leading to the suction side of the oil pump. The screen strains debris from the oil and prevents damage to the oil pump due to contamination in the oil. The oil pan also has a drain plug in order to drain the oil during periodic oil and filter changes.
Oil pumps can be one of three types: simplex, duplex or triplex. A simplex pump has one set of gears that supply oil to the engine. Duplex pumps have two gears that supply oil to the engine and do either scavenging or piston cooling. A triplex pump has three sets of gears that supply oil to the engine and perform scavenging and piston cooling. All of the pumps, or a section of the pump used to supply oil to the diesel engine, are pressure regulated.
A diesel engine for a truck has two oil filters. A full flow lube oil filter will remove any large particles from the oil that could possibly damage engine parts. The success of the engine’s operation depends on being able to quickly filter a large amount of oil without hindering the oil pressure. The full flow filter is connected to the lube pump discharge going to the engine where it can filter the debris at a point of full oil flow and at full pressure. The location of the filter will vary depending on the manufacturer. It may be a direct pump mounting, a removable engine mounting, or an integral part of the engine oil cooler (heat exchanger). The full flow lube oil filter is equipped with an automatic bypass system that will give the engine oil if the filter element becomes incapable of processing enough oil for lubrication.
The second type of filter is the lube oil cleaning filter, or bypass filter. It cleans the fine particles out of the oil. When the filter is new, it can trap and contain most of the solids and impurities in the oil. If the filter is neglected, the filtering process stops and contaminants will remain suspended in the oil traveling throughout the oil system and will cause excessive wear and tear to components over time. Oil and filter changes need to be performed periodically in order to avoid this.
A lube oil cleaning filter is referred to as a Luberfiner, 750 and bypass filter. A smaller version can sometimes be referred to as a 500. The entire supply of oil, over time, will pass through the filter for cleaning. For easy accessibility, the filter is often mounted externally on the frame because it is replaced on a regular basis. It is also sometimes mounted next to the full flow filter in a spin-on configuration.
To regulate the temperature of the oil, an oil heat exchanger is needed. This term was previously referred to as an oil cooler, but today’s heat exchangers not only cool the oil when it is hot, but they heat it when it is cold. It allows heat transfer to take place between the oil and the engine coolant, within the oil heat exchanger. The oil will flow over tubes which carry engine coolant and whichever is a higher temperature it will transfer heat to the other fluid.
For example, while the engine is warming up initially, oil is heated by the engine’s coolant since the engine coolant heats up faster. Conversely, when the engine is hot, the engine coolant will cool the lube oil to maintain proper temperature. Once the engine is at its normal operating temperature, the oil will typically be hotter than coolant and will transfer its heat to the coolant. The coolant will carry the excess heat to the radiator where the heat is lost to the circulating air.
Pressurized oil flow lubricates all the moving parts on a diesel engine. This pressure is created by the oil pump. The suction side of the oil pump gets its supply from the oil pan by the oil pick-up tube. The pressurized oil will leave the pump and flow to the oil cooler, which is a heat exchanger.
After the oil is cooled it continues to the full flow filter and then the main galley of the engine block. The galley is a passage that goes from one end of the cylinder block to the other. Typically, the engine oil pressure gauge is plumbed to the galley. At the main galley there are drilled passages that branch off and direct the oil flow to the various moving parts. The parts include the crankshaft, piston cooling nozzles, camshaft, valve train and air compressor.
After the oil has lubricated the engine, it is drained through the various castings into the oil pan and the process begins all over again.
Key Terms in Lubrication
Flash point is the lowest temperature at which a lubricant gives off vapors that ignite (or flash), but does not continue to burn. However, a flame may be present.
The fire point is the temperature that a lubricant gives off vapors that continue to burn following ignition. Fire points are typically 30° to 50° higher than flash points.
High flash and high fire points indicate that less lubricant was lost by evaporation during usage. Low flash and low fire points sometimes indicate that lubricant has been diluted to a lower viscosity. An undiluted, low-viscosity lubricant will usually give better service than a high-viscosity lubricant that is diluted to a lower viscosity.
A neutralization or acid number will indicate the presence of acid in the petroleum product. It is a measure of the actual weight (mg) of the alkali necessary to neutralize the acid in one mg of lubricant. A good quality, straight mineral oil lubricant is usually neutral (no acid number). A compound lubricant may have a low acid number. Lubricants that are in use may have a higher number caused by heat and water oxidation. Typically, a high neutralization or acid number is not good because acids cause corrosion.
Cooling System
Because a diesel engine produces heat as it is running, it is essential for the engine to have an effective means of cooling it. The cooling system shown in Figure 00-00 consists of a radiator, fan, circulating pump, coolant filter, coolant and thermostat.
The most important maintenance of the coolant system is to assure that proper coolant levels are maintained. Coolant is similar to what is used in automobiles. It is a mixture of water and chemicals that decrease the freezing point and elevate the boiling point. When the coolant level drops, it is replaced with air which could result in the engine overheating. This problem is usually the result of a radiator leaking and is generally discovered after the vehicle is stopped.