Nomen_Oseghale thesis (E.O)

Ehinomhen (Nomen) Oseghale: E-bus: HVAC optimisation of an urban transport vehicle: a CFD model for the
evaluation of internal energy loss
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E-bus: HVAC optimisation of an urban transport vehicle: a
CFD model for the evaluation of internal energy loss
Author: Ehinomhen (Nomen) Oseghale
School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University PO Box 71, Bundoora, Victoria
3083, Australia
Email address:
Normy.ose@gmail.com {Ehinomhen (Nomen) Oseghale}
To cite this article:
Ehinomhen (Nomen) Oseghale, E-bus: HVAC optimisation of an urban transport vehicle: a CFD model for the
evaluation of internal energy loss.
Abstract: The main aim of this work is to minimize the indoor energy gain/loss in an urban transit electric bus.
In summer the battery powered electric bus uses 13kW of total load to run the HVAC system. Therefore reducing the
total energy loss will result in reduction of the total energy used to run the ventilation and air-condition system, also
reduce the bus battery size and save money. This article reports the results of CFD simulation of the bus at steady
state, when the doors are open (transient) and proposed solution to reduce the volume of hot air going into the bus.
CATIA 3D CAD software was used to develop an accurate three-dimensional bus model. Air flow analysis was carried
out using ANSYS CFD-CFX ©. The three main components within the bus structure are ventilation (outlet), diffuser
(inlet), door (opening) and Outside world. The steady state temperature is been simulated and the result is justified,
it takes 4 minutes to get the bus to a steady state temperature (30C to 20C). To validate the transient simulation, an
outside world is created. The outside world is attached to the bus on ANSY CFX and has a constant temperature of
40C and constant 5m/s wind. It was found that more energy goes into the bus when the wind direction is from the
front of the bus. Ventilation and diffusers work together as this provides equilibrium of pressure inside the bus, to
achieve an equilibrium state the inlet mass flow rate was set as 1.11kg/s and outlet vent pressure is 0Pa. Two
solutions were tested, the solutions were applying blower on top of door and turning off the RHS diffuser while
doubling the pressure for the LHS diffusers. This solution is was successful to an extent. The new proposed solution is
to add ‘Mechanical curved pads to outside the doors’; this solution is potentially going to produce more beneficial
results. The plan is to create curved pads across the door (Y direction); these curved pads will open when the door is
open. So when the air blows, the air curves over the curved pads across the door and potentially deflects the air
from going into the bus through the doors. This new proposed solution is been designed and tested.
Keywords: Diffuser, CFD, ANSYS, CFX, CATIA, CAD, Bus, Energy, Door, HVAC, Airflow, Analysis
1. Introduction
In the world today auto companies are
pushing to electrify public buses. A major
challenge is minimizing the total energy
consumption of the HVAC systems in these buses.
In large cities such as Queensland there is a high
demand for lithium ion battery powered electric
busses due to its incredible low energy density
compared to diesel busses. Battery powered
electric bus is an evolutional technology and is still
being improved to suit this day and age. E-bus is a

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bus that is driven by an electric motor and two
tonne battery and its’ energy is obtained as does
by an electric car. Electric busses are of vital
importance socially and economically, its
importance affects public transport uses globally.
The idea of energy loss in public transport arises
from the somewhat frequent door opening of
these busses. The internal temperature humidity
conditions are an important factor for the
comfort, health and safety of passengers and
drivers. To recreate this pattern it is a must that
the physical aspects are accurate. Air flows from
a region at high pressure to a region at low
pressure. As long as there is a pressure difference,
there is an air flow and in this case, the bus
interior is at a low pressure and the exterior is at a
high pressure (initially). Air flows in through the
diffusers. As air molecules accumulate inside the
bus, the pressure inside increases at a certain
rate. At one instant, equilibrium is reached and
the air stops flowing. Now, the air molecules can
only flow out of the bus if the pressure outside
drops to a low value such that, there is a pressure
difference. In this case, the interior is at a high
pressure when compared to the exterior. It's only
in this case that the air molecules flow out of the
bus. For this reasons the automotive industry has
developed ways to model the internal of the bus
and door structure to keep the loss of internal
energy to a minimum. If this goal is achieved, then
the energy loss actual value will be valid.
In this experiment, in order make adequate
progression, with the aid of CFD several
computational simulation scenarios will be carried
out to determine the airflow changes in the bus
internal.
3. CFD model
Simulations are performed with the commercial
CFD software CFX-PRE [3]. Although this software
is used because it provides state of the art grid
generation and flow modelling capabilities,
comparable results could be obtained with any of
the many similar numerical commercial models
available. Three different representations of the
fluid flow in the room are used: laminar flow,
turbulent flow using the standard k–ε turbulence
model, and turbulent flow using the RNG k–ε
turbulence model. In a relatively recent study,
Chen [4] compared the performance of five
different models for simulating simple indoor air
flows and found that the standard and RNG k–ε
predicted actual flow patterns best. The RNG
model was found to perform slightly better than
the standard model in some situations [5] and [7].
The validity of the RNG k–ε model is not yet
assured, however, due to its entirely theoretical
development and lack of widespread application
[8] and [11], but there is particular interest in its
performance with complex indoor air flows.
2. Numerical Analysis
The mathematical model, implemented for the
optimization of the air distribution system, inside
the compartment of a bus, was built using CFD
numerical analysis software (CFX-PRE ©).
The CFD, Computational Fluid Dynamics,
software identifies the method which, through
numerical algorithms, leads to the solution of the
equations which could be the laminar, or a fluid’s
turbulent motion and of the related thermo-
dynamic processes within a specified geometry.
The 3 contributors to the bus internal heat gain
are;
 Radiation (1kW/m^2 @ 12 noon in
summer)
 Convection
 Conduction
3. Existing situation Analysis
To make adequate progress towards achieving the
aims and objectives, certain adequate
experiments have to be taken and resulting data
must be analysed to full potential. Below is the 2
main simulations done to make adequate
progress towards the aim and objectives.

