The crankshaft design depends on the number of cylinders the engine has, and, as you already know, how the cylinders are arranged (in-line, V-type, or horizontally opposed).
1. Four-Cylinder Crankshafts
The crankshaft design determines the firing order of the cylinders by the position of the crankshaft throws and the camshaft. On the four-cylinder in-line engine crankshaft, the throws are all in the same plane. The front and rear throws are 1800 (on the opposite side of the shaft) from the two center throws. This shaft is used with either three or five main bearing journals, depending on engine block construction. With this crankshaft, power is delivered only to the shaft during 1400 of each piston's power stroke. Therefore, there is a power lapse of 400 between each power stroke of the engine which causes the engine to vibrate. The vibration is partially reduced through the use of a heavy flywheel and a vibration damper, which is discussed in depth in paragraphs 2204 and 2205.
Four-cylinder in-line engine crankshaft
2. Six-Cylinder Crankshafts
The power lapse that we just discussed in the four-cylinder, in-line engine is completely eliminated in the six-cylinder, in-line engine due to the arrangement of the throws on the crankshaft. The throws are constructed on three separate planes spaced 1200 apart.
Six-cylinder, in-line engine crankshaft
The 1200 arrangement of the throws not only eliminates power lapses, but also gives the engine power overlap. This condition simply means that each power stroke begins before the previous stroke ends. In the case of the six-cylinder, in-line engine, the power overlap is 200. Though a piston travels through 1200 of its power stroke, the power stroke actually lasts for 1400 of the crankshaft rotation. In other words, as one piston reaches the 1200 of its power stroke, the next piston in the firing order begins its power stroke. This causes two pistons to deliver power to the crankshaft simultaneously during the last 200 of each piston's power stroke. Therefore, unlike in the last 400 of the power stroke in the four-cylinder, in-line engine, the last 400 of the power stroke in the six-cylinder, in-line engine does not have a power lapse because the next piston is carrying the crankshaft through this 400 . Additionally, on the six-cylinder, in-line crankshaft, the number one and six throws, the number two and five throws, and the number three and four throws on the same plane. The crankshaft of this engine may be supported by 3, 4, or 7 main journals depending on the manufacturer.
3. V-Type Engine Crankshaft
Another type of crankshaft is the V-type engine crankshaft. The V-6 engine crankshaft, like the six cylinder, in-line engine crankshaft, has the throws arranged 1200 apart. But, the V-type engine crankshaft only has three throws instead of the six. This is due to the fact that each throw accommodates two pistons. Piston number 1 and 2 are mounted on the first throw, pistons 3 and 4 on the second (center) throw, and pistons 5 and 6 on the third throw. This design of the V-type
engine gives both power overlap and power lapse. Let's examine how the overlap and lapse occurs. As the number one piston begins its power stroke and as the crankshaft reaches 900 of its rotation, the number six piston, on a separate throw, begins its power stroke. This causes both pistons to deliver power to the crankshaft during 500 of crankshaft rotation. The next set of pistons to deliver power are pistons that are on the same throw. This means that the number six piston must reach the end of its down stroke before power can be delivered to the number five piston.Therefore, you have 100 of crankshaft rotation with no power being delivered to the crankshaft. This same condition continues with the remaining three pistons. The flywheel on this engine may be lighter than that of the four-cylinder, in-line engine due to the decreased power lapse. If you added a fourth throw and put each throw on a separate plane and spaced them 900 apart, you would have a crankshaft for a V-8 engine . The power overlap in the V-8 engine is the same as the V-6 engine, but the additional cylinders eliminate the power lapse.
V-8 crankshaft
Someday, you will come across a three-cylinder, in-line engine crankshaft. The three-cylinder, in-line crankshaft is constructed the same way as the six-cylinder, in-line crankshaft with only half as many throws, but with the same power overlap. Very similar to the three-cylinder, in-line crankshaft is the six-cylinder, horizontally opposed crankshaft. The only difference is the length of the crankpins. They are longer to accommodate two connecting rods per crankpin. It's almost time to take a break, but first, can you identify the difference between the four-cylinder crankshaft and the V-8 crankshaft? Awesome! You're right again! On the four-cylinder crankshaft, the throws are all on the same plane, whereas on the V-8 crankshaft, the throws are
on separate planes. Also, don't forget that some V-8 engines have throws on only two planes and look very similar to the four-cylinder, in-line crankshaft. The difference is that the two pistons are mounted on each throw and have longer crankpins. You have found that the number of throws and their length determines the engine that they are designed for. Now that we have discussed crankshafts, power overlap, and power lapse, let's take a look at the flywheel. The flywheel carries the crankshaft through the periods of power lapse,
reduces engine vibration, and helps the engine to operate smoothly.
