08HX-22
Engine Cooling System with a Heat Load Averaging Capability
John Vetrovec
Aqwest, LLC
Copyright © 2008 SAE International
ABSTRACT
There is a need for an automotive engine cooling system
capable of handling increased heat load while, at the
same time, having reduced size and weight. This paper
evaluates a concept for an engine cooling system with a
passive heat accumulator that averages out peak heat
loads. Heat load averaging permits relaxation of the
cooling system requirements and allows substantial
reduction of system size and weight. This also translates
to a smaller coolant inventory allowing for faster engine
warm-up and reduced emissions of harmful pollutants
during a cold engine start.
INTRODUCTION
Recent trends to increase vehicle weight and improve
engine
performance
with
supercharging
or
turbocharging place increased demand on the capacity
of engine cooling systems (ECS). A variety of solutions
are currently pursued to handle increased heat load to
the ECS while, at the same time, attempting to reduce
the cost, size and weight. This is particularly important
for improving fuel economy and reduction of emissions
in automotive vehicles [1]. It has been estimated that
under typical driving conditions, an automotive power
plant generates only about 30% of available power for
90% of the time. The remaining 10% of the time
generally involves relatively short periods, such as
accelerating or climbing steep inclines. Heat load to the
engine cooling system follows a similar pattern.
This paper evaluates the benefits of an ECS with a heat
accumulator that stores excess heat generated during
periods of high heat load, such as acceleration and hill
ascent, and dissipates stored heat during reduced heat
load conditions such as vehicle cruise, deceleration, or
idle. By using advanced phase change materials with a
heat of fusion as high as 339 kJ/kg, the accumulator can
store a large amount of heat in a remarkably compact
and lightweight package. Because the heat accumulator
averages out peak heat loads to the cooling system, the
cooling system requirements may be relaxed and, the
size and weight of the system can be substantially
reduced. This also translates to a smaller coolant
inventory allowing for faster engine warm-up and
reduced emissions of harmful pollutants during a cold
engine start.
SCALING CONSIDERATIONS FOR ENGINE
COOLING SYSTEMS
An automotive internal combustion engine commonly
employs a pressurized cooling system with a circulating
liquid coolant for cooling the engine. Waste heat is
transferred to the coolant in a cooling jacket(s)
surrounding combustion-heated parts of the engine.
Heat absorbed by the coolant is conveyed to the radiator
and dissipated to the environment. The radiator may
operate with a cooling fan, which blows ambient air into
the radiator, thereby promoting heat transfer from liquid
coolant to air.
The design capacity of automotive ECS is traditionally
determined according to the cooling capacity needed for
the most severe operating conditions of the particular
engine installation. These include the conditions of high
engine output, low vehicle speed, and/or hot ambient
temperatures. Heat transfer capacity of the radiator is
dependent on the temperature of ambient air. For
example, in cool temperatures, the radiator can transfer
substantially more heat to ambient air than in hot
ambient conditions. Similarly, higher speed of the
automotive vehicle generates more favorable conditions
for increased heat transfer by the radiator.
Coolant circulation between the engine and the radiator
is controlled by a thermostat regulating the coolant flow
through the radiator so that engine temperature is
maintained near a predetermined “normal” operating
point. However, under heavy load and/or during high
ambient temperature conditions, engine waste heat may
exceed the radiator heat dissipation capacity. If the heat
load is not reduced, coolant temperature will rise. This
may eventually cause an ECS pressure relief valve
(PRV) to open and likely result in a substantial loss of
coolant. To prevent frequent thermal overload, the heat
load handling capacity of a given-size ECS may be
increased by using one of the two principal approaches:
1) increasing the system’s physical size or 2) increasing
the system’s operating temperature.
