transient simulation of coolant peak temperature due to prolonged fan and/or water pump operation...

ORIGINAL

Transient simulation of coolant peak temperature dueto prolonged fan and/or water pump operation after the vehicle iskeyed-off

Suh Chyn Pang • Haji Hassan Masjuki •

Md. Abul Kalam • Md. Ali Hazrat

Received: 5 June 2012 / Accepted: 19 August 2013 / Published online: 29 August 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract Automotive designers should design a robust

engine cooling system which works well in both normal and

severe driving conditions. When vehicles are keyed-off

suddenly after some distance of hill-climbing driving, the

coolant temperature tends to increase drastically. This is

because heat soak in the engine could not be transferred

away in a timely manner, as both the water pump and

cooling fan stop working after the vehicle is keyed-off. In

this research, we aimed to visualize the coolant temperature

trend over time before and after the vehicles were keyed-

off. In order to prevent coolant temperature from exceeding

its boiling point and jeopardizing engine life, a numerical

model was further tested with prolonged fan and/or water

pump operation after keying-off. One dimensional thermal-

fluid simulation was exploited to model the vehicle’s

cooling system. The behaviour of engine heat, air flow, and

coolant flow over time were varied to observe the corre-

sponding transient coolant temperatures. The robustness of

this model was proven by validation with industry field test

data. The numerical results provided sensible insights into

the proposed solution. In short, prolonging fan operation for

500 s and prolonging both fan and water pump operation for

300 s could reduce coolant peak temperature efficiently.

The physical implementation plan and benefits yielded from

implementation of the electrical fan and electrical water

pump are discussed.

List of symbols

F Correction factor

Re Reynolds number

DH Hydraulic diameter, coolant and air side (m)

A Reference area, for coolant and air side (m2)

m Mass flow rates, for coolant and air side

(kg/s)

l Dynamic viscosity (kg/m s)

q Heat transfer (kW)

Tc,in, Tc,out Coolant inlet/outlet temperature (K)

Ta,in, Ta,out Air inlet/outlet temperature (K)

Cp Specific heat of coolant and air (kJ/kg K)

U Overall heat transfer coefficient (W/m2 K)

LMTD Log mean temperature difference

Nu Nusselt number

x Radiator thickness (m)

Kair Thermal conductivity of air (W/m K)

e Thermal effectiveness (%)

1 Introduction

The engine cooling system of a vehicle is crucial as it

ensures that the engine always runs at its optimum oper-

ating temperature. In the water cooling system, coolant

flows in the water jacket and transfers heat away from the

cylinder head and cylinder body. The pump creates a

pressure difference in the coolant circuit and drives the

coolant flow from the water jacket to the radiator. At the

heat exchanger/radiator, hot coolant transfers heat to

cooling air. In short, two of the main heat transfer fluids are

cooling air and coolant.

In conjunction with the computation power of the cur-

rent era, efforts to reduce experimental time, and efforts to

minimize prototype cost, numerical modelling is always a

convenient and economic method to perform studies at the

primary design stage. One dimensional thermo- fluid

S. C. Pang (&) � H. H. Masjuki � Md. A. Kalam �Md. A. Hazrat

Department of Mechanical Engineering, University of Malaya,

50603 Kuala Lumpur, Malaysia

e-mail: [email protected]

123

Heat Mass Transfer (2014) 50:39–56

DOI 10.1007/s00231-013-1223-y

simulation is always utilized to study and to investigate the

system effect of engine cooling circuit, lubrication circuit,

transmission circuit, HVAC system and other thermo-

hydraulic system in vehicles. In this research, a circuit

model which is focusing on engine cooling system is built

with Flowmaster [1]. There are numerous components in

the coolant circuit. The components consist of heat

exchangers, pipe(s), valve, thermostat, heat source, pres-

sure source, flow source, expansion tank, pump, gauge and

etc. After drag- and- drop the components, the components

are linked together. It mimicked an electronic circuit with

different electronic components.