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 Scenario 1: Internal temperature at
steady state
 Scenario 2: Air mixing when door is open
The implementation of a CFD code provides
project validation, with low economic and time
requirements. The comfort can thus be foreseen,
building guidelines which are mostly useful in the
early stages of the system designation, enhancing
the reduction in energy consumption and
improving people‘s wellbeing. The numerical
model of the thermo-fluid dynamic phenomenon
has been carried out on a continuous model.
The governing equations for the indoor air
system are the mass conservation equation and
the Reynolds-averaged Navier–Stokes equations
for three-dimensional fluid flow. In the
simulations, three mathematical representations
are used to describe the air flow in the room.
During experimentation It was assumed that the
indoor flow is turbulent, this resulted in using a
standard k–ε turbulence model. [1,2]
2.1 Boundary conditions and flow
properties
Boundary conditions are necessary; it is used to
specify the value that a certain solution needs to
take along the boundary of the geometry domain.
In the case of the geometry used for the
experiment, below are the appropriate boundary
conditions used.
 Inlet (Diffuser)
 Outlet (Vent)
 Door (Front & Rear)
 Walls
 Windows
2.1.1 CFX setup
Before running the simulation, several important
steps have to be taken. In order for the simulation
solution to produce accurate results the below
conditions has to be set to suit the problem
outcome.
 Gravity
 Energy (ON)
 Viscous
 Material air (ideal gas)
 Air buoyancy density
 Diffuser mass flow rate
 Ventilation pressure
 Initial bus pressure
 Bodywork (Fibre glass)
 Windows (Glass windows)
 General geometry operation condition
 Boundary conditions
 Solution initialization (Initial conditions at
time=0)
 Output request
2.2 Test Environment
There are 2 different geometries, one represents
the bus the other represents the outside world.
To replicate the best results we need to develop
an appropriate geometry/s. In this case our
geometry is a 12.5 meter long bus. Its dimensions
are listed below;
 Height – 2.61 m
 Width (Extrusion) – 2.8 m
 Length – 12.50 m
Fig 1: Model bus top view
Fig 2: Model bus side view
As clearly seen on the model above the back of
the bus has been cut in an angle. This is done to
match the data provided by Bustech © (Bus
provider). This geometry specification has been
implemented in this research.

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Fig 3: Model bus door view
Fig 4: Model bus isometric view
Fig 5: Model bus isometric view
A new addition is introduced to the geometries. In
order to have an accurate result we need to
attach an outside geometry to the bus, with
appropriate spacing between both geometries.
We need to do this in order for the software to
specify what conditions are outside. This will
result in an accurate solution when the bus doors
are open. The outside world has a constant
temperature of 40C and 5m/s wind in X and Y
directions in two separate scenarios.
Outside geometry dimensions shown below;
 Height – 20.0 m
 Width (Extrusion) – 22.0 m
 Length – 40.0 m
Fig 6: Bus and Outside geometries
Fig 6.1: Bus and Outside geometries
In fluid mechanics investigations, sub-scale
models are often used to reduce the cost and
time associated with full-scale systems. In this
experiment full scale model is used to generate
the most accurate result possible matching the
actual situation inside the real life bus. Air flow
data is taken in a full-scale model bus passenger
compartment, which are relatively the exact same
dimensions, curves, edges and placements of
partitions etc. of a typical full-size bus indoor
space. The full scale model is needed so that an
actual solution can be provided to clients
(BUSTECH) after thorough investigation has been
carried out.
In this study, although there is no heating
cooling by the ventilation air, there are several
heating sources inside and outside the bus. Due to
the complications of external heat sources the
most important dimensionless parameter to be
aware of it Reynold number and buoyancy. In
most situations, buoyancy effects from heating
loads influence the structure of the air flow and

