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SELECTION OF ENGINE TYPE

The first step in designing an engine is to determine whether it is, to he a carburetor or a dieselone. Carburetor engines are the most widespread in passenger car and low-capacity trucks, anddiesels in trucks with a capacity over two tons, in dump trucks, trailer tractors and bus. The fuelconsumption in diesels is 30-50 percent lower than in carburetor engines, and for this reason more

and more medium and low-capacity vehicles are being designed with high-speed diesel engines.Diesels cost more manufacture and repair than carburetor engines, because more metalis required per kilowatt, the critical parts are made of alloy steel, and the fuel injection equipment ismuch more expensive. The growth of the compression ratio and speed of carburetor enginesobserved in recent years increased the cost of production owing to the higher mechanical and[thermal loads in the critical elements, and the use of high alloys and more expensive productionmethods. The cost of operation has also eased, because such engines require high-octane petrol,although the fuel consumption is somewhat reduced owing to the high compression ratios. The useof aluminum alloys m passenger cars is constantly increasing. They are used to make many partsof engines. The result is not only a reduction in the weight of the engine, but also a lower price andoperating costs. Among the parts made from aluminum alloys are the main casting (cylinder blockand crankcase), cylinder head, water and oil pumps, oil filter housing, end covers of the generatorand starter, and the intake pipes. The use of such alloys reduce the engine weight by to 35 percent.When cast iron parts with thin walls 3.5 millimeters thick are used, the engine approaches that of anengine with aluminum parts. lather liquid or air cooling will used depends mainly on the climate andoperating condition. In air-cooled engine, the power per liter and per unit of piston area and thebrake mean effective pressure are six to eight per cent lower than in engines with liquid cooling inview of the great losses in the cylinder charge during admission caused by the high temperature ofthe pipes, ducts in the head, cylinder walls and head, and the piston-air cooled engines arc Moredifficult to hop up with respect to die power indices, which are limited by the maximum temperature'of the combustion chamber wails (in the partition between the valves), detonation in carburetorengines, and the thermal stability of the lubricating oils used-Besides/such engines are very noisyin operation .more power is required to drive the fan, and it is difficult to heat the body in coldweather. Air-cooled engines are greater in size than water-cooled engines of the same power thelength of an engine depends on the distance between the cylinder axes. In wilier-cooled engines,this distance is 1.24D, while in air-cooled engines it is 1.46D, because separate cylinder are usedand the surface of the cooling ribs is well developed. The depth of the radiator of in-line water-

cooled engines is 8-12 percent of their length. If the overall dimensions of engines are compared with account taken of the radiatordimensions, an air-cooled engine will be very slightly longer than or of the same length as a water-cooled engines. The width and height of both types of engine are the same. With air cooling, it ismore difficult to accommodate the auxiliary units of the feed and ignition systems, generator andstarter on the engine body owing to the presence of hoods and the danger of overheating. Tochoice of the number and arrangement of the cylinders in a newly designed engine is determinedby:1- The balancing of the forces of inertia of the revolving and reciprocating parts and their moments.2- Uniformity of crankshaft rotation.Requirements to the arrangement in the vehicle due to the necessity of reducing the overalldimension as much as possible and ensuring access to the basic mechanisms and units.4- Obtaining the minimum weight a very rigid main casting (cylinder block and crankcase)

5- 'the kind of cooling. The present tendency is to reduce the length of a motor vehicle engine byemploying a V-type design of the blocks. This also decreases the weight as compared with an in-line engine with the same number of cylinder, liters capacity, power and speed. On anaverage, the length of an engine is reduced by 30 percent and its weight by 20-25 percent.Todecrease the engine height, it is good use a horizontal or V arrangement of the cylinder with anangle between them of 90 or 120 and with L/D < 1. The engine size is minimum when tinsangle is 90. Most widespread are eight-cylinders engine with the cylinder at an angel of 90Powerful engines have up to twelve cylinders arranged in two rows. When the cranks are displacedbe 120 in such engines, the forces of inertia of the reciprocating and revolving parts are wellbalanced. Rotation is most uniform in such engines when the angle between the cylinder is 60.

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Horizontal arrangement of the cylinders is a promising design for the engines of passenger cars,trucks, buses and special types of wheel and crawled-mounted vehicles, since the driver'scab can be arranged directly above the engine, the plat form load carrying area increased, andvisibility and control improved.See Engine Design Program

Linear:

Cylinder liners:" There are two types:1 - the wet type when the out side is in direct Contact with water.2- the dry type is pressed into the cylinder.Dry linero Good resistance to wearo Small amounts of expensive alloy metalso It is finally machined after forcing into the cylinder blocko It is difficult to replace in serviceo No danger of water leakage in the Combustion space or the crank case.o A decrease of heat conduction through the composite wallo In some cases the liner is combined with the water jacket, to prevent leakage.

