H3051
THE UNIVERSITY OF SUSSEX
BEng (final year) and MEng (year 3) EXAMINATION 2023
January 2023 (A1)
Low Emission Vehicle Propulsion
Candidates must attempt THREE out of FOUR questions
Exam Duration: 3 hours
If all four questions are attempted, all will be marked, but the lowest mark
will not be counted in the total for the paper.
Write your answers on A4 paper, scan and save as a single PDF file and
upload to Canvas
PDF file name: candidate number_module title
Examination handout: Formula Sheet (available to download from Canvas)
Read Academic Integrity Statement
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examination or discuss this assessment with others before the end of its 24 hour
window.
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regulations as they relate to this assessment.
H3051 Low Emission Vehicle Propulsion
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Question 1
a) Using Figure Q1-1, suggest a specification for a four-stroke naturally
aspired engine with the maximum power of 250 kW and the maximum
torque of 1000 Nm. The specification will include the suggested stroke of
cylinders, number of cylinders. the bore of pistons and the resulting swept
area. Assume that the maximum power density on the piston must be
2.5 MW/m2 and must be less than or equal to 12 .
Hint: As the best practice, the bore of the pistons should be slightly less
than their stroke.
Figure Q1-1: Maximum torque and power curves of the engine at different rotational
speeds
[10 marks]
b) For the results of (a) and using Figure Q1-2, how much will the volumetric
efficiency of the engine be improved if we use four identical valves (two
valves for intake and the other two for exhaust gas) instead of two identical
valves per cylinder Assume = 0.12 and = 30℃. The specific gas
constant and the heat capacity ratio of air are respectively 287 / /
and 1.40.
Hint: the maximum diameter of the intake valves in a 16-valve structure is
41% of the bore. Using this ratio, you should calculate the equivalent
diameter of an imaginary circular valve with the same area as the sum of
the areas of two intake valves. Theis equivalent diameter will be used to
calculate the Mach number.
/Question continued on next page
0
50
100
150
200
250
0
200
400
600
800
1000
1200
0 1000 2000 3000 4000
Speed (rpm)
Power (kW)
Torque (Nm)
H3051 Low Emission Vehicle Propulsion
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Figure Q1-2 Volumetric efficiency of the engine in terms of Mach number of airflow
[6 marks]
c) Figure Q1-3 illustrates a suggested valve timing for the engine in (a). From
the figure, how long earlier is the exhaust valve open before starting the
exhaust stroke How is this early opening of the exhaust gas helpful in the
high-speed operation of the engine What is the unit of the provided
values
Figure Q1-3 A suggested valve timing for the engine designed in (a)
Also, it is known that exhaust gas recirculation (EGR) reduces NOx by
decreasing the in-cylinder oxygen concentration and temperature. Explain
how adding exhaust gas to the intake manifold decreases these in-cylinder
variables.
[4 marks]
End of Question 1
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7
Valve Flow Mach Number (-)
Volumetric Efficiency (-)
H3051 Low Emission Vehicle Propulsion
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Question 2:
a) A fully loaded passenger car is driving up an incline of 15 degrees at a
constant speed of 60 (km/h). Figure Q2-1 illustrates the BSFC map of the
engine of the car.
Figure Q2-1 BSFC map of engine
Having the car parameters as below, calculate the engine operating point
(engine speed and torque) assuming the driver has selected the most efficient
gear. Assume air with a density of = 1.225 ( / 3
).
The engine is a 3.5 litre, 4-stroke, SI engine with engine operating
range from 1000 ( ) to 6500 ( )
Vehicle Parameters:
o Final Drive ratio = 3.0:1 efficiency 90%
o Gear ratios all with efficiency 93%
1
st = 3.60: 1
2
nd = 2.30:1
3
rd = 1.30: 1
4
th = 1.00:1
5
th = 0.75:1
o Tyres Rolling Radius = 0.33 ( )
o Fully loaded mass = 1700 ( )
o Rolling resistance coefficient = 0.017 ( )
o Cd = 0.27 ( )
o Frontal Area = 1.9 ( 2
)
[10 marks]
b) As in Figure Q2-2, the intake air path consisting of a carburettor and a
compressor can be with either of the following configurations:
a. the carburettor placed before the compressor;
b. the carburettor placed after the compressor.
H3051 Low Emission Vehicle Propulsion
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Compare the cooling effect of fuel evaporation on charge temperature in a
turbocharged spark-ignition engine for these two cases. Explain which
configuration is preferable and calculate the improvement of air mass in
percentage by the preferred configuration rather than the other one for the
same cylinder volume and pressure.
Assume that the air/fuel ratio is 14.7: 1 and the evaporation of the fuel at
this ratio causes a 20 ℃ drop in mixture temperature. The pressure ratio is
1.55 and the corresponding isentropic efficiency of the compressor is 82%.