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must be included, such effects are in this study
when dealing with outside air temperature, for
the purpose of accuracy and simplicity during the
simulation buoyancy was set to 1.2kg/m3 and
radiation heating source was turned off.
The sub-scale model room, as shown in Fig. 1,2,3
is made from fibre glass and has three plane glass
windows which provide adequate optical access;
the bus is 12.5m long, 2.4 m wide, and 2.8 m tall.
Four times 9 width 1.0 m long single inlet, 2.3 m
by 0.46 m outlet vent, both on the bus ceiling,
supply and remove ventilation air, windows on
both sides of the bus and a double door passenger
front door sitting at 1.2 m in width, height at 2.12
m and 3.5 m spacing from the front edge.
2.3 Meshing
Before ANSYS CFX can calculate for a solution, we
need to mesh the geometry so the boundary
condition can act as expected. The boundary
conditions needs less mesh element size in order
for the results to be much more accurate. In other
words the finer the mesh the more accurate the
results. In this case a fine quality mesh was used.
Fig 7: Mesh door view
Fig 8: Mesh side view
Fig 9: Mesh top view
Fig 9.1: Mesh top view (Bus and Outside)
Fig 9.2: Outside inlet 5m/s wind
As seen on fig 6, 7, 8 the diffusers, Doors, inside
bus partitions, Vent and body have been sized
appropriately. Re-sizing these portions result in a
much more accurate simulation results and
airflow.
2.4 Numerical solution procedure
For the model bus, using the ventilation
component constant flow rate of 4800L/s, and the
air buoyancy of 1.2kg/m3 therefore it requires an
inlet mass-flow rate of 1.1 kg/s, the inlet contains
a disturbed turbulent flow. The inlet section is
long enough for the boundary layers to converge,
so the majority of the inlet air velocity profile is
that of turbulent plug flow. The high mass-flow
rate makes the system essentially less sensitive to
small disturbances and thermal gradients that are
assumed to be negligible in the numerical
simulation. The fluctuations of temperature and
pressure in the inlet section are very small; the
inlet is built to behave like a jet like airflow.
The vent pressure was set as 0 Pa, initial
pressure is set to 101325 Pa this will make sure
there is equilibrium pressure in the bus at all
times. Windows are set as fiberglass with a heat
transfer coefficient of 0.96Wm^-2K^-1.
The door settings is a grey area that will be
fixed, the proposed solution for this is to create an
external geometry.
The high sensitivity to upstream disturbances
requires that the pressure regulation and
upstream conditioning of the inlet be closely
monitored. A bypass flow meter helps maintain a

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constant velocity and minimize pressure
perturbations. This bypass flow meter hasn’t been
created yet in this experiment, this could be the
cause of the inaccuracies. For the purpose of this
experiment the initial temperature is set to 30C
while the outside temperature is 40C
3. Results for Existing situation
Analysis
In this work, we set out several goals/objectives.
The passenger comfort and energy change is
analysed when the door is closed and when its
open. The thermo-hygrometric changes are also
investigated, as transient temperature and air
speed gradients, related to the bus stop with open
doors. The opening and closing doors phase
usually lasts for 20-30 seconds and substantially
change the internal thermo-fluid dynamic
conditions by creating strong air speed and
temperature gradients [1]. The passenger comfort
is lost especially in some areas of the vehicle
compartment [1]. Referring to the main aim which
is optimizing the HVAC system, 3 different
solutions have been presented and is still ongoing
testing. Firstly we accomplished a steady state
temperature, from initial temperature of 30C to a
steady state temperature of 20C in a total of 3.5
minutes.
Secondly, we analyse the change in
internal energy when the bus door is open. To do
this the outside boundary conditions and
temperature were set. The constant temperature
is 40C; the temperature was used to represent the
test area in summer (Queensland Australia).
3.1 Scenario 1: Internal temperature at
steady state (Door closed)
Firstly an internal steady state temperature was
calculated and analysed (temperature of bus
internal at steady state (To)). In this case the aim
is to monitor the change in temperature inside
the geometry after X-number of seconds and
when the temperature reached a steady state the
simulation is stopped. In this set-up it is important
to replicate a real life air conditioning system, as p
[the specified total pressure of the air dispensing
from the diffusers should equal the total pressure
of air been extracted by the ventilation system
{Pin = Pout}.
Fig 9.3: ZX Plane contour under diffuser (10
seconds)
Fig 9.3.1: ZX Plane contour under diffuser (60
seconds)
seconds)
seconds) (Steady state)
Fig 9.3.4: 1.5m YX Plane contour (100
seconds)(Steady state)
Fig 9.3.5: 1.5m YX Plane contour (100
seconds)(Steady state)

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Fig 10: Bus at steady state Top view
Fig 11: Bus at steady state side view
Fig 12: Temperature contour legend
Referring to fig 9.3, 9.3.1, 9.3.2 it relatively clear
that as time increases the temperature reduces,
due to the angled up bus internal it is noticeable
to see that the air temperature starts cooling
down from the back seats toward the front of bus.
This pattern is relatively accurate due to buoyancy
and total distance travelled by the cold fluid and
also air mixing modifications. In the bus the
temperature reduces from an initial temperature
of 30C to 19.3C in a span of 252 seconds.
Fig 13: XY Plane contour (1.5 m from bottom of
bus) (10 seconds)
Fig 14 XY Plane contour (1.5 m from bottom of
bus) (60 seconds)
bus) (100 seconds)
bus) (300 seconds)
As seen on fig 13, 14,15,16 at 1.5 meter about the
bus floor it is relatively clear that as time increases
the temperature reduces, due to the angled up
bus internal it is noticeable to see that the air
temperature starts cooling down from the back
seats toward the front of bus. This pattern is
relatively accurate due to buoyancy and total
distance travelled by the cold fluid and also air
mixing modifications. In the bus the temperature
reduces from an initial temperature of 30C to
19.3C in a span of 115 seconds.
Tim
e (s)
Monitor
Point: Back
passenger
1
(Temperatu
re)
Monitor
Point:
Behind
door 1
(Temper
ature)
Monitor
Point:
Disabled
seat
(Temper
ature)
Monitor
Point:
Driver
(Temper
ature)
0 30.0 30.0 30.0 30.0
12 29.1 29.4 29.1 30.0
24 25.6 27.7 27.1 30.0
36 24.3 27.3 25.1 28.7
48 23.7 23.7 23.9 26.8
60 22.8 23.0 24.6 26.2
72 22.5 23.2 23.2 25.5
84 22.3 22.2 22.4 24.3
96 22.2 21.7 21.6 23.5
108 21.7 21.3 21.1 22.8