Wet linero Can be replaced without removing the engine from the caseo They are easily inserted into the guide centering belts of the block, the belts certain the linerstability and geometrical shape during operation.

Piston:-The purpose and types of pistons:The working space of a piston engine is formed by the cylinder, the cylinder cover and the slidingpart called the piston .The piston closes the cylinder hermetically and at the same time changes thevolume of the working space. In engine without cross head the piston is connected directly with dieconnecting rod by means of the judged pin In engines with cross heads the piston is fixed to thepiston rod. The type of the engine determines the shape of the piston and its sealing. Pistons areused in steam engines, compressors, blowers, pumps, combustion engines as well as in different

pneumatic and hydraulic systems etc.Types of pistons :According to their shape, pistons may be divided as follows:1-TRUNK PISTONS2-DISC PISTONS3-DIFFERENTIAL PISTONS4-PLUNGERSPistons must fulfill the following requirements:5-They must prevent the gas or liquid from escaping out of the working space, in internalcombustion engines also excessive penetration oil of lubricating oil into the working space6-They must provide for the transfer of heat delivered to the piston by the gas or liquid into thecylinder walls, i.e. they must possess a good thermal conductivity.7-They must be light in order to ensure small inertia forces of the reciprocating masses. d) Trunkpistons must transmit the normal pressure N on the cylinder.

8- The wear of pistons as well as of cylinder should be as small as possible.See Engine Design Program

Connecting Rod:

connecting rod it is transmits the force acting upon the piston to the crank pin and transforms therectilinear reciprocating motion of the piston into rotating motion .

A connecting rod consists of a shank and two ends usually called the " Big " and " small " ends .materials for connecting rod Steel, carbon steel [su >50 kg/mm2 ] and high - quality carbon steel [sy= 50-80 kg /mm2 ] are used to produce the connecting rod .structural layout of connecting rods

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Short connecting rods are used in high-speed engines ,in which the tendency is toreduce inertiaforce of the moving parts .The section of the connecting rod shank is designed with respect to theengine type and to the kind of the connecting rod material .connecting rod small end The design and shape of the connecting rod small end depend on thesize of the piston pin and on how it is fastened . The external surface of the small end is tapered(fig c) .The length of the small end depends on the allowable pressure on the pin or the conditionsof its end constraints . The holes for splash lubrication of the piston pin should be designed in thetop of the small end , outside the zone of stress concentration .small end construction data :d = piston pin diametera = ( 0.3 - 0.4 ) dfor steel end s = ( 0.09 - 0.15 )dfor Al.alloy end s = (0.2 - 0.3 ) dfor bronze bush ( HP = 160 - 200 )d = 0.08 d = ( 1 - 4 ) mmBush iterferance = 0.03 - 0.12 mmH7/s6 - H7 / u6Side clearance between small end sidesand piston bosses = 1 - 2 mm for each sideconnecting rod big end :

The dimension of the connecting rod big end depend on the diameter and length of the crankpin .The big ends are of split design in multi-cylinder engines . The cap is fastened by means of bolts orstuds which must be tightened sufficiently to make a properly sealed joint at any speed .Withdeveloped crankpins , when the relative dimension (dcp / D )= 0.66 - 0.68 and more , and the pistonwith the connecting rod is to be extracted through the cylinder when taking the engine apart , thebig ends are made with an oblique splitting plane at angle 30 , 45 and 60 to the longitudinal axis ofthe connecting rod shank . This reduces the force of inertia applied to the bolts .connecting rod bolts:Connecting rod bolts are subjected to the action of fluctuating loads . The principal load is the forceof inertia of the reciprocating parts and the centrifugal force of the revolving part of the connectingrod minus the cap mass .To take up the centrifugal forces of the connecting rod big end acting on the bolts , the bolts mustbe well fitted in the big end gap . In the case of highly stressed high - speed units , a serration is

often used between the big end and its cap .In the preliminary calculation , the connecting rod boltdiameter ( db )is related to the pin diameter ( dp ) . With four stroke single - acting engines a diameter db =( 0.18 -0.25 ) dpanalysis of connecting rodThe various element of a connecting rod operate under reversed and fluctuating stresses whosemagnitude changes within broad limits .Analysis of a connecting rod consists in determining thecharacteristic stresses , deformations and factors in the small end , shank , big end , and capbolts .connecting rod small end :The small end of a connecting rod is tensioned by the forces of inertia ( IF )