The ambient air is 30℃, the specific heat capacity and the ratio of air are
respectively , = 1.01 ( . ) and = 1.40 ( ). The ones of the air_xfffe_fuel mixture are also , _ = 1.07 ( . ) and = 1.31 ( ).
Figure Q2-2 Two possible topologies of the intake air path consisting of both
turbocharger and carburettor
[6 marks]
c) The gear ratios are optimised for fuel consumption while delivering the
same power to the wheels. Using Figure Q2-3, explain the illustrated ‘ideal
traction hyperbola’ and the red-hatched area. Also, by sketching a diagram
or by mentioning the calculation formula, explain the ‘slip limit’.
(a)
(b)
H3051 Low Emission Vehicle Propulsion
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Figure Q2-3 Delivered torque to wheels at different gears
[4 marks]
End of Question 2
Vehicle Speed V
1
H3051 Low Emission Vehicle Propulsion
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Question 3:
a) A Diesel truck is being developed with a Diesel Particulate Filter (DPF)
and NOx after-treatment. The DPF needs to be sized to allow a time
between regeneration events of 40 hours of driving at an engine load of
100 ( ). The engine has been developed with the NOx versus soot
trade-off curve in Figure Q3-1. Answer the following questions:
Figure Q3-1: A NOx – Particulates engine out emissions
1. The target tailpipe NOx emission rate is 0.26 ( ). Assuming
the truck meets this target using a NOx after-treatment with 93%
conversion efficiency, what are the particulate emissions from the
engine (Use Figure Q3-1)
2. The backpressure characteristics of the DPF are shown in Figure
Q3-2. Calculate the minimum size of the filter assuming the filter is
regenerated when the back pressure at full load exceeds
0.20 ( ).
Figure Q3-2: Backpressure versus soot loading at full load
[6 marks]
b) Redraw the the phi-T graph of Figure 3-3 (a) in your answer sheet and add
the curve of variation of the in-cylinder temperature during one
H3051 Low Emission Vehicle Propulsion
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thermodynamic cycle. On your drawn curve, mark the equivalent points of
the indicated crank angles a, b, c and d in Figure Q3-3 (b).
Also, which region (A or B) on the phi-T graph for a compression ignition
(CI) engine in Figure Q3-3 (a) corresponds to the area where high NOx
emissions are formed Explain the reason for using a cooler to reduce the
temperature of the exhaust gas before recirculated back into the cylinder.
Figure Q3-3 The Phi-T diagram for a CI engine
[8 marks]
c) Calculate the stoichiometric air/fuel ratio for a type of fuel with the following
gravimetric mixture. Assume that the air is consists of 79% of Nitrogen
( 2
) and 21% of Oxygen ( 2
). The molar mass of Oxygen and Nitrogen is,
respectively, 32 and 28 ( ).
Component
Name
Component The ratio of the
total mass
Molar Mass
(kg/kmol)
Hexane C6H14 43% 6×12+14
Octane C8H18 34% 8×12+18
Cyclohexane C6H12 16% 6×12+12
Benzene C6H6 7% 6×12+6
[6 marks]
End of Question 2
(a)
a b c d
(b)
H3051 Low Emission Vehicle Propulsion
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Question 4:
a) It is known that half of the energy lost from the engine goes to the engine
coolant. Assume that all the energy that passes to the coolant is rejected
by the radiator and the lower heating value for Gasoline is 43 MJ/kg. The
aerodynamic frontal area of the vehicle is 2.3 ( 2
). Answer the following
questions:
1. The following table shows the maximum torque and BSFC of the
engine at specific speeds. Calculate the maximum power being
rejected by the radiator.
Point 1 Point 1 Point 1 Point 1 Point 1 Point 1
Engine Speed (rpm) 1000 2000 3000 4000 5000 6000
Brake Torque (Nm) 170 195 235 210 185 140
BSFC (g/kWh) 280 225 238 250 290 330
2. Calculate the minimum coolant flow rate of the radiator to avoid
overheating under these conditions, assuming the fan is not
running, the radiator is only cooled by ram airflow with the cooling
capacity given by the graph in Figure Q4-1. The cooling capacity is
the rate at which the radiator can remove energy from the coolant
circuit.
Assume the frontal area of the radiator is 9% of the vehicle’s frontal
area and the airflow speed through the radiator is 19 ( ).
Density of air is 1.225 ( 3
).
Figure Q4-1: A graph of cooling capacity versus air flow for different coolant flow rates
[10 marks]
/Question continued on next page
Coolant
flow
rates
H3051 Low Emission Vehicle Propulsion
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a) Compare the thermodynamic efficiencies of the Diesel cycle against the
Otto cycle at the SAME compression ratio and prove that the efficiency of
the former is lower at this condition Hint: Rewrite the efficiency equation of
the Diesel engine using
1 = [(1 + ( 1))
