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
[email protected].
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)
