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Chapter: Mechanical - Automobile Engineering - Engine Auxiliary Systems Ignition System

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Purpose and Types of Lubrication System

· Petroil system · Splash system · Pressure system · Dry-sump system

Lubrication System:

 

Parts require lubrications Crankshaft bearing Piston pin Timing gears Valve mechanism Piston ring and cylinder walls Camshaft and bearings.

 

Purpose of lubrication:

 

·        Reduce friction & wear - by creating a thin film (Clearance) between moving parts

·        Seal power - The oil helps form a gastight seal between piston rings and cylinder walls

 

·        Cleaning - Cleans As it circulates through the engine, the oil picks up metal particles and carbon, and brings them back down to the pan.

 

·        Absorb shock - When heavy loads are imposed on the bearings, the oil helps to cushion the load

 

·        Cooling. - Cools Picks up heat when moving through the engine and then drops into the cooler oil pan, giving up some of this heat.

 

Types Lubrication System:

 

·        Petroil system

·        Splash system

·        Pressure system

·        Dry-sump system

 

Oil change:

 

·        Every 5000Km for four wheeler , Every 2000 Km in two wheeler Ignoring regular oil change intervals will shorten engine life and performance.

 

All internal combustion engines are equipped with an internal lubricating system. Without lubrication, an engine quickly overheats and its working parts seize due to excessive friction. All moving parts must be adequately lubricated to assure maximum wear and long engine life.

 

Purpose of Lubrication;

 

The functions of an engine lubrication system are as follows: Reduces friction and wear between moving parts. Helps transfer heat and cool engine parts. Cleans the inside of the engine by removing contaminants (metal, dirt, plastic, rubber, and other particles).

 

Absorbs shocks between moving parts to quiet engine operation and increase engine life. The properties of engine oil and the design of modern engines allow the lubrication system to accomplish these functions.

Types of Lubrication Systems;

 

Now that you are familiar with the lubricating system components, you are ready to study the different systems that circulate oil through the engine. The systems used to circulate oil are known as splash, combination splash force feed, force feed, and full force-feed.

 

Splash Systems

 

The splash system is no longer used in automotive engines. It is widely used in small four-cycle engines for lawn mowers, outboard marine operation, and so on. In the splash lubricating system, oil is splashed up from the oil pan or oil trays in the lower part of the crankcase.

 

The oil is thrown upward as droplets or fine mist and provides adequate lubrication to valve mechanisms, piston pins, cylinder walls, and piston rings. In the engine, dippers on the connecting-rod bearing caps enter the oil pan with each crankshaft revolution to produce the oil splash.

 

A passage is drilled in each connecting rod from the dipper to the bearing to ensure lubrication. This system is too uncertain for automotive applications. One reason is that the level of oil in the crankcase will vary greatly the amount of lubrication received by the engine. A high level results in excess lubrication and oil consumption and a slightly low level results in inadequate lubrication and failure of the engine.

 

Combination Splash and Force Feed

 

In a combination splash and force feed, oil is delivered to some parts by means of splashing and other parts through oil passages under pressure from the oil pump. The oil from the pump enters the oil galleries. From the oil galleries, it flows to the main bearings and camshaft bearings.

 

The main bearings have oil-feed holes or grooves that feed oil into drilled passages in the crankshaft. The oil flows through these passages to the connecting rod bearings. From there, on some engines, it flows through holes drilled in the connecting rods to the piston-pin bearings. Cylinder walls are lubricated by splashing oil thrown off from the connecting-rod bearings.

 

Some engines use small troughs under each connecting rod that are kept full by small nozzles which deliver oil under pressure from the oil pump. These oil nozzles deliver an increasingly heavy stream as speed increases. At very high speeds these oil streams are powerful enough to strike the dippers directly. This causes a much heavier splash so that adequate lubrication of the pistons and the connecting-rod bearings is provided at higher speeds. If a combination system is used on an overhead valve engine, the upper valve train is lubricated by pressure from the pump.

