Deployment of engine stop-start to 12-V vehicles is starting next year in the U.S., spurred by the EPA’s new five drive-cycle test that includes enough idle-stops to produce a significant effect on fuel economy. However, the feature—well-developed in Europe with re-engineered starters and premium batteries—faces a special hurdle in the U.S.: maintaining cabin cooling with the engine off.
There is some residual cooling from chilled air in the HVAC case, but in perhaps 5 to 15 s the airflow from the air-conditioning registers turns warm, unacceptably so in much of the U.S. during summer weather. Although A/C installations on lower-cost cars have gained in Europe, where peak ambient temperatures typically are lower, A/C is virtually across-the-board in the U.S.
There are three approaches to A/C operation with stop-start that are in the picture. On hybrids and electric vehicles (EVs), a high-voltage electric-motor compressor may be used. All EVs and many hybrids that have some amount of electric-drive operation use one, despite the estimated OE cost of $150 or more. The 12-V “micro-hybrids” with belt-driven A/C and stop-start systems in Europe may offer an “eco” mode or a default, in which the driver can allow the engine and A/C to shut down during idle stops, something typically chosen only in moderate ambient temperatures. An “eco” mode or default may include a temperature-tied algorithm, in which the engine restarts to restore A/C operation if the airflow turns very warm in hot weather. This is the expected approach for the early U.S. stop-start applications, including medium-voltage mild hybrids.
A third approach is use of thermal storage material (TSM) in a special evaporator design, and the first application is a Behr system on BMW 7 Series models in Europe. Although its cost premium for BMW as the pioneer has not been disclosed, it might eventually be lowered to as little as $10 per evaporator in very high-volume production, a knowledgeable engineering executive told AEI.
A competitive design from Delphi is being evaluated by other manufacturers, and Denso has said its design is in development. All three use a TSM, which goes through a phase change, solidifying when chilled (by the operating A/C) and liquefying, to cool air flowing through the evaporator when the A/C is off during an idle-stop. Although the electric-motor compressor is a moderate-cost addition to a hybrid vehicle already engineered in all respects for electric drive, the TSM evaporator still would represent a major cost saving, would be a positive feature on micro and mild hybrids, and for EVs would be expected to provide an increase in hot-weather range.
A Behr study indicates that a vehicle in city traffic is stopped 22% of the time, for an average of 23 s; the two systems provide considerably longer cooling times. The temperature of the air flowing from the A/C registers with the system off during an idle-stop does not necessarily match that with the system running nor is it intended to. Rather, the objective is to keep the flowing air temperature from rising into a discomfort range before the vehicle is moving with the engine and A/C running.
The Behr/BMW system has a pre-evaporator TSM section to provide about 40 s of effective idle-stop cooling at 30°C (86°F), but this is an engineering choice, as the duration could be increased by designing the evaporator to hold more TSM. However, the evaporator is designed primarily for engine-running A/C, so the dual-purpose design has to be balanced for maximum efficiency. The TSM refreezes to 70% at 25°C (77°F) in 30 s, and to 100% in 60 s. So the system can handle stop-start in heavy traffic. The Behr study showed an 8% improvement in fuel economy in city driving.
The Behr pre-evaporator holds the TSM in a middle section of tubing, and during A/C operation the refrigerant flows through outer tubing sections that sandwich it, freezing the TSM to create a “thermal battery.” When the engine stops, the A/C also stops, but the rise in temperature of the cabin airflow (including recirculated air) through the thermal battery section causes the TSM to melt and heat is transferred from the cabin air into the “battery” to provide continued air cooling.
The Delphi system places the TSM chamber in the tank end of the evaporator. When the A/C is stopped, so would the refrigerant flow. But a thermosiphon effect develops in the evaporator tubing as the vaporizing refrigerant rises toward the tank, where it flows through a tubing loop past the TSM chamber to transfer heat, where it cools and condenses, and then continues through the loop. A Delphi system under demonstration road test at 30°C provided more than 60 s of satisfactory system-off-cooling, taking 60 s to resolidify the TSM.
Although the design of the evaporator storage system varies by manufacturer, the state-of-the-art in TSM is thermal wax, used both by Behr and Delphi (Denso has not released details on its approach). The different thermal waxes reportedly are close enough in performance characteristics that the length of the system-off cooling time is primarily a result of the amount of wax used, according to Prasad S. Kadle, Delphi's Director of Advanced Engineering.
In addition to the thermal waxes, inorganic salts are another possible TSM, as they have a higher cold storage capacity. Inorganic salts—despite their greater heat storage capacity, low cost, and nonflammability—raise several concerns that have blocked their acceptance, Kadle said. One is the present need for formulas with additives that degrade, which prevent the salts from meeting the OE 15-year service life. The salts also are corrosive, although in an air-tight container, this is not considered an issue. Engineering work with salts is ongoing, so the industry has not given up on them.
Organic wax is not up against some inherent cold storage limit, and development of advanced waxes with increased storage capacity is possible, which would improve performance and/or enable a reduction in the amount of wax used. The latter could improve packaging of whatever design is used by an evaporator supplier.
Although the waxes have lower heat storage capacity than inorganic salts and are flammable, they freeze easily and the formulations are easily “tuned” for the desired temperature range. And the flammability issue is mitigated by the evaporator design and material, Kadle explained.
How effective the driver perceives the thermal storage cooling is dependent on the actual drive cycle of the car and the ambient temperature, factors which determine the extent to which the TSM melts and refreezes. If the idle-stops come in rapid-fire order and/or are lengthy, the TSM may not fully resolidify and the overall perceived cooling comfort will drop. But as a Delphi study showed, even with repetitive idle-stops including a final one as long as 2 min, the perceived comfort of the passengers with TSM was measurably greater.
Although all vehicle manufacturers to whom AEI spoke have said they are evaluating TSM evaporators, the companies also are looking at changeovers to the low-global-warming HFO-1234yf refrigerant, which entails some system redesign, including a new evaporator. So the likely initial major rollout of TSM is in the MY2015 time frame.