Lubricating Systems

Engine Friction and Lubrication (Part IV)

The main function of lubricating system is to provide sufficient and cool filtered oil to all the moving parts of an engine. The systems are classified as splash lubricating system and pressure feed lubricating system.

We have discussed about basic of Engine friction and lubrication in previous blogs. The link for part I, II and III are provided below.

 Click for the Part I

Click for part II
Click for Part III

1.      Splash Lubrication System

The arrangement of the system is shown in the figure below:

This method generally used for vertical engines with a closed crankcase. The sump is located at the bottom of the crankcase. When the engine crankshaft rotates, the big end of the connecting rod splashes oil by centrifugal action. The connecting rod big end has a hollow pipe called a scoop which is fitted to the bearing cap and pointed towards the direction of the rotation of the crank shaft. The lubricating oil passing through the scoop, lubricate the big end bearing and gudgeon pin bearing. All other parts are lubricated by the splash. Excess oil is collected in the troughs located as shown in the figure and is provided with overflows and collected in the main sump. The level of the oil in trough is maintained constant. The dripping from the cylinders is also collected in the sump. The oil from the sump is recirculated with the help of the pump as shown in figure.

The inability to regulate the quantity of oil splashed against the cylinder wall or inability to keep the oil from getting past the piston head into the combustion chamber, burning with the fuel and passing out with exhaust gases are limitations of this system.

2.      Pressure Feed Lubricating System:

All modern car and bus engines are lubricated by high pressure feed system as shown in the figure given below:

Such a system supplies oil under pressure (2 to 5 bar) directly to the connecting rod bearings, to the camshaft bearings, to the valve gear and to the camshaft drive. Indirect supplies reach the cylinder walls, the gudgeon pin, the distributor and pump drives.

Oil is carried in the sump, a deep tray which closes the bottom of the crankcase and is circulated by the gear pump which sucks from the sump through a strainer as shown in the figure. The pump delivery pressure is controlled by a relief valve and the oil passes through a very fine filter before it reaches the main distributor gallery. From the various bearings, surfaces and gears, the oil simply drains back into the sump.

After lubricating the big end bearings, the oil is fed to the gudgeon pins through the oil way in the connecting rod and further squirted into the cylinder wall.

3.      Charge Lubrication System

This is the simplest method of lubrication and does not require oil filter and oil pump. In this system, the lubricating oil is pre-mixed with the petrol therefore the fuel carries the lubricating oil in the cylinder which helps for lubricating the piston and cylinder. Most of the oil burns with the fuel due to high temperature and burnt oil is carried with the exhaust gases. The lubricating oil cannot be recovered in this system.

This type of lubrication is generally used for two stroke spark ignition engines of scooter and motorcycle. The quantity of lubricating oil mixed with the petrol is 3 to 6% of petrol.

The advantages of this system are listed below:

a.      It does not require separate lubricating system so it is most economical.

b.      There is no risk of failure of lubricating system.

c.      The lubricating oil supplied is regulated at various loads and speeds by the increased fuel flow.

The carbon deposits due to the burning of the oil on the spark plug and on other parts and non-recovery of the oil used are the main disadvantages of this system.

The lubricating system is also classified as wet-sump lubrication and dry sump lubrication system.

1.      Wet Sump Lubricating System:

The arrangement is shown in the figure below:

This is called wet sump as sump is always full of oil. The working is just similar to the pressure feed lubricating system.

Oil is drawn from the sump by an oil pump through an oil strainer. A pressure relief valve is provided which automatically maintains the delivery pressure constant. If the pressure exceeds than the predetermined pressure, the valves opens and allows some of the oil to return to the sump and relives the oil pressure in the system. The oil from the pump goes to the bearings and part of it passes through a filter which removes solid particles from the oil. As all the oil is not passed through the filter, the system is known as by-pass filtering system. Advantages of this system are that a clogged filter will not restrict the flow of oil to the engine.

2.      Dry Sump Lubricating System:

The dry sump lubricating system is shown in the figure given below:

This is known as dry-sump as the sump does not contain oil and all the oil required for lubrication remains in the circulation only. High speed racing cars and military jeeps use this type of lubricating system as the oil in the wet sump is subjected to large back and forth acceleration.

An auxiliary tank is used to supply the oil to the main bearings with the help of the pump. The oil returns back to tray and then returned back to the auxiliary tank by scavenging pump, the capacity of which is always 20 to 30% more than the pressure pump to avoid flooding of the crankcase.

If the filter is clogged, the pressure relief valve opens permitting oil to flow bypassing the filter and reaches the supply tank. The oil is then circulated to the bearings from the supply tank. A separate oil cooler is used to cool the oil to remove the heat from the oil as heating of oil is rapid because of rapid circulation of oil and high speed of the engine.