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120 21.3 21.4 20.9 22.5
132 21.4 21.0 20.7 22.0
144 21.3 20.5 20.5 21.6
156 21.0 20.4 20.4 21.2
168 20.9 20.4 20.3 20.9
180 20.7 20.5 20.1 20.8
192 20.7 20.4 20.1 20.8
204 20.7 20.2 20.2 20.6
216 20.6 20.2 20.2 20.5
228 20.6 20.3 20.1 20.5
240 20.5 20.2 20.1 20.4
252 20.4 20.2 20.1 20.4
Avg 22.4 22.4 22.3 23.6
Table 1: Test points temperature over time.
Graph 1: Test points temperature (C) over time (s).
The graph above shows a realistic trend in
temperature changes when the diffusers are
turned on. It can be seen that the max
temperature is 30C (initial), after 4 minutes the
bus reaches a minimum steady state temperature
of 20C.
Fig 17: Airflow streamline of inlet and outlet (AC
on) (1 second)
on)
on)
As seen on fig 17, 18, 19 the internal airflow
streamline shows the airflow movement and
change in temperature as it moves through the
bus towards the vent. This pattern is relatively
accurate as the diffuser cools the bus the vent
19.00
21.00
23.00
25.00
27.00
29.00
31.00
0 30 60 90 120 150 180 210 240
Temperature(C)
STEADY STATE
Monitor Point:
Back passenger 1
(Temperature)
Monitor Point:
Behind door 1
(Temperature)
Monitor Point:
Disabled seat
(Temperature)
Monitor Point:
Driver
(Temperature)

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sucks out air from the bus and keeps and
equilibrium pressure in the bus passenger cabinet
After thoroughly analysing all data it can be said
that the results and not fully accurate due to
reasons such as; To achieve a more improved
accurate result we, a series of two pressure
regulators should be set incrementally to ensure
that pressure perturbations do not propagate into
the experiment. The flow rate is controlled by a
needle valve, and the flow rate is adjusted until
1.11 kg/s is measured with the LDA system at the
centre of the inlet jet [3].
3.2 Scenario 2: Door Open (Transient
solution)
In this case we introduce a new addition to the
geometry. In order to have an accurate result we
need to attach an outside geometry to the bus
geometry, with appropriate spacing between both
geometries. The door dimensions are standard
and its dimensions are shown below;
Fig 20: Bus door view
Front door:
 h = 2122 mm
 L = 1256 mm
Rear door:
 h = 2122 mm
 L = 870 mm
The aim here is to simulate the airflow inside the
bus when the door is open. To do this we need to
introduce an outside domain, this domain will act
as an outside. This means the door will act as a
real boundary condition, the bus door and the
outside geometry are attached using CGI
interface. The outside domain has a constant 40C.
The outside domain shown below;
Fig 21: Outside domain
3.2.1 Scenario 2: Air mixing when door is
open (Door open)
When the door is open several variables could
determine if the result will be accurate, such
variables includes;
 Initial Internal pressure
 Diffuser pressure
 Outlet vent pressure
 Door setting
 Outside environment pressure
Taking all of the above into consideration, the
following results were simulated on ANSYS CFX ©
3.3 Door Open
Several variables needs to be considered when
the bus doors are open, and various boundary
conditions and initial conditions need to be set.
These variables include;
Test 1: Door Open wind in X-direction
 Initial temperature inside bus – 20C
 Initial temperature outside bus – 40C
 Constant wind velocity and direction –
5m/s towards bus (X direction)
 Bus inlet temperature – 20C (Constant)
 Door open at– 5s
 Door open total time – 75s (1.15 mins)

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Fig 22: Door open top view(X direction)
Fig 23: Door open door view (after 3s) (X
direction)
direction)
direction)
Fig 26: Door open door view (after75s) (X
direction)
Fig 27: Door open top view (after75s) (X
direction)
Figure 22 clearly illustrates the top view of both
the bus and outside domains. It shows that the
outside has a constant 40C degree temperature.
Fig 23 After 3 seconds, it proves that the doors
and outside domains are in perfect sync; this can
be confirmed by looking at the hot air rise above
the colder air, therefore the hot air blows into the
bus through the top of the door and the cold air
escapes through the bottom, this validates the
laws of physics. The hot air has lower buoyancy
(1.12kgm3) while the cold air-conditioned air has
a buoyancy of 1.2kgm3. These buoyancy values
are not constant, the buoyancy value changes
with temperature. Figure 24 shows the
temperature contour the door after 9 seconds.
Figures 25, 26, & 27 shows that after 45 seconds
and 75 seconds respectively, the hot air goes into
the bus through the front door and the keep an
equilibrium pressure the air-conditioned air goes
out through the back door. Figures 28 & 29
displays streamlines at the bus doors, showing air
going in and out of the bus.
Fig 28: 1.5m XY Plane Door open top view (after
0s) (Wind X direction)
Figure 28 shows the bus at a height of 1.5 meters
from the top of bus at time 0 seconds (Door just
opens). Figure 29 clearly shows the results when
the bus doors are open, it can clearly be seen that
the air goes in through the front door at a higher