Figure 1a illustrated a simple and basic coolant circuit

of engine cooling system. On the left hand side, component

no. 2 is radiator while component 3 is pressure source of air

flow and component 4 is flow source of air flow. Coolant

throws out the engine heat (which is absorbed from engine

head and body) to ambient air at radiator. The coolant

flows out from radiator lower hose after dissipated heat at

radiator. The coolant will flow toward engine through

water pump (component 9). The engine is represented by a

solid mass (component 10) and heat source (component

11). Both the solid components are connected to coolant

circuit via thermal bridge (component 12). Engine com-

bustion generates heat, while coolant absorbs engine heat

to ensure engine is not over-heated. After leaving engine,

coolant will pass through thermostat (component 16, 17, 19

and 20). The thermostat plays her role to ensure coolant

flows through by-pass circuit during cold start (coolant will

not flow to radiator). During cold start, the engine heat will

be used to heat up itself. When the coolant temperature

reaches above 85 �C, thermostat will open slowly. Then,

coolant will flow to radiator to dissipate access heat.

The built model will be used to study the time behaviour

of coolant temperature (steady and transient state). In this

research, our main research interest is the coolant peak

Fig. 1 a One dimensional

modelling of coolant circuit for

engine cooling system: 2

radiator, 3 pressure source, 9

pump, 10 solid mass, 11 heat

source, 12 thermal bridge, 16,

17, 19, 20 thermostat, 21 torque

controller; b Hypothesis of

increasing coolant’s

temperature after vehicles’

keying off

40 Heat Mass Transfer (2014) 50:39–56

123

temperature after sudden vehicles keying-off (before keying

off, the vehicle is driven either at high speed or hill climbing

conditions). With the stoppage of fan and mechanical water

pump after vehicles’ keying-off, the coolant temperature is

anticipated to increase tremendously. The coolant mass and

air mass could not dissipate the soaking heat in-time and

thus the energy is stored as sensible heat in coolant mass

(temperature increment). In another words, the heat soak

cannot be dissipated in time after the water pump and the

cooling fan stop operating. Figure 1b illustrated the

hypothesis of this after-boiling phenomenon of coolant after

vehicles’ keying off. Under normal circumstances, the

coolant temperature maintained at 85 �C after the cold-

start. On the other hand, the coolant temperature increases

significantly after vehicles’ keying-off. This phenomenon is

explained as stoppage of fan and pump while the engine

heat is soaking inside coolant circuit.

In the literature, one-dimensional (1D) thermo-fluid

simulation is an excellent option to reflect the effects of

individual components’ (in the coolant path) on the whole

engine cooling system. Eichlseder et al. [2] have modelled

both air flow and coolant flow based on 1D thermo-fluid

simulation with the software KULI. Ning [3] has utilized

AMESim to perform system analysis for engine cooling

systems, mainly for the coolant flow path. Regarding

integrated thermo-fluid simulation, Rok et al. [4] has

integrated Flowmaster and StarCD for passenger cabin

cooling analysis. Kim et al. [5] have also coupled KULI,

Fluent and StarCD, and for automotive air conditioning

analysis including the air flow approaching the condenser.

Bancroft et al. [6] integrated 1D thermal model (of coolant

flow and oil flow) with a 3D model (of air flow) using

Vectis for the study of vehicle thermal management. Ger-

ald [7] integrated the GT-cool thermo-fluid model with

cool3D model. Then, the integrated models were exploited

to study engine cooling systems. Last but not least, a

coupled model of a thermo-fluid model using Flowmaster

(lubrication and coolant circuit) and a Fluent model (under

hood air flow) was established for under-hood thermal

analysis [8].

Though, individual thermo-fluid model [2, 3] or inte-

grated thermo-fluid model [6–8] are established to model

the engine cooling circuit. However, limited research is

carried out to study the transient phenomenon of coolant

temperature after vehicles’ keying-off. Though Piccione

et al. conducted experiment to study the coolant phenom-

enon after sudden keying-off. Extensive experiment-based

research on the cooling system after shutdown was carried

out to predict the after-boiling phenomenon [9–11]. The

experimental results provided us with a bench-mark plat-

form for our numerical results. Besides, Lucic et al. and

Sateesh et al. conducted experiment study about the

nucleate boiling phenomennon [13, 14]. While Du et al.

[15] conducted analytic study about after-boiling effect.

Finally, Pang et al. [12] conducted detailed review of

research related to engine cooling system and provided

insights about the after-boiling effect.

The focus of the current research is the use of a 1D

thermo-fluid model [1] to simulate coolant temperature

after a vehicle is suddenly keyed-off. Using the model, it

was possible to observe how the coolant temperature

fluctuated over time with heat flow, air flow, and coolant

flow. Furthermore, the peak temperature of the coolant

after keying-off could be determined. In order to ensure the

robustness of this numerical model, the numerical result

was validated with industry field data. Besides, the

numerical model was further tested with prolonged fan or/

and prolonged water pump operation after keying-off in

order to prevent the coolant temperature from exceeding its

boiling point and jeopardizing engine life. Finally, the

numerical results provided sensible insights into the pro-

posed solution.

