Lubrication Systems and Methods of Lubrication used in IC Engine | Thermal Engineering

Lubrication Systems and Methods of Lubrication used in IC Engine!

Introduction to Lubrication in I.C. Engine:

The machine parts which have relative motion rub against each other. The rubbing surfaces are never perfectly smooth, even when they are highly polished. If they are observed under a microscope, small depressions and elevations can be seen. These depressions and elevations get interlocked when machine parts are under pressure and give rise to a resisting force called frictional force.

Due to friction, parts may get worn out and become useless. Friction produces heat which, in this case, represents loss of work. To reduce this wear and loss of power caused by friction certain foreign substances are introduced between the rubbing surfaces which keep them apart. These substances are called lubricants.

The lubricants used for machine parts may be classified as solid, such as graphite; semi-solid such as heavy greases, and liquid. Mineral oil is the most commonly employed liquid lubricant and is used in general purpose machineries and engines. It is obtained after refining petroleum oil.

The surfaces requiring lubrication in the machines may be divided into two classes:

(1) Sliding

(2) Rotating

In case of sliding surface such as piston/cylinder the lubricating oil is scraped off by the motion of the oil control ring. The lubricating oil is scraped off by the motion of the oil control ring unless this is prevented by slightly rounding off the corners. While rotating surfaces due to their motion helps lubrication.

Therefore a sliding surface is a difficult to lubricate than a rotating surface as the former does not assist in forming the thin film of oil between surfaces are grooved to assist in distributing the oil and thereby they maintain the continuous film of lubricant.

Main parts to be lubricated in case of I.C. engines are:

(1) Cylinder walls

(2) Crank shaft main bearings

(3) Big end bearing of a connecting rod i.e. crank pin

(4) Small end bearing of a connecting rod i.e. gudgeon pin

(5) Cam faces where they engage with the tappets

(6) Push rod guides in case of overhead valve engines

(7) Rocker arm pin in case of overhead valve engines

(8) Valve guides

(9) Timing gears i.e. cam shaft driving gears

(10) Cam shaft bearings.

There are many ways of lubricating the above-mentioned parts of I.C. engines according to design, type and the speed of the engines.

Coefficient of Friction:

To know the lubricating action the knowledge of frictional forces is necessary. The frictional force depends on the factor called coefficient of friction which is equal to the ratio of the force required to cause the surfaces to just slide over each other to the normal forces pressing them together. The coefficient of friction can be reduced to a very small value by these of lubricants. This also in turn reduces the power absorbed due to frictional resistance.

In case of journal bearing supplied with the necessary amount of oil the coefficient of friction varies directly as:

(1) Dimensions of bearing journal d and I, where d and I are the diameter and length of the journal respectively.

(2) R.P.M. of the shaft N.

(3) Load N = P x d x I, where P is the bearing pressure -intensity and the product d x l is known as the projected area of the bearing.

(4) Diametral clearance, between journal and bearing, c/d.

 

From the above equation it can be seen that for any given bearing c/d is constant, so for the particular bearing varies as which is ZN/P known as the bearing characteristic number. Fig. 13-60 shows the sketch of journal bearing.

 

But for the same bearing working at a particular load and speed the coefficient of friction varies as the absolute viscosity of the forming the film between the rubbing surfaces i.e. µ α Z.

Viscosity:

The viscosity of a lubricant is taken as the measure of friction in oil as the various layers of oil slide over each other. The absolute viscosity Z is defined as the tangential force required to move, at a velocity of one centimetre per second, a plane surface having an area of one cm² parallel to another plane from which it is separated by a layer of oil one cm thick.

The unit of absolute viscosity is the poise being equal to one dyne second per cm². The centipoise 1/100 of the poise and the absolute viscosity of water at 20°C is equal to one centipoise.

Where it is a matter of comparing oils among themselves a determination of comparative or relative viscosity will be satisfactory. The relative viscosity consists of noting the time of flow of a fixed volume through a standard capillary tube; thus we speak of Saybolt or Redwood seconds depending on, which instrument is used to measure the time of flow.

