|
|||||||
|
|
|
|||||
|
|
|||||||
Introduction to Lubrication Systems
The lubrication and ventilation systems used on modern internal combustion engines are excellent examples of good engineering design. The lubrication system must be designed so it will deliver a large amount of oil to some of the parts while providing mere drops to other parts in the engine. Too much oil in some places can cause as much trouble as too little. If, for example, the lubrication system delivered as much oil to the cylinder walls as it does to the main bearings, the rings could not wipe off the excess oil fast enough. Some of the excess oil would get by the rings into the combustion chamber where it would foul up the spark plugs, gum up the chamber with carbon, and burn during the power stroke to be lost through the exhaust pipe. Too much oil in the wrong places will also cause oil seals to fail and valves to stick.
The crankcase ventilation system (also known as the breathing system) plays an important part in the life of an engine. It has the responsibility of removing the vapors that form in the crankcase. If not removed, these vapors form sludge when mixed with the engine oil and destroy the lubricating qualities of the oil.
What are these vapors and where do they come from? Well, part of the vapors are plain water which collects on the walls of the crankcase the same way water forms on the outside of a glass of iced tea. As you know, when the air containing water vapors strikes the cold outside of an iced tea glass, the vapors condense and form drops of water on the glass. The same thing happens in an engine. When the engine is running, the air in the crankcase starts to heat up. When the heated air strikes the cooler sides of the oil pan and crankcase, the moisture in the air condenses and falls as drops of water into the engine oil.
Excess fuel from the combustion chamber can travel by the piston rings and collect in the crankcase oil along with the water. Even with an engine in good condition, some unburned fuel gets by the rings into the crankcase, especially during the time the engine is being cranked or is being operated while it is cold. The driver usually has the choke pulled out or is pumping the accelerator to supply the engine with more fuel. Much of the fuel that is not burned winds up in the crankcase where it dilutes the oil.
We cannot prevent the water and fuel
from getting into the lubricating oil, but we can remove most of both
of them through the crankcase ventilation system.
OPERATION OF LUBRICATION SYSTEMS
The lubrication system has the job of supplying all of the moving parts in an engine with oil. As you know, the oil helps to reduce friction, clean the moving parts, and cool them by carrying off part of the heat. In the case of the pistons and rings, the oil helps to seal off the combustion chamber and helps to prevent the burning gases from entering the crankcase.
There are four types of lubrication systems used in internal combustion engines. They are the splash, the combination splash and force feed, the force feed, and the full-force feed.
The splash, combination splash, and force-feed systems are no longer used in automotive engines in this country. They are, however, used in such things as lawnmower engines, outboard motors, and motorcycles, so we will discuss them briefly.
FIGURE 51. SPLASH-TYPE LUBRICATION SYSTEM.
Notice on the splash system there is a dipper on the bottom of the connecting rod. This dipper splashes the oil all over the inside of the engine. All of the moving parts are lubricated by the oil splashed on them by the dipper.
A variation of the splash system is used in the two-stroke-cycle engine. Here the lubricating oil is mixed with the gasoline in the fuel tank. During the compression stroke of the piston, the reed valve opens and a fresh charge of oil, fuel, and air enters the crankcase. The fuel, air, and oil moves through the intake port into the cylinder when the piston moves down on its power stroke. The oil in the fuel lubricates the moving parts. This method of lubricating the engine is very common on small two-stroke-cycle engines, especially those used on motorcycles and motorboats. This system is known as the vapor lubrication system.
The amount of oil mixed with the fuel in the two-stroke-cycle engine must be carefully controlled. Too little oil will cause rapid wear of the engine parts. Too much oil will cause carbon to form in the combustion chamber, foul up the spark plugs, and clog the exhaust ports. Spark plug fouling is a very common problem with this type of engine.
An advantage of this type of lubrication system is that the engine can be tilted or operated in any position. If the engine was operated on its side or in an upside down position, it would not last long. This is because the oil in the crankcase would flow away from the dipper and could not be splashed over all of the moving parts.
| FIGURE 52. | COMBINATION SPLASH AND FORCE-FEED LUBRICATION SYSTEM. |
The combination splash and force-feed systems of engine lubrication were used on some American-made vehicles until the middle 1950s. An engine using this system still relies on dippers on the connecting rods to lubricate the connecting rod journals, the cylinder walls, and the pistons and rings. Other parts, such as the main bearings, valves, camshaft, and timing gears are lubricated by oil supplied by a pressure pump.
Compare the lubrication system shown with the one above. Notice the parts that have been added to provide oil under pressure to some parts of the engine. Item A is an oil pump. It is usually driven by a gear on the camshaft. The pump picks up the oil through a pickup tube in the engine oil pan B. Item C is the oil passage leading from the oil pump to the main bearings and to other engine parts that are supplied oil under pressure. Notice one oil passage leads to a small jet or nozzle that can spray the oil directly into the dipper on the connecting rod and into the tray or trough directly below dipper D. With this arrangement, the dipper does not have to reach the oil in the pan. You can imagine what would happen if the oil level in the pan was a bit too low for the dipper to reach. With the oil spray jet, the connecting rod bearings and cylinder walls will be properly lubricated even if the oil level is low in the oil pan.
FIGURE 53. FORCE-FEED LUBRICATION SYSTEM.
Shown here is the force-feed lubrication system. Notice there are no dippers on the connecting rods. Instead, the crankshaft has drilled passageways leading from each of the main bearing journals to the connecting rod journals. Oil is delivered to the main bearing by the pump. Part of the oil travels through the drilled passageways to the connecting rod journals.
On some engines, the oil leaking out around the connecting rod bearings is thrown on the cylinder walls by the spinning crankshaft.
On other engines, a small squirt hole is drilled through the rod to spray oil on the piston and cylinder wall.