Engine Friction and Lubrication.

Introduction:

Almost all machine components of an I.C. Engines have relative motion and rub against each

other. The lubrication is required to reduce this rubbing action and minimize the frictional losses and increase the life of engine. The purpose of lubrication in IC Engine is generally two fold. It reduces the frictional losses caused by rubbing action between different machine components having relative motion between them and removes the heat generated inside the engine by absorbing them.

The power developed inside the engine is known as I.P. but the power available at the crankshaft (B.P) is always less than I.P. This is because, a part of power generated by engine is lost in bearing, cylinder and piston, gears and many other parts due to friction. It is impossible to remove all the frictional losses but it can be minimized by using lubrication between the parts which have relative motion between them.

Increase in friction ultimately dissipated as heat to the cooling water and it further increases the pump and fan power requirements also.

The frictional resistance between two or more moving parts having relative motion is mostly dependent on lubricating oil properties, surface condition, material of the surface, rate of relative motion, nature of relative motion and quality of lubricating oil.

The purpose of this blog is to discuss about the nature of friction, selection of lubricating oils and methods of lubrication by which frictional losses will be reduced and mechanical efficiency of the engine can be increased.

Sources of Losses:

Total frictional loss can be defined as the difference between IP and BP. This includes direct mechanical friction, throttling valve losses through inlet and outlet valves, pump losses, blowdown losses and many others.

1.      Direct Frictional loss

It included bearing losses, as main bearing, cam shaft bearing, piston and cylinder friction loss and many moving parts. The frictional losses are comparatively higher in reciprocating IC engines.

2.      Pump loss

A considerable amount of power is used during intake and exhaust processes in 4 stroke IC Engines. This loss is negligible in 2 stroke engines as the incoming fresh air charge is used for scavenging the exhaust gases and charging the cylinder.

3.      Blow by losses

This loss is because of leakage of combustion products past the piston from the cylinder into the crank case. This loss depends upon inlet pressure and compression ratio. This loss increases directly with compression ratio but reduced with an increase in the engine speed.

4.      Throttling valve losses

The standard practice for sizing the exhaust valve is to provide smaller exhaust area than the inlet valve area. This increases the pumping loss as smaller area resists more the flow of exhaust gases. This increase with increase in speed of the engine rapidly if the valve size, valve timing and valve flow coefficient are not designed properly.

The inlet throttling occurs due to restrictions imposed by air cleaner, carburettor, venturi, throttle valve, inlet manifold, and the inlet valve. All of above add in pressure loss. Similarly some pressure loss occurs during exhausting the burned gases.

5.      Combustion chamber pressure loss

This loss occurs with pre-combustion chamber. This loss occurs due to the pumping work required to push the air into pre-combustion chamber through small orifice. This is dependent upon orifice size, and speed, which increases with increasing engine speed.

6.      Power loss to drive the auxiliaries

Some part of power generated by engine is taken out to drive the auxiliaries like water pump, oil pump, fuel pump, cooling fan and generator. This is also considered as loss as part of engine power developed is used for these purposes.

Theory of Lubrication

Lubrication reduces the power required to overcome the friction between moving parts by interposing a oil film between the parts requiring lubrication. If the film between mating parts is such that it will not allow the mating parts to touch each other, the chances of wear is reduced and wear reduction depends upon the oil film strength. The oil film should not possess higher strength as the power required to move the mating parts will be considerably high. The oil film should not be so thin so it will wash out because of the shear force acting between mating parts. The oil film always offers resistance to the shearing force when relative motion of the mating surfaces occurs.

Generally in application, the viscosity of lubricating oil is measured in poise or strokes. The load or force to the moving surface will try to force the lubricating oil film out of the space between the mating surface and ultimately results in metal to metal surface contact. Thus in order to maintain the oil film between mating surfaces, continuous supply of lubricants must be available to fill the space.

Functions of Lubrication

As mentioned earlier, the primary function of lubrication is to keep the moving parts sliding freely over each other and thus reducing the frictional losses. Apart from this, the following are also the main functions of the lubricating system:

1.      Cooling

The lubricating agent that comes in contact between mating surfaces also absorbs the heat and cool then. This cooling action takes place simultaneously with lubrication. During cooling process, the oil carries the heat which may promote oxidation.

2.      Cleaning

In IC Engines, the piston rings and bearings of gudgeon pin are exposed to the burned gases and they are contaminated by the carbon deposits. The lubricating oil helps to wash out these deposits and clean them. This washing action of oil further stops the forming of sludge inside the cylinder.

3.      Sealing

The two sides of the piston are continuously subjected to very high and low pressure and because of this, there is always possibility of leakage of air fuel mixture. The oil film between the piston and cylinder forms a seal proof layer by filling the minute leakage paths and surface irregularities on cylinder, piston and piston rings.

4.      Noise reduction

The noise level is decreased as the friction between mating parts is reduced due to presence of lubricating agent.

In short, the main functions of lubricating agent are, to provide thin film between two metallic mating parts to primarily reduce friction, to remove heat generated inside the engine, to reduce the possible wear and tear  of metallic parts like cylinder, piston and valves  to keep the engine clean from deposits and sludge, to control the corrosion due to oxide formation and rusting, to improve the combustion sealing and to provide balance between high and low temperature oil thickening.

Some examples of lubrication are:

Engine oils: SSE 10W, SAE 20, SAE 40, SAE 50, SAE 80.

Gear oils: SAE 40, SAE 80, SAE 90.