So from lubrication view point, the viscosity is one of the important factor. It is affected by temperature, the higher the temperature the lower will be the viscosity of the oil.

So, for the proper functioning of the lubricant, viscosity of the oil should be selected in such a way that it should not go down below a certain minimum value at the highest temperature at which the bearing is likely to operate. A lower viscosity than this will cause seizing of the bearing while running. A higher viscosity than this will be safe and tolerable but gives higher coefficient of friction and so greater power loss due to it.

Properties of a Lubricant:

For the proper working as well as for the life of the bearing the lubricant used must possess certain properties. Also these properties should be permanent under the condition of use.

Where gravity flow or flow through pipes under a small driving head is used to supply lubricant to a bearing and the temperature of working of the bearing is low, if proper lubricant is not used then flow of the oil may not be sufficient or it may completely cease.

For this purpose the chill point test of the oil should be taken to avoid the difficulty of selecting a proper lubricant. Chill point is the temperature at which the bulk viscosity passes through a certain arbitrarily set standard viscosity.

Also the ideal lubricant should not have any tendency to start rusting or corroding and so it should be chemically neutral. It should not change by oxidation and should not react with water or form emulsion with it. It will increase the lubricating oil consumption.

Methods of Lubrication:

The way and the manner in which the oil is supplied to I.C. Engine influence the efficiency of the choice of the lubricant. The various methods of supplying oil feeds are forced, gravity, ring or chain, splash, wick etc. The forced, the gravity and the wick feeds are more economical.

The systems used are:

(1) The Forced Type Feed System:

In this system oil is supplied to different bearings under pressure which is greater than atmospheric. The pressure is generally developed by a separate pump. In this system the oil is supplied, as directly as possible, to bearing to maintain the pressure. This system is used with enclosed type of machines. The oil leaking from the bearings is returned to the pump for re-circulation. Fig. 13-65 shows forced feed system.

(2) The Gravity Type Feed System:

In this system, oil is not supplied under direct pressure; only head due to gravity forces oil to the bearings. It is used for lubricating most of the external moving parts such as main bearings, crossheads and crankpins of simple steam engines, etc. For supplying oil in this system mostly drop feed oilers are used. They can be fitted at moderate cost.

(3) Drop Feed Oiler:

It is also known as sight feed oiler. Fig. 13-66 shows the drop feed oiler. It consists of a glass cut, with a metal bottom which has a drip hole at the centre. The hole is closed by a needle valve which may be opened and closed by means of a snap lever.

The rate of feed is adjustable by means of a screw, which slightly raises or lowers the needle, and can be seen through a glass window. The drop feed oilers are generally started and stopped by hand but some oilers operate automatically. They are not very reliable and not much efficient.

(4) Ring or Chain Type Feed System:

The horizontal bearings are satisfactorily lubricated by this type of feed system. Fig. 13-67 shows a ring type lubricator. The oil ring rests on the journal where a portion of the bearing shell has been cut away. The bearing housing forms an oil reservoir in which oil is maintained at a level sufficiently high to have the lower end of the rings submerged. One or two rings rest on the top of the journal and their bottoms dip in the oil.

When the journal rotates, the oil ring rotates with the journal and carries oil from the bath to the top of the bearing. The oil then distributed to the bearing by the oil grooves. The surplus oil flows to the ends of the bearing and drops back into the oil reservoir. A guard may be provided to keep the ring in place.

Chains may be used in place of oil rings for lower speed bearings, because they will carry more oil.

This lubricator is very satisfactory in assisting hydrodynamics lubrication. It is automatic and reliable. But it is not used at high speeds because the oil will be thrown off due to centrifugal force and slippage will occur in case of the oil rings. Also at very slow speeds it is not very useful due to insufficient amount of lubricant carried by the ring or chain.

(5) Wick Type Lubricator:

Fig. 13-68 shows the wick type lubricator. This lubricator operates on capillary action. It consists of a cup having a central tube and a shank which can be screwed into the oil hole. Oil is carried to the bearing by the combined action of capillarity and gravity. For better action the distance ‘a’ should be less than 5 cm and distance should be greater than 5 cm.

