Charge Air Subcooling—Refrigerant-based charge air cooling for improved driving dynamics

MAHLE has already presented indirect and cascaded indirect charge air cooling, two methods for cooling charge air as effectively as possible. The temperature that can thereby be achieved is physically limited to remain above ambient temperature. By linking the coolant and refrigerant circuits in the charge air cooling system for the first time, MAHLE has broken through this barrier and achieves a substantial improvement in vehicle dynamics without affecting passenger comfort and without incurring higher fuel consumption.

Charge air that has been heated due to compression has several disadvantages for combustion: higher susceptibility to knocking, greater nitrogen oxide formation, and generally an increased thermal load on engine components. The density of the charge air also falls as the temperature rises, thus reducing the maximum quantity of oxygen allowed in the combustion chamber. Doubling the boost pressure from a starting point of 1 bar and 25°C leads to an increase in charge air density of only 50 percent. Cooling the charge air downstream to about 15 K above ambient temperature increases the density by another 40 percent. Subcooling the charge air, e.g., to 15 K below ambient temperature, would increase the charge air density by another 21 percent, which would correspond to a 200 mbar increase in boost pressure.

The air conditioning circuit in a vehicle is generally designed for rapidly cooling off when outside temperatures are high. Afterward, it is throttled back and operates with a reduced power consumption. Accordingly, after the initial cooling down, there is a surplus of capacity, often as much as 4 kW. Already now, the compressor’s power output is reduced in response to sudden step changes in load in order to minimize losses in the powertrain. Owing to the high thermal inertia of the refrigerant circuit, this is barely perceptible to the driver.

Instead, the surplus capacity can be transferred to the charge air subcooling system. To this end, the new iCAS (integrated Charge Air Subcooling) intake pipe from MAHLE is connected in parallel with the air conditioning evaporator via a chiller (refrigerant/coolant heat exchanger) with an expansion valve. The chiller feeds the integrated heat exchanger—the charge air subcooler—with “subcooled” coolant. A bypass valve allows switching between the subcooling function and regular direct charge air cooling.

The iCAS system from MAHLE has been tested on the test bench in a steady-state turbocharged 1.0 L three-cylinder gasoline engine with a charge air temperature of 10°C. The measurements indicate a torque improvement of 16 to 19 percent at the low end between 1,100 and 1,300 rpm in comparison with a charge air temperature of 40°C. Boost pressure can thereby be reduced by 200 mbar and the ignition angle can be optimized for efficiency by shifting it forward by 3° to 5° CA.

When evaluating fuel consumption, however, the additional power consumption of the air conditioning compressor must also be considered. The climate conditions in particular will affect the actual benefit with respect to responsiveness and fuel savings. Measurements show that at an ambient temperature of 25°C and relative humidity of 50 percent, the power consumption of the air conditioning compressor is about 0.85 kW for iCAS (30 K subcooling) and is thus less than a quarter of the power gained from the iCAS (at 1,300 rpm).

With an optimized design and suitable “loading strategy,” the ratio can be improved even further: while the iCAS is regeneratively conditioned during braking phases, the power consumption of the air conditioning compressor can be decoupled from the iCAS at the right time. The improved performance at 1,300 rpm thanks to iCAS can thus be increased to above 22 percent.

The MAHLE iCAS was installed in a compact-class demonstrator vehicle in order to verify the improved dynamics and to test the loading strategy based, for example, on braking energy recuperation. In a first test at 30°C ambient temperature (800 W/m2 of solar load) and 50 percent air humidity, an acceleration from 30 km/h to 50 km/h (in fourth gear) was simulated in a climatic wind tunnel. With the iCAS loaded and the compressor decoupled, the vehicle reaches the target speed 0.7 seconds earlier than the variant without iCAS. At the same time, the required boost pressure is achieved significantly sooner.

Especially in order to demonstrate the potential of braking energy recuperation via the air conditioning compressor, and thus feeding the energy into the iCAS, a hypothetical driving profile was simulated in the climatic wind tunnel. The vehicle accelerated from a constant 30 km/h to 130 km/h. After driving steadily at this speed, the vehicle braked down to 30 km/h again. The results of this test show that the energy recuperated via the fully engaged air conditioning compressor during moderate braking deceleration of–2 m/s2 can compensate for the energy consumed by the iCAS system during the acceleration phase. At more intense accelerations or shorter, more intense braking decelerations, for example, this energy can only be partially compensated for. Nevertheless the test demonstrates that it is possible and beneficial to condition the iCAS via recuperative braking energy, even within an NDEC or a WLPT cycle.

One of the fundamental requirements for acceptance of iCAS is that passenger comfort is not noticeably affected by the system. Therefore, the controls for both systems—air conditioning and charge air cooling—must be closely coordinated.

When maximum cooling down is required for high ambient and interior temperatures, therefore, the interior evaporator has priority over the iCAS. Only when the required discharge temperature of the HVAC system has been reached can the iCAS chiller be shortly engaged to avoid any negative influence on the evaporator’s temperature. Subsequently, the chiller is repeatedly engaged depending on the dis-charge temperature. This maintains the variance in the evaporator temperature within a range of ±3°C, and thus imperceptible to the driver, while also fully loading the iCAS system within two to four minutes during the cooling down phase. By then, both circuits have reached their respective target temperatures. The air conditioning compressor can now be throttled back in order to maintain the interior temperature.

In this controlled operating condition, MAHLE makes use of the thermal inertia of the refrigerant circuit in order to harmonize the different requirements of the iCAS and the interior air conditioning: on the one hand, the constant cooling of the vehicle interior, and on the other hand the power consumption of the iCAS cooling system, which depends on the driving power presently required. Owing to the thermal inertia, the air conditioning compressor can be decoupled from the refrigerant circuit for about 15 to 20 seconds with no perceptible detriment to the interior air conditioning. This duration is generally sufficient for loading the iCAS in a central European climate.

By linking the previously independent systems for air conditioning and engine cooling, MAHLE has been able to further improve the performance of turbocharged gasoline engines. In particular, drivability at low speeds—also known as lowend torque—can be massively improved. Fuel efficiency is increased by utilizing the surplus energy from the air conditioning system. Despite the increased power, the iCAS system does not lead to higher fuel consumption, especially thanks to the potential for recuperative preconditioning during braking phases. Air conditioning comfort in the vehicle interior is not noticeably affected.

  • The climate control and engine cooling systems are interconnected in order to further improve the performance of turbocharged gasoline engines without degrading passenger comfort.
  • Low-end torque in particular is massively increased, with improved fuel efficiency.
  • Recuperative preconditioning during braking phases especially raises the performance of the system.