New compact and fuel economy cooling system SLIM *

Transcription

1 論文 New compact and fuel economy cooling system SLIM * Hara, Junichiro; Iwasaki, Mitsuru; Meguriya, Yuichi Calsonic Kansei Corp. KEYWORDS - engine, cooling, radiator, condenser, charge air cooler ABSTRACT A new engine cooling system is proposed that alters sub-radiator function according to driving conditions without valves and actuators. This system enables substantial fuel consumption improvement by: 1.Simplified heat exchanger configuration located in the front end of a vehicle with only two layers instead of three, which leads to 30% reduction in pressure drop of air flow. 2.Cooling fan power and air-conditioning compressor load reduction associated with above mentioned air flow improvement. This performance improvement mechanism of the new system called SLIM (Single Layer Integrated cooling Module) is studied in various aspects such as air pressure drop in comparison with full water-cooled condenser systems. Furthermore, by extending the concept to CAC (Charge Air Cooler) with a focus on the use of optimum heat transfer efficiency, a new 2-way CAC structure is proposed. 30% improvement in heat exchange performance over conventional water cooled CAC is confirmed. INTRODUCTION New generation powertrain such as hybrid, clean diesel or downsizing turbo requires an additional heat exchanger. Accordingly pressure drop of heat exchangers is likely to increase, which further leads to increased cooling fan load and worsened fuel consumption. The proposed system focuses on integration of various cooling functions to meet various driving conditions. By analyzing load situations of condenser for air conditioning system and subradiator for CAC or motor and inverter for hybrid vehicle, it was found that the loads for each of the systems are complementary. Thus it is conceivable to reduce the number of heat exchanger layers if the function of subradiator can vary depending on the driving conditions. Consequently cooling fan power can also be reduced. A compact cooling system based on this concept is called SLIM (Single Layer Integrated cooling Module) and has been studied in a wide range of vehicle operation cases. Additionally it is worth noting that emission regulation trend will require larger capacity CAC, although it might become more difficult to increase its capacity without enlarging the surface area of heat exchangers (1-3). The SLIM concept is found to be also effective to CAC performance improvement. STRUCTURE Figure 1 shows a schematic of a SLIM system compared with a conventional system. As indicated in the figure, AC (Air Conditioning) condenser is divided into water-cooled and air-cooled ones. The function of sub-radiator is cooling the water-cooled condenser and a CAC. The water-cooled condenser is placed in the sub-radiator tank. The coolant of sub-radiator is below 60ºC. Refrigerant of the AC system that flows out of the AC compressor flows into the water-cooled condenser. Then the AC refrigerant goes to the air-cooled condenser. In the condenser part in gas area, the AC refrigerant is cooled by water flow which has higher heat transfer coefficient than air flow condenser part. And by using an air-cooled condenser, the AC refrigerant can be cooled to near the temperature of air flow. The volume of water-cooled condenser is small (less than 150cm 3 in this case) but the heat dissipation capacity can be as large as 2kW. * This paper is published with permission from FISITA (International Federation of Automotive Engineering Societies) and was originally presented at FISITA 2012 World Automotive Congress in Beijing, China November

2 CALSONIC KANSEI TECHNICAL REVIEW vol Figure 1. Comparison of conventional cooling system and SLIM Figure 2. Water-cooled condenser PRINCIPLE IN OPERATION The thermal loads of water-cooled CAC and water-cooled condenser changes in various driving scenes (see Figure 3). In idling condition, engine speed is low and the heat dissipation of the CAC is minimal. The vehicle speed is zero and the load of AC condenser is very high. In case of high speed driving and hill-climbing condition, the load to CAC is high while the load to AC condenser is low. System operations in such various driving scenes have been studied with 1-D simulation tool and actual test drives. Based on the simulation and the vehicle experimental data, it was confirmed that SLIM can cope with superimposed condition of the heat loads of CAC and AC condenser. 74