(6) Splash Type Feed System:

It is employed to lubricate number of bearings in an enclosed casting. It is mostly used with vertical and horizontal high speed engines of smaller power and having a closed crank chamber. It requires little attention other than the maintenance of the correct oil level. But large initial expenses are necessary to provide an oil tight chamber.

The closed crank chamber contains oil at such a level that the crank pin dips into the oil at each revolution and the splash takes place. The oil settles on the parts inside the chamber and chamber walls. Oil wells or pockets, cast on the inside of the casting, collect the oil and lead it through various channels to the parts requiring lubrication. Surplus oil is returned to the chamber.

Oil Filter:

Repeated use of a lubricant is desirable for economy, but oil gets contaminated in course of its use. Dirt and sludge are formed, which are harmful to the bearing. To purify this oil for re-using it is filtered. The filtering may be continuously carried out by fitting a filter in the lubricating system.

This can be easily done in case of forced feed type system of lubrication. Filtering may be done periodically by removing the oil from the system and filtering it and again putting it into use. This method is used where it is not easy to fit the filter in the lubricating oil circuits.

The filters generally consist of cloth filters, closely spaced, through which oil must pass. Oil is sometimes heated before passing through the oil filters thereby reducing the viscosity of the oil and increasing the flow through the filter.

Where oil filtering is separately done centrifuges are used to speed up filtering. After a long use oil is so much contaminated that it cannot be useful even after filtering and at that time it is replaced by fresh oil.

Closed Crankcase Engines:

Mostly smaller power vertical engines have a closed crankcase and they are lubricated as follows:

At the bottom of the crankcase an oil sump is formed. It is always filled with oil upto a definite level which can be easily noted by the help of a dip stick provided for the purpose. The lubrication of all the above mentioned parts takes place by the splash.

The mist of oil formed due to splash settles on the cylinder walls as well as on the open surface in the crankcase. The mist will lubricated the cylinder walls, timing gears and the cam surfaces. Then the oil mist which has settled on the inner surface of the crankcase is drained in the pockets which lead the oil to different bearings.

Oil is supplied in this manner to the main bearings, push rod guides, valve rod guides, rocker pins and camshaft bearings. The small end and the big end bearing of the connecting rod are lubricated by a scoop, which is a hollow pipe leading to the big end bearing and bent in the direction of rotation of the crankshaft and fitted on the big end bearing cap.

During the splash the scoop also dips in the oil which is forced through it to the big end bearing. A hole is also drilled from big end bearing to the small end bearing and so the oil from big end bearings is applied to small end bearing for its lubrication.

In large high speed vertical engines, the lubrication of cylinder walls, timing gears and cam faces is carried out by splash but lubrication of the rest of the parts is mostly carried out by forced lubrication. The pump supplying oil is located in the oil sump. The oil is led to the different bearings through the holes drilled in the crank shaft and the engine body.

Though lubrication of different engines differs with the designs but for the vertical engines combination of the forced and splash method is mostly used to lubricate all the parts.

Lubrication of the Horizontal Engines:

For high speed and high power engines lubricating oil is led to different parts by a force from oil pump and complicated lubricating methods are to be sought depending upon the speed, service and type of the engine.

For slow and medium speed I.C. engines of the horizontal type, one of the commonly employed methods is described below:

Main bearings and camshaft bearings are lubricated by ring lubrication. The big end bearing is supplied with oil by the centrifugal force developed by the rotation of the crank. One method of lubricating big end bearing is shown in fig. 13-69.

In this case, the oil is supplied in a ring fitted concentric with the shaft on the crank web. It is forced from there to the big end bearing through the hole drilled through the web and the crank pin.

The cylinder and the small end bearings are lubricated as follows:

Fig. 13-70 shows one method of lubricating them. The oil is supplied through a hole in the cylinder walls. It falls on the moving piston and thereby spreads on the cylinder walls. There is a hole in the piston just above the hole in the bearing of the small end. When these two holes come in line with the hole in the cylinder wall, the drops of oil fall directly in small end bearing and it gets lubricated. This occurs at every stroke of piston.