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rate and velocity that the back door. Reason for
this is due to the fact that the vent is located close
to the front door. Therefore the air that goes into
the bus is relatively sucked through the vent to
keep an equilibrium pressure and stable
temperature inside the bus. Figure 30 & 31 shows
the progression of the airflow in respect to time
45 seconds and 75 seconds respectively.
Figure 32, 33 & 34 shows the bus at a height of
2.1 meters from the top of bus at time 9, 45 and
75 seconds respectively.
Tim
e (s)
Back
pass
enge
r (C)
Behi
nd
rear
door
(C)
Behi
nd
front
door
(C)
Disa
bled
seat
(C)
Drive
r (C)
Outsi
de
(C)
0 20.0 20.0 20.0 20.0 20.0 40.0
3 20.0 20.0 20.0 20.0 20.0 40.0
6 20.0 20.0 20.3 20.1 20.7 40.0
9 20.0 20.0 20.3 20.9 20.7 40.0
12 20.0 20.0 20.3 22.3 24.2 40.0
15 20.1 20.0 20.2 23.5 27.1 40.0
18 20.4 20.0 21.2 22.8 24.5 40.0
21 20.9 20.1 21.8 22.1 24.5 40.0
24 21.7 20.1 22.3 22.7 27.6 40.0
27 22.2 20.1 23.2 23.8 27.5 40.0
30 22.1 20.3 23.5 24.0 27.1 40.0
33 22.5 20.4 23.2 24.9 26.5 40.0
36 23.4 20.4 23.5 25.3 26.2 40.0
39 24.0 20.5 23.7 24.6 26.6 40.0
42 24.5 20.5 23.6 24.5 26.6 40.0
45 24.6 20.6 23.5 24.4 26.5 40.0
48 24.2 20.7 24.2 24.4 27.6 40.0
51 23.6 20.8 24.8 24.2 27.1 40.0
54 23.3 21.0 26.4 24.0 26.4 40.0
57 23.2 21.1 25.8 24.3 26.4 40.0
60 23.3 21.2 23.5 24.4 27.4 40.0
63 23.3 21.3 22.7 24.2 28.4 40.0
66 23.4 21.3 23.1 24.9 27.5 40.0
69 23.4 21.3 23.7 26.0 27.0 40.0
72 23.4 21.3 24.1 26.2 27.0 40.0
75 23.9 21.4 24.0 25.5 29.2 40.0
Avg 22.4 20.6 22.8 23.6 25.8 40.0
Table 2: Test points temperature over time when
door is open for 75 seconds (Wind in X-direction).

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Table 2 shows that the driver hits a maximum
temperature of 29.9C at time 75 seconds. The
passenger behind the rear door has a record low
average temperature of 20.6C followed by the
back passenger 22.4C. The driver records the
highest temperature of 25.8C. This temperature
change inside the bus proves the fact that energy
is been lost and gained when the doors are open.
It also solidifies the outside domain working as
intended.
Test 2: Door Open wind in Y-direction
 Initial temperature inside bus – 20C
 Initial temperature outside bus – 40C
 Constant wind velocity and direction –
5m/s towards bus (Y direction)
 Bus inlet temperature – 20C (Constant)
 Door open at– 5s
 Door open total time – 75s (1.15 mins)
Fig 35: Door open door view (after 9s) (Y
direction)
Fig 36: Door open door view (after75s) (Y
direction)
Fig 37: Door open top view (after75s) (X
direction)
As seen on figures 35, 36 & 37 it can be seen
that the result’s looks as expected. At 9
seconds the hot 5m/s air will blow in through
the rear door that and the makes it way to
the front door where the vent is located.
0s) (Wind Y direction)
15.00
20.00
25.00
30.00
35.00
40.00
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
Temperature(C)
AC ON Door Open (Wind from back)
Monitor Point: Back
passenger 1 (Temperature)
Monitor Point: Back
passenger (Temperature)
Monitor Point: Behind
door 2 (Temperature)
front door (Temperature)
Monitor Point: Disabled
seat (Temperature)
Monitor Point: Driver
(Temperature)
Monitor Point: Outside
(Temperature)

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Overall looking at the temperature contour, and
tables it can be seen that the internal
temperature is slightly higher when the wind
comes from the Y-direction (Back of bus).
Tim
e (s)
Back
pass
enge
r (C)
Behi
nd
rear
door
(C)
Behi
nd
front
door
(C)
Disa
bled
seat
(C)
Drive
r (C)
Outsi
de
(C)
0 20.0 20.0 20.0 20.0 20.0 40.0
3 20.0 20.0 20.0 20.0 20.0 40.0
6 20.0 20.0 20.0 20.2 20.0 40.0
9 20.3 20.0 20.0 23.0 20.0 40.0
12 20.1 20.0 22.0 24.3 21.2 40.0
15 20.3 20.9 27.0 23.1 25.9 40.0
18 21.9 22.5 29.7 22.4 28.6 40.0
21 25.3 22.7 29.4 23.8 30.4 40.0
24 26.8 22.4 29.2 25.3 31.9 40.0
27 26.9 21.2 29.8 25.4 32.3 40.0
30 26.4 20.5 28.9 24.7 32.2 40.0
33 25.6 20.5 27.6 25.1 32.5 40.0
36 25.2 20.7 27.2 25.5 32.1 40.0
39 25.3 20.8 27.0 25.7 31.4 40.0
42 25.5 20.8 26.8 25.6 31.2 40.0
45 25.4 20.8 26.4 25.5 31.5 40.0
48 25.4 20.9 25.9 25.7 31.7 40.0
51 25.6 21.1 25.4 26.2 31.2 40.0
54 25.7 21.3 24.9 26.2 29.7 40.0
57 25.7 22.1 24.7 25.7 28.6 40.0
60 25.7 22.1 24.4 26.1 28.9 40.0
63 25.6 21.8 23.9 26.3 29.6 40.0
66 25.4 21.9 23.8 25.7 29.8 40.0
69 25.2 22.0 25.5 25.5 30.1 40.0
72 25.4 22.1 27.1 26.1 30.0 40.0
75 25.7 22.2 26.9 26.0 29.8 40.0
Avg 24.2 21.2 25.5 24.6 28.5 40.0
door is open for 75 seconds (Wind in Y-direction).