IMPLICATIONS OF A LARGER ECS - Increasing the
physical size of the cooling system may be
accomplished, for example, by increasing the size of the
radiator core, capacity of the coolant pump (water
pump), capacity of the cooling fan, or some combination
of these. However, available space in the automotive
engine compartment is becoming very scarce in-part
IMPLICATIONS OF HIGHER ECS TEMPERATURE Increasing the ECS operating temperature is a wellknown approach to boosting ECS thermal handling
capacity without increasing its physical size. By
permitting a higher temperature difference between
coolant and ambient air at the radiator core, heat
dissipation capacity of the radiator is significantly
increased. Operating temperature of the cooling system
is also related to its operating pressure, which should be
held at a sufficiently high level to prevent the coolant
from boiling. In particular, the operating temperature of
many cooling systems for automotive engines in current
production is about 100 degrees Centigrade (100°C)
[215 degrees Fahrenheit (215°F)].
Drawbacks to increasing the ECS operating temperature
include reduced lifetime of cooling system components
such as the radiator core, radiator hoses and water
pump seals. In addition, higher coolant pressure may
have adverse effects on heat transfer at certain critical
points in the engine, particularly in systems where a
significant amount of (liquid-to-vapor) phase-change
cooling occurs. For example, the most efficient cooling
occurs at the engine cylinder wall when coolant
conditions are conducive to nucleate boiling. An
increase in the operating pressure of a given system
elevates the coolant boiling point and impedes nucleate
boiling, thereby decreasing the heat transfer from the
cylinder wall to the coolant. This may lead to occurrence
of hot spots in the engine, which may accelerate
component fatigue, cause detonation, and excessive
NOx emissions.
This situation calls for an ECS capable of increased heat
load handling capacity without increasing the size and
temperature of the system.
NEW ENGINE COOLING SYSTEM DESIGN
A novel ECS concept has been developed to provide
increased capacity and overload handling capability. The
ECS includes a heat accumulator, which receives and
stores heat from the coolant at times of elevated engine
heat load and returns the stored heat back to the coolant
during reduced heat load conditions. In particular, the
accumulator stores excess engine waste heat during
vehicle acceleration or hill climbing, and dissipates
stored heat during vehicle cruise, deceleration, or idle.
The new ECS shown in Figure 1 has a conventional
layout with a radiator, water pump, thermostat, radiator
bypass, occupant heater and control, and separate
circuit branches for the engine block and head [2]. The
new element is a heat accumulator installed downstream
of the radiator. The heat accumulator contains a phase
change material (PCM) in thermal contact with the
coolant. PCM is a material that changes in heat content
when undergoing a reversible solid-liquid phase
transformation.
Pressure
Relief
Heater
Control
Thermostat
Fan
Cylinder Head
Cabin Heater
Radiator
due to downsizing of vehicle body, aerodynamic styling,
and a recent trend to add a supercharger. This situation
makes a large radiator rather unattractive. An enlarged
water pump and/or cooling fan add parasitic losses and
reduce the overall engine system efficiency. Finally,
larger volume of cooling fluid in the ECS negatively
impacts warm-up characteristics of the engine, which
translates to increased cold start emissions.
Cylinder
Block
Heat
Accumulator
Water Pump
Diverter
Valve
Engine
Figure 1: Engine cooling system (ECS) with a heat
accumulator
PHASE CHANGE MATERIALS (PCM) – The PCMs,
synonymously known as latent thermal energy storage
materials, are commonly used for thermal energy
storage. Absorption of the necessary quantity of energy
by a solid PCM results in melting. The energy absorbed
by the PCM to change phase at its characteristic melting
temperature is known as the latent heat of fusion. The
latent heat of fusion stored in the liquid state is released
on resolidification.
Desirable PCMs have a high latent heat of fusion, high
thermal conductivity, low supercooling, and the ability to
cycle thermally from solid to liquid and back to solid
many times without degradation. ("Supercooling" refers
to a discrepancy between the temperature at which
solidification (freezing) initiates and the melting
temperature of a given PCM when cooled and heated
under quiescent conditions. In some instances,
supercooling can be suppressed by additives.) Other
considerations for PCM selection include melting
temperature, density, packaging, toxicity and cost.
References [3] and [4] include comprehensive lists of
PCMs and their properties. In addition to a variety of
commercial applications, PCMs have been successfully
used aerospace applications requiring long lifetime and
high reliability [5].