2 Methodology

2.1 Modelling of thermo-fluid model

Flowmaster [1], 1D thermo-fluid software was used for

coolant circuit modelling. The software allowed us to

include numerous cooling components in a coolant circuit

and to link them together. It mimicked an electronic circuit

with different electronic components. A simplified coolant

circuit consisting of the engine mass, engine heat source,

thermostat, radiator, reservoir expansion tank, water pump,

pump speed control, pipes, and air flow source were built.

The model was used to study the effects of individual

components on the whole engine cooling system and the

interaction between components. Then, it was necessary to

set the parameters of each and every component in detail.

For the radiator, the parameters were coolant flow area,

coolant hydraulic diameter, airside flow area, airside

hydraulic diameter, curve of coolant side pressure drop

versus flow rates, curve of airside pressure drop versus flow

rates, and so on. Radiator heat transfer data at a given air

flow and coolant flow are available in tabular form, as

shown in Table 1. These data were normalized and con-

verted to the Reynolds number and Nusselt number by

applying Eqs. (1–10).

Equations (1) and (2) normalize coolant flow and air

flow to their respective Reynolds numbers. The Reynolds

number is a function of fluid mass flow rate, hydraulic

diameter, dynamics viscosity, and fluid flow area.

Recoolant ¼_mDH

lA

� �coolant

ð1Þ

Heat Mass Transfer (2014) 50:39–56 41

123

Reair ¼_mDH

lA

� �air

ð2Þ

Equations (3) and (4) relate heat transfer to the fluid

mass flow and fluid temperature drop.

q ¼ _mcCp;c Tc;in � Tc;out

� �ð3Þ

q ¼ _maCp;a Ta;out � Ta;in

� �ð4Þ

Equation (5) calculates the overall heat transfer

coefficient after the ‘‘log mean temperature difference’’

(LMTD) has been computed.

U ¼ q

FA � LMTD; where

LMTD ¼½ Tc;in � Ta;out

� �� Tc;out � Ta;in

� ��

lnTc;in�Ta;outð ÞTc;out�Ta;inð Þ

ð5Þ

Once the overall heat transfer coefficient is known, the

Nusselt number can be derived using Eq. (6).

Nu ¼ Ux

Kair

ð6Þ

Equation (7) indicates the maximum heat transfer at the

radiator while Eq. (8) evaluates the thermal effectiveness

of the radiator.

qmax ¼ ð _mCpÞmin Tc;in � Ta;in

� �ð7Þ

e ¼ q

qmax

e� 1 ð8Þ

Equations (9) and (10) compute the thermal

effectiveness as a function of the fluid temperature.

e ¼ ðTa;out � Ta; inÞðTc;in � Ta;inÞ

; when ðmCpÞmin ¼ ðmCpÞa ð9Þ

e ¼ ðTc;in � Tc;outÞðTc;in � Ta;inÞ

; when ðmCpÞmin ¼ ðmCpÞc ð10Þ

Finally, the radiator heat performance was converted

successfully in the form of a Nusselt surface, as shown in

Fig. 2.

Table 1 Radiator heat transfer data from local automobile

manufacturer

Water (kg/s) Air (kg/s) Heat (kW)

0.67 0.54 19.0

0.67 1.08 27.0

0.67 1.62 31.0

0.67 2.16 32.0

0.67 2.42 34.0

0.67 2.70 35.0

1.01 0.54 20.0

1.01 1.08 29.0

1.01 1.62 34.5

1.01 2.16 38.0

1.01 2.42 37.5

1.01 2.70 39.0

1.35 0.54 20.5

1.35 1.08 31.5

1.35 1.62 38.5

1.35 2.16 42.5

1.35 2.42 44.0

1.35 2.70 46.0

Fig. 2 Radiator heat transfer,

the Nusselt surface [1]


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2.2 Heat transfer analysis and heat transfer

characteristic

In the evaluation of the overall heat transfer coefficient,

some mathematic computations are required. With Eq. (4),

the outlet air temperature could be computed with the

known inlet air temperature and engine heat.

q ¼ _maCp;a Ta;out � Ta;in

� �ð4Þ

The two variables (coolant inlet temperature and coolant

outlet temperature) could be computed with Eqs. (3) and

(9) with a known thermal effectiveness.

