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Engine Balance (Automobile)

2.8.

Engine Balance

Due to the presence of the number of reciprocating parts, like piston, connecting rod, etc.

which move once in one direction and then in other direction, vibration develops during

operation of the engine. Excessive vibration occurs if the engine is unbalanced. It is, therefore,

necessary to balance the engine for its smooth running. The vibration may be caused due to

design factors or may result from poor maintenance of the engine. In order to minimize the

vibration, attention must be given to the following parameters :

(£) Primary balance

(«) Component balance

(Hi) Firing interval

(iv) Secondary balance.

2.8.1.

Primary Balance

When a piston passes through TDC and BDC, the change of direction produces an inertia

force due to which the piston tends to move in the direction in which it was moving before the

change. This force, called the primary force, increases with the rise of the engine speed, and

unless counteracted produces a severe oscillation in the vertical plane, i.e., in line with the

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Fig. 2.40. Direction of primary force for single cylinder.Single-cylinder. Figure 2.40 illustrates the primary inertia forces developed in a single-cylinder engine. The diagram shows the direction and magnitude of the force for one revolutionof the crankshaft, in which the upward direction has been considered as positive. Thus at TDC,the deceleration of the reciprocating masses (piston assembly and one-third of the connectingrod) produces an upward force on the engine.At BDC a similar force is also generated but the direction of the force is downwards. Theeffect of these two forces is such that when the engine is running it oscillates up and down at afrequency equal to the engine speed, causing vibration.This vibration of the engine can be reduced byadding counter-balance masses at A and B to exertan outward force with the rotation of the crankshaft.Also by varying these two masses, the outward forcecan be made to equalize the inertia forces Fi andF2. It may be noted that in positions other than thedead centers the counter-balance masses themsel-

ves produce an out-of-balance force. This is un-desirable because it only shifts the plane ofvibration from the vertical to the horizontal. There-fore, the counter-balance mass used on a single-cylinder engine is set to balance only half thereciprocating mass. As a result, vibration in thevertical and horizontal planes is expected in a singlecylinder engine. To withstand this vibration all nutsand bolts used on vehicles propelled by single-cylinder engines should be adequately locked.Four-cylinder. The crank throw layout on a four-cylinder in-line engine and the directionof the primary forces are shown in Fig. 2.41. Primary balance is achieved in this arrangement,because the forces on the two pistons at TDC equal the forces on the pistons at BDC.The crankshaft throws (as shown in Fig. 2.41) are arranged so that forces acting on pistons1 and 2 develop the opposite turning moment (couple) on the shaft axis to that produced by theforces on pistons 3 and 4. The opposing couples introduced by this crankshaft layout preventthe rocking action of the engine and consequently minimize fore and aft vibration of the engine.Counter-balance masses are added to the crankshaft to reduce the bending action on thecrankshaft produced by the couples, and the high load on the center main bearing. Also, fivemain bearings are used to support the crankshaft, instead of the three commonly used in thepast, so that a stiffer construction is obtained which is essential for the high-speed operations


of modern engines.Three-cylinder. Consideration of balancing of a three-cylinder in-line unit is usefulbecause it is used as a ‘straight’ in-line engine and also it forms the back unit for both the in-linesix and V-six cylinder engines. Figure 2.42 illustrates the crankshaft layout and the primaryforces when piston 1 is at TDC. In this case the crank throws are set at 120 degrees; thereforethe large force at each of the dead centers is balanced by the two smaller forces on the other twopistons. These smaller forces are caused due to acceleration or deceleration of the piston as itapproaches or leaves the end of the stroke.

Fig. 2.41. Primary forces for four cylinders.

Fig. 2.42. Primary forces for three cylinders.2.8.2.

Component Balance

To minimize vibration, all components that rotate at high speeds must be balanced. This isspecifically important for large heavy components such as a flywheel and clutch assembly. Eventhough these two parts are balanced individually within allowable limits, the mating of eachpart with the crankshaft axis is essential so that they ‘run true’. Various location devices suchas spigots, registers and dowels are used to obtain mating of these components.Ideally the balancing of both the crankshaft and flywheel assembly as one unit is desirablebecause it avoids the ‘build-up of tolerances’. Vibration occurs when ‘heavy spots’ of each partare positioned so that they all act in the same direction. High cost associated with duringmanufacturing as well as repair generally rules out the use of this one-piece balancing method


on mass-produced vehicles. Reciprocating masses should also be balanced to achieve goodprimary balance. All parts that move in this manner should have nearly equal weight.Balance of components should cover both static balance and dynamic balance. The staticbalance can be carried out by placing the shaft and/or component on two horizontal ‘knife-edges’,so that when released the heaviest part moves to the bottom. Dynamic balance requiresexpensive equipment, which rotates the part at high speed and indicates the extent and locationof the heavy spots. Imbalance is normally corrected by removing metal by drilling one or moreholes in the component at the heavy point.2.8.3.

Firing Interval

The angle turned by the crankshaft between power strokes of a multi-cylinder engine shouldbe regular to achieve maximum smoothness. Also if the more cylinders are fired during the 720degrees period of the four-stroke cycle, the lower is the variation in the output torque, and thesmoother is the flow of power to the road wheels (for details refer sections 2.4.2 and 2.6).2.8.4.