 

Force Feed

 

A somewhat more complete pressurization of lubrication is achieved in the force-feed lubrication system. Oil is forced by the oil pump from the crankcase to the main bearings and the camshaft bearings. Unlike the combination system the connecting-rod bearings are also fed oil under pressure from the pump. Oil passages are drilled in the crankshaft to lead oil to the connecting-rodbearings.

The passages deliver oil from the main bearing journals to the rod bearing journals. In some engines, these opening are holes that line up once for every crankshaft revolution. In other engines, there are annular grooves in the main bearings through which oil can feed constantly into the hole in the crankshaft. The pressurized oil that lubricates the connecting- rod bearings goes on to lubricate the pistons and walls by squirting out through strategically drilled holes. This lubrication system is used in virtually all engines that are equipped with semi floating piston pins.

 

Full Force Feed

 

In a full force-feed lubrication system, the main bearings, rod bearings, camshaft bearings, and the complete valve mechanism are lubricated by oil under pressure. In addition, the full force-feed lubrication system provides lubrication under pressure to the pistons and the piston pins.

 

This is accomplished by holes drilled the length of the connecting rod, creating an oil passage from the connecting rod bearing to the piston pin bearing. This passage not only feeds the piston pin bearings but also provides lubrication for the pistons and cylinder walls. This system is used in virtually all engines that are equipped with full-floating piston pins.

 

 

Four-stroke Spark-ignition Engine

 

In a four-stroke engine, the cycle of operations is completed in four strokes of the piston or two revolutions of the crankshaft. During the four strokes, there are five events to be completed, viz, suction, compression, combustion, expansion and exhaust. Each stroke consists of 180° of crankshaft rotation and hence a four-stroke cycle is completed through 720° of crank rotation. The cycle of operation for an ideal four-stroke SI engine consists of the following four strokes:

 

i.       Suction or intake stroke;

 

ii.     Compression stroke;

 

iii.  Expansion or power stroke and

 

iv.  Exhaust stroke.

 

Working principle of a Four Stroke SI Engine

 

i.  Suction or Intake Stroke: Suction stroke starts when the piston is at the top dead centre and about to move downwards. The inlet valve is open at this time and the exhaust valve is closed. Due to the suction created by the motion of the piston towards the bottom dead centre, the charge consisting of fuel-air mixture is drawn into the cylinder. When the piston reaches the bottom dead centre the suction stroke ends and the inlet valve closes.

 

Compression Stroke: The charge taken into the cylinder during the suction stroke is compressed by the return stroke of the piston. During this stroke both inlet and exhaust valves are in closed position. The mixture that fills the entire cylinder volume is now compressed into the clearance volume. At the end of the compression stroke the mixture is ignited with the help of a spark plug located on the cylinder head. In ideal engines it is assumed that burning takes place instantaneously when the piston is at the top dead centre and hence the burning process can be approximated as heat addition at constant volume.

 

During the burning process the chemical energy of the fuel is converted into heat energy producing a temperature rise of about 2000 °C. The pressure at the end of the combustion process is considerably increased due to the heat release from the fuel.

 

iii. Exhaust Stroke: At the end of the expansion stroke the exhaust valve opens and the inlet valve remains closed. The pressure falls to atmospheric level a part of the burnt gases escape. The piston starts moving from the bottom dead centre to top dead centre and sweeps the burnt gases out from the cylinder almost at atmospheric pressure.

 

The exhaust valve closes when the piston reaches T.D.C. at the end of the exhaust stroke and some residual gases trapped in the clearance volume remain in the cylinder. Residual gases mix with the fresh charge coming in during the following cycle, forming its working fluid.

Each cylinder of a four stroke engine completes the above four operations in two engine

 

revolutions, one revolution of the crankshaft occurs during the suction and compression strokes and the second revolution during the power and exhaust strokes. Thus for one complete cycle there’s

 

only one power stroke while the crankshaft turns by two revolutions.

 

Consumption of lubricating oil is high in two-stroke engines due to higher temperature.

 

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