q ¼ _mcCp;c Tc;in � Tc;out

� �ð3Þ

Fig. 3 Time based setting: a basic scenario; b with fan operation prolonged by 100 s; c with fan operation prolonged by 200 s; d with fan

operation prolonged by 300 s; e with fan operation prolonged by 400 s; f with fan operation prolonged by 500 s


123

e ¼ ðTa;out � Ta; inÞðTc;in � Ta;inÞ

; when ðmCpÞmin ¼ ðmCpÞa ð9Þ

With all the known fluids’ temperature, LMTD could be

computed using Eq. 11 and thus overall heat transfer

coefficient could be computed using Eq. 5.

LMTD ¼½ Tc;in � Ta;out

� �� Tc;out � Ta;in

� ��

lnTc;in�Ta;outð ÞTc;out�Ta;inð Þ

ð11Þ

U ¼ q

FA � LMTD;

In this study, we would like to model the heat soak that

accumulates in the engine cooling system after the vehicle is

suddenly keyed-off. The vehicle was driven at 60 km/hour

(aggressive hill climbing) before it was slowed down (at

2,500 s) and keyed-off (at 2,800 s). Similarly, air flow and

coolant flow were slowed down (at 2,500 s) and reached their

minimum (at 2,800 s), as shown in Fig. 3a. However, there

was a thermal lag during which the engine heat decreased

linearly (at 2,800 s), 300 s after the air flow and coolant flow

had decreased (at 2,500 s). The engine heat reached its

minimum level at 3,200 s. The thermal lag represented the

amount of heat soak after keying-off. The higher amount of

heat soak is indicated by the higher level of thermal lagging.

Figures 3a and 4a show the basic scenario which will later be

compared and validated with industry field data.

The numerical model was further tested with different

scenarios, in which the length of the prolonged period of

fan operation after keying-off was varied. The aim of

prolonged fan operation was to prevent the coolant tem-

perature from exceeding its boiling point. The time-based

settings of different test scenarios are plotted in Fig. 3b–f.

Lastly, the numerical model also simulated scenarios in

which both the operation of the fan and the operation of the

water pump were prolonged concurrently, as summarized

in Table 2. The aim was also to reduce the coolant tem-

perature after the vehicle was keyed-off. The prolongation

Fig. 4 Time based setting: a basic scenario; b with fan and water pump operation prolonged by 100 s; c with fan and water pump operation

prolonged by 200 s; d with fan and water pump operation prolonged by 300 s


123

period was varied in the range of 100, 200, and 300 s

respectively. The time-based settings of different test sce-

narios are plotted in Fig. 4b–d. Table 3 summarized all the

heat transfer characteristic of current research.

3 Results and discussion

3.1 Coolant temperature behaviour with prolonged fan

operation

Figure 5 shows the coolant temperature trend for the basic

scenario, which was tested in the numerical model and

physical field test (industry data). At an early time step of

this time-based simulation, the coolant temperature

increased slowly from its initial temperature, which was

20 �C. During this cold start period (\900 s), the thermo-

stat was closed as coolant looped inside the by-pass circuit

(without going through the radiator). The coolant temper-

ature increased continuously after absorbing the engine

heat. After the coolant temperature reached the thermostat

opening temperature (85 �C) at 900 s, the thermostat

opened and coolant flowed slowly through the radiator.

When coolant flow started to pass through the radiator, the

coolant transferred excess heat to the ambient air through

the radiator tube and fin. From 1,200 to 2,500 s, the system

was in a steady state and the coolant temperature was fairly

stable. The system became unstable after the vehicle was

slowed down and keyed-off. Thus, the transient state of the

coolant temperature was the main research interest in our

study. The coolant temperature increased significantly after

the vehicle was slowed down (2,500 s) and then keyed-off

(2,800 s). After the vehicle was keyed-off, the coolant

temperature increased slowly and reached a peak of 133 �C

(at about 3,200 s). This highest temperature after keying-

off is referred to as ‘‘coolant peak temperature’’. The

coolant took a long time to cool down again. When the

vehicle was driven in severe conditions like hill climbing

and was keyed-off, the coolant temperature in the radiator

upper hose tended to increase drastically. This was because

stoppage of the pump and fan after keying-off led to

insufficient heat dissipation at the radiator.