For use in ECS heat accumulator, the PCM should have
a melting temperature Tm somewhat higher than the
normal coolant operating temperature T0 but lower than
the temperature at which a coolant PRV opens. PCM
performance should be stable over the expected lifetime.
Suitable PCM can be of an inorganic type such as
eutectic mixtures of salts and salt hydrites or organic
type such as organic acids and sugar alcohols. Dibasic
and monobasic acids [6] with higher molecular weight
are largely nonhygroscopic and noncorrosive, which
makes them very good candidates for an ECS heat
accumulator. In particular, PCM suitable for ECS with a
normal coolant operating temperature T0 in the vicinity of
100°C
includes cross-liked polyethylene (PE),
magnesium chloride hexahydrate (MgCl2.6H2O), eutectic
solution E117 [7], benzoic acid (C6H5COOH), and
erythritol (C4H10O4), Figure 2. While erythritol exhibits
significant supercooling, this effect can be cured with
additives [8].
Melting
Temp.
[degC]
MgCl2.6H2O
116
Latent
Heat
[kJ/kg]
125146
169
Eutectic solution E117
117
~170
0.7
Erythrytol (C4H10O4)
118
339
0.326 (L)
Benzoic acid (C6H5COOH)
122
138
PCM Type
Cross-linked PE
110-115
Thermal
Conductivity
[W-m/degC]
0.3
0.6 (L)
Figure 2: Properties of selected PCM [4]
HEAT ACCUMULATOR – The heat accumulator
consists of a housing containing encapsulated PCM,
Figures 3 and 4. The housing can be made of rubber or
high-temperature plastic and configured as a cylindrical
vessel with an inlet and outlet for connection to ECS
plumbing. PCM is packaged in small capsules, which
may be in a form of long cylinders washed by the
coolant flow. Since most PCMs are rather poor thermal
conductors, this approach beneficially provides a large
area for heat transfer between the coolant and the PCM,
and a short path length for heat conduction. The
diameter and length of the capsules defining the heat
transfer area must be balanced against a pressure drop
introduced to the coolant loop. Heat transfer between
the coolant and PCM can be enhanced by adding fins to
the interior surface of the capsules. Material for the
capsules should be compatible with the PCM, have a
high corrosion resistance, and a good thermal
conductivity. For example, the capsules can be made
from metal extrusions and have either formed or welded
end caps to produce a hermetically sealed package.
Cross-linked PE does not turn into liquid when going
through phase change and it is frequently used without
encapsulation [4]. Long-term stability of this PCM when
directly exposed to engine coolant environment must be
ascertained.
Housing
Coolant
Flow
Capsule
Fin
PCM
made from common materials, making it conducive to
high-volume production at low cost.
INSTALLATION – As shown in Figure 1, the heat
accumulator is installed in the ECS loop just
downstream of the radiator and upstream of the radiator
bypass line. Preferably, the accumulator replaces a
portion of ECS flexible lines. Such an approach places
very little demand for additional space in the engine
compartment. During cold engine start, the coolant flow
bypasses the accumulator. As a result, thermal inertia
of the accumulator does not impede engine warm-up.
OPERATION – During normal operation, the ECS
functions in a traditional way with the thermostat
regulating the coolant flow through the radiator to
maintain the normal coolant operating temperature T0. At
this time, the PCM in the heat accumulator is in a solid
state. Whenever the coolant temperature rises above
the normal operating temperature T0 and exceeds the
PCM melting temperature Tm (such as caused by
increased engine waste heat load), the PCM gradually
melts and removes heat from the coolant. Conversely,
when the engine heat load is reduced to normal levels,
coolant temperature drops below the PCM melting
temperature Tm, the PCM gradually solidifies. The heat
stored in the accumulator is returned back to the
coolant, which dissipates it through the radiator. In this
manner, the accumulator averages out certain peak heat
loads to the ECS. As a result, the ECS requirement for
real-time rejection of heat to environment can be
reduced to handle only an average rather than a peak
heat load. The analysis below shows that by adding
such a heat load averaging capability, the size and
weight of ECS can be substantially reduced.