14 | P a g e
temperature of 28.5C, (10.5% more that wind in
X- direction) at time 75 seconds. The passenger
behind the rear door has a record low of an
average temperature of 21.2C, (3.9% more than
wind in X-direction), Followed by the back
passenger 24.2C, (8.5% more than wind in X-
direction). The driver records the highest
temperature of 28.5C. This temperature change
inside the bus proves the fact that energy is been
lost and gained when the doors are open. It also
solidifies the outside domain working as intended.
Comparing wind in both X and Y directions shows
a common trend, the hottest passenger in both
scenarios is the driver followed by the passenger
behind the front door, then disabled seat, back
passenger & behind front door passenger.
3.4 Wind from back vs Wind from
front (Original setup)
Below is the average change in temperature when
the door is open with wind in both X and Y
directions respectively.
Test Point X-
direction
(C)
Y-
direction
(C)
%
Difference
(C)
Back
passenger
22.4 24.2 7.72
Behind rear
door
20.6 21.2 2.87
Behind
front door
22.8 25.5 11.18
Disabled
seat
23.6 24.6 4.15
Driver 25.8 28.5 9.94
Table 4: Test points average temperature
percentage difference over time when door is
open for 75 seconds (Wind in X&Y-direction).
Table 4 shows that the back passengers achieved
the lowest temperature in both scenarios and
they also record the lowest temperature
difference. Although the drivers have the highest
average temperature for both test scenarios, they
have a 9.9% difference in average temperature
(second highest % difference). The highest
temperature difference is recorded at the
passenger behind the front door. This figure
15.00
20.00
25.00
30.00
35.00
40.00
0 12 24 36 48 60 72
Temperature(C)
AC ON Door Open (Wind from Front) Monitor Point: Back
passenger 1
(Temperature)
Monitor Point: Back
passenger
(Temperature)
door 2 (Temperature)
front door
(Temperature)
Monitor Point:
Disabled seat
(Temperature)
(Temperature)
Monitor Point:
Outside (Temperature)

15 | P a g e
proves that the model works as expected.
Now with these concrete results of the change
in temperature inside the bus when the door is
open, innovative and realistic solutions are
simulated. Several ideas failed and 2 proposed
solutions produced positive results.
4. Solution 1 (Change diffuser
mount position)
The aim here is to reduce the average
temperature change inside the bus when the door
is open for a total time of 75 seconds. The tested
solution is to turn off the RHS diffusers and extend
the LHS diffusers forward coinciding with the front
door, essentially the diffusers runs across the
front door. Only the LHS diffuser is turned on and
its mass flow rate increases from 0.555kg/m^2 to
1.11kg/m^2. This idea of turning off the RHS
diffusers is to see if there is a noticeable
difference when the LHS diffusers mass flow rate
is increased, (RHS diffusers = 0kg/m^2, LHS
diffusers = 1.11kg/m^2). Applying more air-
conditioned air and pressure to the door area
should reduce the total energy going into the bus
Test 3: Diffuser extension Door Open wind in
X-direction (Solution)
opens). Figure 46 clearly shows what happens
when the bus doors are open, it can be seen that
the air goes in through the front door at a higher
rate and velocity than the back door. Reason for
this is due to the fact that the vent is located close
to the front door. Therefore the air that goes into
the bus is relatively sucked through the vent to
keep an equilibrium pressure and stable
temperature inside the bus. Figure 47 & 48 shows
the progression of the airflow in respect to time
45 seconds and 75 seconds respectively.