In Fig. 5, the numerical results are compared and vali-

dated with field data from the automotive industry. It was

reasonable to conduct this comparison and validation as the

numerical model’s parameters (engine heat, coolant flow,

and air flow) were set according to vehicle field test

Table 2 Summary of all variations of simulation scenarios

Scenarios Prolonged period

of fan (s)

Scenarios Prolonged period

of fan and pump (s)

1 0 1 0

2 100 2 100

3 200 3 200

4 300 4 300

5 400

6 500

Table 3 Heat transfer characteristic of current research

Parameters Value

The thermal effectiveness, e 70–100 %

The engine heat, q 0–30 kW

The heat transfer area, A 0.33 m2

Corrections factor, F 0.8

The overall heat transfer coefficient, U 0–5.5 kW/m2 K

Fig. 5 Comparison between

numerical results and real-time

field results


123

Fig. 6 Coolant temperature at radiator upper hose and lower hose

during the slowing down and keyed-off period: a basic scenario;

b with fan operation prolonged by 100 s; c with fan operation

prolonged by 200 s; d with fan operation prolonged by 300 s; e with

fan operation prolonged by 400 s; f with fan operation prolonged by

500 s


123

conditions. The deviation between numerical and industry

result data was in the range of ±7 %.

It is not desirable for the coolant to reach boiling tem-

perature, as this could result in fatal and permanent damage

to the engine water jacket and coolant flow path. After the

numerical model had been validated, it was utilized to

investigate the feasibility of prolonging fan operation and

to check whether this helps reduce the coolant peak

Fig. 7 Air flow, coolant flow, and dissipated heat at radiator: a basic scenario; b with fan operation prolonged by 100 s; c with fan operation

prolonged by 200 s; d with fan operation prolonged by 300 s; e with fan operation prolonged by 400 s; f with fan operation prolonged by 500 s


123

temperature. This is the advantage of numerical modelling:

we were able to save time and cost in developing a pro-

totype and conducting experiments. This is crucial espe-

cially during the conceptual design stage. Figure 6 displays

the coolant temperatures in the upper and lower radiator

hoses over time, for different scenarios. The graph in

Fig. 6a is similar to that in Fig. 5, and presents the trend in

coolant temperature for the basic scenario. However,

Fig. 6b–f display the coolant temperature trend for other

scenarios of prolonged fan operation. The coolant peak

Fig. 8 Air temperature rise, inlet temperature difference, and thermal

effectiveness of radiator: a basic scenario; b with fan operation

prolonged by 100 s; c with fan operation prolonged by 200 s; d with

fan operation prolonged by 300 s; e with fan operation prolonged by

400 s; f with fan operation prolonged by 500 s


123

Fig. 9 Coolant temperature difference of radiator: a basic scenario; b with fan operation prolonged by 100 s; c with fan operation prolonged by

200 s; d with fan operation prolonged by 300 s; e with fan operation prolonged by 400 s; f with fan operation prolonged by 500 s


123

temperature decreased after the implementation of pro-

longed fan operation. Prolonging the radiator fan operation

for an additional 100, 200, 300, 400, or 500 s reduced the

coolant peak temperature by 13, 22, 26, 30, or 33 �C,

respectively, from 133 �C. Figure 6 displayed both coolant

inlet temperature and coolant outlet temperature allows us

to observe how both temperatures interact with each other

(different heat transfer will result in different coolant

temperature, the gap between the two graphs). There are

two curves in the study as one curve indicates hot fluid inlet

temperature into radiator. After heat dissipation in radiator,

the fluid temperature decreases thus another curve indicates

cold fluid outlet temperature from radiator. The inlet hose

is located at upper part while the outlet hose is located at

lower part as this obeys the rule of natural convection. The

hot fluid will move upwards while cold fluid will move

downward. In evaluation of the vehicles’ hardware fatigue

life, the red curve is suggested to use. Some noise/

fluctuation in the temperature is observed during the

unsteady state of simulation. This might due to the

changing of simulation conditions/input variables in every

second, from one second to the next second. The transient

input variable results the unsteady output variable. How-

ever, the fluctuation/noise will disappear after the simula-

tion state becomes steady state.