Capsule
PCM
Fin
Coolant
Housing
Figure 4: Heat accumulator cross-section
perpendicular to coolant flow
Coolant
Flow
Figure 3: Heat accumulator cross-section parallel to
coolant flow
Capsules are attached to holders (not shown) for proper
spacing and to keep them from being dislodged by the
flow. The accumulator is simple, compact, and can be
PERFORMANCE MODELING
A model was developed to simulate the ECS
performance. Parameters used in the case study below
are purely exemplary and do not relate to any existing
ECS design. Many PCMs (namely organic types) do not
have a sharp solid-to-liquid transition. To accommodate
gradual release of latent heat, PCM was modeled as a
material with a variable specific heat having a Gaussian
temperature distribution, namely
(1) cp (T) = cp0 + L12 (2π)-½ Tw-1 exp (-(T-Tm)2/(2Tw2))
where L12 is the latent heat, cp0 is the constant part of
specific heat, Tm is the melting temperature, and Tw is
the e-folding half-width of the temperature distribution.
Integrating the temperature-dependent part of the cp
over a temperature range yields the latent heat L12.
When heated from temperature T1 to T2, the amount of
heat stored in the PCM is
T2
(2)
Q = mPCM ∫ cp,PCM (τ)dτ
T1
The fraction of PCM melted at temperature T can be
calculated as
(3)
fmelt (T) = erf ((T-Tm)/Tw)
where erf is the error function. PCM parameters used in
the heat accumulator for the case study below generally
correspond to erythrytol with anti-supercooling additive,
and are listed in Figure 5. Thermal mass of the heat
accumulator structure was ignored in the analysis.
ignores the sensible heat acquired by PCM between
temperatures Ta and TTS. Also ignored is the spatial
distribution of temperature of the coolant in the flowing
loop. For the sake of simplicity, radiator heat transfer
coefficient HR is assumed to depend only on the
difference between the coolant temperature and the
temperature of ambient. (This means that HR is assumed
to be independent of vehicle speed).
A case study was conducted to compare the
performances of three ECS configurations: a full size,
down sized, and down sized with a heat accumulator. A
simple scenario was designed to demonstrate the heat
accumulator utility under relevant conditions. In general,
it was assumed that the down-sized ECS is 1/3 smaller
than the full-size ECS. Parameters of these systems are
shown in Figure 6. The model, which is in MS Excel®,
tracks the time evolution of bulk coolant temperature Tc
of each system in response to a variable heat load
dQ/dt. Coolant temperature Tc is calculated by
integrating eq. (5) in the time domain.
The thermostat response function was modeled as
Fullsize
DownSized
DownSized
w/HA
Units
Coolant fill
10
7
7
kg
Radiator heat xfer coef. , HR
3
2
2
kW/°C
100
100
100
°C
fTS(T) = ½ + ½ tanh(α(T – TTS))
where TTS is the temperature at which the thermostat is
50% open and α is the slope coefficient. It was assumed
that at any given time, 1/3 of the total coolant fill was in
the radiator loop.
PCM Parameter
Symbol
Value
Units
-
Erythrytol
-
Specific heat
cp0
2
kJ/kg-°C
Melt temperature
Tm
110
°C
Melt band half width to 1/e
Tw
1
°C
Latent heat
L12
339
kJ/kg
Mass
mPCM
5
kg
Total latent heat storage capacity
Qstor
1.7
MJ
Type
Figure 5: Heat accumulator parameters used
in the case study
Heat load was related to coolant temperature Tc using
an approximation
Thermostat setting, TTS
Figure 6: ECS parameters used in the case study
The simulation started with each system at an ambient
temperature of 40ºC to represent a cold engine start. At
time t = 0, a time-variable heat load dQ/dt with the profile
shown in Figure 7 was applied to each system.
