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# Engine design

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### Engine design

1. 1. ENGINE DESIGN (Source: Tractors and Automobiles, by V.Rodichev & G.Rodicheva, Mir Publishers, Moscow)1. Operation of Multi-cylinder Engines The cycle of operations of four-stroke engines is completed in two turns of the crankshaft. Withsuch an operating cycle, the crankshaft receives energy from the piston only during one half its turnwhen the piston moves on the power stroke. During the remaining three half turns, the crankshaftcontinues to revolve by inertia and, aided by the flywheel, it moves the piston on all itssupplementary strokes – exhaust, intake, and compression. Therefore, the crankshaft of a single-cylinder engine operating on the four-stroke principle revolves no uniformly: it accelerates on thepower stroke and decelerates on the supplementary strokes of the piston. Furthermore, the single-cylinder engine usually produces little power and features excessive vibration. For this reason,automobiles are powered by multiple-cylinder engines.Fig.1. (a) Schematic diagram gram and (b) firing-order of a four-cylinder four-stroke engine For a multi-cylinder engine to run uniformly, the power strokes of its pistons must be spacedrotationally at one and the same crank angle (i.e., they must occur at regular intervals, called thefiring intervals). To find this angle, the duration of the engine cycle, expressed in degrees ofcrankshaft rotation, is divided by the number of the engine cylinders. For example, in a four-cylinder four-stroke engine, the power stroke occurs every 180˚ (720˚/ 4), i.e., every half turn of thecrankshaft. The other strokes in this engine occur also every 180˚. Therefore, the crankshaft throws(or crank throws) of four-cylinder four-stroke engines are spaced at 180˚, i.e. they lie in a singleplane. The crank throws of the first and fourth cylinders are arranged on one side of the crankshaft,and those of the second and third cylinders, on the opposite side. Such a shape of the crankshaft
2. 2. provides for even firing intervals and a good engine balance, since all the pistons simultaneouslyreach their extreme positions (two pistons reach their TDC at the same time as the other two reachBDC). The order in which like piston strokes occur in the engine cylinders is known as the firing order.The firing order of the four-cylinder engines is usually 1-3-4-2. This means that after the piston inthe first cylinder has completed its power stroke, the next power stroke occurs in the third cylinder,then in the fourth cylinder, and finally, in the second cylinder (Fig.1). When selecting a firing order for a particular engine, designers try to distribute the load on thecrankshaft as uniformly as possible. Multi-cylinder engines may have an in-line or a two-bank (V-type) cylinder arrangement. In anin-line cylinder engine, all the cylinders are arranged vertically in a straight line, while in a V-typeengine, the cylinders are arranged in two banks set at an angle to each other. V-type engines aremore compact and less heavy than their in-line cylinder counterparts. In a six-cylinder four-stroke engine, like piston strokes occur at 120-degree intervals. Therefore,its crank throws are spaced in pairs in three planes with an angle of 120˚ between them (Fig.1 a). Inan eight-cylinder four-stroke engine, like piston strokes occur every 90˚, and so the crank throwsare arranged crosswise with an angle of 90˚ between them (Fig.1 b). With an eight-cylinder four-stroke engine, eight power strokes occur for every two revolutions of the crankshaft, which makesfor very smooth running of the engine. Modern six- and eight-cylinder automotive engines use V-type cylinder arrangements. The firing order of eight-cylinder four-stroke engines is 1-5-4-2-6-3-7-8 and that of six-cylinder ones, 1-4-2-5-3-6. Knowing the firing order of an engine, one can correctly connect the ignition wires to the sparkplugs and adjust the valves. a) b)Fig.2. Crank-throw arrangements in (a) V-6 and (b) V-8 type engines
3. 3. 2. Crank Mechanism2.1. Engine Framework The engine framework serves as an enclosure and a support for all the component parts of theengine mechanisms and systems. The framework of automotive engines is formed by a number of components that are rigidly heldtogether. Depending on the engine type and power output, these structural components do havesome constructional differences, but in principle they are similar in all engines. The main structural component of a multi-cylinder engine is the cylinder-block-and-crankcaseunit.THE CYLINDER-BLOCK-AND-CRANKCASE UNIT (Fig.3) of most in-line cylinder engines is aone-piece box-like casting, this combination generally being termed monoblock construction. Toimprove its rigidity and divide it into several compartments, or chambers, the unit is fabricated withinner partitions, or bulkheads. Horizontal partition or lower deck of the cylinder block 2 divides theunit into approximately equal halves: the upper half—cylinder block 1 – and the lower half –crankcase 3. The cylinder block houses cylinder liners, or sleeves, that tightly fit into bores in theupper and lower decks of the block, the upper deck being usually referred to as the cylinder deck.Solid vertical partition 6 passing along one of the sides of the cylinder block separates push-rod, ortappet, chamber 7 from the water (coolant) jacket. Fig.3. Schematic diagram of the cylinder- block-and-crank-case unit of and in-line engine. 1 – cylinder block; 2 – horizontal partition (lower deck); 3 – crankcase; 4 – crankcase partitions (bulkheads); 5 – camshaft bearing bore; 6 – vertical partition; 7 – push-rod (tappet) chamber.