Figure 7 shows precisely the heat transfer at the radiator

during the slowdown and keying-off period (2,000–

4,000 s). Comparing Fig. 7a–f, the areas under the heat

transfer curves show an increasing trend from figure to

figure. During the period with prolonged fan operation, a

higher amount of heat was able to dissipate from the

radiator. Hump-shaped heat transfer curves (indicated by

arrows) can be observed in Fig. 7b–f for that period. The

value of maximum heat transfer and actual heat transfer of

the radiator were pushed higher by the prolonged air flow

supply after vehicle was keyed-off. As air specific heat was

Fig. 10 Coolant temperature at radiator upper hose and lower hose

during the slowing down and keyed-off period: a basic scenario;

b with fan and water pump operation prolonged by 100 s; c with fan

and water pump operation prolonged by 200 s; d with fan and water

pump operation prolonged by 300 s


123

a quarter of water specific heat, air was usually the fluid

with the minimum specific heat ð _mCpÞ and air could limit

the maximum heat transfer. However, during the period of

prolonged fan operation, the fluid with the minimum spe-

cific heat ð _mCpÞ switched from air to coolant. The fluid

with the minimum specific heat ð _mCpÞ would always pre-

dominate, which would limit the maximum heat transfer

and constrain the actual heat transfer by thermal effec-

tiveness. This suggested that heat transfer at a high air flow

rate was limited by coolant flow during this period. In

short, continued air flow supply (during prolonged fan

operation) increased the radiator heat capacity and

increased the amount of heat dissipated by radiator. As a

result, when a higher amount of heat was dissipated from

the coolant to the ambient air, coolant temperatures

decreased in the upper and lower hoses of the radiator.

Figure 8 plots the rise in air temperature, the fluid inlet

temperature difference (ITD), and the thermal effectiveness

of the radiator for all the different scenarios. In Fig. 8b–f, it

can be observed that the air temperature differences during

the period of prolonged fan operation (2,500–3,300 s) were

lower. This was because the air mass flow rates were higher

during this period, as shown in Eq. (4). Secondly, during

the period of prolonged fan operation, the effectiveness of

the radiator was approximately 100 % and the minimum

specific heat ð _mCpÞ was that of the coolant. This could be

explained by referring to Eqs. (3) and (10): in order for the

small flow of coolant to transfer an adequate amount of

heat, the coolant outlet temperature was pushed to the limit

(equivalent to air inlet temperature). Thirdly, the average

value of air temperature rise (DTair) after keying-off was

higher in the basic scenario compared to the prolonged fan

Fig. 11 Air flow, coolant flow, and dissipated heat at radiator: a basic scenario; b with fan and water pump operation prolonged by 100 s; c with

fan and water pump operation prolonged by 200 s; d with fan and water pump operation prolonged by 300 s


123

operation scenarios. This was attributed to the fact that

average heat transfer (after keying-off) was higher in the

basic scenario compared to the scenario in which fan

operation was prolonged by 100 s (average 3.8 kW vs.

average 3.4 kW, respectively). With a lower air mass flow

after keying-off, the air temperature difference was rather

sensitive to heat flow. Thus, a graph of the trend in air

outlet temperature should be similar to the graph of the

difference in air temperature, as the air inlet temperature

was fixed at 20 �C. The air outlet temperature in the basic

scenario could be as high as 100 �C. Lastly, the fluids inlet

temperature difference curve mimicked the coolant inlet

temperature curve, as the air inlet temperature was fixed.

All the three variables plotted in Fig. 8 could form Eq. (9).

Figure 9 demonstrates the coolant temperature differ-

ence over time for all the scenarios. During the period of

prolonged fan operation, the difference in coolant

temperature was larger due to the lower coolant mass flow

rate, as indicated in Eq. (3). Figure 9 also shows how the

coolant temperature difference affected the coolant tem-

perature graph. In Fig. 9, the severe change in coolant

temperature drop from radiator inlet and radiator outlet is

observed during the keying-off period. This is because the

air mass flow is remained while coolant mass flow is

reduced. Thus, with the reduced coolant flow, coolant

temperature drop is greater to compensate for the reduced

mass flow.

3.2 Coolant temperature behaviour with prolonged fan

and water pump operation

In order to reduce coolant peak temperature, prolonging the

operation of the cooling fan and pump concurrently is

proposed and tested. In Fig. 10a, the coolant peak

Fig. 12 Air temperature rise, inlet temperature difference, and thermal effectiveness of radiator: a basic scenario; b with fan and water pump

operation prolonged by 100 s; c with fan and water pump operation prolonged by 200 s; d with fan and water pump operation prolonged by 300 s


123

temperature of the basic scenario is 133 �C. In Fig. 10b–d,

operation of both fan and pump is prolonged for 100, 200,

and 300 s, respectively. It can be observed that their cor-

responding coolant peak temperatures are 123, 115, and

105 �C. Prolonging the operation of both the fan and the

pump concurrently shortens the prolongation period (to

decrease the coolant temperature to the same level, i.e.