16 | P a g e
Ti
me
(s)
Back
passen
ger (C)
Behi
nd
rear
door
(C)
Behi
nd
front
door
(C)
Disa
bled
seat
(C)
Drive
r (C)
Outsi
de
(C)
0 20.0 20.0 20.0 20.0 20.0 40.0
3 20.0 20.0 20.0 20.0 20.0 40.0
6 20.0 20.0 20.0 20.0 20.6 40.0
9 20.0 20.0 20.3 20.5 23.2 40.0
12 20.0 20.2 21.7 21.6 25.6 40.0
15 20.0 21.1 23.0 21.9 25.5 40.0
18 20.0 22.4 22.7 22.9 27.4 40.0
21 20.0 23.1 22.5 23.3 27.6 40.0
24 20.0 23.0 22.8 23.9 26.8 40.0
27 20.1 22.7 23.3 26.4 26.8 40.0
30 20.1 23.0 23.7 26.1 27.0 40.0
33 20.2 23.6 23.5 24.1 27.1 40.0
36 20.3 23.8 22.9 23.4 26.8 40.0
39 20.9 23.4 22.6 24.0 26.4 40.0
42 21.6 23.4 22.7 24.9 26.1 40.0
45 21.3 24.1 23.1 25.4 26.0 40.0
48 21.1 24.7 23.3 25.7 25.9 40.0
51 21.1 24.8 23.3 25.8 25.9 40.0
54 21.2 25.1 23.3 25.8 25.9 40.0
57 21.5 26.0 23.6 25.8 26.0 40.0
60 22.0 26.1 23.9 25.9 26.3 40.0
63 22.2 25.7 24.1 26.3 26.6 40.0
66 22.3 25.8 24.0 27.3 26.7 40.0
69 22.4 25.8 23.6 28.3 26.9 40.0
72 22.3 26.1 23.6 28.5 26.9 40.0
75 22.1 26.7 24.1 27.2 26.9 40.0
Av
g
20.9 23.5 22.7 24.4 25.6 40.0
door is open for 75 seconds (Wind in X-direction).
Table 4 shows that the disabled seat passenger
hits a maximum temperature of 25.6C at time 72
seconds. The passenger behind the rear door has
a record low of an average temperature of 20.9C
followed by the back passenger 22.7C. The
disabled seat passenger records the highest
temperature of 25.6C. This temperature change
inside the bus proves the fact that energy is been
lost and gained when the doors are open. It also
solidifies the outside domain working as intended.
15.0
20.0
25.0
30.0
35.0
40.0
0 12 24 36 48 60 72
Temperature(C)
LHS Diffuser only. Wind from back
Monitor Point: Back passenger
(Temperature)
Monitor Point: Behind door 1
(Temperature)
Monitor Point: Behind front
door (Temperature)
Monitor Point: Disabled seat
(Temperature)
(Temperature)
(Temperature)

17 | P a g e
Test 4: Diffuser extension Door Open wind in
Y-direction
opens). Figure 46 clearly shows what happens
when the bus doors are open, it can be seen that
the air goes in through the back door at a higher
rate and velocity than the front door. Reason for
this could be due to the fact that the wind is
coming from the back of the bus. Figure 47 & 48
shows the progression of the airflow in respect to
time 45 seconds and 75 seconds respectively.
Tim
e (s)
Back
pass
enge
r (C)
Behi
nd
rear
door
(C)
Behi
nd
front
door
(C)
Disa
bled
seat
(C)
Drive
r (C)
Outsi
de
(C)
0 20.0 20.0 20.0 20.0 20.0 40.0
3 20.0 20.0 20.0 20.0 20.0 40.0
6 20.0 20.0 20.0 20.7 20.0 40.0
9 20.0 20.0 20.0 25.6 20.2 40.0
12 20.0 20.0 22.3 28.6 21.1 40.0
15 20.9 22.1 24.0 26.1 22.4 40.0
18 21.8 25.0 24.5 24.3 25.6 40.0
21 22.2 25.0 24.0 24.4 30.5 40.0
24 22.1 25.0 24.4 25.6 29.4 40.0
27 22.8 26.0 25.3 25.7 27.1 40.0
30 23.8 25.4 24.9 25.9 27.1 40.0
33 22.9 24.5 24.4 26.4 26.3 40.0
36 21.6 24.1 24.3 26.7 25.3 40.0
39 22.1 24.3 24.0 26.7 25.3 40.0
42 23.0 24.6 24.2 25.4 25.6 40.0
45 23.0 24.7 24.5 24.9 26.2 40.0
48 23.0 24.8 25.0 25.8 28.7 40.0
51 23.3 24.9 25.2 26.4 28.6 40.0
54 23.5 24.6 24.7 26.4 27.5 40.0
57 23.4 24.7 24.0 26.1 26.4 40.0
60 23.3 24.8 23.7 25.7 25.4 40.0
63 23.4 24.8 23.5 25.2 25.3 40.0
66 23.8 25.0 23.7 25.0 26.0 40.0
69 24.2 25.3 24.0 25.0 26.0 40.0
72 24.5 25.5 23.6 25.2 26.1 40.0
75 25.0 25.3 23.5 25.5 26.4 40.0
Avg 22.4 23.9 23.5 25.1 25.3 40.0
door is open for 75 seconds (Wind in Y-direction).

18 | P a g e
Graph 5: Test points temperature (C) over time(s)
.
temperature of 30.5C after 21 seconds. The back
passenger has a record low of an average
temperature of 22.4C followed by the passenger
behind front door 23.5C. The driver records the
highest temperature of 30.5C. This temperature
change inside the bus proves the fact that energy
is been lost and gained when the doors are open.
It also solidifies the outside domain working as
intended.
3.4 Diffuser extension solution vs
Current data
Below is the average change in temperature when
the door is open with wind in both X and Y
directions respectively for the current accurate
simulation results and the solution (diffuser
extension).
Wind from Front (X-direction):
Test Point X-
direction
(C)
(Current)
X-
direction
(C)
(Solution)
%
Difference
(C)
Back
passenger
22.4 20.1 10.82
Behind rear
door
20.6 23.5 13.15
Behind
front door
22.8 22.7 0.44
Disabled
seat
23.6 24.4 3.33
Driver 25.8 25.6 0.78
open for 75 seconds (Wind in X-direction).
Table 6 shows the passenger behind the front
door recorded the lowest temperature change of
0.44%. The highest temperature difference is
recorded at the passenger behind the rear door
13.15%. This figure proves that the model works
as expected.
Now with these concrete results of the change
in temperature inside the bus when the door is
open, innovative and realistic solutions are
15.0
20.0
25.0
30.0
35.0
40.0
1 5 9 13 17 21 25
Temperature(C)
LHS Diffuser only. Wind from back
Monitor Point: Back
passenger (Temperature)
Monitor Point: Behind door
1 (Temperature)
Monitor Point: Behind door
2 (Temperature)
Monitor Point: Behind front
door (Temperature)
Monitor Point: Disabled
seat (Temperature)
(Temperature)
(Temperature)

