Faculty of Engineering & Transport

THERMODYNAMICS I -  SAMPLE TEST 2

EXAMINATION CONDITIONS
National Module EA714
TAFE Subject 7760B
Time Allowed 2 hours
Calculators may be used.
Attempt all questions
Show all working
TOTAL MARKS    100
This test is worth 60% of your assessed mark.
Marks available are shown in [square] brackets.

DATA & FORMULAE

TABLE I (General & Constants)

P  =  F * v  =  W / t 
Cp Air  1005 J/kgK
W  =  Energy  =  F * s 
 R(air) = 287  J/Kg/K 
Q = m * Cv * D T
Cp Water 4.19 KJ/kgK
F  =    M * a 
 Density Air 1.2 kg/m3
Q    =   M * L 
Ice Latent Heat Fusion 335 KJ/kg.
R   =   Cp  - Cv
 Carbon = Atomic Mass 12 
g   =   Cp/Cv
 Hydrogen = Atomic Mass  1 
P*V  =  M*R*T
 R = 8314/M 
W  =  (P1V1 - P2V2)/(n-1)
Air Pressure = 101.3 kPa
W  = P1V1Ln(V2/V1)
Pi = Pm*L*A*N
T2/T1 = (V1/V2 )(n-1)
Pb  =  T * w
P2/P1 = (T2/T1 )(n/(n-1))
Pth  =  mf * CVf
hc = 1 - Tl/Th
hith =  Pi/Pth
hbth =  Pb/Pth
hbi =  Pb/Pi

TABLE II (Gases)

Process
p,V,T
Work W
Internal Energy E
Heat Q
Constant Pressure
V1/T1 = V2/T2
p(V2 - V1) 
mCv(T2 - T1) 
mCp(T2 - T1)
Constant Volume
p1/T1 = p2/T2
0
mCv(T2 - T1)
mCv(T2 - T1)
Isothermal
p1V1 = p2V2
P1V1*Ln(V2/V1)
0
P1V1*Ln(V2/V1)
Polytropic
p1V1n = p2V2n
(p1V1 - p2V2)/(n-1) 
mCv(T2 - T1)
W + DE
Adiabatic
p1V1g = p2V2g
(p1V1 - p2V2)/(g -1)
mCv(T2 - T1)
0

Compression Ratio is the ratio of max. cylinder volume to minimum volume.
 

Question 1.{20}
A Tank of Diameter 3 m and height 6 m contains acetylene (C2H2) at a pressure of 20 KPa (gauge) and a temperature of 25 deg.C.  The tank is cooled until the temperature reaches zero degrees centigrade.   Calculate..
(a)  The mass  of Acetylene in the tank                        [10]
(b)  Final pressure shown on the gauge.                       [4]
(c)  Heat extracted from the Acetylene                        [4]
(d)  Change in internal energy of the Acetylene.           [2]
Take Cp for Acetylene to be 1.8 KJ/Kg/K
{54kg, 111 kPa, 2.0 MJ, 2.0 MJ}

Question 2{20}
Air at 1.2 MPa (abs) and a temperature of 150 deg.C is expanded adiabatically to a pressure of 150 kPa (abs) and then compressed isothermally to it’s original volume.  Draw the process on a P - V diagram and calculate:

{0.1012 m3/kg, -39C, 664kPa, 136 kJ/kg}

Question 3{30}
A Six cylinder four stroke engine has bore 75 mm and stroke 70 mm and gave the following results when tested at 3300 RPM
Mean Effective Pressure

The net brake load was  450 N
Brake arm   300 mm long.
Fuel Consumption  20 litres/hour
Assuming fuel has a calorific value of 35 MJ/Kg and Relative Density 0.85, determine {56.3 kW, 46.6 kW, 83%, 0.365 kg/kWh, 33.9%, 28.2%}

Question 4{19}
Draw the constant volume (Otto) cycle on a P - V diagram.            [3]
For a theoretical engine operating on this cycle drawing in air at 30 deg.C and 95 Kpa with a compression ratio of 8.5 : 1 and a swept volume of 1.1 litres, determine:
a) Temperature & pressure after compression,                                 [9]
b) Work done during the compression stroke.                                  [7]

(440 deg.C; 402 J)

Question 5.{11)
An internal combustion engine has an overall efficiency of 30%.   It produces 100kW brake power, and the maximum & minimum cycle temperatures are 1430C and 200C.  The calorific value of the fuel used is 42 MJ/kg. Determine
a)    The maximum (carnot) efficiency possible.                        [3]
b)    The quantity of heat rejected per second.                         [4]
c)    The mass of fuel consumed per hour.                                [4]

(72.2%; 233 KW; 28.5Kg/hr)
 

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