105 �C). Coolant helps to transfer heat away from the

engine body while air helps to carry heat away at the heat

exchanger. When coolant and air work together, the max-

imum benefits are yielded; q and qmax are greater. When

only prolonged fan operation is used without prolonged

water pump operation, the limited coolant flow to the heat

exchanger constrains the potential of air to dissipate the

higher heat.

Figure 11 allows the mechanisms which decrease the

coolant temperature to be studied and compared. When

operation of both the cooling fan and the water pump is

prolonged, the fluids flow and heat flow at the radiator are

extended. As a result, a larger amount of heat is dissipated

away at the radiator and thus the coolant temperature is

reduced. The operation of the cooling fan and water pump

is a dominant factor in the radiator’s heat flow. When the

cooling fan and water pump operation was prolonged until

2,800 s, the heat flow at the radiator also persisted until

2,800 s. The total amount of heat dissipated at the radiator

could be equivalent to the area under the heat flow curve.

When operation of the cooling fan and water pump is

prolonged, the area under the heat flow curve is larger and

thus the total heat dissipated at the radiator is larger. The

Fig. 13 Coolant temperature difference of radiator: a basic scenario; b with fan and water pump operation prolonged by 100 s; c with fan and

water pump operation prolonged by 200 s; d with fan and water pump operation prolonged by 300 s


123

final coolant temperature is lower as the heat energy stored

in the coolant circuit is less after dissipation.

In Fig. 12, the ITD, air temperature rise, and thermal

effectiveness are plotted. It can be seen that in the basic

scenario, as shown in Fig. 12a, the highest thermal effec-

tiveness occurs during the transient keying-off period. This

is attributed to the higher remaining heat in the coolant

circuit in the basic scenario. When cooling fan and water

pump operation is prolonged, a higher portion of the heat is

dissipated during the prolonged period and thus less resi-

dent heat remains inside the coolant circuit. As a result, the

ITD and thermal effectiveness were lower during the

transient keying-off period. In Fig. 12d, the thermal

effectiveness is lowest among the scenarios.

In Fig. 13, the coolant temperature difference remains

low when the water pump is still running. The coolant

temperature difference increases after the water pump is

keyed-off and when the coolant flow is limited. The

slightly different heat flow during the keying-off period

does not have a significant impact on the coolant temper-

ature difference. This might be due to the larger specific

heat of water. However, the slightly different heat flow

during the keying-off period obviously does impact on the

air temperature difference.

3.3 Implementation of electrical fan and electrical

water pump

3.3.1 Implementation of electrical water pump as a main

pump

With this option, the mechanical water pump is removed

and its housing is kept. An electrical water pump (EWP)

weighing 900 g is mounted on the radiator bottom hoses.

The operation of the water pump and electrical fan are

controlled by a digital controller which allows the target

Fig. 14 Installation of

electrical water pump as main

pump

Fig. 15 Installation of

electrical water pump as

auxiliary pump


123

temperature (75, 80, 85, or 90 �C) to be set. The digital

controller reads the signal from the thermal sensor located

at the radiator upper hoses. Once the target temperature is

reached, the digital controller gives a signal to the EWP to

step in. The electrical fan will step in at the target tem-

perature plus 3 �C. A conventional belt-driven water pump

will sap power from the engine (8–10 kW, *5 %). By

installing an EWP, this parasitic power could be eliminated

(as parasitic power is equivalent to cubed engine speed).

The electrical water pump consumes power in the range of

only 36–120 W. The conventional mechanical water pump

runs at a speed proportional to engine speed, while the

electrical pump runs at a desired speed proportional to

coolant temperature. In this option, the thermostat and

bypass circuit is removed. The EWP is wired directly to the

battery, so that it can continue to run after keying-off

(normally 2 min). Figure 14 shows a modification of the

current circuit to replace the mechanical pump with the

EWP as the main pump.

3.3.2 Implementation of electrical water pump

as an auxiliary pump

In this option, there are no major changes but an additional

EWP and thermal switch are installed. The EWP acts as an

auxiliary pump assisting the main mechanical pump. A

temperature bulb is placed inside the radiator upper hoses.