Secondary Balance

The inertia forces considered during the study of primary balance are based on a pistonmovement, called simple harmonic motion (SHM). This type of reciprocating movement isillustrated in Fig. 2.43A. Let a point P travels at a constant speed around a circle of diameterAB, and another point N moves in a straight path from A to B. The point N is said to move insimple harmonic motion if it always keeps at the foot of the perpendicular NP. The velocity ofpoint N varies as it travels across AB and this is represented by the graph (Fig. 2.43B).When the movement of an engine piston is compared with SHM, it can be seen that duringthe first 90 degrees rotation of the crank from TDC, the piston covers a greater distance andwithin the range 90-180 degrees it covers a smaller distance in the given time (Fig. 2.43C). Thiscauses the following situations.(a) The piston travels more than half the stroke during movement of the crank from TDCto the 90 degrees position.(b) Considering the piston initially at TDC, the relative piston velocity for each 90 degreesof crank movement is fast, slow, slow, and fast.(c) The piston dwell, which is the angular period where piston movement is small inrelation to crankshaft motion, at BDC is much greater than at TDC.id) The inertia force at TDC is much greater than at BDC.

This last point demands for the engine balance if vibration is to be reduced. The study ofengine balance requires the analysis of secondary balance, which involves the differencebetween actual piston movement and the ideal SHM.


Fig. 2.43. SHM and actual piston motion.Figure 2.44 presents the primary force produced by SHM, and also the secondary forcerequired to be added or subtracted to correspond the actual motion. It is observed that thefrequency of the secondary force is twice the speed of the crankshaft. The information providedby this graph can be used to obtain the direction of the secondary force and this has been addedto the diagram of the engine’shown in Fig. 2.45. The result indicates a four-cylinder in-lineengine has very good primary balance but has poor secondary balance. This imbalance producesa vibration in the vertical plane at a frequency twice the speed of the crankshaft. In the pastthis vibration has been tolerated and soft rubber engine mountings have been used to preventtransmission of the engine vibration to the remainder of the vehicle.

Fig. 2.44. Graph of secondary force.

Fig. 2.45. Direction of primary and secondary forces.In the three- and six-cylinder in-line units, and V-six, the secondary forces balance out, and


this is one reason why the six-cylinder in-line engine was used extensively in the past. Nowadaysfour-cylinder in-line units for engines up to about 2 liters capacity are preferred, because ofpromising economy resulting from lower frictional losses. When it is combined with the use ofsimpler engine management systems, a higher power to weight ratio can be obtained. Inaddition, the short and stubby crankshaft used on a four-cylinder unit does not produce severetorsional vibration problems associated with longer shafts.

Secondary Harmonic Balancer.

The use of a secondary harmonic balance is an effective method of eliminating secondaryforces. Frederick Lanchester used this method first time in 1911 to balance four-cylinderengines. Even though this device was very effective, the use of soft rubber mountings instead

of a damper continued for cost reasons. In 1975 Mitsubishi of Japan produced a secondarybalancer, in several ways similar in principle to the Lanchester type. The engines using thisarrangement are much smoother in operation.The principle of a secondary balancer is illustrated in Fig. 2.46. Two counter-balance shaftshaving offset masses are driven by the crankshaft at twice crankshaft speed. One counter-balance shaft is rotated clockwise and the other anti-clockwise. Both shafts are timed to thecrankshaft so that when the piston is at TDC both masses exert a downward force.

Fig. 2.46. Principle of secondary balancer.To counteract the secondary force on the engine, the balancer exerts an opposing force onlywhen it is necessary. For four-cylinder in-line engines this is a maximum when at 0, 90, 180,and 270 degrees rotation of the crankshaft. In Fig. 2.46A and C, this balancing force isdownwards and upwards respectively. In Fig. 2.46B and D, the two masses of the balanceroppose each other causing a neutral effect. The engine attains a state of balance in these neutralpositions.Mitsubishi Motors ‘Silent Shafts’ arrangement incorporates twin counter-balancing shaftswith one shaft higher up the engine than the other (Fig. 2.47). This shaft arrangement dampsthe vertical vibration and also the secondary rolling couple, produced when the crankshaft isrotated by the force of combustion.The upper shaft rotates in the same direction as the crankshaft and the vertical spacing ofthe shafts is 0.7 times the length of the connecting rod. This arrangement of the counter-balancemasses sets up a couple, which opposes the rolling couple. Balance of the rolling couplethroughout the complete engine load range is not possible. Therefore a shaft position isoptimised to minimize the unabalnced couple during the most frequently encountered road loadconditions. The rolling couple of a balanced four-cylinder engine, with this arrangement providesa better result than that of a six-cylinder unit.


Fig. 2.47. Secondary balancer (Mitsubishi Motors).

Fig. 2.48. Secondary balancer as fitted to Porsche engine.The Porsche 944 engine (Fig. 2.48) installs a double-sided toothed belt, to drive thecounter-balance shafts. The balancer system on this engine reduces the noise level by 20 dB.

When the secondary vibration, especially at high engine speed is minimized, it provides areduction of the ‘boom’, which is felt and heard in the passenger compartment. In addition, adecrease in secondary vibration lengthens the life of engine auxiliaries such as emission control

equipment, electrical and fuel supply components and management systems, which incorporateelectronic devices.

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