3 New compact and fuel economy cooling system SLIM * Figure 3. Heat dissipation in various driving scenes IMPROVEMENT OF HEAT TRANSFER In SLIM system, two condensers are used : (i) a small water cooled condenser is placed in a low temperature subradiator; and (ii) a conventional air cooled condenser. Table 1. Heat transfer for various cooling systems *Italic style shows optimum choice. 1 ; In the calculations, the are ratios have been omitted 2 ; Air side heat transfer coefficient is dependent on the airside face velocities Temperatures of fluid used for above calculations Outside temperature 35ºC Refrigerant temperature inlet 69.5ºC Sub RAD water temp. inlet 1 59ºC Refrigerant saturation temperature 60ºC Sub RAD water temp. inlet 2 50ºC (gas area uses inlet1, refrigerant two phase area uses inlet2) 75

4 CALSONIC KANSEI TECHNICAL REVIEW vol The water cooled condenser essentially cools the superheated refrigerant gas to saturated conditions; and the second condenser (air cooled condenser) further cools it through two-phase condensation process. In both heat exchangers, heat transfer coefficient has been optimized as indicated in the following Table 1. According to equation 1, heat dissipation is decided by temperature difference and heat transfer coefficient. In Table 1, temperature difference and heat transfer coefficient are shown for different system configurations. Data in italic letter shows optimum heat transfer coefficient. As shown in this table, performance of SLIM system is the same as full water-cooled condenser in gas area; and in two phase area, SLIM is superior to the others. In this case the SLIM system shows the highest performance. PRIMARY FACTORS OF IMPROVEMENT OF FUEL CONSUMPTION In SLIM, the reduction of pressure drop or air flow is essential. By this effect, the followings are achieved. 1) Reduction of cooling fan power due to the lower air flow resistance 2) Power reduction of AC compressor by enhancing heat transfer performance of condenser The reduction of power of the cooling fan and AC compressor consequently leads to improved fuel mileage. APPLICATION TO VEHICLES SLIM system was applied to several prototype vehicles. The examples are shown as follows. (Figure 4): Table 2. Application for vehicles AC means Air-conditioning. JC08 is Japanese regulation on fuel consumption. In each vehicle, the following advantages of the SLIM system were confirmed. (1) Improved fuel consumption (2) Weight reduction (3) Compactness of heat exchangers 76

5 New compact and fuel economy cooling system SLIM * ANALYSIS OF IMPROVEMENT OF FUEL CONSUMPTION The two contribution factors in fuel mileage have been studied as follows. (1) Reduction of cooling fan load As shown in Figure 4, when the discharge pressure of AC system is the same as a conventional case, the cooling fan load of SLIM can be reduced by about 150W. (2) Thermal storage effect of water During the acceleration, the flow rate of refrigerant in the AC system and accordingly discharge pressure increase. Larger thermal storage effect of water than air, however, is utilized in SLIM to keep the temperature change moderate in such conditions. A part of the AC condenser is cooled by water in the SLIM system, which lowers the refrigerant temperature than conventional systems. Thus the AC compressor load is reduced and engine can avoid large BSFC (Brake Specific Fuel Consumption) area. Figure 4. Reduction of fan load Figure 5 Moderation of pressure rise at vehicle acceleration 77

6 CALSONIC KANSEI TECHNICAL REVIEW vol ANALYSIS OF OPTIMAL RATIO OF SUB-RADIATOR Because air-cooled condenser and sub-radiator are placed in vertical position, the optimal ratio of the height of subradiator to total height is necessary to be considered. This ratio is constrained by the minimum compression ratio of refrigerating cycle, which is in various cases below 40%. It is necessary to cool CAC even in the highest speed in this ideally proportional sub-radiator. In our vehicle experiments, we can confirm that sub-radiator cools CAC enough for compact cars. Figure 6 shows three cooling types of AC condenser. Conventional air-cooled condenser (at left hand side) SLIM (combined air-cooled and water-cooled condenser) Full water-cooled condenser (at right hand side) (4,5) SLIM can choose the optimal heat transfer mechanism, and then the discharge pressure of AC system is the lowest. The lower pressure brings the vehicle fuel mileage to be better. AMESim which was developed by LMS Corporation as 1-D simulation for analyzing a refrigerant cycle to reduce the number of prototypes and experiments (6,7). Figure 6. Optimum ratio of sub-radiator height ENLARGEMENT OF HEAT DISSIPATION OF CAC In Φ -T map (equivalence ratio - temperature diagram), to reduce NO x, low temperature combustion is necessary. EGR (Exhaust Gas Recirculation) including CO 2 has larger specific heat than N 2 and O 2 molecules. Then enlarged capacity of EGR cooler is required. When the ratio of EGR is increased, a lot of unburned matter is generated. So, cooled flesh air with oxygen is necessary. Therefore to enlarge heat dissipation of CAC is necessary. 78