The temperature bulb triggers the thermal switch and turns

on the EWP. The coolant temperature determines the

operation of the EWP. It may be observed that there are two

thermal switches, one for the electrical fan and one for the

EWP. This provides flexibility to control the fan with the fin

air temperature and to control the water pump with the

coolant temperature. However, this option does not remove

the parasitic power of the mechanical pump towards the

engine. The EWP is wired to the battery in order to ensure

its operation after keying-off. Figure 15 shows a schematic

drawing of the addition of an EWP as an auxiliary pump.

Installation of EWP is provides great benefit to us.

Firstly, installation of the EWP will eliminate heat soak

after sudden vehicle keying-off. Table 4 summarizes the

advantages and disadvantages of the two proposed options

with the existing design. Firstly, installation of the EWP

will eliminate heat soak after sudden vehicle keying-off.

Secondly, the EWP consumes power in the range of

36–120 W. Meanwhile releasing the belt-driven mechani-

cal pump from engine could save another 8–10 kW (5 %)

for other useful work, vehicles’ power and vehicles’ torque.

Installation of electrical fan and electrical water pump

would be a trend for now and near future, as this is an era

for digital controller, sensors, robotics and automatics

system. The operations of fan and pump will be triggered

by temperature sensor and the reactions of the system will

be programmed by digital controller. Proposal mentioned

Table 4 Comparison of options available in order to replace mechanical water pump

Now

Mechanical water pump

Option 1

Electrical water pump as main pump

Option 2

Electrical water pump as auxiliary

pump

Modification NIL Remove existing mechanical pump Keep existing mechanical pump

Remove thermostat and bypass circuit

Install EWP (80L/min) Install EWP (80L/min)

Install Digital Controller Install Thermal Switch

Cost Mechanical pump @36USD EWP (12 V)@260USD EWP (12 V)@260USD

Digital Controller@250USD Thermal Switches@85USD

Mechanical Pump@36USD

Benefits Cheap and robust a) Eliminate heat soak after keying-off

b) Increase power and torque by 8-10 kW

(*5 %)

By removing belt-driven mechanical

pump, full power will be used to drive

wheels. When pump speed double,

power drawn by pump will be eight-

folds. While EWP

power = 9A*12 V = 108 W

c) Increases cooling

EWP allows coolant flow to be increased

at idle or low speed driving. While flow

driven by mechanical pump is

proportional to engine speed

Can eliminate heat soak after keying-

off, thus can prolong engine life

Assists main pump in engine cooling


123

above could be implemented for automobile application

and adapted for industry applications.

4 Conclusions

One dimensional thermo-fluid simulation was an accurate

and efficient way to model the transient coolant tempera-

ture after the vehicle was keyed-off. In the model, the

coolant temperature trend over time could be observed.

The numerical result was validated with industry field data

with some tolerance error. The coolant temperature trend

was an interaction between heat flow, air flow, and coolant

flow. Other parameters like air temperature difference,

coolant temperature difference, and thermal effectiveness

of the radiator were also readable as a research output. The

coolant peak temperature for the current study (basic sce-

nario) was 133 �C. The coolant temperature was improved

to approximately 100 �C when fan operation was pro-

longed by an additional 500 s. The local heat transfer

behaviour during the fan operation prolongation period

(2,800–3,300 s) is illustrated graphically. The air flow after

keying-off was able to increase the rate of heat removal at

the heat exchanger, thus reducing the coolant temperature.

The implementation of prolonged fan operation is

uncomplicated due to direct wiring of the electrical fan to

the battery with the help of a thermal switch. The coolant

temperature was improved to approximately 105 �C when

fan and water pump operation was prolonged by an addi-

tional 300 s. The local heat transfer behaviour during the

fan and water pump prolongation period (2,800–3,300 s)

was illustrated graphically. The coolant flow and air flow

were able to increase the rate of heat removal at the heat

exchanger more efficiently. The implementation of the

electrical fan and EWP in the engine cooling system was

feasible thanks to the installation of a digital controller or

thermal switches. This could eliminate the heat soak in

engine cooling system. When the mechanical water pump

was replaced by an EWP, this could save 8–10 kW of

parasite power for useful work and vehicles’ torque.

Acknowledgments We would like to express our gratitude to the

Ministry of Higher Education (MOHE), University of Malaya (UM.C/

HIR/MOHE/ENG/60) and University Malaya Research Grant

(RG145-12AET) for providing us with the research funding.

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