Oil is supplied to the ring in case of big end bearing and to the cylinder hole in case of Small end bearing by a separate drop feed lubricator fixed in position or if a force pump is fitted, it gives the oil through pipes. The rest of the parts, also, are supplied with oil by drop feed oiler or by force pump.

Minor parts, which do not require lubrication continuously, are lubricated by hand lubrication.

Fig. 13-71 shows the gear pump used for supply of oil high pressure to lubrication system.

For complete lubrication of any particular I.C. engine, the manufactures supply the instructions. The methods of lubrication of each part are also explained there.

Types of Lubrication System:

There are three types of lubrication system used in I.C. Engine:

(1) Wet sump

(2) Dry sump

(3) Mist lubrication.

(1) Wet Sump Lubrication System:

The wet sump lubrication system, the lubricating oil is drawn from the engine sump which contains the oil. The oil is placed in the sump and it is drawn by pump through the strainer. The sump contains the lubricating oil and supplies continuously to system. Fig. 13-72 shows wet sump lubrication system.

There are three types of wet sump lubrication systems used as below:

(i) Splash system

(ii) Semi-pressurised

(iii) Full pressurised.

(i) Splash System:

Fig. 13-73 shows the splash type lubrication system used for small and slow speed engines. The system provides lubrication to caps, crank pin bearing and main pin bearings which are placed in the lubricating oil of the engine sump. When the connecting rod is in the lowest position, it dips in the oil troughs and thus directs the oil through the holes in the caps to the big end bearings.

The splash of the oil by the cap of the big end bearing results into lubrication of the cylinder walls, crank shaft and piston rings etc. The surplus oil splashed eventually flows back to the oil sump. The constant level of oil is maintained in the sump.

This system is suitable for low and medium speed engine having low CR engines.

(ii) Semi-Pressurised System:

This is combination of splash and pressure lubrication systems. In this case the pressurized oil is supplied to the engine crank shaft, cam shaft etc. The oil is drawn from the oil sump by use of oil pump and filter. The big end bearings and main pin bearings are lubricated by pressurized oil. The small end bearings, piston pin and piston rings are lubricated by the splash of the oil. An oil pressure gauge is used to know the pressure of the oil in the line. This system can be used for the high speed and high CR engines.

(iii) Pressurised Lubrication System:

Fig. 13-74 shows the pressurized lubrication system. In this system, the oil is drawn from the sump by gear type of oil pump. The oil from the oil pump at very high pressure is supplied to the main pin bearings and from the main pin bearing of the crank shaft. It is supplied to the crank pin bearing by the groove cut in the crank shaft.

The oil is at high pressure which is supplied to bearings which results into hydro-dynamic lubrication of the bearings, which supports the higher loads. The oil from the big end of the connecting rod flows at high pressure to the small end of connecting rod through the groove cut in the connecting rod.

Then the oil at high pressure flows through the surface of the piston pin towards the cylinder walls resulting into splash of oil on the walls which lubricated the piston rings. Therefore, the complete lubrication system is controlled by high pressure oil.

 

This system can be used for high speed and load engine developing very high power.

Dry Sump Lubrication System:

In this system, the engine sump remains dry and the oil is placed in the tank near to the sump. The oil is drawn from the tank and supplied by pump to the lubrication system. Fig. 13-75 shows the dry sump lubrication system.

 

Mist Lubrication System:

This system is used in the two stroke engine for lubrication. In this lubrication system, the oil in certain proportion of fuel is pre-mixed in the fuel tank and lubricating oil is supplied along with the fuel to the crank case of the engine. The lubricating oil gets deposited on the surfaces of the main pin, crank pin and piston pin bearing and piston rings.

This provides lubrication to rubbing parts of the engine and the fuel enters in the engine cylinder from the crank case. The disadvantage of this is that the lubricating oil along with the fuel may enter the engine cylinder. Fig. 13-76 shows the dry sump lubrication system.