19 | P a g e
simulated. Several ideas failed and 2 proposed
solutions produced positive results.
Wind from Front (Y-direction):
Test Point Y-
direction
(C)
(Current)
Y-
direction
(C)
(Solution)
%
Difference
(C)
Back
passenger
24.2 22.4 7.72
Behind rear
door
21.2 23.9 11.97
Behind
front door
25.5 23.5 8.16
Disabled
seat
24.6 25.1 2.01
Driver 28.5 25.3 11.89
open for 75 seconds (Wind in Y-direction).
Table 7 shows the disabled seat passenger
recorded the lowest temperature change of
2.01%. The highest temperature difference is
recorded at the passenger behind the rear door
11.97%. This figure proves that the model works
as expected. Now with these concrete results of
the change in temperature inside the bus when
the door is
5. Solution 2 (Mechanical
addition to outside of doors
when open)
This solution is potentially going to produce more
accurate results. The plan is to create a curve pad
across the door (Y direction); this curved pad will
open when the door is open. So when the wind
blows the air curves across the door and
potentially less air goes into the bus through the
doors.
6. Conclusion
The main aim is to use modern simulation
techniques and sound engineering principles to
minimise energy wastage through the HVAC
system of an electric bus. Due to the energy
density difference, this work is applicable for
lithium ion electric bus rather than diesel buses.
The geometry used for this study has all necessary
components (Front and Rear doors, Ventilation
unit and diffusers). This geometry equals modern
day busses.
The research undertaken so far suggests that
the results are accurate. This article showcases
the results from this study. With the main aim in
mind, three objectives were required to be
completed. Firstly a successfully simulation was
carried out to investigate the airflow inside the
bus when the doors were closed (steady state).
This result established the current steady state
temperature and air quality inside the bus in real
life scenario. We assumed the initial temperature
is 30C while the outside temperature is 40C
constant. Turning on the air diffusers released
cold air-conditioned air into the bus; this air
refrigerates the bus to a steady state of 20C in a
total of 3.5mins. Also noted is the ventilation unit
as it works as expected; it constantly sucks out
excess contaminated air from inside the bus and
returns the air as cool air-conditioned air. This is
known as the refrigeration cycle. To simulate and
achieve the best results, a second geometry
needed to be designed; this geometry acts as an
outside world. It has a constant temperature of
40C and wind in both X (Front of bus) and –X
(Back of bus) directions, replicating different
weather conditions. This outside domain enabled
simulating the airflow with the bus doors open
possible and an accurate result was achieved.
Analysing the resultant data from the airflow
simulation of the change in energy inside the bus
when the door is open proves that there is a
possible solution to be implemented. Several
solutions were tested to reduce the volume of hot
air going into the bus. Such solution include
turning off the RHS diffusers and doubling the

20 | P a g e
mass flowrate of the LHS diffusers, so the cold air
mixes the hot air before it travels far into the bus.
As shown in this article that solution didn’t
produce significant result. To achieve the aim,
geometry modifications needed to be applied to
the bus. The proposed solution is to create curve
pads across the doors (Y direction); these curved
pads will open when the door is open. So when
the air blows it curves over the curved pads across
the door and potentially deflects the air from
going into the bus through the doors. This new
proposed solution is been designed and tested. At
this point in time a definite conclusion cannot be
made due to the errors already encountered.
However based on these results, the internal
steady temperature proves to be accurate; also
the second scenario (door open) proves to be
accurate. The new solution involves Mechanical
addition to outside of doors when open, this
solution is potentially going to produce more
accurate results.
Furthermore, looking back at all the time and
expertise applied in the simulations it can be
noted that three out of four objectives have been
successfully achieved and a realistic idea of the
next proposed solution has been design and
undergoing simulation/testing. The ultimate goal
of reducing the energy wastage inside the electric
bus is relatively within reach.
Nomenclature
𝐾 𝑃 =
[𝑃𝐶] 𝐶
× [𝑃 𝐷] 𝑑
[𝑃𝐴] 𝑎 × [𝑃𝐵] 𝑏
𝑚′
= 𝜌 × 𝑉′
𝑚′
= 𝜌 × 𝑣 × 𝐴
𝜌 =
𝑃
𝑅 × 𝑇
References
[1] Roberto de Lieto Vollaro. Indoor Climate
Analysis for Urban Mobility Buses: a CFD
Model for the Evaluation of thermal
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Environmental Protection and Policy . Vol.
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10.11648/j.ijepp.20130101.11
[2] Posner J.D., Buchanan C.R., Dunn-Rankin
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sting/how-far-will-your-energy-go-an-
energy-density-comparison/
[8] A. Golnik and G. Elert. (2003). Energy
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http://www.che.sc.edu/centers/RCS/desc
_e_and_p.htm
[10] Absolute, Dynamic and Kinematic
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at:
http://www.engineeringtoolbox.com/dyn
amic-absolute-kinematic-viscosity-
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[11] WhatIs.com. 2016. What is computational
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omputational-fluid-dynamics-CFD.
[Accessed 27 May 2016].
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