50
40
Heat Load [kW]
(4)
ECS Version
Parameters
30
20
10
(5)
dQ/dt = fTS(Tc).HR(Tc-Ta) +
+ [(⅔ + ⅓fTS(Tc)).mccpc (Tc-Ta) +
+ fTS(Tc).mPCM cp,PCM(Tc)] dT/dt
where dQ/dt is the heat load, Ta is the ambient
temperature, HR is the bulk heat transfer coefficient of
the radiator, mc and cpc are, respectively, the mass and
specific heat of the coolant, t is time. The first term on
the right side of eq. (5) represents the heat transferred
by the radiator to environment, the second term is the
heat stored as sensible heat in the coolant, and the last
term is the heat deposited in the PCM. Equation (5)
0
0
200
400
600
800
1000
1200
Time [sec]
Figure 7: Applied time-varying heat load profile
The model integrated eq. (5) with a time step of 2
seconds (sec) and at each step calculated the coolant
temperature Tc. The initial 300-sec period with a 10-kW
heat load represented an engine warm-up. As expected,
the temperature of the two down-sized systems reached
the normal operating temperature in about 2/3 of the
time required for the full-size system, Figure 8.
the full-size system rapidly cooled to its normal operating
temperature near 100ºC. The down-sized ESC with a
heat accumulator remained at 110ºC as the PCM
gradually solidified and transferred its stored heat to the
radiator. At t = 1030 sec, all of the PCM became
solidified and soon after that the system returned to its
normal operating temperature.
160
Temperature [deg C]
140
120
100
80
Full Size
60
Down-Sized
40
Down-Sized w/Heat Accum.
20
0
0
200
400
600
800
1000
1200
Time [sec]
Figure 8: Temperature responses of full-sized, downsized, and down-sized system with a heat accumulator
At t = 300 sec, the heat load was increased to 20 kW. All
three systems showed good handling of the 20-kW load
with only a slight temperature increase. At t = 400 sec,
the heat load was increased to 30 kW for 200 sec. The
down-sized ECS without a heat accumulator failed to
reject the 30-kW heat load at near Tc = 100ºC and its
temperature showed a steady rise. In a practical
situation, this system would overheat and cause the
PRV to open. Temperature of the full -size system rose
to about 107ºC and stabilized as the system continued
to dissipate heat in real time. Temperature of the downsized ECS with a heat accumulator rose to about 110ºC,
which was the PCM melting temperature Tm. At about
420 sec, the PCM started to melt (Figure 9) and about
45% of the PCM was melted by the end of the 200 sec
period of 30-kW heat load.
Results of this study indicate that a down-sized ESC
with a heat accumulator performs comparably to a fullsized ECS under specific heat load conditions, and it
can outperform a full-size ECS in handling of high
transient loads. At the same time, the down-sized ESC
with a heat accumulator offers a reduced radiator size
and engine warm-up time while weighing about the
same as the full-size system. The additional cost of
fabricating and installing the accumulator is expected to
be more than offset by cost savings due to a smaller
radiator and newly available volume in the engine
compartment.
Weight of the heat accumulator can be reduced by about
half by placing the accumulator onto a special bypass
section of the cooling loop and isolated by a 3-way
valve, Figure 10. In this approach, the accumulator
remains at ambient temperature until its use. The 3-way
valve, which can be operated thermostatically or under
computer control is set to open just below the PCM
melting temperature Tm. Starting from ambient
temperature, the accumulator receives sensible heat of
about cp,PCM.mPCM (Tm – Ta), which is comparable in
magnitude to the latent heat L12.
