# Machine Dynamics Questions and Answers – Forces on the Reciprocating Parts of an Engine Neglecting Weight of the Connecting Rod

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This set of Tricky Machine Dynamics Questions and Answers focuses on “Forces on the Reciprocating Parts of an Engine Neglecting Weight of the Connecting Rod”.

1. The net force acting on the crosshead pin is known as __________
a) Crank pin effort
b) Crank effort
c) Piston effort
d) Shaft effort

Explanation: Crank pin effort is the force acting along the direction perpendicular to the crank. Crank effort is the torque acting on the crank, piston effort is the force acting on the piston or the crosshead pin.

2. Piston effort acts along the line of stroke.
a) True
b) False

Explanation: Piston effort is the force acting along the line of action of the stroke on the crosshead pin or the piston.

3. In a horizontal engine, reciprocating parts are accelerated when the piston moves from _______
a) TDC to BDC
b) BDC to TDC
c) Midway to TDC
d) BDC to midway

Explanation: The reciprocating parts of a horizontal engine are accelerated during the period when the piston moves from inner dead center to outer dead center.

4. In a horizontal engine, reciprocating parts are retarded when the piston moves from _________
a) TDC to BDC
b) BDC to TDC
c) Midway to TDC
d) BDC to midway

Explanation: The reciprocating parts of a horizontal engine are accelerated during the latter half of the stroke, i.e when the piston moves from inner dead center to outer dead center and retards when the piston moves from outer dead center to inner dead center.

5. When the piston is accelerated, the piston effort is given by which of the following the equation?
a) F(L) – F(I)
b) F(L) + F(I)
c) F(L) ± F(I)
d) F(L) – F(I) + R(f)

Explanation: During acceleration, piston effort is the sum total of the net load on piston and inertia forcer. The presence of resistance force also effects the expression.

6. In the presence of frictional resistance, the expression for piston effort is _________
a) F(L) – F(I)
b) F(L) + F(I)
c) F(L) ± F(I) – R(f)
d) F(L) – F(I) + R(f)

Explanation: Piston effort depends on the net piston load and inertia force, in presence of frictional resistance and additional term of R(f) is subtracted from the overall expression.

7. Crank effort is the product of crank pin radius and _______
a) Thrust on sides
b) Crankpin effort
c) Force acting along connecting rod
d) Piston effort

Explanation: Crank effort also known as turning moment or torque on the crank shaft is the product of the crankpin effort a.k.a FT and the crank pin radius a.k.a r. It is applied at crank pin perpendicular to the crank.

8. In a horizontal engine, the weight of the reciprocating parts also add/subtract to the piston effort.
a) True
b) False

Explanation: The weight of the reciprocating parts of the engine add or subtract to the piston effort only in a vertical engine and it depends on the motion of the piston.

9. For the given data of an Internal combustion engine : Mass of parts = 180 kg bore = 175 mm, length of stroke = 200 mm, engine speed = 500 r.p.m., length of connecting rod = 400 mm and crank angle = 60° from T.D.C, find the inertia force.
a) 17.56 N
b) 19.2 N
c) 18.53 N
d) 18.00 N

Explanation: Ratio of length of connecting rod and crank n = l/r = 2*200/(200÷2) = 4,
We know that inertia force is m.ω2.r(cosθ + cos2θ/ n)
inserting values will give F = 18.53 N.

10. The crank-pin circle radius of a horizontal engine is 300 mm. The mass of the reciprocating parts is 250 kg. When the crank has travelled 30° from T.D.C., the difference between the driving and the back pressures is 0.45 N/mm2. The connecting rod length between centres is 1.2 m and the cylinder bore is 0.5 m. If the engine runs at 250 r.p.m. and if the effect of piston rod diameter is neglected, calculate the net load on piston.
a) 88000 N
b) 90560 N
c) 78036 N
d) 88357 N

Explanation: Net piston load is given by F(l) = (p1-p2)πD2÷4
p1-p2 = 0.45 N/mm2
D = 500 mm
Therefore F(l) = 88357 N.

11. From the data given:
mass of the reciprocating parts = 250kg
difference between the driving and the back pressures is 0.45 N/mm2
The connecting rod length between centres is 1.2 m and the cylinder bore is 0.5 m.
engine runs at 250 r.p.m & 30° from T.D.C.
Find the piston effort.
a) 32.4 kN
b) 35.2 kN
c) 37.3 kN
d) 40.2 N

Explanation: Net piston load is given by F(l) = (p1-p2)πD2÷4
Fp = Fl – Fi
Fi = m.ω2.r(cosθ + cos2θ/n)
= 51020 N
Therefore, Fp = 37.3 kN.

Sanfoundry Global Education & Learning Series – Machine Dynamics.

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