4. Flywheel Designs
The two designs of flywheels are the friction clutch and the fluid coupling
Flywheel designs
The flywheel used with an automatic transmission is made of thin metal containing a ring gear forthe starter motor to engage. It is merely a connection between the engine and the fluid couplingof the transmission. The fluid coupling carries the crankshaft through the power lapse. On theother hand, the flywheel used with a standard transmission is constructed of a much heavier metal.The actual weight of the flywheel depends upon the amount of vibration that the engine producesdue to the difference in power overlap and power lapse. Most standard transmission and automatic transmission flywheels are interchangeable on the samecrankshaft so that you have a choice of transmissions. But, the transmission and flywheel must bedesigned by the same manufacturer for that particular engine size and type. Engine vibration isproduced not only by the differences in the power overlap and the power lapse, but also by theelasticity of the crankshaft itself. To compensate for the additional vibration, a smaller wheelknown as a vibration damper is mounted onto the front of the crankshaft.
5. Vibration Dampers
Vibration dampers are used to dampen out the crankshaft torsional vibration, the twisting actionin the crankshaft caused
by the sudden application of power. The weight of the flywheel tends toresist the sudden impulse of power applied to the crankshaft. This causes the crankshaft to actually twist. The purpose of the damper is to eliminate this twisting action. Vibration dampersare usually constructed of a small wheel with a larger wheel (balancer weight) mounted around itscircumference through a rubber mounting (damping rings).
Refer now to the image below. As the inner wheel is forced to suddenly turn with a jerk, the balancerweight tends to lag behind. The flexible rubber mounting first allows this to happen, until itstretches to its capacity. Then it pulls the balancer weight around with such force that it tends topass the inner wheel and pull it. This continuous passing and lagging damps out the crankshaft vibration.
A typical vibration damper
6. Pistons
Whether it's a diesel or gasoline engine, the basic piston structure remains about the same. Somepiston variations may depend on the type of engine being discussed. shows a pistoncut-away (not all pistons are equipped with
reinforcing ribs).
Piston structure
Some pistons used in diesel (compression-ignition) engines form a combustion chamber becauseof the head design; therefore, the pistons will be of a concave head design. Some diesel pistons are flat and some have thicker heads for
additional strength and are cooled by an oil jet thatshoots oil on the underside of the piston head. Not all diesel pistons
are cooled in this manner.You will find that these pistons vary somewhat in their design. Gasoline-engine pistons also vary in design. Some are flat on the top and some may have ribs caston the underside of the piston head for cooling and reinforcement. Some pistons for both the gasoline and the diesel engine are relieved (cut flat) around the piston pin hole to allow for expansion and to reduce weight. Most piston pin bosses (the area immediately surrounding the piston pin hole) are centered; however, some are offset about 1/16 inch to either the compressionthrust side or the power thrust side to reduce a condition known as a piston slap (rock) in the cylinders. This condition is a result of an uneven distribution of pressure on the top of the piston when the gases are ignited. The offset hole tends to hold the piston flat against the cylinder wall under this uneven distribution of pressure. The portion of the piston below the piston rings is known as the skirt. The piston is kept in alignment by the skirt, which is usually cam ground and elliptical as viewed in the cross-section. This elliptical shape permits the piston to fit the cylinder, regardless of whether the piston is coldor at working
temperature. Its narrowest diameter is at the piston pin bosses, where the metal isthickest. At its widest diameter, the
piston skirt, is thinnest. As the piston expands from heatduring operation, it becomes round because the expansion is proportional to the thickness of the metal. You will find a seemingly unending variation in the design of piston skirts. These designs are desirable to keep the piston as light as possible and to prevent excessive expansion during engine operation.
The three basic piston skirt designs
7. Piston Ring Design
The two types of rings used are compression and oil. Let's discuss the compression rings first. Most compression rings
have the same general design. You will find that the primary differencein ring design is on the outer edges of the ring. These design differences are easily distinguishedby a cross-sectional view of the ring below.
Various compression ring designs
The design of the piston ring depends upon the amount of surface contact desired with thecylinder wall. There are many
ring designs--rectangular, beveled edge, rectangular with innergroove curved, rectangular with inner groove square, rectangular outer groove scraper type and rectangular with additional steel rail for scraper effect. The most common rings used are the rectangular ring and the rectangular with a grooved inner edge. These rings give full face contact with the cylinder wall with less pounds per square inch(psi) exerted. The more pressure exerted by the rings on the cylinder wall, the more drag is created on the engine. Oil control rings may be of a two-, three-, or four-piece construction. The two-piece ring israrely used in modern vehicles, so it will not be discussed in this study unit. The three-piece oilring is probably the most common ring you will
find. It consists of two steel rails, separated by aventilated steel segment. The four-piece oil ring consists of two steel rails separated by acast-iron center section, which resembles the old cast-iron oil ring mentioned earlier, and a springsteel expander.
Various oil control ring designs
8. Connecting Rods
The connecting rods are of an I-beam construction with a piston pin hole at the upper end, asaddle at the lower end, and a separate bearing cap connected to the connecting rod by two boltsor studs and nuts.
Typical Connecting Rod
Opposed- and V-type cylinders require a connecting rod with the saddle offset to accommodate the opposing pistons because the cylinders are slightly offset in relation to one another.
Offset connecting rod saddle used in opposed and V-type engines.
The saddle of some connecting rods may be cut at an angle to facilitate the removal and installation of the piston assembly. Aside from these two variations, there is little, if
any, variation in design.
Angled connecting rod saddle
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