Pressure
Relief
Heater
Control
Thermostat
Cabin Heater
Radiator
Fan
1.0
Cylinder
Block
0.8
PCM Fraction Melted
Cylinder Head
0.6
3-Way
Valve
0.4
By-Pass
Heat
Accumulator
Water Pump
Diverter
Valve
Engine
Figure 10: Engine cooling system (ECS) with a heat
accumulator on a bypass
0.2
0.0
0
200
400
600
800
1000
1200
Time [sec]
Figure 9: Fraction of PCM melted as a function of time
At t = 600 sec, the heat load was increased to 40 kW for
100 sec. This load was beyond the capacity of the fullsize ECS and its coolant temperature showed an
immediate rise. The down-sized ECS with a heat
accumulator remained at a stable temperature of about
110ºC (Figure 8) as its PCM continued to melt (Figure
9). At the time t = 700 sec when the heat load was
reduced back to 10 kW, about 95% of the PCM has
been melted. With the heat load reduced back to 10 kW,
By using the approach in Figure 10, it is possible to
reduce the amount of PCM in the accumulator by about
one-half and still obtain the same performance as with
the heat accumulator placed directly in the cooling loop
shown in Figure 1. After the PCM has been re-solidified,
the accumulator is cooled back to ambient temperature
by vehicle slip stream air. For this purpose, the
accumulator may have external fins to promote heat
transfer.
CONCLUSION
A new ECS concept using a passive heat accumulator
for heat load averaging was presented. The heat
accumulator stores excess heat generated during
periods of high heat load such as acceleration and hill
ascent, and dissipates stored heat during reduced heat
load conditions such as vehicle cruise, deceleration, or
idle. By using advanced phase change material(s), the
accumulator can store a large amount of heat in a
remarkably compact, lightweight, and low-cost package.
The accumulator can be configured to replace a portion
of ECS coolant lines. As a result, the new concept
allows significant down-sizing of ECS volume and weight
while 1) offering a heat load-handling performance
comparable to a full-size ECS under typical heat load
conditions, and 2) outperforming a full-size ECS in
handling of high-transient loads. In addition, smaller
coolant inventory in the down-sized ECS permits faster
warm-up time during cold engine start, thereby reducing
emissions of harmful pollutants.
REFERENCES
1. N. S. Ap and M. Tarquis, “Innovative Engine Cooling
Systems Comparison,” SAE Paper No. 2005-011378.
2. ”Engine Cooling,” in Automotive Handbook, 6th
edition, Robert Bosch GmbH, 2004.
3. B. Zalba et al., “Review on Thermal Energy Storage
with Phase Change: Materials, Heat Transfer
Analysis and Applications,” Applied Thermal
Engineering, vol. 23, pp251-283, 2003.
4. S. D. Sharma et al., “Latent heat storage materials
and systems: A review,” Intl. J. of Green Energy, vol.
2, pp 1-56, 2005.
5. D. G. Gilmore ed., “Spacecraft Thermal Control
nd
Handbook,” Phase-Change Materials, Ch. 11, 2
ed., Aerospace Press, 2002.
6. Lane et al., “Dibasic acid based phase change
material compositions,” U.S. Patent No. 5,755,988.
7. EPS Ltd., Slough, Berkshire, United Kingdom.
8. Kakiuchi et al. in U.S. Patent No. 5,785,885.
CONTACT
John Vetrovec is the founder and president of Aqwest, a
science and technology innovation company in Larkspur,
CO, USA; www.aqwest.com. John has 28 years
experience in the aerospace industry at TRW Defense &
Space Technologies (now Northrop Grumman Space
Technologies) in Redondo Beach, CA and The Boeing
Company in Canoga Park, CA. During his aerospace
career, he led R&D projects in rocket engines,
spacecraft, plasma physics, vacuum systems, thermal
management, high-energy lasers, electro-optics, and
missile interceptors. In 2006, he retired as a Technical
Fellow from Boeing and started Aqwest. John’s
professional interests include adapting selected
aerospace technologies to commercial uses. John holds
a BA and an MA in mathematics, and MSEE in electrooptics, all from UCLA. He is an author of over 57
technical publications and has 42 patents issued or
pending.
John
can
be
reached
at
jvetrovec@aqwest.com.
DEFINITIONS, ACRONYMS, ABBREVIATIONS
ECS
HA
HDPE
PE
PCM
PRV
sec
- Engine cooling system
- Heat accumulator
- High-density polyethylene
- Polyethylene
- Phase change material
- Pressure Relief Valve
- second(s)