7 New compact and fuel economy cooling system SLIM * In usual, CAC can be cooled by air flow, engine coolant or low temperature coolant. Air-cooled CAC can provide lower gas temperature of outlet of CAC. When a vehicle is in an acceleration state, gas temperature of water-cooled CAC is lower than air-cooled one because of thermal storage effect of water as coolant. Low temperature coolant brings the gas temperature of CAC to be lower. Regarding the temperature difference of coolant and air, heat dissipation of low temperature radiator is small. Because heat dissipation is determined by temperature difference and heat transfer coefficient, the limiting factor of CAC is found to be the heat dissipation of low temperature radiator. Therefore, the CAC is divided, and one side is cooled by engine coolant and the other is cooled by low temperature radiator, that is a sub-radiator. In this divided CAC system, called 2-way CAC system, the total heat dissipation of is increased compared with a conventional CAC system by above 30%. Figure 7. 2-way CAC system Figure 8. Performance of radiators COMPARISON WITH FULL WATER-COOLED CONDENSER SYSTEM Comparison with full water-cooled condenser system with water-cooled CAC has been studied. The heat dissipation of AC condenser and CAC were estimated for vehicle idling, constant speed 40km/h and hill climbing conditions. In each case, the heat dissipation of CAC is assumed to be the same, the amount of which is based on experimental data with actual vehicle with 1.6L turbo-charged petrol engine. The results are shown in Figure 9. 79

8 CALSONIC KANSEI TECHNICAL REVIEW vol In the figure the bars on the left hand side graph are for a hill-climbing driving condition. Full water-cooled condenser system shows lower performance than SLIM in this condition. This is due to high temperature of subradiator outlet water temperature that cools CAC. The center pair of the bars shows an idling condition case. Since temperature of coolant of a sub-radiator is high, in the case of full water-cooled condenser system, its heat dissipation of AC condenser is inferior to SLIM. The right hand side bars corresponds to a constant speed driving condition at 40km/h. SLIM s performance is superior in this condition as well. In every driving condition, consequently, SLIM shows better performance than full water-cooled condenser systems. Figure 9. Comparison with full water-cooled condenser system CONCLUSION A new concept cooling system SLIM is proposed that enables substantial improvement both in fuel economy and space and weight savings. This concept allows optimal choice of the best heat transfer efficiency within air-cooled and water cooled systems depending on driving and environmental conditions that widely vary. Advantages of the SLIM system is extensively studied and confirmed in a wide range of the cases. The points of the system are: Sub-radiator function that can be altered by temperature difference of coolant and target heat loads, (Above-mentioned) switching performed automatically without valves and actuators, Fuel consumption improvement by 3-5% by load reduction of cooling fan power and AC compressor power, and Improvement in spatial efficiency of engine cooling module by 40%. (for turbo-charging petrol and diesel engine vehicles) 80

9 New compact and fuel economy cooling system SLIM * In addition, a new 2-way CAC system is conceived along the SLIM concept to achieve 30% improvement in heat exchange performance. Considering future emission regulation for diesel engines etc., this 2-way CAC system has a potential to cope with various requirements in a flexible way. REFERENCES (1) Iwasaki, M. Development of new energy saving cooling system SLIM Automotive Summit, Nov (2) Peuvrier, O. Development of compact cooling system SLIM, VTMS 2011 (3) Mathur, G.D. Development of an Innovative Energy Efficient Compact Cooling System SLIM, SAE April, 2012 (4) AP, N.S., UltimateCooling System Application for R134a and R744 Refrigerant SAE July, (5) Malvicino, C. Dual level vehicle heat rejection system VTMS10, Mar (6) Yagisawa, K. From AC to complete Vehicle Thermal Management Using LMS Imagine Lab. AMESim International LMS Engineering Simulation Conference, Oct (7) Rozier, T. From the modeling of AC system to the complete Vehicle Thermal Management using one single platform Haus der Technik