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# Machine Dynamics MCQ – Coefficient of Fluctuation of Energy

In the figure given below, the areas BbC, CcD represent _______

a) Power generated

b) Power lost

c) Fluctuation of energy

d) Change in momentum

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Answer: c

Explanation: The variations of energy above and below the mean resisting torque line are called fluctuations of energy. The areas under the following curves: BbC, CcD, DdE and so on represent fluctuations of energy.

The fluctuation of energy is the variation of energy above the line of minimum torque.

a) True

b) False

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Answer: b

Explanation: The given statement is false because the variations of energy above and below the mean resisting torque line are called fluctuations of energy. It is represented by the enclosing curves above and below the mean line.

From the turning moment diagram shown below, the maximum speed will occur at point

a) p

b) b

c) q

d) c

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Answer: c

Explanation: The maximum engine speed occurs at point q because the flywheel absorbs energy when the crank moves from point p to q. This results in a gain of speed.

From the turning moment diagram shown below, the minimum speed will occur at point

a) p

b) b

c) q

d) c

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Answer: a

Explanation: The minimum engine speed occurs at point p because the flywheel gives out some of its energy when the crank moves from point a to p. This results in loss of speed

Coefficient of fluctuation of energy is ratio of _______

a) Mean fluctuation energy to work done per cycle

b) Maximum fluctuation energy to work done per cycle

c) Minimum fluctuation energy to work done per cycle

d) Mean fluctuation energy to power generated per cycle

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Answer: b

Explanation: The ratio of the maximum fluctuation of energy to the work done per cycle is known as the coefficient of fluctuation of energy. Mathematically, coefficient of fluctuation of energy (CE) = Maximum fluctuation of energy/Work done per cycle.

Work done per cycle is calculated as _______

a) Tmean × θ

b) Tmin × θ

c) Tmax × θ

d) Tmax × θ/2

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Answer: a

Explanation: The work done per cycle (in N-m or joules) may be obtained by using the following relations: Work done per cycle = Tmean × θ, where Tmean = Mean torque, Mean torque is same as the mean resisting torque.

For a 4 stroke IC engine, the angle θ assumes a value equal to _______

a) π

b) 2π

c) 4π

d) π/2

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Answer: c

Explanation: For a 4 stroke internal combustion engine, a power stroke takes place every 2 revolution therefore the total angle turned is equal to 4π.

To calculate power generated, product of maximum torque and angular speed is taken.

a) True

b) False

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Answer: b

Explanation: To calculate power generated, product of mean effective torque and angular speed is taken.

P = Tmean × ω

For a 4 stroke IC engine, Tmean = 1875 N-m, then find the work done per cycle.

a) 23.56 kJ

b) 11.78 kJ

c) 5.89 kJ

d) 2.94 kJ

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Answer: a

Explanation: For a 4 stroke internal combustion engine, a power stroke takes place every 2 revolution therefore the total angle turned is equal to 4π. Hence θ=4π

therefore work done per cycle = 23.56 kJ.

A shaft fitted with a flywheel rotates at 250 r.p.m. and drives a machine, the torque fluctuated about 18750 N-m, find the power required to drive the machine.

a) 49.125 kW

b) 24.56 kW

c) 12.28 kW

d) 6.14 kW

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Answer: a

Explanation: ω = 2π × 250/60 = 26.2 rad/s,

P = Tmean ×ω

= 1875 x 26.2

= 49.125 kW.